Allelochemicals

For thousands of years insects and plants have been locked in a battle for which survival is the ultimate prize. For insects, plants constitute food sources for growth and development, and in some cases, sites for reproduction. On the other hand, plants attempt to counter insects feeding on their tissues (herbivory) so that their own vigorous growth and development will occur and lead to reproductive success. The consequences of this warfare are great and for humankind the outcomes of these battles may be of major economic significance in terms of the production of various foods. The welfare of various human populations can be threatened if hordes of ravenous insects consume specific crops that are the mainstays of these populations. But plants do not take this “lying down.” Over thousands of years, plants have done everything possible to make life miserable for insects. On the other hand, insects have returned the favor many times over. In recent geological time, plants and insects have changed their “spots” (plant-feeding strategies), resulting in an incredible point-counterpoint relationship of these organisms that is characterized by some remarkable developments.

As far as nutrients are concerned, different kinds of plants (species) are fairly similar and can provide an insect herbivore with the basic nutrients required for growth and development. These compounds (chemicals) are called primary compounds because they are required for the insect’s growth, development, and reproduction. All insects require these compounds and in theory they should be readily available from a wide variety of plant species. But most species of insects, rather than feeding on many different kinds of plants, limit their plant menu to a relatively small number of plant species (monophagy), most of which are related. Significantly, the limited preferences that insects have for their food plant species are due to non-nutritive compounds that usually vary from one plant group to another. These compounds are not related to the primary compounds identified with growth and development, and it is apparent that these plant-derived compounds (allelochemicals) generally have functions related to other species of organisms. These compounds are obviously not primary compounds but rather secondary compounds (non-nutritive) whose manufacture has been described as secondary metabolism. Indeed, these allelochemicals appear to be responsible for both the associations and non-associations that insects have with specific groups of plant species. In essence, it would be no exaggeration to state that the host-plant preferences of insects really reflect the ability of an insect species to either tolerate, or be repelled by, an allelochemical. Allelochemicals are not mysterious compounds but rather are a very important part of the everyday world, especially in terms of human food preferences. In a sense, the strong food preferences exhibited by insects are not so different from those of humans, with one striking exception. Many insect species are “locked in” to specific food plants, and these insects will reject a foreign plant species and die in the absence of their normal food plant. On the other hand, there is little evidence that human beings will subject themselves to starvation if their favorite foods are not readily available.

A World of Allelochemicals

Fruits and vegetables possess characteristic odors and tastes that create desire (preference) for these foods. Significantly, these tastes and smells are not identified with the primary compounds responsible for plant growth and development, such as sugars, fats, and proteins. Therefore, the plant has invested in producing a variety of chemicals that will not help it grow or reproduce. While an onion may possess a distinctive odor and taste for both insects and humans (not necessarily the same odor and taste for both), this fact hardly justifies the onion spending its energy and resources to produce an onion fragrance. On the other hand, if the taste and odor of onions combine to make this vegetable distasteful and repellent to most plant-feeding insects, then these allelochemicals perform a very vital function. In essence, it is generally believed that these secondary compounds are responsible for protecting plants from herbivores and possibly pathogens as well. A brief examination of some well-characterized allelochemicals offers a means of examining these compounds as agents of defense both as toxins and as repellents.

Oleander, which has a very limited number of herbivores, is extremely toxic because of the presence of allelochemicals that are somewhat related to cholesterol. The odor of the plant probably constitutes an early-warning system that makes potential herbivores aware of the danger of feeding on this plant. The same can be said for the tobacco plant which, like oleander, does not have too many insect herbivores. Leaves of the tobacco plant are quite toxic, but in some South American populations young children become addicted to the nicotine in the leaves before they are ten years old. Nitrogen-containing compounds (alkaloids) produced by opium poppies are powerful repellents for a wide range of insect species, and there is no doubt that compounds such as morphine and heroin, which are powerful human narcotics, were evolved to deter herbivores rather than to function as narcotics for humans.

Alkaloids such as nicotine have been adapted to function as insecticides, and a variety of plant products such as derris, rotenone, ryania, and sabadilla are also used as insecticides in different cultures. In some cases, allelochemicals such as prunasin in cherry leaves cause poisoning in livestock. Not to be outdone, humans have frequently utilized the alkaloid strychnine to murder people. However, it would be a mistake to lose track of the fact that, human abuses notwithstanding, these allelochemicals were evolved as plant protectants long before humans appeared. Obviously, allelochemicals do not provide plants with absolute protection against herbivores. Indeed, probably all plants containing allelochemicals are fed upon by insects, and in many cases these herbivores are only found on a limited number of host plant species. For example, monarch butterfly caterpillars are limited to the milkweed species. Bark beetles limit their attacks to pines and related conifers, developing in environments that are rich in toxic turpentines. These insects have breached the chemical defenses of their hosts, and in so doing, they have “captured” specific kinds of food plants that are either repellent or highly toxic to most other species of insects. Guaranteed these “forbidden fruits,” these herbivores should have to share their food resources with very limited numbers of competitors. Barring an ecological disaster does not devastate the populations of their host plant species, this specialization should have much to recommend it. On the other hand, many insect species choose a lifestyle which is characterized by feeding on a variety of unrelated plant species.

Insects like the monarch butterfly and bark beetles that are restricted to a limited number of related plant species are referred to as specialists.

These herbivores have become resistant to the toxic effects of their host plant allelochemicals, and in many cases they appear to be completely immune to the plant toxins they ingest. In the case of monarch caterpillars feeding on milkweeds, it has been demonstrated that these larvae actually grow more rapidly on milkweed plants containing the highest concentration of toxins.

Indeed, allelochemical concentrations may be generally quite high, often averaging 5–10% of the dry weight of the plant. By contrast, plantfeeding generalists feed on a wide range of plant species, often unrelated. However, in general, these herbivores select plant species in which the concentrations of allelochemicals are not too high, enabling them to process low levels of a wide variety of plant toxins.

The immunity of specialists to the toxic effects of the allelochemicals in their diets demonstrates that for these insects these compounds can no longer be considered poisons. Surprisingly, the basis for this important allelochemical resistance, which has great economic significance, was only understood about thirty years ago.

Sequestration and its Consequences

Insects such as the monarch butterfly store compounds in their tissues that render them unpalatable to predators. These compounds, the cardenolides, were ingested by the larvae from their milkweed food plants, and retained in their bodies into the adult stage. The storage of these milkweed compounds is called sequestration, and constitutes a widespread phenomenon among specialists feeding on allelochemical-rich plants. In a sense, sequestration represents the insect’s success in utilizing the plant’s chemical defenses for its own purposes. Indeed, sequestration can be regarded as a form of detoxication since potentially toxic compounds are removed from the circulation and stored in the tissues.

Sequestration has been detected in at least seven orders of insects including species of toxic grasshoppers, aphids, lacewings, beetles, wasps, butterflies and moths. In general, these insects are brightly (= warningly) colored, a characteristic described as aposematic. Armed with the toxins from their food plants, large insects such as brilliantly colored grasshoppers move very slowly, as if to advertise their poisonous qualities to the world. Obviously the term toxic is relative, since these insects routinely sequester these allelochemicals during normal feeding. However, since these specialists are physiologically adapted for ingesting these compounds, their ability to tolerate these allelochemicals is really not surprising. On the other hand, non-adapted species (e.g., predators) would certainly encounter toxic reactions if they ingested these toxic plant products.

The fates of allelochemicals, which are usually present in mixtures, are not at all predictable aſter ingestion by an adapted herbivore. Although many compounds are sequestered immediately aſter ingestion, others may be metabolized before being stored, or even eliminated aſter being metabolized. In other cases selected allelochemicals in a mixture may be absorbed and sequestered whereas other compounds in the mixture may be eliminated immediately. An examination of the options for initially processing ingested allelochemicals emphasizes the versatility of specialists in treating the toxic compounds produced by their food plants.

Sequestration of Insect Toxins by Vertebrates: A Significant

“Allelochemical” Phenomenon

It has become evident that the allelochemical relationship of insects and plants is paralleled by a similar relationship of amphibians and insects. It is now recognized that the sequestration of ingested toxic insect compounds by vertebrates differs little from this phenomenon in insect herbivores and plants. In essence, a variety of insect toxins is sequestered by amphibians and these compounds have similar protective functions for frogs and insects (see Allelochemicals as phagostimulants). Frogs exploit insect allomones (defensive compounds) as if they were animal “allelochemicals,” and it seems worthwhile to emphasize this congruency in examining the scope of allelochemistry.

Frogs in the genus Dendrobates contain mono-, di-, and tricyclic alkaloids which are clearly of ant origin. The alkaloids, termed pumiliotoxins, appear to be products of ant species in the genera Brachymyrmex and Paretrechina and constitute the only known dietary source of alkaloids of these frogs, not unlike the specialist insects feeding on narrow plant diets enriched with allelochemicals. The same phenomenon has been described for the myrmicine ant Myrmicaria melanogaster which synthesizes ten alkaloids. Some of these alkaloids have previously been identified in a dendrobatid frog and a toad.

Neurotoxic steroidal alkaloids, the batrachotoxins, have been isolated from New Guinea birds in the genera Pitohui and Ifrita. These compounds are among the most toxic natural substances known, and they are not produced by captive birds, suggesting a dietary source. Recently, the batrachotoxins were identified in beetles in the genus Chloresine (Melyridae) which are normally fed on by the bird species. Since the genus Chloresine is cosmopolitan, it is the possible source of some of the avian alkaloids found in birds in different areas.

Vertebrate sequestration of alkaloids from insects has only recently been explored. Clearly this chemical storage has a common denominator with sequestration of alkaloids by insects (see Allelochemicals as pheromonal precursors) and should be examined as a paradigm of comparative physiology. Clearly, insects are pivotal to both systems, either as food for vertebrates or food for insects, with sequestration the major common feature.

Initial Processing of Allelochemicals by Specialists

Once an adapted insect has ingested an allelochemical, a menu of options is available for processing it. An insect species may utilize a variety of adaptive strategies for processing a single compound that is characteristic of the host plant defense.

Immediate Allelochemical Excretion

Some insects essentially fail to absorb ingested allelochemicals from the gut. These compounds are excreted directly and are concentrated in the feces. A lymantriid moth larva that is a specialist on the coca plant, which is the source of the alkaloid cocaine, rapidly excretes this compound with only traces being found in the blood. However, cocaine may still have defensive value for the larva as part of an oral regurgitate that is externalized when the larva is disturbed.

Three different species of moth larvae that feed on tobacco plants rapidly excrete nicotine, a very toxic and reactive alkaloid. There is no evidence that nicotine is absorbed from the gut of any tobacco feeder, but as is the case for the moth larva excreting cocaine, nicotine in oral or anal exudates constitutes an excellent defensive compound.

Allelochemical Metabolism

Many insect specialists rapidly metabolize ingested allelochemicals which are then sequestered, or in some cases excreted. Nicotine, which is both highly reactive and very toxic, is converted to a non-toxic metabolite called cotinine by both tobacco-feeding insects and those that are not tobacco feeders. Since cotinine has virtually no toxicity to insects, it is probable that its production from nicotine constitutes true detoxication.

Cabbage-feeding insects feed on plants that are rich in sinigrin, a compound that yields a highly toxic mustard oil when metabolized.

Although sinigrin can be sequestered without generating the reactive mustard oil in a variety of cabbage-feeding species, cabbage butterflies (whites) actually break down sinigrin and sequester the highly reactive mustard oil. For these butterflies, the mustard oil is more suitable for storage than sinigrin.

Larvae of the tiger moth Seirarctia have evolved a novel strategy for coping with the toxic effects of MAM, a compound derived from cycasin which is a constituent in the cycad leaves upon which they feed. When larvae encounter MAM, they convert it to cycasin which is absorbed through the gut wall before being sequestered. Since the enzyme that produces cycasin or MAM is only found in the gut, once cycasin crosses the gut wall into the blood prior to sequestration there is no chance of MAM being generated from cycasin.

Many species of moths, butterflies, and grasshoppers feed on plant species that produce extremely toxic compounds known as pyrrolizidine alkaloids. These alkaloids, present as mixtures, are frequently sequestered by these specialist insects and, in some cases, metabolized plant compounds are the preferred storage forms. For example, larvae of the tiger moth (Tyria species) feed on ragwort and primarily sequester the alkaloid seneciphylline, although this compound is present in the plant as the N-oxide. Conversely, the grasshoppers of the Zonocerus species convert the ingested alkaloid monocrotaline to its N-oxide before sequestering the compound.

Insects feeding on milkweed metabolize the toxic cardenolides (steroids) produced by these plants, converting them into compounds that can be readily sequestered. The milkweed bug (Oncopeltus species) oxidizes cardenolides as a mechanism for converting these steroids into compounds that can be efficiently sequestered. Similarly, larvae of the monarch butterfly store metabolized cardenolides in tissues aſter oxidizing these compounds into suitable chemical forms for sequestration.

Selective Biomagnification of Allelochemicals in Tissues

There is little indication that the profiles of insectstored allelochemicals in any way mirror those of their host plant. In a sense, each insect species treats ingested allelochemicals distinctively, so that a compound totally excreted by one species may constitute the main sequestration product of another.

The very toxic grasshopper Poekilocerus bufonius sequesters only two of the cardenolides that it ingests from its milkweed food plant. Similar selectivity is shown by moth larvae (Syntomeida species) which sequester oleandrin, the main steroid found in the leaves of oleander. On the other hand, a variety of other insects feeding on oleander leaves do not sequester oleandrin.

Similar unpredictability characterizes the sequestration of pyrrolizidine alkaloids by moth larvae. Tiger moths (Amphicallia species) sequester the alkaloids crispatine and trichodesmine, whereas the main alkaloid present is crosemperine. Another tiger moth (Tyria species) concentrates senecionine in its tissues in spite of the fact that this compound is a trace constituent in the leaves. Tyria is no less curious as a sequestrator because it stores jacobine, jacozine, and jacoline as minor constituents in adults, yet these three compounds are major alkaloids in the leaves.

The Diverse Functions of

“Captured” Allelochemicals

While highly concentrated allelochemicals may constitute a major deterrent to non-adapted insects, these compounds can represent a real treasure trove for species for which these plant products are non-toxic. Indeed, in the course of exploiting for their own protection compounds that are repellent or toxic to most insect species, specialists have gone beyond the point of simply being resistant to allelochemicals. In many cases, a variety of specialist species have utilized the rich allelochemical pool that is available in order to develop a menu of remarkable functions.

Insect Sequestration of Bacterial Compounds and their Glandular Secretion

Prokaryotes (bacterial types) are almost everywhere and their widespread association with insects is certainly well established. But the bases for these diverse bacteria-insect relationships are, for the most part, terra incognita. However, very recent research suggests one very surprising function for bacteria in insect glands.

All major types of metabolism evolved in prokaryotes and the success of these organisms was both cause and effect of changing environments on earth. If these bacteria are sequestered in insect secretory glands, their great metabolic abilities could be utilized to biosynthesize bacterial allelochemicals which could be used as potent defensive compounds. This possibility appears to have been realized as a product of the virtual ubiquity of both insects and their biosynthetically versatile prokaryotes.

Predaceous diving beetles (Dytiscus species) are distinguished by their ability to produce defensive steroids, some of which are novel animal products that are limited to species of diving beetles. Furthermore, insects do not synthesize cholesterol which in insects must be obtained from exogenous sterols. However, it now appears that the surprising steroidal versatility of dytiscids may reflect the biosynthetic elegance of bacteria rather than insects.

Adult diving beetles may contain concentrations of at least 10 bacterial species, mostly detected in a variety of organs. Culturing individual bacterial species resulted in the identification of diverse steroids that had previously been characterized in the prothoracic defensive glands of the adults. The steroid-rich secretions of these glands function as vertebrate deterrents that can cause emesis of fish that swallow these beetles. If the dytiscid-bacterial association is typical of a variety of insect species and their bacterial symbiotes, then a multitude of insect-bacterial relationships may require reevaluation of possible examples of insect sequestration of bacterial allelochemists.

Non-pathogenic bacteria are commonly housed in insects and, in a sense, these prokaryotes are sequestered by their insect hosts. Furthermore, if the bacteria synthesize toxic compounds which may be externalized from a defensive gland (prothoracic glands of dytiscids), then the bacterial products may be regarded as bacterial allelochemicals that have been sequestered. Indeed, bacterial compounds of symbiotic bacteria of insects clearly constitute an unrecognized group of allelochemicals.

Additives in Defensive Glands

Milkweed bugs (Oncopeltus species) add cardenolides, derived from their milkweed host plants, to their thoracic defensive gland secretion which considerably enhances the deterrency of their secretion. Similarly, a warningly colored generalist, the lubber grasshopper (Romalea guttata) incorporates a large number of allelochemicals derived from a variety of plant species into its thoracic gland secretion. This grasshopper generally feeds on plants with low concentrations of allelochemicals, but if it is fed high concentrations of plants with known repellents (e.g., onion), the odorous secretion can be highly deterrent.

Another toxic grasshopper, Poekilocerus bufonius, utilizes allelochemicals as the mainstay of its defensive secretion. This aposematic (very warningly colored) insect sequesters two of six cardenolides from its milkweed diet which are the major irritants in the secretion when it is sprayed at adversaries. Utilization of allelochemicals as defensive gland constituents is particularly pronounced in the swallowtail larvae of Atrophaneura alcinous, which feed on leaves that are rich in toxic aristolochic acids. Seven aristolochic acids are sequestered by the larvae and all are transferred to the defensive gland in the head. The acids are concentrated in the gland and are the major deterrents for birds.

Regurgitation and Defecation of Allelochemicals

The intestines of stimulated grasshoppers can discharge ingested plant products which may serve as repellents for predators. Regurgitated allelochemicals can effectively repel ants, as is the case for anal discharges from the hind gut. When tactually stimulated, the milkweed bug, Oncopeltus fasciatus, also defecates a solution containing repellent allelochemicals. In this case, they are cardenolides ingested from their milkweed food plant.

Allelochemicals as Tissue Colorants

The cuticular (skin) coloration of many insects is diet-dependent and is highly adaptive since it enables the insect to respond in a positive way to its background color. Diet-induced changes may result in the insect being cryptic (background matching), whereas aposematic species can be background contrasting. Background quality, which is of great survival value, appears to be controlled by allelochemicals that are widespread in the diets of moths, butterflies and true bugs. These insects are particularly sensitive to the carotenoids (e.g., tomato red) that fortify their host plants.

If the large white butterfly, Pieris brassicae, is reared on its normal diet of cabbage leaves, the pupae are green and contrast with their background. This toxic insect contains high concentrations of carotenoids, and the carotenoid lutein is concentrated in the cuticle. On the other hand, if these insects are reared on an artificial diet lacking carotenoids, they possess a turquoise-blue coloration and exhibit no response to background. In the absence of carotenoids, these insects are quite conspicuous on their background and could be readily detected by predators.

Allelochemicals as Inhibitors of Toxin Production

Some plant toxins are present in plants in an inactive form only to be converted to toxic compounds aſter ingestion by herbivores. This is particularly true for many cyanogens (cyanidecontaining toxins) that generate cyanide when the leaf surface is broken as would occur with a plant feeder. It now appears that cyanogenesis (producing cyanide) in damaged leaves may be inhibited by allelochemicals that are compartmentally isolated from the cyanogens in the intact leaves.

Leaves of papaya, Carica papaya, contain two cyanogens that yield hydrogen cyanide aſter enzymatic attack. However, tannins, which are widely distributed in plants, inhibit the release of cyanide caused by the action of enzymes that attack the cyanogens. Insects attacking plants containing cyanogens may have adapted tannins to prevent cyanide release, a strategy that may be suitable for other plant groups that yield toxic products aſter leaf damage.

Allelochemicals as Pheromonal Precursors

Bark beetles (Scolytidae) in the genera Dendroctonus and Ips convert the hydrocarbons produced by their pine hosts into alcohols that are utilized as either aggregation or sex pheromones (communication compounds) by the attacking beetles. Similarly, butterflies in the family Nymphalidae and moths in the family Arctiidae convert pyrrolizidine alkaloids (PAs) into sex pheromones that are especially critical during courtship. The PAs may be collected from damaged plants by males to be transformed into sexual pheromones that constitute the key to reproductive success. For these males, the allelochemicals (PAs) are identified with reproductive fitness.

Allelochemicals as Structural Paint

Some insects actually “paint” structures with ingested compounds possessing considerable biological activity. Larvae of the parsnip webworm, Depressaria pastinacella, apply ingested allelochemicals to silk-webbed flowers that serve as housing units. The applied compounds are derived from wild parsnip, a food plant that is rich in highly toxic furanocoumarins. These compounds are sequestered in the silk glands before being applied to the flowers in which the larvae reside. Since the larvae are quite sensitive to ultraviolet light, the presence of UV-absorbing furanocoumarins on their silken housing is highly adaptive. In addition, because these allelochemicals possess pronounced antimicrobial activity against bacteria and fungi, their presence on the silk can act as a major barrier to pathogens.

Allelochemicals as Metabolites in Primary Metabolic Pathways

Some specialist herbivores metabolize the characteristic allelochemicals in their host plants into compounds that are of major significance in growth and development. In essence, these specialists exploit their food plants by utilizing not only their primary nutrients for growth and development, but their allelochemicals as well.

Larvae of the bruchid beetle, Carydes brasiliensis, develop exclusively on seeds of a legume (pea family) that contains canavanine, a foreign amino acid related to arginine. Canavanine is highly toxic when incorporated into proteins by non-adapted herbivores. On the other hand, larvae of C. brasiliensis metabolize canavanine into products of great metabolic significance. Large amounts of ammonia are generated for fixation into organic compounds, and an amino acid is produced from canavanine for ready metabolism. Thus, the very toxic allelochemical of the legume has been thoroughly exploited by the beetle larvae as a source for key nutrients.

Beetle larvae in the genus Chrysomela also convert a toxic allelochemical into a metabolite with considerable importance in growth and development. These larvae feed on leaves of willow (Salix), a rich source of salicin, a toxic metabolite. Metabolism of salicin yields a very effective defensive compound that is sequestered by the larvae in defensive glands. In addition, this metabolism generates enough glucose to account for about one-third of the daily caloric requirements of the larvae. Salicin should be regarded as an allelochemical nutrient.

Allelochemicals as Agents of Sexual Development

Tiger moths in the genus Creatonotus feed on plant species that produce high concentrations of pyrrolizidine alkaloids (PAs). These compounds are converted to sex pheromones by the males. Additionally, these allelochemicals control the development of important secondary sexual characters called coremata. The coremata are eversible andraconial (male) organs that are the source of the volatile sex pheromones of the males, and their degree of development is controlled by the amount of PAs ingested by the developing larvae. In effect, PAs are functioning as male hormones that regulate both sex pheromone production and development of the coremata.

Allelochemical Discharge from Nonglandular Reservoirs

Some insects sequester ingested allelochemicals in non-glandular reservoirs that can be evacuated upon demand. Gregarious larvae of the European pine sawfly, Neodiprion sertifer, sequester toxic turpentine terpenes in foregut pouches. These pinederived compounds can be discharged upon demand to function as highly effective predator deterrents. Similarly, lygaeids such as the milkweed bug, Oncopeltus fasciatus, sequester cardenolides from their milkweed hosts in dorsolateral spaces on the thorax and abdomen. Significantly, high concentrations of cardenolides are stored in these spaces, resulting in a concentrated deterrent discharge which repels potential predators.

Allelochemicals as Defensive Agents of Eggs

Insects ingesting allelochemicals often utilize these compounds as protectants for the next generation of insects. These plant compounds may be sequestered in the eggs in order to provide a formidable defense against predators and pathogens. The insect embryo must be resistant to the toxic effects of the allelochemicals that have been sequestered in the reproductive system. For example, chrysomelid beetle adults feeding on willow and poplar sequester the toxic allelochemical salicin which is used to fortify the eggs. Salicin has different functions in the embryo and the larvae. For the embryo, salicin is a deterrent toxin which can kill ants. For the young larvae, salicin is converted to salicylaldehyde, a powerful repellent that is not frequently encountered in insects. A wide variety of allelochemicals are sequestered in insect eggs which includes pyrrolizidine alkaloids, aristolochic acids, cannabinoids, quinones, cardenolides and mustard oils. It is evident that the females of a large number of species have appropriated their host-plant defenses (allelochemicals) for protection of their eggs.

Allelochemicals as a Copulatory Bonus

Females may obtain allelochemicals suitable for their own protection and that of their eggs from the seminal ejaculate. For example, males of ithomiine butterflies gather pyrrolizidine alkaloids (PAs) from flowers and decomposing foliage and about half of the PAs are channeled to the spermatophore (sperm packet) that is transferred to the female during copulation. Since the females are rarely found feeding on alkaloid sources, the copulatory bonus ensures these toxic allelochemicals will be available to protect both the female and her eggs. It is also very significant that the resistance of the spermatozoa to the known toxic effects of the pyrrolizine alkaloids enables copulatory bonus strategy to be highly adaptive.

Allelochemicals as Synergists for Pheromones

The intimate relationship of specialist insects their food plants is exemplified by the turnip aphid, Lipaphis erysimi, and its alarm pheromone. aphid is typical of many aphid species. Paired glands near the tip of the abdomen secrete an alarm pheromone that causes both adults and larvae to disperse and drop off of the food plant. The alarm pheromones synthesized by the aphids are key communications chemicals that enable these insects “abandon ship” when threatened by a predator. Surprisingly, (E)-B-farnesene, the major alarm for a large variety of aphid species, is only weakly active when secreted by the turnip aphid. However, the activity of this pheromonal secretion is increased appreciably by allelochemicals that act as powerful synergists for the major alarm pheromone. synergists are derived from typical food plant compounds that have been modified by the aphids.

Allelochemicals as Phagostimulants

The close relationship of insect specialists their allelochemicals is further demonstrated some species of sawflies and chrysomelid beetles which feed on very bitter food plants. Adults the turnip sawfly, Athalia rosae, feed on the surface of a plant that is not a larval food plant. Compounds in the leaf surface that are responsible for their bitter taste are powerful phagostimulants for

A. rosae. In addition, these bitter compounds incorporated into the cuticle, thus providing these sawflies with a cuticular set of “armor” to protect against aggressive predators.

Similarly, species in three genera of chrysomelid beetles utilize cucurbitacins, compounds found in their squash and pumpkin hosts, as phagostimulants that are biomagnified in their bodies. The beetles are rendered distasteful and, as is the case for the sawflies, the allelochemicals possess dual roles that both induce ingestion and promote sequestration of highly distasteful compounds.

Allelochemicals as Inducers of Detoxifying Enzymes

Both generalist and specialist insects can encounter a diversity of allelochemicals with varying degrees of toxicity. For generalists this is particularly true since a generalist diet can sample a wide variety of plant species containing a large diversity of allelochemicals. On the other hand, specialists may encounter fewer allelochemicals but it is likely that these compounds will be at high concentrations. In the case of both feeding modes, it is obviously necessary to possess mechanisms for blunting the toxic properties of the ingested allelochemicals. Detoxication would appear to constitute the key process for neutralizing the toxicities of ingested allelochemicals. The enzymes chiefly identified with converting allelochemicals into less toxic compounds are the mixed-function oxidases, particularly cytochrome P-450.

Mixed-function oxidases metabolize fat- soluble toxins into water-soluble ones that can be excreted. The level of these enzymes may determine the tolerance of an insect for a particular allelochemical. For a generalist ingesting a large diversity of allelochemicals derived from many plant species, the induction of a variety of these oxidases would promote the possibility of detoxifying many kinds of plant compounds. For a specialist, fewer oxidases at very high levels would enable the herbivore to detoxify the very high concentrations of allelochemicals in its restricted food plants.

Mixed-function oxidases play a key role in protecting the southern armyworm, Spodoptera eridania, from a host of allelochemicals. Unrelated plant compounds rapidly induce enzymatic in creases of 2 to 3-fold in larvae. Significantly, the rise in P-450 activity is immediate and proceeds rapidly over much of its course during the first few hours. These results strongly suggest that P-450 induction is critical to allelochemical tolerance.

Allelochemicals as Allomonal Precursors

In some cases, insects have produced powerful repellents from allelochemicals in their food plants and have thus exploited the plant’s defensive chemistry in a very efficient way. Such a strategy is particularly adaptive because the insect has benefited both nutritionally and defensively from feeding on its host.

Host plant exploitation is particularly pronounced in some chrysomelid beetle larvae in the genera Chrysomela and Phratora. The larvae feed on willow and poplar leaves, both of which contain salicin, a well known feeding deterrent for non-adapted species. The beetle larvae convert salicin to salicylaldehyde and glucose, utilizing the former for defense and the latter for growth. For these chrysomelid larvae, the conversion of salicin to salicylaldehyde is doubly beneficial. Very little energy is used to synthesize salicylaldehyde, compared to what is required to produce other defensive compounds that must be totally synthesized. Because salicylaldehyde is a far more effective repellent than salicin, the beetle larvae receive very important double bonus by converting the allelochemical into a compound that can be readily stored and secreted from the defensive glands.

Allelochemicals as Communicative “Jamming” Agents

In theory, plant species could reduce or eliminate herbivory if the plants generated volatile compounds identical to or similar to the pheromones utilized by herbivores as signals. If these signals were behaviorally disruptive, feeding could be appreciably diminished, to say the least.

The wild potato, Solanum berthaultii, has effectively “jammed” the pheromonal alarm signal of its potential aphid herbivore. (E)-B-farnesene, an alarm pheromone of the aphid Myzus persicae, is also produced by wild potatoes, resulting in repellency and dispersion of the aphids. In effect, the potato has exploited the aphid’s herbivory by utilizing a highly disruptive compound that has been evolved by aphids as a warning signal.

Quenchers of Phototoxic Allelochemicals

Diverse plant species produce photo-activated compounds that are highly toxic to insects aſter digestion. In essence, these compounds generate highly toxic species of oxygen that attack key biochemicals such as nucleic acids. On the other hand, if the herbivore simultaneously ingests allelochemicals that are effective quenchers of toxic oxygen species along with the phototoxins, then survival and prosperity are possible. The availability of these allelochemical antioxidants has enabled some insect species to utilize food plants that are “forbidden fruits” for most herbivores.

Larvae of the tobacco hornworm feed on a variety of plant species that contain the phototoxin α-terthienyl, a constituent of many species of asters (Asteraceae). However, the additional ingestion of

β-carotene reduces mortality from 55% (controls) to 3% (+carotene) during 48 h. β-carotene, an effective quencher of toxic oxygen species, is concentrated in the tissues of the larvae where it can serve as a potent antioxidant for photoactivated toxins found in its food plant.

Antibiotic Functions of Allelochemicals

The demonstrated range of allelochemicals against insect-associated viruses, fungi and bacteria makes it probable that these arthropods have commonly exploited plant compounds as key elements in their phytochemical defenses.

A compound commonly produced by conifers is a-pinene, which inhibits diverse microorganisms including the insect pathogen Bacillus thuringiensis. Along with several related compounds, α-pinene reduces the infectivity of B. thuringiensis for larvae of the Douglas fir tussock moth, Orgyia pseudotsugata. At concentrations approximating those found in fir needles, a-pinene increases the 50% lethal dose for B. thuringiensis by 700-fold.

A pathogenic fungus, Nomuraea rileyi, frequently attacks lepidopterous (moth) larvae such as the corn earworm, Helicoverpa zea. However, the pathogenicity to this larva can be reduced if the moth ingests a tomato alkaloid, α-tomatine. If the larvae ingest α-tomatine prior to exposure to fungal conidia, it increases larval survivorship considerably. The alkaloid is a further asset to H. zea because it is quite toxic to larval parasites of the corn earworm.

The pathogenicity of viral pathogens of H. zea can also be compromised by host plant allelochemicals. Chlorogenic acid, a common plant compound, is oxidized to chlorogenoquinone by plant enzymes, and this oxidation product binds to a nuclear polyhedrosis virus. Binding to this baculovirus results in a reduction in digestibility and a decrease in infectivity. Furthermore, it appears that the liberation of infective virons in the midgut, which is a requirement for successful infection, is impaired by the binding of chlorogenoquinone to the baculovirus.

Specialists and Generalists: Two Selected Case Studies

Although specialists and generalists may be highly efficient sequestrators, the storage characteristics of both groups differ considerably. Some insights into how these insects manipulate the allelochemicals in their diets have been provided by recent studies of the fates of a variety of ingested plant chemicals. An analysis of these studies demonstrates that the particulars of sequestration are, if nothing else, very unpredictable.

The Monarch Butterfly, Danaus plexippus

The monarch is a specialist that feeds exclusively on different species of milkweeds. Milkweeds contain steroids called cardenolides, which are somewhat related to vertebrate hormones such as testosterone. These compounds are toxic and highly emetic, vomiting oſten following their ingestion by non-adapted species.

Polar (water soluble) cardenolides are sequestered in the large volume of gut fluid possessed by the larvae. Sequestration is much more efficient from plants with low level cardenolide concentrations than with high concentrations. Significantly for the monarch, and not necessarily for other milkweed feeders, it is the large volume of gut fluid that makes it possible to feed and develop on these plants.

The cardenolide-rich gut fluid, which may exceed one-third of the larva’s total liquid volume, is withdrawn at pupation to become part of the hemolymph (blood) pool, stored primarily under the wings. Subsequently, the wing scales (bird predators beware), along with the hemolymph, become the richest sources of cardenolides in the body aſter being withdrawn from the gut fluid. The volume of gut fluid decreases before pupation only to increase again before pupal molting. Again, gut fluid diminishes during pupal development only to increase again in the new adult. The cardenoliderich gut fluid is again converted to hemolymph during adult development so that very little remains to be lost when the newly developed adult evacuates accumulated waste products from its gut.

The polar cardenolides in the gut fluid clearly are the source of the defensive compounds manipulated by the monarch at all stages. The larval and pupal exuviate (cast skins) eliminated after molting are an excretory form for the cardenolides, as is the case for these compounds in the wing scales. Excretion notwithstanding, the ability of all life stages to manipulate the cardenolide pool is quite pronounced. This is evident in 2-day-old pupae that contain low concentrations of cardenolides in the gut fluid but high concentrations in the hemolymph. However, before wing expansion in the newly developed adult, the cardenolide level in the hemolymph is at its lowest, only to increase to the highest level in any life stage.

The presence of high levels of cardenolides in the blood of the adult demonstrates that these compounds are not locked in tissues but rather are circulating freely, possibly to be utilized upon demand. The warningly colored (aposematic) adult monarch utilizes a defensive system based on compounds that it did not ingest as an adult. Although the complexities of cardenolide sequestration in this species are evident, it is highly significant to understand these ingested steroids are an extraordinarily dynamic state.

The Lubber Grasshopper, Romalea microptera (also known as R. guttata)

This large grasshopper found in the southeastern United States is quite conspicuous because of it red, black and yellow coloration. It is one of the most aposematic (warningly colored) species in its habitat. This brightly colored grasshopper is especially distinctive because it is a generalist that feeds on a very wide range of plants belonging to a variety of species. Lubber grasshopper is known to feed on 104 plant species belonging to 38 families, many of which produce toxic allelochemicals. Both immature and mature grasshoppers are capable of causing emesis in predators such as lizards, demonstrating that all stages of these insects are protected from at least some predatory vertebrates.

Immature individuals of R. microptera produce defensive compounds that cause emesis in both lizard and bird predators. Additionally, mature and adult grasshoppers secrete defensive compounds from paired tracheal (respiratory) glands in the metathorax. These glands only become active near the adult period and their secretion can be extremely repellent to small predatory insects such as ants. At least 50 compounds are produced by the defensive glands, the secretions varying intraspecifically, so that components of females of the same age and population sometimes differ by 70-fold, with some compounds being absent in certain individuals. However, in addition to the compounds synthesized in the metathoracic glands, a number of allelochemicals are sequestered in these glands as a reflection of an individual grasshopper’s diet. Indeed, the composition of the metathoracic gland secretion of each grasshopper appears to be unlike that of any other grasshopper, since no two of these generalist grasshoppers have identical diets from which to sequester allelochemicals. For a predator, each lubber secretion may be sufficiently distinctive to make it impossible to learn an olfactory pattern that clearly identifies the prey as lubber grasshopper.

Lubber grasshopper is unusual in being a polyphagous (eating many plant species) insect species that sequesters allelochemicals. In general, monophagous (feeding on one group of plant species) and stenophagous (feeding on a limited range of plant species) insect herbivores characteristically sequester plant compounds, but not generalist feeders. Furthermore, if R. microptera is presented with a restricted diet (specialist feeding mode), the number of compounds in the secretions and their concentrations are reduced, and the relative composition of the secretion is markedly different from that of field-collected grasshoppers. Significantly, if grasshoppers are presented with only a single-host plant as a food source, they frequently feed readily, sequestering host-plant volatiles, and exhibit no immediate ill effects. Lubbers feeding only on wild onion sequester a large number of onion volatiles which impart a strong onion odor to the secretion. The secretion is a powerful repellent to hungry ants and is considerably more active than the secretions of fieldcollected grasshoppers. Compounds in other single-plant diets (e.g., catnip) produce secretions that are similarly active.

The secretion of lubber grasshopper clearly has both a dietary and an individual origin that correlates with great variations in secretory components. The possibility that these grasshoppers can temporarily switch to a monophagous feeding mode in the presence of a preferred host plant is not unreasonable, and can result in a secretion with a high a concentration of sequestered allelochemicals as is characteristic of some specialist insects.

Lubber grasshopper is quite unpalatable and emetic to a variety of vertebrates, especially birds. Diverse bird species have been demonstrated to vomit aſter ingestion of these grasshoppers, presumably as a consequence of Romalea-synthesized toxins that fortify their bodies. While Romalea would appear to be completely defended against birds, as is oſten the case, the best defense has been overcome by a better offense. Shrikes, predatory birds that impale their insect prey on spines or even barbed wire, capture lubber grasshoppers and impale them. However, the birds wait for about 48 h before they remove and eat the grasshoppers. Though shrikes “store” all their food in this manner, in all likelihood the emetic toxin(s) produced by Romalea decomposes during the time the grasshopper is impaled.