Adaptation of Indigenous Insects to Introduced Crops

Host range expansion, or adaptation of insects to new crops, is a world-wide phenomenon that has been observed repeatedly and extensively. It is particularly well documented in North America, where forests and prairies consisting of indigenous plants were planted extensively to introduced cultivated crops only aſter European emigrants arrived in the eighteenth century. Although many new insect pests were also accidentally introduced from Europe into Canada and the United States, many species of native insects adapted to the new crops and became economically important pests (Table 2).

Prior to widespread introduction of cultivated crops, some species of native insects fed on a wide range of plants and therefore might be expected to accept the new crops readily. Although polyphagous insects such as grasshoppers (Orthoptera: Acrididae), wireworms (Coleoptera: Elateridae), and cutworms (Lepidoptera: Noctuidae) readily accepted corn, wheat and other crops, not all of the species within these groups became agricultural pests. For example, several hundred species of cutworms and grasshoppers are present, but only about a dozen species in each group have achieved regular pest status.

Other native insect species had a narrower host range, and therefore adapted to a more narrow range of crops, or perhaps only a single crop. In the south the boll weevil, Anthonomus grandis grandis Boheman (Coleoptera: Curcullionidae), originally fed on native plants in Mexico related to cultivated cotton, and dispersed northward into the new cotton belt of the southeastern United States as cotton was planted extensively. In the north, the wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), fed on hollowstemmed wild grasses, and within 10 years aſter tillage began in Alberta, wheat was damaged from the Canadian prairie provinces south into Montana and North Dakota. In the eastern United States the apple maggot, Rhagoletis pomonella (Walsh) (Diptera: Tep hritidae), expanded its host range to included newly introduced cultivated fruits, especially apples. The Colorado potato beetle, Leptinotarsa decemlineata Say (Coleoptera: Chrysomelidae) originally fed on solanaceous weeds in Mexico or the southwestern United States, and quickly spread across the United States on potatoes once it gained access to potato acreage. These are just a few examples of insects accepting new hosts, and the same phenomenon is well documented for important tropical crops such sugarcane and cacao, and other crops, as they were introduced and cultivated in various locations around the world.

Some of the native insect populations originally occurred at low levels because their host plants were relatively sparse. Large acreage of new monocrop habitats therefore resulted in an abundant food supply, excellent survival, and eventually in pest population outbreaks. Also, the native environments were relatively stable, and supported a wide range of beneficial insects suppressed herbivorous insects. However, soil tillage within the agricultural environments produced highly disturbed systems, and pests with high fecundity were not effectively suppressed by predators and parasitoids.

America’s native insects displayed considerable plasticity in acquiring new hosts. This trend has been noted everywhere agriculture is practiced, and we can expect the number of pests to increase with time, and especially with the area planted each particular crop, as indigenous species adapt to imported host plants or crops are exposed to additional potential pests in new geographic areas. However, species accrual occurs most rapidly soon aſter plant introduction, and the number of species feeding on a plant (species richness) does not increase indefinitely, leveling off aſter less than 300 years if there is not an increase in crop acreage.

We can observe insects with both broad and narrow host selection behavior expanding their host range to include introduced crop plants. This is not surprising for generalist species, which feed broadly on many plants, but it is quite interesting when insects with a narrow host range adopt new hosts. In such cases the species with a narrow host range usually are pre-adapted to accept the foreign crops because they feed on plants in the same family as the introduced crop. North America possesses close relatives to nearly all the introduced crops among its indigenous flora, so it is not surprising that insects associated with the native plants would adapt to the introduced crops. The presence of secondary plant metabolites (allelochemicals) such as alkaloids, terpenoids, and cyanogenic glycosides oſten serves to keep non-adapted insects from feeding extensively on plants, but may serve as chemical cues or stimulants for insects that are adapted. Thus, insects that specialize on cruciferous weeds and crops are attracted to allylisothiocyanate, and insects that feed on cucurbitaceous weeds and crops are attracted to cucurbitacin.

Host selection behavior by insects is not a static situation, nor is it as simple as the singlechemical scenario presented above. It is constantly evolving in response to various biotic characteristics such as herbivory, and even to crop cultural practices. Some natural selection of insect strains may have occurred during the adaptation from native to introduced plants. In the northern Great Plains, wheat matures earlier in the season than wild grasses. Therefore, aſter a century, adult wheat stem sawflies are now active nearly a month earlier than previously, and now are more effective in utilizing wheat.

Changes in farming practices that have also impacted populations of native insect pests in croplands. Originally, horses were used for farming, and oats were needed for their feed. Later, the horses were replaced by tractors, and the need for oats was reduced. Oats are resistant to wheat stem sawflies, and when oats was eliminated from the cropping system, the vast acreages of wheat resulted in a population explosion of the sawflies. More recently, canola has been included in the Canadian Prairie Provinces, and populations of grass-feeding insects are somewhat disrupted by the presence of a non-host, cruciferous crop. Other water and soil conservation practices such as alternate-year summer fallow, strip cropping, and chemical fallow have affected the prevalence of both pest and beneficial insects.

Beneficial insect populations were also impacted by tillage and cultural practices, and changes in the chemical constituency of crop plants. Parasitoids have complex host searching behavior that begins with finding plant environments in which their hosts could occur. Therefore, it was necessary for the parasitoids to learn that the new crops could be sources of hosts. In the case of the wheat stem sawfly, only two of the known parasitoid species have currently adapted from wild grasses to wheat. Parasitoids may be more favored by one plant cultivar than another, or less favored by a crop than a similar weed. The availability of food for the adult parasitoid or predator, either nectar from blossoms or extra floral nectaries or pollen from blossoms, is often implicated in differential survival of beneficial insects among different plants.

Overall, adaption by herbivorous insects to new host plants is a dynamic and widespread phenomenon. Though sometimes it is difficult to determine whether it is the change in the constituency of the host plant that accounts for insect acceptance, or it is some other factor such as widespread host plant availability that accounts for insect abundance, it is clear that the relationship between insects and plants is not static, resulting in a continuing stream of new pest problems for crop plants.

Table 2 Examples of American insect pests that have adapted to introduced crops