16.5. Biological control
Regulation of the abundance and distributions of species is influenced strongly by the activities of naturally occurring enemies, namely predators, parasites/parasitoids, pathogens, and/or competitors. In most man- aged ecosystems these biological interactions are severely restricted or disrupted in comparison with natural ecosystems, and certain species escape from their natural regulation and become pests. In biological control, deliberate human intervention attempts to restore some balance, by introducing or enhancing the natural enemies of target organisms such as insect pests or weedy plants. One advantage of natural enemies is their host-specificity, but a drawback (shared with other control methods) is that they do not eradicate pests. Thus, biological control may not necessarily alleviate all economic consequences of pests, but control systems are expected to reduce the abundance of a target pest to below ET levels. In the case of weeds, natural enemies include phytophagous insects; biological control of weeds is discussed in section 11.2.6. Several approaches to biological control are recognized but these categories are not discrete and published definitions vary widely, leading to some confusion. Such overlap is recognized in the following summary of the basic strategies of biological control.
Classical biological control involves the importation and establishment of natural enemies of exotic pests and is intended to achieve control of the target pest with little further assistance. This form of biological control is appropriate when insects that spread or are introduced (usually accidentally) to areas outside their natural range become pests mainly because of the absence of natural enemies. Two examples of successful classical biological control are outlined in Boxes 16.2 and 16.4. Despite the many beneficial aspects of this control strategy, negative environmental impacts can arise through ill-considered introductions of exotic natural enemies. Many introduced agents have failed to control pests; for example, over 60 predators and parasitoids have been introduced into north-eastern North America with little effect thus far on the target gypsy moth, Lymantria dispar (Lymantriidae) (see Plate 6.7). Some introductions have exacerbated pest problems, whereas others have become pests themselves. Exotic introductions generally are irreversible and non-target species can suffer worse consequences from efficient natural enemies than from chemical insecticides, which are unlikely to cause total extinctions of native insect species.
There are documented cases of introduced biological control agents annihilating native invertebrates. A number of endemic Hawai’ian insects (target and non-target) have become extinct apparently largely as a result of biological control introductions. The endemic snail fauna of Polynesia has been almost completely replaced by accidentally and deliberately introduced alien species. The introduction of the fly Bessa remota (Tachinidae) from Malaysia to Fiji, which led to extinction of the target coconut moth, Levuana iridescens (Zygaenidae), has been argued to be a case of biological control induced extinction of a native species. However, this seems to be an oversimplified interpretation, and it remains unclear as to whether the pest moth was indeed native to Fiji or an adventitious insect of no economic significance elsewhere in its native range. Moth species most closely related to L. iridescens predominantly occur from Malaysia to New Guinea, but their systematics are poorly understood. Even if L. iridescens had been native to Fiji, habitat destruction, especially replacement of native palms with coconut palms, also may have affected moth populations that probably underwent natural fluctuations in abundance.
At least 84 parasitoids of lepidopteran pests have been released in Hawai’i, with 32 becoming established mostly on pests at low elevation in agricultural areas. Suspicions that native moths were being impacted in natural habitats at higher elevation have been con- firmed in part. In a massive rearing exercise, over 2000 lepidopteran larvae were reared from the remote, high elevation Alaka’i Swamp on Kauai, producing either adult moths or emerged parasitoids, each of which was identified and categorized as native or introduced. Parasitization, based on the emergence of adult parasitoids, was approximately 10% each year, higher based on dissections of larvae, and rose to 28% for bio- logical control agents in certain native moth species. Some 83% of parasitoids belonged to one of three biological control species (two braconids and an ichneumonid), and there was some evidence that these competed with native parasitoids. These substantial non-target effects appear to have developed over many decades, but the progression of the incursion into native habitat and hosts was not documented.
A controversial form of biological control, sometimes referred to as neoclassical biological control, involves the importation of non-native species to control native ones. Such new associations have been suggested to be very effective at controlling pests because the pest has not coevolved with the introduced enemies. Unfortunately, the species that are most likely to be effective neoclassical biological control agents because of their ability to utilize new hosts are also those most likely to be a threat to non-target species. An example of the possible dangers of neoclassical control is provided by the work of Jeffrey Lockwood, who campaigned against the introduction of a parasitic wasp and an entomophagous fungus from Australia as control agents of native rangeland grasshoppers in the western USA. Potential adverse environmental effects of such introductions include the suppression or extinction of many non-target grasshopper species, with probable concomitant losses of biological diversity and existing weed control, and disruptions to food chains and plant community structure. The inability to predict the ecological outcomes of neoclassical introductions means that they are high risk, especially in systems where the exotic agent is free to expand its range over large geographical areas.
Polyphagous agents have the greatest potential to harm non-target organisms, and native species in tropical and subtropical environments may be especially vulnerable to exotic introductions because, in comparison with temperate areas, biotic interactions can be more important than abiotic factors in regulating their populations. Sadly, the countries and states that may have most to lose from inappropriate introductions are exactly those with the most lax quarantine restrictions and few or no protocols for the release of alien organisms.
Biological control agents that are present already or are non-persistent may be preferred for release. Augmentation is the supplementation of existing natural enemies, including periodic release of those that do not establish permanently but nevertheless are effective for a while after release. Periodic releases may be made regularly during a season so that the natural enemy population is gradually increased (augmented) to a level at which pest control is very effective. Augmentation or periodic release may be achieved in one of two ways, although in some systems a distinction between the following methods may be inapplicable. Inoculation is the periodic release of a natural enemy unable either to survive indefinitely or to track an expanding pest range. Control depends on the progeny of the natural enemies, rather than the original release. Inundation resembles insecticide use as control is achieved by the individuals released or applied, rather than by their progeny; control is relatively rapid but short-term. Examples of inundation include entomopathogens used as microbial insecticides (section 16.5.2) and Trichogramma wasps, which are mass reared and released into glasshouses. For cases in which short-term control is mediated by the original release and pest suppression is maintained for a period by the activities of the progeny of the original natural enemies, then the control process is neither strictly inoculative nor inundative. Augmentative releases are particularly appropriate for pests that combine good dispersal abilities with high reproductive rates — features that make them unsuitable candidates for classical biological control.
Conservation is another broad strategy of biological control that aims to protect and/or enhance the activities of natural enemies. In some ecosystems this may involve preservation of existing natural enemies through practices that minimize disruption to natural ecological processes. For example, the IPM systems for rice in south-east Asia encourage management practices, such as reduction or cessation of insecticide use, that interfere minimally with the predators and parasitoids that control rice pests such as brown plant-hopper (Nilaparvata lugens). The potential of biological control is much higher in tropical than in temperate countries because of high arthropod diversity and year-round activity of natural enemies. Complex arthropod food webs and high levels of natural biological control have been demonstrated in tropical irrigated rice fields. Furthermore, for many crop systems, environmental manipulation can greatly enhance the impact of natural enemies in reducing pest populations. Typically, this involves altering the habitat available to insect predators and parasitoids to improve conditions for their growth and reproduction by the provision or maintenance of shelter (including overwintering sites), alternative foods, and/or oviposition sites. Similarly, the effectiveness of entomopathogens of insect pests sometimes can be improved by altering environmental conditions at the time of application, such as by spraying a crop with water to elevate the humidity during release of fungal pathogens.
All biological control systems should be underpinned by sound taxonomic research on both pest and natural enemy species. Failure to invest adequate resources in systematic studies can result in incorrect identifications of the species involved, and ultimately may cost more in time and resources than any other step in the biological control system. The value of taxonomy in biological control is exemplified by the cassava mealybug in Africa (Box 16.4) and in management of Salvinia (Box 11.3).
The next two subsections cover more specific aspects of biological control by natural enemies. Natural enemies are divided somewhat arbitrarily into arthropods (section 16.5.1) and smaller, non-arthropod organisms (section 16.5.2) that are used to control various insect pests. In addition, many vertebrates, especially birds, mammals, and fish, are insect predators and their significance as regulators of insect populations should not be underestimated. However, as biological control agents the use of vertebrates is limited because most are dietary generalists and their times and places of activity are difficult to manipulate. An exception may be the mosquito fish, Gambusia, which has been released in many subtropical and tropical waterways worldwide in an effort to control the immature stages of biting flies, particularly mosquitoes. Although some control has been claimed, competitive interactions have been severely detrimental to small native fishes. Birds, as visually hunting predators that influence insect defenses, are discussed in Box 14.1.
