Biological control

Outbreaks of pests occur repeatedly and so does the need to apply pesticides. But biologists can sometimes replace chemicals by managing resistance another tool that does the same job and often costs a great deal less -biological control (the manipulation of the natural enemies of pests). Biological control involves the application of theory about interactions between species and their natural enemies (see Chapters 10, 12 and 14) to limit the population density of specific pest species. There are a variety of categories of biological control.

The first is the introduction of a natural enemy from another geographic area - very often the area in which the pest originated prior to achieving pest status - in order that the control agent should persist and thus maintain the pest, long term, below its economic threshold. This is a case of a desired invasion of an exotic species and is often called classical biological control or importation.

By contrast, conservation biological control involves manipulations that augment the density or persistence of populations of generalist natural enemies that are native to the pest's new area (Barbosa, 1998).

Inoculation is similar to introduction, but requires the periodic release of a control agent where it is unable to persist throughout the year, with the aim of providing control for only one or perhaps a few generations. A variation on the theme of inoculation is 'augmentation', which involves the release of an indigenous natural enemy in order to supplement an existing population, and is also therefore carried out repeatedly, typically to intercept a period of rapid pest population growth.

Finally, inundation is the release of large numbers of a natural enemy, with the aim of killing those pests present at the time, but with no expectation of providing long-term control as a result of the control agent's population increasing or maintaining itself. By analogy with the use of chemicals, agents used in this way are referred to as biological pesticides.

Insects have been the main agents of biological control against both insect pests (where parasitoids have been particularly useful) and weeds. Table 15.3 summarizes the extent to which they have been used and the proportion of cases where the establishment of an agent has greatly reduced or eliminated the need for other control measures (Waage & Greathead, 1988).

Table 15.3 The record of insects as biological control agents against insect pests and weeds. (After Waage & Greathead, 1988.)

Insect pests

Weeds

Control agent species

563

126

Pest species

292

70

Countries

168

55

Cases where agent has become established

1063

367

Substantial successes

421

113

Successes as a percentage of establishments

40

31

Probably the best example of 'classical' biological control is itself a classic. Its success marked the start of biological control in a modern sense. The cottony cushion scale insect, Icerya purchasi, was first discovered as a pest of Californian citrus orchards in 1868. By 1886 it had brought the citrus industry close to the point of destruction. Ecologists initiated a worldwide correspondence to try and discover the natural home and natural enemies of the scale, eventually leading to the importation to California of about 12,000 Cryptochaetum (a dipteran parasitoid) from Australia and 500 predatory ladybird beetles (Rodolia cardinalis) from Australia and New Zealand. Initially, the parasitoids seemed simply to have disappeared, but the predatory beetles underwent such a population explosion that all infestations of the scale insects in California were controlled by the end of 1890. Although the beetles have usually taken most or all of the credit, the long-term outcome has been that the beetles are instrumental in keeping the scale in check inland, but Cryptochaetum is the main agent of control on the coast (Flint & van den Bosch, 1981).

This example illustrates a number of important general points. Species may become pests simply because, by colonization of a new area, they escape the control of their natural enemies (the enemy release hypothesis) (Keane & Crawley, 2002). Biological control by importation is thus, in an important sense, restoration of the status quo for the specific predator-prey interaction (although the overall ecological context is certain to differ from what would have been the case where the pest and control agent originated). Biological control requires the classical skills of the taxonomist to find the pest in its native habitat, and particularly to identify and isolate its natural enemies. This may often be a difficult task - especially if the natural enemy has the desired effect of keeping the target species at a low carrying capacity, since both the target and the agent will then be rare in their natural habitat. Nevertheless, the rate of return on investment can be highly favorable. In the case of the cottony cushion scale, biological control has subsequently been transferred to 50 other countries and savings have been immense. In addition, this example illustrates the importance of establishing several, hopefully complementary, enemies to control a pest. Finally, classical biological control, like natural control, can be destabilized by chemicals. The first use of DDT in Californian citrus orchards in 1946-47 against the citricola scale Coccus pseudomagnoliarum led to an outbreak of the (by then) rarely seen cottony cushion scale when the DDT almost eliminated the ladybirds. The use of DDT was terminated.

Many pests have a diversity of natural enemies that already occur in their vicinity. For example, the aphid pests of wheat (e.g. Sitobion avenae or Rhopalasiphum spp.) are attacked by biological control: the use of natural enemies in a variety of ways cottony cushion scale insect: a classic case of importation ...

... illustrating several general points conservation biological control of wheat aphids coccinellid and other beetles, heteropteran bugs, lacewings (Chrysopidae), syrphid fly larvae and spiders - all part of a large group of specialist aphid predators and generalists that include them in their diet (Brewer & Elliott, 2004). Many of these natural enemies overwinter in the grassy boundaries at the edge of wheat fields, from where they disperse and reduce aphid populations around the field edges. The planting of grassy strips within the fields can enhance these natural populations and the scale of their impact on aphid pests. This is an example of 'conservation biological control' in action (Barbosa, 1998).

'Inoculation' as a means of biological control is widely practised in the control of arthropod pests in glasshouses, a situation in which crops are removed, along with the pests and their natural enemies, at the end of the growing season (van Lenteren & Woets, 1988). Two particularly important species of natural enemy used in this way are Phytoseiulus persimilis, a mite that preys on the spider mite Tetranychus urticae, a pest of cucumbers and other vegetables, and Encarsia formosa, a chalcid parasitoid wasp of the whitefly Trialeurodes vaporariorum, a pest in particular of tomatoes and cucumbers. By 1985 in Western Europe, around 500 million individuals of each species were being produced each year.

'Inundation' often involves the use of insect pathogens to control insect pests (Payne, 1988). By far the most widespread and important agent is the bacterium Bacillus thuringiensis, which can easily be produced on artificial media. After being ingested by insect larvae, gut juices release powerful toxins and death occurs 30 min to 3 days later. Significantly, there is a range of varieties (or 'pathotypes') of B. thuringiensis, including one specific against lepidoptera (many agricultural pests), another against diptera, especially mosquitos and blackflies (the vectors of malaria and onchocerciasis) and a third against beetles (many agricultural and stored product pests). B. thuringiensis is used inundatively as a microbial insecticide. Its advantages are its powerful toxicity against target insects and its lack of toxicity against organisms outside this narrow group (including ourselves and most of the pest's natural enemies). Plants, including cotton (Gossypium hir-sutum), have been genetically modified to express the B. thuringiensis toxin (insecticidal crystal protein Cry1Ac). The survivorship of pink bollworm larvae (Pectinaphora gossypiella) on genetically modified cotton was 46-100% lower than on nonmodified cotton (Lui et al., 2001). Concern has arisen about the widespread insertion of Bt into commercial genetically modified crops, because of the increased likelihood of the development of resistance to one of the most effective 'natural' insecticides available.

Biological control may appear to be a particularly environmentally friendly approach to pest control, but examples are coming to light where even carefully chosen and apparently successful introductions of biological control agents have impacted on nontarget species. For example, a seed-feeding weevil (Rhinocyllus conicus), introduced to North America to control exotic Carduus thistles, attacks more than 30% of native thistles (of which there are more than 90 species), reducing thistle densities (by 90% in the case of the Platte thistle Cirsuim canescens) with consequent adverse impacts on the populations of a native picture-winged fly (Paracantha culta) that feeds on thistle seeds (Louda et al., 2003a). Louda et al. (2003b) reviewed 10 biological control projects that included the unusual but worthwhile step of monitoring nontarget effects and concluded that relatives of the target species were most likely to be attacked whilst rare native species were particularly susceptible. Their recommendations for management included the avoidance of generalist control agents, an expansion of host-specificity testing and the need to incorporate more ecological information when evaluating potential biological control agents.

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