Biological pest control

The terms ‘biological control’ and its abbreviated synonym ‘biocontrol’ have been used in different fields of biology, most notably entomology and plant pathology. In entomology, it has been used to describe the use of live predatory insects, entomopathogenic nematodes, or microbial pathogens to suppress populations of different insect pests. In plant pathology, the term applies to the use of microbial antagonists to suppress diseases as well as the use of host-specific pathogens to control weed populations. In both fields, the organism that suppresses the pest or pathogen is referred to as the biological control agent (BCA).

Natural enemies of pests

Parasites, pathogens, and predators are the primary groups used in biological control of insects and mites. Most parasites and pathogens, and many predators, are highly specialized and attack a limited number of closely related pest species.

Parasites

A parasite is an organism that lives and feeds in or on a host. Insect parasites can develop on the inside or outside of the host’s body. Often only the immature stage of the parasite feeds on the host. However, adult females of certain parasites (such as many wasps that attack scale insects and whiteflies) feed on and kill their hosts. Although the term ‘parasite’ is used here, true parasites (e.g., fleas and ticks) do not typically kill their hosts. Species useful in biological control, and discussed here, kill their hosts; they are more precisely called ‘parasitoids’. Most parasitic insects are either flies (order Diptera) or wasps (order Hymenoptera). It is important to note that these small to medium-sized wasps are incapable of stinging people. The most common parasitic flies are the typically hairy Tachinidae. Adult tachinids often resemble houseflies. Their larvae are maggots that feed inside the host.

Pathogens

Natural pathogens are microorganisms including certain bacteria, fungi, nematodes, protozoa, and viruses that can infect and kill the host. Populations of some aphids, caterpillars, mites, and other invertebrates are sometimes drastically reduced by naturally occurring pathogens, usually under conditions such as prolonged high humidity or dense pest populations. Some beneficial pathogens are commercially available as biological or microbial pesticides. These include Bacillus thuringiensis, entomopathogenic nematodes, and granulosis viruses. Additionally, some microorganism by- products, such as avermectins and spinosyns, are used in certain insecticides; but applying these products is not considered to be biological control.

Predators

Predators kill and feed on several to many individual preys during their lifetimes. Many species of amphibians, birds, mammals, and reptiles prey extensively on insects. Predatory beetles, flies, lacewings, true bugs (order Hemiptera), and wasps feed on various insect pests or mites. Most spiders feed entirely on insects. Predatory mites feed primarily on spider mites.

Distinguishing pests from natural enemies

Proper identification of pests, and distinguishing pests from natural enemies, is essential for effective biological control. Carefully observe the mites and insects on your plants to help discern their activity. For example, some people may mistake syrphid fly larvae for caterpillars. However, syrphid fly larvae are found feeding on aphids and not chewing on the plant itself. If you find mites on your plants, observe them with a good hand lens. Predatory mites appear more active than plant- feeding species. In comparison with pest mites, predatory mites are often larger and do not occur in large groups.

Table 3: Some pests and their common natural enemies

Examples of biological agents

Table 4 shows selected biological control agents (BCA) available on the market against plant pathogens. Different countries have different regulations about who is allowed to use these products. It may be necessary to take an exam in order to be able to purchase these products. Also, not all of these products might be available in every country.

Table 4: Selected biological control agents (BCA)

Common green lacewings (Chrysoperla carnea)

Named lacewings after the delicate wing venation of adults, or the aphid lion after the voracious appetite of its larvae, Chrysoperla carnea is an active predator of many soft-bodied arthropods and their eggs. Various species of the genus Chrysoperla are mass-produced in several countries for use on both outdoor and protected crops. The third instar larva is extremely voracious and can consume an aphid or a whitefly pupa in less than a minute. The larvae are cannibals and when young may eat unhatched eggs, other larvae, and even adults if food becomes scarce. In the presence of mixed prey, green lacewings attack aphids first, followed by thrips and spider mites. They are also known to feed on young caterpillars and moth eggs, mealybugs, scale insects, whitefly larvae, and pupae. Plants with dense foliage are best suited to these predators, particularly when there is an even spread of prey through the canopy. Lacewing larvae are useful on organic crops where pesticide restrictions necessitate a more generalist predator to control many pest species. C. carnea are more tolerant of low humidity than other lacewing species.

Figure 20: Lacewing predatory larva (left) and adult (right)

Figure 21: Life cycle of lacewing (K. Kos, with copyright permission)

The whitefly parasitoid Encarsia formosa

Encarsia formosa was discovered in England and successfully first used in 1926. Within two years, 250,000 parasitoids had been reared for use on nurseries around England, France, and subsequently Canada. This species is now commercially available in many countries. Adult females are 0.6 mm long with a black head and thorax, a yellow abdomen, and translucent wings. The most obvious sign of Encarsia spp. activity is the presence of black ‘scales’ on leaves. These are the pupal stages of the parasitoid and are formed inside the pupae of the whitefly. Adult wasps are attracted to the host whitefly ‘scale’ (so-called because the larval stage of whitefly is mostly immobile and resembles miniature scale insects) by volatile compounds given off from whitefly honeydew. Adults feed on the honeydew. Usually, a single egg is laid that passes through three larval stages, during which time the whitefly scale remains white and develops normally. When fully developed the whitefly scale turns black as the parasitoid pupates. The pupae remain attached to the leaf and the adult emerges some 10 days later from a hole cut through the puparium with a special ‘tooth’. E. formosa is introduced to crops as black scales stuck to cards from which adults emerge a few days later.

Figure 22: Encarsia formosa laying an egg on whitefly (left) and presence of black ‘scales’ (right)

Figure 23: Synchronized life cycle of Encarsia formosa with the lifecycle of whitefly (K. Kos, with copyright permission)

Entomopathogenic nematodes

Entomopathogenic nematodes are also called eelworms or roundworms. These minute organisms are relatively simple — bilaterally symmetrical, elongated, and tapered at both ends. The species described here are facultative parasitoids (i.e., they can live as saprophytes as well as parasitoids). Although found in nature, they can be mass produced on artificial diets using a liquid medium fermentation type process and are commercially used as biological control agents. Unlike plant pathogenic nematodes, these entomopathogenic species have symbiotic bacteria in their alimentary tract. These produce a toxin and it is this that is the lethal agent. Once the nematode has entered the host and is feeding on its hemolymph (the fluid, analogous to the blood in vertebrates, that circulates in the interior of the arthropod body), it defecates a small pellet containing the pathogenic bacteria which, under the right temperature conditions, will kill the host after only 2-3 days. The nematodes then reproduce in the soup of bacteria and hemolymph, leaving the cadaver as third- stage infective larvae. These are unusually resistant to adverse environmental conditions and can survive for several months.

Figure 24: Life cycle of a steinernematid or heterorhabiditid nematode (Drawing by A.E. Burke)

Predatory mites

These are small, fast moving mites that can be specific predators, such as Phytoseiulus persimilis, or more generalist in their diet, such as many of the Amblyseius species. All deposit eggs close to the intended prey that hatch as six-legged nymphs, pass through two moults, and then develop as eight- legged adults. Location of prey is usually by kairomones (semiochemical) released by the prey faeces, plant damage or, in the case of spider mites, by their webbing that produces an attractant and arrestment stimulus in the predator that retains them in close vicinity of their host prey. Most predatory mites are capable of surviving on relatively low numbers of prey and can increase rapidly to provide adequate levels of control before any major outbreak occurs.

Predatory mites are found throughout the world and several are in commercial production for mass release, particularly for protected crops. However, their use on outdoor crops is increasing, especially on edible crops where post-harvest intervals of many pesticides restrict or even prevent chemical intervention. Many of the Amblyseius species can be mass produced on a bran-based diet that may be packaged along with a factitious host mite in paper sachets for ease of distribution and improved establishment on a crop.

Figure 25: Adult predatory mite eating phytophagous mite (left), controlled release system (CRS) sachet placed in a commercial crop to introduce amblyseid predatory mites (right)

Figure 26: Life cycle of predatory mites, family Phytoseiidae (K. Kos, with copyright permission)

Parasitoid wasp (Aphidius colemani)

Aphidius colemani is a small, black wasp, 4-5 mm long, that inserts a single egg into a host aphid. All other life stages occur within the aphid. The appearance of a golden-brown mummy indicates the presence of these parasitoids on a crop. In general, this parasitoid attacks the smaller aphid species. This species is commercially available in many countries. Aphidius spp. are good at host location and can provide reasonable levels of control if introduced early when pest numbers are low. However, if aphids are established in colonies, A. colemani will take some time to make an impact on the pest population, so predators or a selective pesticide should be considered. The mummy stage is tolerant to most short-persistence pesticides, but those such as synthetic pyrethroids have long residual activity and may kill the adult as it emerges from the aphid mummy. Banker plants of cereals infested with a specific aphid are useful in crops where a continuous supply of parasitoids is required.

Figure 27: Adult parasitoid wasp (Aphidius colemani) ovipositing in an aphid (left). Aphids parasitized by Aphidius colemani: mummy stage (right)

Figure 28: Parasitoid wasp (Aphelinus mali) for biological control of aphids (Eriosoma lanigerum) (K. Kos, with copyright permission)

Copyright © Partners of the Aqu@teach Project. Aqu@teach is an Erasmus+ Strategic Partnership in Higher Education (2017-2020) led by the University of Greenwich, in collaboration with the Zurich University of Applied Sciences (Switzerland), the Technical University of Madrid (Spain), the University of Ljubljana and the Biotechnical Centre Naklo (Slovenia).