Biological control occurs when pests are controlled by non-pest organisms that inhabit the surrounding crop(s), or nearby habitat. It is an important component of an IPM system as biological control agents have a number of advantages:
- In many cases they are already present and merely require appropriate management strategies (e.g. using selective insecticides where possible) to improve effectiveness
- Many are host-specific, allowing targeting of specific pests, and reducing the risk of pest resurgence
- Some may reduce or prevent population increases in secondary pests
- They provide the opportunity to manage pests with high levels of tolerance to chemical products
Biological control agents include (click link to go to section):
- Beneficial arthropods (predators or parasitoids)
- Disease-causing organisms (pathogens).
- Vertebrate insectivores, such as birds and bats
Predators
Insect predators consume several-to-many prey over the course of their development. Some or all life stages can be predatory. They are free living, and are usually as big as or bigger than their prey. Many have modified body parts to assist predatory movement e.g. well developed mouthparts.
Predators may be generalists, feeding on a wide variety of prey, or specialists, feeding on only one or a few closely related species. They can have chewing mouth parts (e.g. ladybugs, ground beetles, preying mantis) or piercing-sucking mouthparts (e.g. predatory bugs, lacewing larvae) which suck out the body contents after injecting powerful toxins or digestive enzymes that immobilise the prey.
Predators can be transient (e.g. ladybirds) or residential (e.g. predatory mites). Some actively search for prey (e.g. parasitoid wasps), and some lie in wait (e.g. spiders).
Main predator groups include:
- spiders
- predatory bugs
- predatory mites
- lacewings
- beetles
- hoverflies
Benefits of predators
- They often attack different life stages of the pest, and even different pest species.
- Many predators are also able to supplement their diet by feeding on alternative food sources, such as nectar, pollen and fungi.
- They are often voracious feeders and more robust than parasites.
Limitations of predators
- They usually require higher populations of their prey to work effectively and once they have ‘cleaned them up’ may move on.
- It is often hard to directly demonstrate predator impact as they may leave no prey carcases to count and they contribute to a complex of natural control factors in field situations.
To improve the abundance and diversity of beneficial insects and vertebrate predators, consider native vegetation on the farm when developing a pest management strategy.
Parasitoids
A parasite is an organism that lives on or in the body of another organism (the host) during some part of its lifecycle e.g. parasitic mites. However, the parasitic beneficial insects found in grain production systems are usually parasitoids, which (unlike a true parasite) ultimately sterilises, kills and/or consumes its host. Because parasitoids spend most of their life cycle developing within their prey, they are less visible than predators, and their performance may be underestimated as a result.
Most beneficial insect parasitoids are wasps or flies. Unlike predators that immediately kill or disable their prey, pests attacked by parasitoids die more slowly. Some hosts are paralysed, while others may continue to feed or even lay eggs before they die.
Parasitoids are usually highly specialised or host specific. They tend to be smaller than the host and only the female searches for the host to lay her eggs on or in. Immature parasitoids remain on or in host; sometimes multiple adults may emerge from one pest. Adults are free-living and mobile and often require an alternative food source such a pollen or nectar. All life stages can be highly susceptible to insecticides, but adults are especially vulnerable. Immature parasitoids will usually die if their host is killed
Types of parasitoids include:
- Egg parasitoids e.g. Trichogramma & Telenomus wasps
- Larval parasitoids e.g. Netelia producta (Orange caterpillar parasite) & Tachinid flies
- Larval – pupal parasitoids e.g. Heteropelma scaposum (Two-toned parasite)
- Pupal parasitoid –Ichneumon promissorius (Banded caterpillar parasite)
Some parasitoid species attack one stage but do not emerge from their host until much later. An example of this is the larval-pupal parasitoid Heteropelma sp. wasp that lays its eggs inside the helicoverpa caterpillar, but the adult wasp does not emerge until after the caterpillar has pupated.
Parasitoids often complete their life cycle more quickly and increase their numbers faster than many predators and can therefore be the more dominant and effective natural enemy of some pest insects, but their presence may not be obvious. To determine the extent of parasitism, insect pests may need to be dissected or reared to see if adult parasitoids emerge. When monitoring for parasitism:
- Look for evidence of an exit hole in the host
- Dissect samples (may be difficult with very small insects)
- Rear pests in an insect proof cage to see if parasitoids emerge
- Check for deformed insects e.g. caterpillars or mummified/bloated insects such as aphids
- Look for wasp cocoons near caterpillars
Importance of parasitoids in insect management
- Pest management with parasitoids costs nothing
- At low pest densities, parasitoids can suppress infestations to below economic thresholds
- Parasitoids reduce the number of pests surviving to the next generation
- They are compatible with other biological control agents (diseases and predators)
- Parasitoids are host specific
- Some parasitoids affect feeding behaviour e.g. parasitised larvae eat less than healthy ones
- Parasitoids are efficient host searchers – they can find hosts even when pest densities are low
Benefits of parasitoids
- They are usually extremely well adapted to their natural host and are very good at finding them even when densities of the host are relatively low
Limitations of parasitoids
- They are often (but not always) host-specific and often will attack only one species of pest.
- They are generally more delicate than predators and hence more vulnerable to pesticides.
- Parasitoids can be parasitised by other parasitoids. This is known as hyperparasitism – a natural and common occurrence which can reduce the effectiveness of some beneficial species. Little can be done to manage hyperparasitism
Pathogens
Pathogens are disease-causing microorganisms that infect the pest, usually eventually killing it. Pathogen groups include:
- Fungi
- Viruses
- Bacteria
- Nematodes
Fungi
Fungal diseases are most common in moist environments, and may be more prevalent in wetter than average seasons. Fungal spores germinate and hyphae penetrate the insect skin (cuticle), grow in the insect’s body, and may release toxins. Infected insects become covered in hyphae (often appears as white fur) and fruiting bodies develop. The carcass typically becomes anchored to the plant surface by the hyphae.
Useful fungal genera include Beauveria, Metarhizium, Hirsutella, Entomophthora, Nomuraea and Verticilium.
Unfortunately, consistent mass production of fungi as a biopesticide is difficult as specific temperature and humidity conditions are required for successful insect control, and the formulations have a limited storage life outside the host. However, a few fungal pathogens that are being pursued as commercial preparations for application in field situations e.g. Metarhizium for black field crickets.
Viruses
Viral diseases are often highly specific. Viral proteins damage the insect’s gut lining.
They are more easily commercially produced and applied than fungal preparations. The most commonly available is the caterpillar-specific nuclearpolyhedrosisvirus (NPV).
Bacteria
Bacterial cells are not usually utilised directly, but a toxin derived from bacteria (Bacillus thuringiensis (Bt)) is widely used. Generally specific to caterpillar pests, formulations are also available for some beetles and mosquito larvae. Bt toxins have also been incorporated into some genetically modified crops, dramatically reducing the number of externally-applied sprays per season.
Nematodes
Entopathogenic nematodes are microscopic worm-like organisms that actively search for insect hosts, enter the host body through natural openings and release bacteria that digest the insect. The nematode then feeds on the bacteria/insect slurry. When the dead insect’s body ruptures it releases more nematodes.
Commercially-produced nematodes are mostly targeted at horticultural pests, although there is a species available (Steinernema carpocapsae) for armyworm control. There are also commercial products available internationally (e.g. Nemaslug® and Nemassist® that use nematodes for the control of slugs and snails
Further information
The Good Bugs website provides information on commercial insect, mite and nematode bio-control agents currently available in Australia and New Zealand.