Farmers can help to keep the balance in their favour in trying not to harm predatory arthropods such as ladybirds, spiders and hover fly larvae which feed on plant-eating pests such as aphids and caterpillars. Some of the ways to do this are:
- Use control pesticide products only when necessary and then not broad spectrum ones (note that most botanical pesticides are broad spectrum)
- If control products are used – use them selectively
- Growing flowering plants which provide nectar and pollen to farmers’ friends such as adult parasitoid wasps, hover-flies and ladybird beetles adults by having living fences (hedges) around the crop to provide shelter and refuge for farmers’ friends should be encouraged. These are called refugia, and examples include beetle banks (grassy areas near crops) flowering plants and unsprayed field edges.
- Mulching to provide refugia for ground-living farmers friends such as predatory beetles
- Learning about insects, studying them in the field.
Conservation of natural enemies
When crops are grown, it inevitably disturbs the natural balance, especially where the crop is a monoculture i.e. all one species of plant. However, the beneficial effect of predators and parasitoids continue to be critically important. If they are correctly managed, they will help prevent some of the pest problems which farmers encounter. An example of how farmers can help to keep the balance in their favour is to try not to harm predatory insects such as ladybirds, spiders and hoverfly larvae which feed on plant-eating pests such as aphids and caterpillars. These predators can be found on most crops together with parasitoid wasps (and occasionally parasitoid flies) which lay their eggs in/on pests. In IPM systems which aim to minimise dependence on pesticides, it is essential that the farmer can distinguish these natural enemies from pests and can use farming techniques which will conserve and encourage natural enemies.
Like humans, insects also suffer from diseases which can weaken or kill them. Types offungal, bacterial and viral pathogens which only infect insects and are safe for humans and animals have been identified. Some of these are commercially produced as biopesticides and some can be prepared on the farm.
Augmentation and inundation with natural enemies
Sometimes there are predators and other natural enemies present which are feeding on the pests, but they are not able to control them effectively, particularly those pests that are capable of breeding very quickly. Farmers can augment the number of natural enemies by bringing them in from outside the field, for example, ladybird beetles or parasitised aphids which contain young parasitoids. Some types of natural enemies can be specially bred in large numbers, and then released onto the crop in order to attack and control the pest.
The natural enemies inundate the pest population. Many of the advances in this technique have been against pests of crops which have been economically important for a long time, such as cotton. For example, the egg parasitoid wasp called Trichogramma has been bred in laboratories to allow huge numbers to be released when eggs of the African bollworm are present on the crop. The same bollworm can be a serious pest of tomatoes, so it may be possible to use the same biocontrol tactics developed for cotton. Aphid parasitoids could theoretically be produced and released in a similar way, but these technologies are often not yet available to small-scale vegetable growers.
Using insect pathogens in pest control sprays
Naturally-occurring pathogens (fungi, bacteria and viruses) that kill insects can be obtained from diseased insects and incorporated into sprays applied to control the pest. This is a type of inundation. Pathogen-based sprays are not yet widely available for vegetable pests except for Bt (Bacillus thuringiensis) a bacterium which kills larvae of moths and butterflies (caterpillars). One example under development is a virus which kills diamondback moth caterpillars – a serious pest of brassicas. The pathogen is called Plutella xylostella granulovirus (PlxyGV). The pathogen has the important advantage of being highly specific. In other words it does not harm other arthropods such as natural enemies so it works together with the natural processes which limit pest numbers. This contrasts with most pesticides which also kill natural enemies. Farmers sometimes use a type of home-made biopesticide – they gather diseased pests, crush and mix them with water, then spray the liquid onto the crop. The fungi, bacteria or viruses which were infecting the collected pests will infect other pests in the crop and kill them.