The baculoviruses are the most important group of virus diseases of insects. They have to be ingested to act and are usually very host specific and are mainly important in moth caterpillars. Several have been exploited as biocontrol agents, notably the granuloviruses of codling and summer fruit tortrix moths.
Bacillus thuringiensis (Bt) is the most important pathogenic bacteria but it normally is only found in orchards in association with sprays of Bt products.
- The pathogenicity of Bt, which has to be ingested to act, is due mainly to the production of a crystal protein toxin.
- Bt can be mass produced by fermentation and has long been available as a microbial biological control agent.
- The most widely used strain in orchards is active against caterpillars and has been widely used for control of winter and tortrix moth caterpillars.
Insect pathogenic fungi are common in the environment particularly in soil. The flora is dominated by a small number of species.
- They are known to be important mortality factors in overwintering larvae for codling and tortrix moths.
- There have been several attempts to exploit them for control of orchard pests but they have a requirement for high humidity and adequate temperatures for spore germination.
- They generally infect the insect through the cuticle.
- They may be adversely affected by sprays of fungicides applied for scab and mildew control in orchards but such effects have not been adequately investigated.
Insect pathogenic nematodes are also important natural enemies of some pests, notably overwintering codling moth larvae. Nematodes are soft-bodied invertebrate animals that need surface moisture to move and survive. Suitable conditions for them occur only very transiently on the aerial parts of fruit trees.
- They have been exploited for control of pests which live in galleries (larvae of apple clearwing moth), on the trunk of the tree close to the soil (e.g. codling moth) or in the soil.
- They can be mass produced in culture on artificial substrates and several biocontrol formulations are commercially available principally for control of soil pests of protected crops.
- They are not generally used for biocontrol on orchards.