The objective of biological control is to encourage naturally-occurring populations of predators and parasites to thrive. This includes the cultural step of making manure as dry and as hospitable as possible.
Using biological control, manure removal is carried out according to a staggered schedule, so that the predator and parasite populations are not decimated.
Removing portions of the manure over a period of time, preferably in the cooler season when fly numbers are lowest, preserves a portion of the parasite and predator population to reinvade the new manure.
Sometimes attempts are made to augment naturally occurring parasite populations by releasing additional individuals.
Such augmentation can result in increases in the parasitism rate and reduced number of flies. However, at this time it is not possible to set quantitative guidelines as to when to release and what species and numbers to use under varying conditions.
If augmentation is attempted, it should only be carried out after taking the cultural and manure management steps necessary to maximize the naturally occurring populations of predators and parasites.
Chemicals for fly control should be used carefully and selectively to preserve the predator and parasite populations.
There are several products on the market, which are selective. For example, the adulticide thiamethoxam and the larvicide cyromazine have a very good efficacy against the adults and fly larvae, respectively, whereas they exhibit a very low toxicity to beetles and mites.
Further selectivity can be achieved, according to the mode of application of the insecticides. Rather than routine spraying of all of the manure with broad spectrum insecticides which will kill the predators and parasites, some selectivity can be accomplished by spraying only those areas where fly larvae are unusually abundant.
This spot treatment with larvicides will only have a minor adverse effect on the overall parasite and predator populations.
Treatments directed against the adult flies (adulticiding) should be applied to the surfaces of the buildings with a coarse spray to minimize drift and treatment of the manure.
Residual surface treatments should be directed to those areas of the structures where adult flies are observed to rest. These are usually partitions and upper parts of the structure where flies rest for long periods at night.
Spotting of the surfaces by regurgitation and faeces from flies indicates fly resting areas.