Insects can be infected by certain viruses which can help reducing insect pest populations. For example some caterpillars are very sensitive to Baculoviruses. Virus infested caterpillars can be easily recognized in the field. In the beginning they often change to a whitish color and become less active. Often they migrate to the higher parts of plants where they are then found hanging dead at the underside of leaves. The color has changed to black and they are filled with a smelling liquid.
It is possible to encourage the further spreading of the disease among other caterpillars in the field.
For about half a hectare, first collect about 20 diseased caterpillars and grind them up with a bit of water. Filter this mixture through a piece of cloth and dilute the liquid with enough water to spray the crop. After 3 or 4 days the effect will become visible as caterpillars become diseased and die.
To avoid too much damage by the caterpillars, this type of treatment should be carried out as early as possible, when the caterpillars are still young. Good results have been obtained against the Cabbage semi-looper (Trichoplusia ni) and the African bollworm (Helicoverpa armigera).
In some countries, including Thailand, viruses are cultured commercially and formulated to be sold as biological insecticides.