Parasitic flies resemble houseflies but are somewhat hairier. Adults may be seen resting on foliage or on flowers in which they feed. The flies lay their eggs on the skin of caterpillars. The eggs hatch and larvae eat into the caterpillar where they develop. Aphid parasites are very important in the pecan ecosystem.
Pathogens occur naturally and can be a very significant factor in reducing insect populations.
Insect diseases may be caused by bacteria, fungi, nematodes, protoza, rickettsiae, or viruses. All of these live abundantly on and in the bodies of insects. However, in nature, insect disease epidemics depend upon rather precise conditions of temperature, humidity, and host density. It is quite common to see insects in an orchard that have been killed by disease. Fungal pathogens often reduce pecan aphid populations very rapidly.