They are aggressive and not particularly finicky about what's on their plate. Parasites are organisms which infest other organisms for food and shelter. Like deer ticks on deer or malaria in people, parasites live off the host's blood and good graces, sapping strength and vigor. In the insect world, however, parasites not only make their host sick, they often kill the host outright. This key difference makes parasites in cranberry beds potentially potent biological pesticides.
Some parasites can literally rid the field of a particular pest.
Parasites primarily belong to two groups, the flies and the stingless wasps. They can attack egg, larval, or pupal stages or some combination. There are a large number of parasitic wasps and a smaller number of parasitic flies that attack the larvae of many of our leaf-feeding caterpillars like Sparganothis fruitworm, fireworms and spanworms. They all have naturally-occurring parasites. One group of stingless wasps is Trichogramma.