Insects for the biocontrol of weeds: predicting parasitism levels in the new country.

Parasitism in the new country can be a major problem with insects used as weed biocontrol agents, with some otherwise successful agents parasitised so heavily that their impact is negligible. This increases the difficulties involved in choosing the best agent. Can we predict parasitism levels? Are certain taxonomic or habitat groups more liable to parasitism in the new country? The crofton weed gall fly, Procecidochares utilis, is heavily parasitised in some countries, such as India, South Africa, New Zealand, and Australia, but not in others, such as Hawaii and China. Why is the similar gall fly, Cecidochares connexa, not parasitised in Indonesia? Why do no parasitoids attack the pseudococcid, Hypogeococcus festerianus, in Australia despite a large number of native and introduced parasitoids attacking pseudococcids? The various theories are discussed in an attempt to discover some guidelines for predicting parasitism levels in different countries.