Host plant water deficit stress impairs reproduction and development of the galling fly ( Parafreutreta regalis ), a biological control agent of Cape-ivy ( Delairea odorata )

Drought leading to water deficit stress is known to reduce performance of galling insects. The shoot tip-galling fly Parafreutreta regalis has been released for biological control of Cape-ivy (Delairea odorata) in California. Lack of moisture during the dry season causes wilting of Cape-ivy shoots, and subsequent reduced host quantity and quality could influence the fly?s ability to multiply and establish. We imposed water deficit stress on potted Cape ivy plants, then measured the plant?s and insect?s response to water deficit compared to fully-watered plants. Water deficit stress was imposed during female oviposition (short-term), gall development (long-term), or full insect life cycle (long-term). Plants subjected to long-term water deficit showed declines in shoot growth, leaf area, and total biomass. In choice tests, 72% fewer galls were found on water deficit stressed plants, but there was no difference in the numbers of insects per gall. In no-choice tests, long-term water deficit reduced the number and size of galls by up to 51% and 39%, respectively, and puparia inside the galls were up to 35% smaller. Long-term water deficit also increased the time required for the insect to complete its life cycle (egg to adult) by about 10 days, and up to 80% fewer adults emerged, decreasing the fly?s rate of increase by up to five-fold. Our study shows that P. regalis will oviposit and develop successfully on water deficit-stressed plants, but declines in female preference, insect development and abundance could inhibit fly field establishment and/or population growth under climatic drought conditions.