The parasite‐host relationship between Encarsia formosa (Hym., Aphelinidae) and Trialeurodes vaporariorum (Hom., Aleyrodidae)

(For the first and second part of this paper) The greenhouse whitefly, Trialeurodes vaporariorum (Westwood), is an important pest of a number of glasshouse crops. Since about 1970 the pest is controlled on an increasing area and in an increasing number of countries by introduction of a natural enemy, the parasite Encarsia formosa Gahan. In 1979 biological control of the greenhouse whitefly was applied in 11 countries on about 1200 hectares. After several years of application of E. formosa we started a research project on population dynamics of host and parasite. The reasons were:1 to find a quick and reliable sampling method, both for growers and suppliers of parasites; 2 to test the effectiveness of the Dutch method of introducing E. formosa;3 to find an explanation for the usually worse control results in small glasshouses compared with results obtained in large houses; 4 to test Huffaker's 1958 hypothesis that coexistence of predator and prey is possible only if special requirements of space and dispersal capacity of both prey and predator are fulfilled.The changes in numbers and distribution of greenhouse whitefly and its parasite were followed for 16 weeks in a glasshouse of 6480 m 2 containing 18,000 tomato plants. Two methods were used: weekly counts of pupae on all infested plants and a stratified random sampling program whereby 0.6 % of the plants were checked each week. Results obtained with the random sampling did not reflect the actual numbers and the distribution of both organisms at all, and this method can therefore not be recommended to growers and suppliers of parasites (E ggenkamp ‐R otteveel M ansveld et al. 1978). Data of the absolute counts and the distribution maps constructed with these data revealed that:1 the introduction scheme used in the Netherlands was successful (the parasites are introducted 1 to 4 weeks after the pest has been seen, 4 parasite introductions with an interval of two weeks, with a planned total of 6 parasites per plant; in our study the total number of parasites that was released appeared to be 9.8). No whitefly damage was observed throughout the cropping period. This control result is no exception but the rule for houses where biocontrol is applied. 2 the distribution of both the whitefly and parasite was clustered though the degree of clustering decreased strongly in the course of time. It changed gradually towards a regular pattern during the last weeks. This was caused by migration of whitefly adults, which resulted in an increase of the number of infected plants but a decrease in the number of pupae per infected plant, so the total number did not increase. 3 the dispersal and searching capacity of the parasite are very good. The parasites covered distances of 10 m at least and they were able to find even a single host pupa on a plant that grew amidst many uninfested plants. 4 in H uffaker's 1958 experiment subpopulations of a phytophagous mite could be completely eliminated by a predatory mite, while at the same time new subpopulations of the phytophagous mite developed on other fruits. Complete extinction of patches (patch: a group of whiteflies and/or parasites separated from other such groups by a 3 m wide space without whiteflies or parasites) did not occur. However, within patches whiteflies and parasites regularly migrated to other plants. The whitefly subpopulation on one plant sometimes disappeared through complete parasitism, while a new subpopulation developed on another plant. From patches or plants where all hosts are parasitised, the parasites migrate to the new whitefly subpopulations. As a result of this, the percentage parasitism per plant varied between 0 and 100 %. The average percentage parasitism was rather constant (around 50 %), so the heterogeneity in the glasshouse leads to an “overall‐stability” while the events taking place on one plant were quite dramatic.