Distribution of greenhouse whiteflyTrialeurodes vaporariorum(Homoptera, Aleyrodidae) andEncarsia formosa(Hymenoptera, Aphelinidae) in a greenhouse tomato crop: implications for sampling

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Abstract
Vertical and horizontal distributions of whitefly puparia in a Dutch commercial greenhouse tomato crop were studied over 16 weeks in order to test sampling techniques. In the period Week 7–8,2270 puparia infested 137 out of the 18 144 plants, whereas in Week 15–16,2005 puparia infested 1024 plants. Calculation of spatial autocorrelations showed that at both times infested plants were grouped along rows with a much lower probability of them being alongside one another in adjacent rows. The seventh leaf layer (seven leaves up from the base of the plant) was the most heavily infested in Week 7–8 and the distribution of the infestation was the same as for the whole plant. It is concluded that presence-absence sampling for whitefly puparia and for estimates of parasitism should be restricted to the zone of three-five leaf layers with most puparia and involve the examination of a single leaf taken from widely spaced plants in each row of a small greenhouse or in areas of infestation in a large greenhouse.