2.2. Greenhouse experiments
The effects of the predatorsA. swirskiiandE. ovalisonwestern flower thrips, greenhouse whitefly and a combina-tion of these two pests was examined in a greenhouseexperiment in 12 separate compartments (each 18 m2) thateach contained two tables (1·3 m) on which cucumberplants were grown. The experiment was set-up as a split-plot experiment with four replicates. Each block containedone compartment with thrips, one compartment withwhiteflies and one compartment with thrips and whiteflies.In each compartment,A. swirskiiwas released on plants onone table andE. ovalison the other table. Note that we didnot include control treatments in which the pest species hadno predators. Hence, we have no data on the effects of thepest species on each other through the shared host plant.The plants in the treatments with both pest species hadsuch low damage levels that exploitative competitionbetween the two pests was improbable, certainly when weconsider the large leaf size of a cucumber crop (in this treat-ment 250–450 cm2). However, the two pests possiblyaffected each other through induced plant responses (Kar-ban and Carey, 1984). This will be the subject of forthcom-ing research. Secondly, it should be realized that thepredator treatments are strictly not independent, becausethrips and whiteflies were able to migrate between thetwo tables in a compartment. This migration might resultin an underestimate of the control capacity of the best per-forming predator, and an overestimate of the capacity ofthe other predator. However, for analyzing results weassumed the predator treatments to be statisticallyindependent.Each greenhouse compartment had a small closedentrance corridor without windows and was equipped withan air pressure system in order to minimize contaminationwith organisms from outside. Plants were at the fifth-leafstage when the experiments started, and roots were preven-tively treated with Propamocarb againstPythiumspp. Fourplants were placed on two pieces of a rock wool substrateslab on each table. The experiment started in week number12. The rock wool slabs were continuously immersed in anutrient solution that was automatically supplied onceper day. Plants were cultivated vertically up to a 1.5 m highcrop supporting wire. Side-shoots were removed until thetop of the plant reached the crop supporting wire. Lateron, plant shoots and side shoots grew down over the cropsupporting wire. Contamination of treatments was avoidedby applying insect glue to the wires supporting the cropand by keeping the plants isolated in the water layer onthe tables.Pests were introduced one day after the cucumber plantswere planted. Forty adult female thrips, collected from theculture using an aspirator, were released on each table. Atotal of 120 adult whiteflies were released per table. Thepopulation of whiteflies contained on average 42% females.The same numbers were released in the treatment with bothpests. Predatory mites were released seven days after