Amir Grosman, Eric de Groot and Gerben Messelink
Biological control of pests in greenhouses is sometimes supported by sponaneously occuring natural enemies, such as soil-dwelling predatory beetles or hunter flies. The aim of this study was to determine the effects of these predators on western flower thrips in chrysanthemum and how population densities of the predators can be increased. De hunter fly Coenosia attenuata was not able to significantly reduce thrips populations in cages, but there was a minor reduction in pest densities. Densities of the predatory beetle Atheta coriariae could be increased by applying fertilisers, proteins (bird feed) or yeast to to soil. However, increased densities of soil-dwelling predators did not enhance thrips control in a greenhouse trial. This may have been the results of food saturation of the predators or the pupal behaviour of thrips. More research is needed to optimize soil enrichment with food in such a way that thrips control will be enhanced.