Biological control of thrips, spider mites and whiteflies goes well in several, but not all crops, because of the sometimes poor establishment of predatory mites. The project “standing army” of Wageningen UR Greenhouse Horticulture aims to understand the underlying mechanisms for these observed differences and to work on methods to improve predator establishment, particularly in ornamental crops. It has been clear that a lack of food is often the main reason for poor establishment of predatory mites. This can be solved by providing pollen or other food sources to crops.
However, it is not only the lack of food that causes problems, as establishment can be poor even when food is provided. An experiment on potted plants showed that predatory mites reached up to 10 times higher densities on Spathiphyllum than on Athurium, even when food was sufficiently added to both plants. This could be explained by the presence of shelters for predatory mites on Spathiphyllum, which provide a better microclimate and increase survival of vulnerable predator stages. Some plants provide these shelters very clearly, also called acarodomatia, or little houses for predatory mites (see photo). These domatia not only provide a better microclimate, but also offer shelter against predators of predatory mites. Further research will focus on the role of these domatia in the control of thrips and how a lack of domatia in crops can be compensated by offering artificial shelters.