Update on the role of Phytoseiidae as basis for a new integrated spider mite control strategy in Flemish apple cultivation

European red mite, Panonychus ulmi Koch, is an important pest in all major fruit growing regions. Predatory mites belonging to the family Phytoseiidae, play a key role in the control of phytophagous mites in European apple orchards, both Tetranychidae and Eriophyidae. The predatory mite Typhlodromus pyri Scheuten has been the most important natural enemy of the above phytophagous mites. This predator is found in orchards and vineyards and has, in comparison with other predatory mites, a stable presence in the orchard because the species is able to survive on alternative food sources such as pollen and plant sap, and if necessary, mycelia and spores of several fungi. The importance of T. pyri in European apple orchards was enlarged by the spread of organophosphate and carbamate resistant populations. However, recently there are more spider mite problems observed in apple orchards, sometimes in spite of the fact that Phytoseiidae are observed. Therefore a thorough update of the fragile balance between P. ulmi and Phytoseiidae is required.

We investigate four hypotheses that could explain this growing spider mite problem:

1) There is a shift to more biodiversity within the species complex of phytophagous mites.

2) New cultivation systems affect the spider mite numbers or composition of their complex directly or indirectly through a change in the complex of Phytoseiidae.

3) Due to differences in biology and behavior or intraguild interactions, T. pyri is outcompeted by other Phytoseiidae.

4) The current semi-selective agents (25-50% killing of natural enemies) have, because of sublethal effects or because of the timing of their application, a greater impact on mite populations than was initially thought.

To carry out the project aims, the research is divided into four work packages. Work package 1 aims at a qualitative and quantitative description of the species complex of phytophagous mites and the predatory mites in apple. The second work package investigates if the complex of Phytoseiidae differs within recently introduced cultivation systems such as orchards with hail nets, orchards with different pruning system and orchards where no predatory mites were released. In the third work package behavioral differences (biology, predation and hibernation) and intraguild predation are examined between T. pyri and the other main Phytoseiidae. Finally, in work package 4 the effects of pesticides on Phytoseiidae are examined.

Based on our findings, we will present an updated strategy for integrated control of spider mites in apple, which possibly will go beyond the interests of the Belgian apple and fruit economy. If other Phytoseiidae have indeed become crucial as natural enemy and they would fundamentally differ from T. pyri in their susceptibility to pesticides, the European strategy for estimating the unwanted side effects of plant protection products has to be adapted.