Influence of leaf trichomes on predatory mite density and distribution in plant assemblages and implications for biological control

Non-glandular trichomes on leaves or stems are strongly positively correlated with the abundance of many predatory phytoseiid mite species. Some perennial fruit crops have few or no trichomes and overcoming this limitation of leaf habitat for natural enemy mites could improve biological control of pest mites in agricultural crops. This study evaluated whether juxtaposing plants with and without trichomes will promote more abundant natural enemies throughout an ensemble of plants. We first asked whether the pattern of increased phytoseiid retention with higher trichome densities on a single leaf “scales up” to larger plantings. For three patch sizes (1, 4 or 16 plants) of two grape varieties inoculated with phytoseiids, no strong effect of patch size on Typhlodromus pyri Scheuten abundance was observed in the trichome-rich Baco plantings or in the trichome-free Dechaunac plantings. No predator populations established in Dechaunac plantings of any size. Second, we asked whether the addition of a plant with leaf trichomes within a row (“array”) of plants lacking leaf trichomes can increase phytoseiid density throughout arrays. In both a greenhouse and a field experiment, predatory mites moved onto and established on a neighboring plant only if that plant had trichomes; phytoseiids rapidly dispersed from trichome-free plants even when the plant was the one inoculated with predators. The overall number of phytoseiids decreased over time in ensembles that included both trichome-rich and trichome-free plants in the greenhouse and the field experiments, clearly indicating that for biological control programs to be successful, all plants must express trichomes.