Interactions between foliage- and soil-dwelling predatory mites and consequences for biological control of Frankliniella occidentalis

The predatory mites Neoseiulus barkeri (Hughes) (Acari: Phytoseiidae) and Stratiolaelaps scimitus Womersley (Acari: Laelapidae) are indigenous species used for control of Frankliniella occidentalis Pergande (Thysanoptera: Thripidae) in China. The present study investigated the interactions between these two predators and evaluated the effects of their combined releases to control F. occidentalis in cucumber and eggplant greenhouses during a two-year period. Releases of N. barkeri and S. scimitus were made on the surface of leaves and on soil adjacent to plant roots at a density of 100 per plant, respectively. Although releases of either N. barkeri or S. scimitus alone significantly reduced both adult and larval F. occidentalis densities, combined releases of N. barkeri and S. scimitus greatly improved F. occidentalis control. However, some negative interactions between the two predators did occur. The mean levels of adult and larval F. occidentalis suppression on cucumber plants were 62 and 51 %, and the mean levels of adult and larval F. occidentalis suppression on eggplant plants were 66 and 66 %. Results also revealed that N. barkeri and S. scimitus successfully established populations in the target microhabitats. Moreover, predators established populations more easily on cucumber compared to eggplant crops. These results highlight the potential use of N. barkeri in combination with S. scimitus for control of F. occidentalis.