Interactions between the entomopathogenic fungus Beauveria bassiana and the predatory mite Neoseiulus barkeri and biological control of their shared prey/host Frankliniella occidentalis

• Either B. bassiana or N. barkeri significantly suppressed thrips populations.
• Simultaneous application of B. bassiana and N. barkeri caused non-additive effects.
• Conidia adhered to N. barkeri lead to an infection of exposed thrips.
• Thrips mortality caused by mite dissemination of the conidia decreased within 48 h.

Interactions between multiple natural enemy species are relevant to biological control of agricultural pests. The first part of the study evaluated the effects of single and combined applications of the entomopathogenic fungus Beauveria bassiana and the predatory mite Neoseiulus barkeri for controlling western flower thrips, Frankliniella occidentalis on greenhouse cucumber. Results revealed that a separate application of either B. bassiana or N. barkeri significantly reduced both larval and adult F. occidentalis populations. The control effects from the fungal application occurred more rapidly than those attributable to predation during the 7-week experiment. However, the thrips population fluctuations and non-additive effect of the B. bassiana and N. barkeri combination treatment suggest possible negative interactions may have occurred between the fungus and the predators. The second part of the study investigated the interaction between B. bassiana and N. barkeri in the laboratory. We confirmed the presence of B. bassiana conidia on mites by scanning electron microscopy and bioassays. Though a benefit of this procedure was dissemination of the pathogen to the target pest resulting in 77.5% mortality of adult thrips, mites also spent a considerable amount of time engaged in self-grooming behavior. Any potential benefits of fungal dissemination by the predatory mites were possibly weakened by increased mite grooming time, which likely reduced the searching activity and predation rates of N. barkeri. Overall, our results suggest that simultaneous application of B. bassiana and N. barkeri would not be recommended for effective biological control.

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