Combined use of Beauveria bassiana and the predatory mite Neoseiulus californicus to suppress Tetranychus urticae on greenhouse-grown sweet pepper

Predatory mites and invertebrate-pathogenic fungi are important biological control tools for managing populations of the two-spotted spider mite, Tetranychus urticae, in horticultural field crops. Here, we evaluated strategies for the combined use of a mite-pathogenic strain of Beauveria bassiana and Neoseiulus californicus to suppress T. urticae populations on sweet pepper plants. First, we compared the performance of both biocontrol agents, applied alone or in combination, under greenhouse conditions. A decline in T. urticae populations was observed 7 days after the release of the predatory mite (53-62%), the application of fungal conidia (77-81%), and their combined use (73-81%). However, in the combined treatment, N. californicus abundance was lower on plants sprayed with B. bassiana than on unsprayed plants. We then evaluated a temporal selectivity strategy in lab and greenhouse conditions, releasing the predatory mite at varying intervals after fungal application, to improve T. urticae suppression. Intervals of 6 days between B. bassiana conidia application and N. californicus release, under both laboratory and greenhouse conditions, substantially reduced the negative effects of the fungus on the predatory mite. These findings indicate that the combined use of these biological control agents requires a well-defined sequential application strategy to maximize T. urticae suppression by the predatory mite.