Impact of Melittobia acasta on Megachile rotundata managed populations during fall and spring

Tiny Melittobia acasta females readily enter Megachile rotundata cells to lay eggs on the cocooned prepupa. Up to 200 offspring develop on the host; female offspring leave the host cell to find new hosts. For managing commercial M. rotundata populations in the fall, bee cells remain in nesting boards at ambient temperatures until removed and then stored at constant wintering temperatures. We examined the ability of M. acasta to successfully parasitize hosts and to infest new hosts depending on when the bee cells are moved to cold storage. We also examined the timing of wasp emergence from host cells during spring incubation and whether the use of dichlorvos insecticide prior to adult bee emergence (standardly used for protection against Pteromalus venustus) during early incubation prevented the wasps from infesting new host cells. Results thus far suggest that M. acasta emerges as soon as one day after the start of incubation, that wasp infestation is reduced when bee cells are placed into cold storage in early fall, and that adding dichlorvos a few days earlier than recommended is a simple and effective strategy to avoid heavy M. acasta infestations during incubation.