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Megachile rotundata F. is the primary commercial pollinator for alfalfa seed production in North America. Managed M. rotundata populations are susceptible to several mortality factors including attack by parasitoids. One such parasitoid, Melittobia acasta Walker, is a multivoltine wasp whose infestations can decimate bee stocks. Details of M. acasta life history using M. rotundata as a host are needed to develop control strategies. Our objectives were (i) to describe the M. acasta life cycle using M. rotundata prepupae as hosts and (ii) to determine the M. acasta developmental base temperature and propose a degree-day model. First, 150–300 M. acasta adults were introduced to 60 M. rotundata prepupae (10–20 wasp females/4 bee prepupae) upon which they oviposited. Progeny development (at 30 °C) was monitored through adulthood. We identified 12 distinct phases of the M. acasta life cycle that were observed among samples over an average of 19.5 days. Second, vials each containing a M. rotundata prepupa with M. acasta eggs were positioned across a temperature gradient bar (2 vials per temperature). In repeated trials, wasp development was tracked from egg to adult where a mean development time for 30 °C was found to be 13 days. A linear regression analysis determined the lower developmental temperature threshold to be 8.55 °C. Application of this base temperature in a degree-day model revealed an average of 305.8-degree-day accumulation from egg to adult. These results provide a framework to assist bee managers in devising M. acasta control strategies and timing their implementation. [ABSTRACT FROM AUTHOR]

Melittobia acasta Walker is one among other hymenopterous parasitoids of Megachile rotundata F. Commercial M. rotundata populations are employed to pollinate North American alfalfa for seed production. This wasp can be prolific when using M. rotundata as a host and can reduce or destroy bee stocks. Hundreds of M. acasta female offspring can develop in a single M. rotundata cell and disperse to infest other cells, producing thousands of more parasitoids. In this study, we determined (i) upon what bee life stages M. acasta females choose to lay eggs and if those eggs ultimately become adults and (ii) M. acasta female longevity when exposed to various resources within M. rotundata cells. We found that M. acasta females lay eggs on M. rotundata prepupae and pupae and that those eggs can hatch and survive to adulthood. Eggs are not laid on early instar bee larvae; eggs laid on adults do not survive. Average female life span is 5 days without feeding, 8–9 days if a pollen–nectar provision is available while the bee develops through larval stages, and 34 days if the wasp can feed on prepupal hemolymph. Wasp females can emerge from bee cells several days after trays of cells are taken to fields. Therefore, adult females could survive long enough for new bee offspring to become prepupae. Our findings support a better understanding of host life stage preference and the longevity of M. acasta females that can inform the timing of the implementation of possible control measures. [ABSTRACT FROM AUTHOR]