Life cycle of Melittobia acasta (Hymenoptera: Eulophidae) using Megachile rotundata (Hymenoptera: Megachilidae) as a host

Authors: ANDERSON, ALAN - Utah State University; RAMIREZ, RICARDO - Utah State University; CREECH, EARL - Utah State University; Pitts Singer, Theresa

Submitted to: Annals of the Entomological Society of America
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/12/2023
Publication Date: 4/22/2023
Citation: Anderson, A.R., Ramirez, R.A., Creech, E.J., Pitts Singer, T. 2023. Life cycle of Melittobia acasta (Hymenoptera: Eulophidae) using Megachile rotundata (Hymenoptera: Megachilidae) as a host. Annals of the Entomological Society of America. 116(4):207-218. https://doi.org/10.1093/aesa/saad011.
DOI: https://doi.org/10.1093/aesa/saad011

Interpretive Summary: The alfalfa leafcutting bee (ALCB) is the most important commercial pollinator for alfalfa seed production in North America. Managed ALCB populations suffer from several mortality factors including attack by parasitoids. A parasitoid paralyzes a host (often a larva) and lays eggs on it; the parasitoid offspring feed on the host. One such parasitoid is Melittobia acasta, a tiny wasp that can have multiple generations per year and whose offspring consume bee larvae. Wasp population numbers can increase greatly and quickly so that managed bee stocks can be decimated. Details of M. acasta life history using ALCBs as host are needed to develop tools for their management. Objectives were 1) to describe the M. acasta life cycle using M. rotundata as a host and 2) to determine the M. acasta temperature at which wasps can develop on bee larvae and propose a mathematical model (degree day model) that can predict wasp growth rates. First, 150-300 M. acasta adults were introduced to 60 ALCB prepupae (10-20 wasp females / four bee prepupae, i.e., large larvae in individual cocoons) upon which they laid eggs. Progeny development (at 30°C) was monitored through adulthood. We identified 12 distinct phases of the M. acasta life cycle that averaged 19.5 days. Second, vials each containing an ALCB prepupa with M. acasta eggs were positioned across a metal bar across which were temperatures that gradually and incrementally increased. In repeated trials, development of wasps was tracked from egg to adult. Using statistical analysis, it was determined that the lower development temperature threshold was 8.5°C. Applying 8.5°C as the base temperature in a degree day model showed an average of 305.8-degree day accumulation from egg to adult. These results provide a framework to assist bee managers for developing M. acasta control strategies and timing their implementation.

Technical Abstract: 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 is Melittobia acasta Walker, a multivoltine wasp whose egregious infestations can decimate bee stocks. Details of M. acasta life history using M. rotundata as host are needed to develop tools for their management. Our objectives were 1) to describe the M. acasta life cycle using M. rotundata as a host and 2) 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 / four bee prepupae in a vial) 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 averaged 19.5 days. Second, vials each containing a M. rotundata prepupa with M. acasta eggs were positioned across a temperature gradient bar (two vials per temperature). In repeated trials, development of wasps was tracked from egg to adult. Using linear regression analysis, it was determined that the temperature for the lower development threshold was 8.5°C. Applying 8.5°C as the base temperature in a degree day model showed an average of 305.8-degree day accumulation from egg to adult. These results provide a framework to assist bee managers for developing M. acasta control strategies and timing their implementation.