Effect of storage conditions on host egg suitability and the reproductive fitness of Oobius agrili (Hymenoptera: Encyrtidae), an egg parasitoid of the invasive emerald ash borer (Agrilus planipennis Fairmaire)

Authors: QUINN, NICOLE - University Of Massachusetts, Amherst; ROBERTSON, REBECCA - University Of Massachusetts, Amherst; Duan, Jian

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/14/2024
Publication Date: 9/24/2024
Citation: Quinn, N., Robertson, R., Duan, J.J. 2024. Effect of storage conditions on host egg suitability and the reproductive fitness of Oobius agrili (Hymenoptera: Encyrtidae), an egg parasitoid of the invasive emerald ash borer (Agrilus planipennis Fairmaire). Environmental Entomology. 1-8. https://doi.org/10.1093/ee/nvae081.
DOI: https://doi.org/10.1093/ee/nvae081

Interpretive Summary: The emerald ash borer (EAB) is a serious invasive forest pest in the United States and Canada. The parasitic wasp (Oobius agrili) attacks EAB eggs and was one of several natural enemies introduced from the pest’s native northeast Asian range to the U.S. for EAB biocontrol. Methods for raising O. agrili in the laboratory have been developed, but its mass production often depends on the continuous production and storage of freshly laid EAB eggs, as well as the availability of wasps that can parasitize them. The purpose of this study was to determine optimal environmental conditions for long-term storage of host eggs and wasps to improve the timing of egg and wasp availability. Findings of the study showed that EAB eggs could be stored for approximately 30 days without significant reduction of their viability and suitability for egg parasitism at low storage temperatures (1.7 – 12.8oC) under high (~99.9%) and medium (~ 74.4%) relative humidity. For wasp larvae inside host eggs, neither storage temperature or humidity had any significant effects on adult wasp emergence, longevity, and realized lifetime fecundity for storage durations of 90 to 270 days. When storage durations were over 120 days, however, both adult wasp longevity and number of offspring produced declined approximately 20 - 30% across all temperature and humidity treatments. This this information is useful for mass-production and storage of the egg parasitoid for biological control releases.

Technical Abstract: Oobius agrili Zhang and Huang (Hymenoptera: Encyrtidae) is an important egg parasitoid of the emerald ash borer (EAB), Agrilus planipennis Fairmaire (Coleoptera: Buprestidae). This parasitoid (along with two species of larval parasitoids) was introduced in 2007 to the United States for biological control. To support biocontrol releases against EAB, methods for laboratory-rearing O. agrili have been developed but its mass-production in the laboratory often depend on the continuous production and storage of freshly laid EAB eggs as well as diapaused parasitoid progeny (inside parasitized EAB eggs). The purpose of this study was to determine optimal environmental conditions for long-term storage of host eggs as well as diapaused parasitoid progeny. Fresh host eggs and diapaused parasitoid progeny were stored at two low storage temperatures (1.7o C and 12.8o C) and three levels of relative humidity (low ~31%, medium ~74%, and high ~99.9%) for various length of time (15 – 270 days) and then evaluated for host egg suitability and the reproductive fitness of stored parasitoid progeny. EAB eggs could be stored for approximately 30 days without significant reduction of their viability and suitability to O. agrili parasitism at low storage temperatures (1.7 – 12.8oC) under high (~99.9%) and medium (~ 74.4%) relative humidity. For diapaused O. agrili larvae inside host eggs, neither storage temperature (1.7 or 12.8 oC) or humidity (31.4 ~ 99.9%) had any significant effects on adult parasitoid emergence, longevity, and realized lifetime fecundity for storage durations of 90 to 270 days. When storage durations were over 120 days, however, both adult parasitoid longevity and realized fecundity declined approximately 20 - 30% across all temperature and humidity treatments. Relevance of findings to mass-production and storage of O. agrili for biocontrol is discussed.