Influence of host age on critical fitness parameters of Spathius galinae (Hymenoptera: Braconidae), a new parasitoid of the emerald ash borer (Coleoptera: Buprestidae)

Authors: WATT, TIMOTHY - University Of Delaware; Duan, Jian

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/29/2014
Publication Date: 8/4/2014
Citation: Watt, T.J., Duan, J.J. 2014. Influence of host age on critical fitness parameters of Spathius galinae (Hymenoptera: Braconidae), a new parasitoid of the emerald ash borer (Coleoptera: Buprestidae). Journal of Economic Entomology. 107(4):1320-1329.

Interpretive Summary: The parasitic wasp (Spathius galinae) is a newly discovered natural enemy of the invasive emerald ash borer (EAB) that has killed tens of millions of North American ash trees since it was discovered in 2002 in US. This natural enemy has been recently imported to the US and is being considered for introduction to the US for biological control of the invasive emerald ash borer. To aid in the development of laboratory rearing protocols, we determined the optimal ages and/or stages of the emerald ash borer larvae for rearing this parasitic wasp. We found that this wasp attacked 5 and 7-week old EAB larvae more frequently than 3.5-week and 10-week old ones. When attacked by this wasp, larger EAB larvae (7 and 10-week old) produced more offspring (˜6.7 offspring per attacked EAB larva), and the offspring that emerged from these attacked-EAB larvae had larger body sizes and more female-biased sex ratios. To increase the efficiency in rearing this parasitic wasp for biocontrol releases against EAB, we thus recommend that large (7-week old) EAB larvae should be used to effectively rear this parasitic wasp. These findings are expected to improve the implementation of EAB biocontrol programs via effective mass production of this biocontrol agent for field releases.

Technical Abstract: Spathius galinae Belokobylskij and Strazenac (Hymenoptera: Braconidae) is a recently discovered gregarious idiobiont larval ectoparasitoid currently being evaluated for biological control against the invasive emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae) in the United States. To aid in the development of laboratory rearing protocols, we assessed the influence of various emerald ash borer stages on critical fitness parameters of S. galinae. We exposed gravid S. galinae females to emerald ash borer host larvae of various ages (3.5, 5, 7, and 10 wk post egg oviposition) that were reared naturally in tropical ash logs, or to field-collected, late stage emerald ash borers (J-larvae, prepupae, and pupae) that were artificially inserted into green ash logs. When exposed to larvae in tropical ash logs, S. galinae attacked 5 and 7-wk hosts more frequently (68 - 76%) than 3.5-wk (23%) and 10-wk (12%) hosts. Subsample dissections of the these logs revealed that 3.5, 5, 7 and 10-wk host logs contained, respectively, mostly 2nd, 3rd, 4th, and mature, non-feeding J-shaped larvae (“J-larvae”) that had already bored into the sapwood for diapause. No J-larvae were attacked by S. galinae when naturally reared in tropical ash logs. When parasitized by S. galinae, 7 and 10 wk hosts produced the largest broods (˜6.7 offspring per parasitized host), and the progenies that emerged from these logs had larger anatomical measurements and more female-biased sex ratios. When exposed to emerald ash borer J-larvae, prepupae, or pupae artificially inserted into green ash logs, S. galinae attacked 53% of J-larvae, but did not attack any prepupae or pupae. We conclude that large (4th instar) emerald ash borer larvae should be used to rear S. galinae.