Influence of host stage on attack rate, sex ratio, progeny size and fitness of Spathius galinae, a new parasitoid of emerald ash borer, Agrilus planipennis

Spathius galinae Belokobylskij and Strazenac (Hymenoptera: Braconidae) is a newly discovered gregarious larval ectoparasitoid currently being evaluated as a biological control agent of the invasive Emerald Ash Borer (EAB), Agrilus planipennis Fairmaire (Coleoptera: Buprestidae). In the laboratory, we presented gravid female parasitoids with naturally infested larvae, concealed in tropical ash (Fraxinus uhdei) logs, of age classes 3.5-, 5-, 7-, and 10wk-old, which corresponded with increased host size. We found that host size (as measured by age class) had a significant influence on the attack rate of foraging parasitoids, with small host larvae (3.5wk-old, 1st- 2nd instars) parasitized 26% of the time, and large host larvae (7wk-old, 4th instars) parasitized 76% of the time. Small larvae (3.5wk-old larvae) produced significantly fewer and more male-biased progenies than 7wk-old larvae, and the anatomical measures of individual fitness (length of left hind tibia, body, and female ovipositor) were greater for individuals emerging from older larvae. Most of the larvae in the 10wk-old age class had excavated a chamber in the inner sapwood and ceased feeding (termed “J-larvae’), and were not parasitized by S. galinae in naturally infested logs. However, when these J- larvae were removed and artificially inserted into grooves made in the host plant, they were parasitized by S. galinae and progeny was produced. In contrast, S. galinae never attacked artificially inserted pre-pupae or pupae, which completely cease feeding activities. Findings from our study indicate that parasitoid attack rate, progeny size, and sex ratio (female-male) are positively correlated with host size and that actively feeding late instar larvae (~7wk old) , but not J-larvae, pre-pupae or pupae, should be used to rear S. galinae with naturally-infested host plants most efficiently in the laboratory. When artificially inserted into the host plant, however, J-larvae may still be suitable to S. galinae production. Field releases of this parasitoid should furthermore be correlated with the seasonal timing of actively feeding late EAB larvae in the wild.