Biology and laboratory rearing of Spathius galinae (Hymenoptera: Braconidae), a parasitoid of the invasive emerald ash borer, Agrilus planipennis (Coleoptera: Buprestidae)
Date
2014
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Publisher
University of Delaware
Abstract
Emerald ash borer (Coleoptera: Buprestidae) is a highly invasive pest of North American ash. As part of an effort to control this pest, Spathius galinae (Hymenoptera: Braconidae), a newly discovered parasitoid of emerald ash borer, is petitioned for environmental release. To gather basic biological information about S. galinae and to assist its mass-rearing, the following studies were conducted. First, effects of emerald ash borer (host) stage on parasitoid fitness were investigated by presenting different size larvae (developing naturally in logs), pre-pupae, or pupae to parasitoids. Results showed large larvae were parasitized significantly more often than small larvae, and broods with more individuals and fewer males were produced when large larvae were utilized; progeny fitness was positively associated with host size. Naturally developing larvae were not parasitized by S. galinae once they bored into the host plant sapwood (becoming J-shaped), although they were parasitized when removed from pupal chambers and re-inserted into a new host plant. No pre-pupae or pupae were parasitized. Secondly, effects of temperature (15, 20, 25, 30, 35°C) on developmental and reproductive parameters were studied. At 15°C, parasitoid progeny developed to fifth instars (cocoons) but entered diapause; at 35°C parasitoid eggs became desiccated and did not hatch. Between 20 -- 30°C development time (egg to adult) was inversely associated with temperature; sex ratio was unaffected by temperature. When newly emerged females, living at these temperatures, were provided hosts throughout their lifetime, parasitism occurred at all temperatures except 35°C, but parasitism rates and fertility were greatest at 25°C; survival was inversely associated with temperature. Life table analysis revealed the highest net reproductive rate ( R0 =25.7) and capacity for increase (rc =0.09) at 25°C. Finally, effects of parasitoid-host group structure on parasitoid fitness were studied by varying the number, or density, of parasitoids and hosts exposed together in cages. With only one host, parasitism was positively associated with increasing numbers of female parasitoids; brood size, sex ratio, and fitness were unaffected. With a 1: 1 parasitoid: host ratio, but varying the parasitoid-host density, parasitism increased with greater densities of parasitoids, and more progeny per female parasitoid were produced when there were higher densities of parasitoids and hosts.