Ovipositional behavior of the egg parasitoid Gryon pennsylvanicum (Hymenoptera: scelionidae) on two squash bug species Anasa tristis and Anasa armigera (Hemiptera: coreidae)

Authors: Cornelius, Mary; Vinyard, Bryan; Mowery, Joseph; Hu, Jing - Hu

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/31/2020
Publication Date: 10/14/2020
Citation: Cornelius, M.L., Vinyard, B.T., Mowery, J.D., Hu, J.S. 2020. Ovipositional behavior of the egg parasitoid Gryon pennsylvanicum (Hymenoptera: scelionidae) on two squash bug species Anasa tristis and Anasa armigera (Hemiptera: coreidae). Environmental Entomology. https://doi.org/10.1093/ee/nvaa118.
DOI: https://doi.org/10.1093/ee/nvaa118

Interpretive Summary: Parasitoids have been used effectively as biological control agents in a variety of vegetable crops. Egg parasitoids are tiny wasps that are particularly effective as biological control agents due to their ability to prevent the emergence of the pests. Gryon pennsylvanicum is the primary egg parasitoid of squash bugs, but little is known about its occurence and impact on parasitized pests. This study examined the egg-laying behavior of the parasitoid on egg masses of the common squash bug (a serious pest of squash) and the horned squash bug (a minor pest of squash). This study evaluated how parasitoid numbers and their access to food (dilute honey) influenced their ability to parasitize egg masses of the two species in laboratory tests. Effects of parasitoid density were complex, but access to dilute honey did not significantly influence parasitism rates on horned squash bug egg masses, and only increased parasitism on large common squash bug egg masses with 20-25 eggs. Overall, parasitism rates were higher on horned squash bug egg masses than on common squash bug egg masses and parasitoids were more likely to emerge successfully from horned squash bug eggs than from common squash bug eggs. Parasitoids spent the same amount of time probing eggs of the two host species, but they spent significantly more time drilling into common squash bug eggs than horned squash bug eggs. Measurements taken using transmission electron microscopy determined that squash bug eggs had significantly thicker shells than those of horned squash bug. It is possible that this difference accounts for the lower rates of parasitism and parasitoid emergence and for the increased time spent drilling into squash bug eggs compared with horned squash bug eggs. This study increases our understanding of the interaction between parasitoids and pests and provides the basis for future studies that will improve biological control programs in squash fields.

Technical Abstract: his study examined the ovipositional behavior of Gryon pennsylvanicum Ashmead (Hymenoptera: Scelionidae) on egg masses of two squash bug species Anasa tristis DeGeer and Anasa armigera Say (Hemiptera: Coreidae) by evaluating how parasitoid density and access to honey water influenced parasitism rates on egg masses of different sizes in laboratory tests. When the parasitoid density was increased to two parasitoids per egg mass, parasitoid emergence increased on larger egg masses without decreasing on smaller egg masses. However, when there were three parasitoids per egg mass, parasitoids were less likely to emerge successfully from smaller egg masses. Continual access to nutrition did not significantly influence parasitism rates on A. armigera egg masses, and only increased parasitism on A. tristis egg masses with 20-25 eggs. Overall, parasitism rates were higher on A. armigera egg masses than on A. tristis egg masses and parasitoids were more likely to emerge successfully from A. armigera eggs than from A. tristis eggs. Parasitoids spent the same amount of time probing eggs of the two host species, but they spent significantly more time drilling into A. tristis eggs than A. armigera eggs. Measurements taken using transmission electron microscopy determined that the average width of the epicuticle and exocuticle of the egg chorion was significantly greater for A. tristis eggs than for A. armigera eggs. It is possible that this difference accounts for the lower rates of parasitism and parasitoid emergence and for the increased time spent drilling into A. tristis eggs compared with A. armigera eggs.