Impact of the Egg Parasitoid, Gryon pennsylvanicum (Hymenoptera: Scelionidae), on Sentinel and Wild Egg Masses of the Squash Bug (Hemiptera: Coreidae) in Maryland
Seasonal changes in egg parasitism and predation rates on sentinel (laboratory-reared) and wild (naturally occurring) egg masses of the squash bug, Anasa tristis (De Geer), were evaluated in squash fields in Maryland from June through September in 2013 and 2014. Rates of egg parasitism were significantly higher on wild egg masses than on sentinel egg masses. Squash bug nymphal emergence was significantly higher on sentinel egg masses than on wild egg masses. Between the first week of July and the first week of September of both survey years, squash bug nymphs emerged from 24.2% of wild eggs compared with 46.2% of sentinel eggs and parasitoids emerged from 55.7% of wild eggs compared with only 21.8% of sentinel eggs. Sentinel egg masses significantly underestimated the rate of natural egg parasitism. The egg parasitoid, Gryon pennsylvanicum (Ashmead), was responsible for over 99% of parasitism of squash bug eggs. There was a significant negative correlation between parasitoid emergence and nymphal emergence, suggesting that parasitoids were able to suppress squash bug populations. The average rate of parasitoid emergence peaked on wild egg masses on the fifth week of July at 72.8%, whereas the average rate of nymphal emergence from wild egg masses was <20% from the fifth week of July until the first week of September. These results demonstrate that G. pennsylvanicum was able to efficiently track wild squash bug eggs throughout the season and that it has the potential to be an effective biological control agent of the squash bug in Maryland.
