An important group of beneficial insects that help control soybean aphids are tiny parasitic wasps (also known as aphid parasitoids). These wasps do not sting or harm humans, livestock, or any insects besides aphids. The biology of these parasitic wasps is like something out of a science-fiction movie. The female wasps inject their eggs into aphids. The larvae that hatch from the eggs then develop inside of their aphid hosts, eventually killing the aphid, and later emerging as winged adult wasps. Aphids attacked by these parasitic wasps become “mummies,” which are the slightly swollen, brown- or black-colored bodies of the dead aphids. When managing soybean aphid, use of scouting and the economic threshold (250 aphids per plant) will help reduce insecticide inputs and conserve these aphid-killing wasps.
In recent years, the community of parasitic wasps attacking soybean aphid has changed and expanded. In particular, a new species of parasitoid called Aphelinus certus arrived in Minnesota around 2011 and has been increasing in abundance. To determine the importance of this and other parasitic wasp species in different regions of Minnesota, entomologists at the University of Minnesota are collaborating with scientists from the Plant Protection Division of the Minnesota Department of Agricultural to conduct a statewide survey of the parasitic wasps attacking soybean aphid.
In 2014, over 200 soybean fields were sampled from throughout Minnesota from July 18 to August 29. In each field, the numbers of aphids and aphid mummies (parasitized aphids) were recorded from a sample of plants and a subsample of the aphid mummies were sent to the laboratory to determine the species of parasitic wasps. We found that the percentage of aphids attacked (parasitized) by the parasitic wasps per field ranged from 0 to 88%. Densities of soybean aphids and parasitic wasps, as well as the percentage of aphids parasitized increased over the season. Over 90% of aphid mummies were parasitic wasps in the genus Aphelinus (family Aphelinidae), and the remaining ~10% of mummies were parasitic wasps from the family Braconidae (subfamily Aphidiinae). Over the coming months, these parasitic wasps will be identified to species level, so we can confirm whether or not they are the recently introduced Aphelinus certus or some other species. This will also allow us to estimate the abundance and diversity of hyperparasitoids (i.e., parasitic wasps that attack other parasitic wasps) in soybean fields.
To help us understand how the community of parasitic wasps changes over time, the survey is being repeated in the summer of 2015. Overall, these data will provide a better understanding of the complex dynamics of beneficial insects that influence soybean aphid populations in Minnesota, so we can better incorporate them into soybean aphid management programs.
Collaborators from MDA include Jean Ciborowski, Tiffany Pahs and Kathy Kromroy and several field staff. This work is partially supported by the MnDRIVE Global Food Venture Grad Fellowship Program and the Minnesota Soybean Research & Promotion Council.
|Figure 1: Aphids parasitized by Aphelinidae (Aphelinus species) (top, black mummies) |
and Braconidae (lower, tan mummies) wasps
Credit: Bruce Potter
|Figure 2: Summary of 2014 survey of soybean aphid parasitoids. Dots represent sample locations. Mummies from parasitoids in the genus Aphelinus were found in counties highlighted in blue.|