Project Number: 8010-22000-029-09-S
Project Type: Non-Assistance Cooperative Agreement
Start Date: Sep 1, 2015
End Date: Aug 31, 2020
To (1) release and establish two parasitoid species from China, Aphelinus glycinis and Aphelinus rhamni; (2) determine the impact of these parasitoids together with soybean resistance on the abundance and dynamics of soybean aphid, and develop an integrated resistance management (IRM) program involving natural enemies and combinations of soybean resistance genes; and (3) test for virulent genotypes of soybean aphid in the field.
To establish the parasitoids Aphelinus glycinis and Aphelinus rhamni on soybean aphid, we will release large numbers at 2-4 locations in Iowa. At the Beneficial Insect Introductions Research Unit, Newark, DE, we can rear over 240,000 parasitoids every 3 weeks or 640,000 over the growing season, using protocols we have already developed. We will release parasitoids in fields where host plant resistance trials are being carried out. Breeders have developed several isogenic lines with one, two and three Rag genes. We will test the impact on parasitism and soybean aphid abundance of isogenic lines with either no resistance or combinations of Rag1, Rag2, and Rag3 genes, including a triple pyramid of Rag1+Rag2+Rag3, on soybean aphid. The set of plots with each resistance genotype will be replicated, and either naturally or artificially infested with soybean aphid. We will release A. glycinis and A. rhamni in half of the replicates and measure the impact on soybean aphid densities. This will allow us to determine the interaction between Rag genes and parasitoids in their impact on soybean aphid abundance. We will monitor A. glycinis by sampling in soybeans at the release sites and in buckthorn stands near release sites. We will collect data on soybean aphid and parasitoid densities and the impact of the parasitoid with whole-plant samples for soybean and branch samples for buckthorn. We will sample aphids and parasitoids randomly along transects across each field on each sample date. To estimate aphid and mummy density, we will photograph leaves for later counting, and we will also collect aphids and mummies from plants. We will pool healthy aphids by field and rear them to determine whether they were parasitized and by which species. We will develop a model for testing multiple approaches to insect resistance management (IRM) for Rag-genes. We will test the hypotheses that (1) parasitoids slow the development of virulence to Rag genes, and (2) including a refuge for avirulent biotypes to survive will lower the frequency of virulence to Rag genes in a soybean aphid population. Pyramiding resistance genes into a single cultivar is a form of IRM because the redundant killing of more than one resistance gene educes the frequency of virulent biotypes. Natural enemies can affect the evolution of resistance and have been observed to delay insect resistance to Bt crops. Using previous results and those from the experiments in this proposal, an IRM model will be modified to more accurately describe the soybean aphid-soybean system. With an IRM, we can prescribe methods to delay the build-up of virulent biotypes to Rag-genes in the US. We have adapted a simple deterministic model to investigate how soybean aphid biology and management affect the frequency of virulence. Using data collected under the second objective of this project, we will further modify this model to investigate how host plant resistance affects the frequency of virulent genotypes. We will use the results of our model to identify the optimal release strategy for Rag genes with varying levels of mortality from aphid natural enemies.