Predators and alate immigration influence the season-long dynamics of soybean aphid (Hemiptera: Aphididae)

The identification of key species within multi-predator assemblages suppressing agricultural pests is paramount to the incorporation of predators into pest management plans. Generalist predators have strong impacts on population growth of soybean aphids (Aphis glycines, Hemiptera: Aphididae) in North America, but their relative influence may vary seasonally, and potentially interact with immigration of alate aphids following initial field colonization. Here we present the results of a season-long study in Minnesota that used cage manipulations to estimate weekly impacts of predation on field populations of soybean aphids, while concurrently quantifying alate aphid and predator densities. We used generalized linear models based on field level aphid and predator counts, and aphid counts on caged and uncaged experimental plants, to show that Harmonia axyridis (Coleoptera: Coccinellidae) was negatively associated with A. glycines abundance, even under alate immigration levels associated with aphid outbreaks, suggesting potential regulation of aphid populations. In contrast, two other common predators, Coccinella septempunctata (Coleoptera: Coccinellidae) and Orius insidiosus (Hemiptera: Anthocoridae), did not show patterns of association with aphid dynamics. We also show that A. glycines populations only have significant positive associations with the number of immigrating alatae on uncaged experimental plants that had higher alate densities than the field average. The negative effect of H. axyridis on aphid populations was also observed on uncaged experimental plants, suggesting that even high levels of alate aphid immigration did not disrupt predator suppression. These results add to the growing body of evidence demonstrating that even low numbers of certain generalist predators can effectively suppress pest populations in agroecosystems. In particular, our findings suggest that the abundance of both H. axyridis and alate aphids are critical to soybean aphid seasonal dynamics.