Abstract
Coccinellid communities across North America have experienced significant changes in recent decades, with declines in several native species reported. One potential mechanism for these declines is interference competition via intraguild predation; specifically, increased predation of native coccinellid eggs and larvae following the introduction of exotic coccinellids. Our previous studies have shown that agricultural fields in Michigan support a higher diversity and abundance of exotic coccinellids than similar fields in Iowa, and that the landscape surrounding agricultural fields across the north central U.S. influences the abundance and activity of coccinellid species. The goal of this study was to quantify the amount of egg predation experienced by a native coccinellid within Michigan and Iowa soybean fields and explore the influence of local and large-scale landscape structure. Using the native lady beetle Coleomegilla maculata as a model, we found that sentinel egg masses were subject to intense predation within both Michigan and Iowa soybean fields, with 60.7% of egg masses attacked and 43.0% of available eggs consumed within 48 h. In Michigan, the exotic coccinellids Coccinella septempunctata and Harmonia axyridis were the most abundant predators found in soybean fields whereas in Iowa, native species including C. maculata, Hippodamia parenthesis and the soft-winged flower beetle Collops nigriceps dominated the predator community. Predator abundance was greater in soybean fields within diverse landscapes, yet variation in predator numbers did not influence the intensity of egg predation observed. In contrast, the strongest predictor of native coccinellid egg predation was the composition of edge habitats bordering specific fields. Field sites surrounded by semi-natural habitats including forests, restored prairies, old fields, and pasturelands experienced greater egg predation than fields surrounded by other croplands. This study shows that intraguild predation by both native and exotic predators may contribute to native coccinellid decline, and that landscape structure interacts with local predator communities to shape the specific outcomes of predator-predator interactions.
Highlights
In many areas of the U.S, human-mediated disturbances have altered the landscape, resulting in a matrix of agricultural and urban land uses containing fragmented patches of semi-natural habitats
Area of soybean fields where experiments were conducted Perimeter of soybean fields where experiments were conducted Average abundance of soybean aphid present within each site Average abundance of all potential egg predators collected in sweep samples+average abundance of all potential egg predators collected on yellow sticky card traps Average abundance of exotic potential egg predators collected in sweep samples+average abundance of exotic potential egg predators collected on yellow sticky card traps Simpson’s Index of landscape heterogeneity, calculated at a radius of 2 km surrounding the study sites Principal component 1 interpreted from Principal Components Analysis Principal component 2 interpreted from Principal Components Analysis Principal component 3 interpreted from Principal Components Analysis
Egg masses of C. maculata were subject to significant predation in soybean fields across Michigan and Iowa after 48 h of field exposure (F1,54 = 45.7, P,0.001) (Figure 1)
Summary
In many areas of the U.S, human-mediated disturbances have altered the landscape, resulting in a matrix of agricultural and urban land uses containing fragmented patches of semi-natural habitats These landscapes support altered food webs which contain both accidently and intentionally introduced species at multiple trophic levels. Evaluating competitive interactions occurring between native and introduced species is critical to understanding potential threats to the stability of native predator biodiversity and biocontrol services. The soybean aphid Aphis glycines Matsumura, a native of Asia, was first detected in the U.S in July of 2000. Both the primary and secondary host plants of the aphid; common buckthorn, Rhamnus cathartica L. Both the primary and secondary host plants of the aphid; common buckthorn, Rhamnus cathartica L. (Rhamnaceae) and the cultivated soybean, Glycine max
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