Natural enemies affect field distribution and population growth of the soybean aphid Aphis glycines Matsumura (Homoptera: Aphididae) in Michigan
Alejandro C. Costamagna, email@example.com and Douglas A. Landis, firstname.lastname@example.org. Michigan State University, Department of Entomology, 204 Center for Integrated Plant Systems, East Lansing, MI
The soybean aphid Aphis glycines Matsumura (Homoptera: Aphididae) is an invasive species from Asia, now established in 21 states. We tested the effect of natural enemies on soybean aphid populations at within plant, whole plant, and field scales. We conducted a grid sampling (50 patches of 10 x 10 m) in a soybean field naturally colonized by soybean aphid from June to August 2003. Field patches were classified initially as high (> 10 aphids / plant), medium (5 – 10 aphids / plant), and low density (< 5 aphids / plant), and ranked again for each sampling. The effect of natural enemies was assessed by enclosing plants with 1) fine screen mesh bags supported by tomato cages, versus 2) sham cages in which openings allowed natural enemy movement, and 3) totally open controls. Significantly more aphids were obtained after 2 weeks in the exclusion cages over the sham and open treatments. High-density patches had more aphids than medium and low-density patches in the exclusion cages, but not in the sham or open treatments, indicating that natural enemies (almost exclusively predators) reduced aphid populations to similar levels, despite initial aphid density. This was further confirmed at the field level, where the rank of the patches changed from week to week, without the presence of continuous ‘hot spots’ of aphid increase. At the within plant level, the proportion of aphids at each node was not affected by exposure to predation, indicating the absence of refuges from predation at this spatial scale.
Species 1: Homoptera Aphididae Aphisglycines (soybean aphid) Species 2: Coleoptera Coccinellidae Harmoniaaxyridis (multicolored Asian lady beetle) Species 3: Coleoptera Coccinellidae Coccinellaseptempunctata (sevenspotted lady beetle) Keywords: predation, refuges