Pan trapping for soybean aphid (Homoptera: Aphididae) in Minnesota soybean fields

Soybean aphid, Aphis glycines Matsumura, can be an economically important pest in soybean, Glycine max (L.) Merrill. In 2002 and 2003, we evaluated the use of nonattractive pan traps to estimate soybean aphid densities in commercial soybean fields. A regression analysis indicated a strong relationship between log-transformed pan trap counts and whole-plant counts (r² = 0.70). To measure soybean aphid dispersion as characterized by each sampling method, Taylor's power law a and b values were calculated. The mean-to-variance ratios for pan trapping (a = 0.85, b = 1.63, r² = 0.96) were comparable to whole-plant counts (a = 1.67, b = 1.99, r² = 0.93). The precision and cost (i.e., time) of each sampling method was evaluated using relative net precision. Both sampling methods were generally precise throughout the season (<0.25). However, the relative net precision of whole-plant counts (4.21) was greater than for pan traps (0.67), and therefore whole-plant counts are a more cost effective sampling method. We also evaluated trap placement within fields during the initial colonization period. Trap catches did not differ among directions within the field or at the edges of fields versus the interior of fields. This lack of a preference for alates to land at field edges may preclude the use of border sprays, which have proven effective in other systems.