Population dynamics and sampling of the soybean aphid

The soybean aphid, Aphis glycines Matsumura, can be an economically important pest in Midwestern soybean, Glycine max (L.) Merrill. Soybean production has been altered since the discovery and establishment of this invasive insect in 2000. In 2001--2003, we evaluated the spring colonization of soybean aphid to better understand the population dynamics and make treatment recommendations. In a state-wide study of Minnesota, spring colonization began during the early vegetative stages and lasted approximately two weeks. The production of summer migrants started prior to the flowering of soybean and continued until pod set. On a relatively smaller scale, non-attractive pan traps were evaluated as a tool for describing the landing behavior of alatae aphids. Spring colonizing alates did not preferentially land at commercial soybean field edges. The precision and cost of pan trapping was compared to whole-plant counts. Both sampling methods were generally precise over the season, yet pan trapping may be too time-consuming for most research. The spatial and temporal within-field distribution of soybean aphid was examined using geostatistics. Variograms and kriged density contour maps also indicated soybean aphids did not have a landing preference during initial colonizing in early vegetative soybean. However, yield analyses indicate one well-timed insecticide application at the economic threshold can reduce loss. As a result of these small scale studies, site-specific management is not recommended as a tool to suppress soybean aphid populations. Two sampling plans were created as a method to estimate soybean aphid densities in commercial soybean. An enumerative (i.e., counting) sampling plan was developed for reaching desired precision; 310 plants are required for ecological research and 38 plants are required for pest management purposes. A binomial sequential (i.e., presence/absence) sampling plan was developed for making treatment decisions. A minimum of 11 plants are required to make a management decision using 40 aphids per plant as a tally threshold. Finally, a preliminary description of the mitrochondrial DNA (deoxyribonucleic acid) provided evidence of the soybean aphid being genetically distinct from other closely related aphids and has not yet developed biotypes throughout native and introduction populations.