Biotic and Abiotic Influences on Within-Plant Distribution of Soybean Aphid (Hemiptera: Aphididae: <i>Aphis glycines</i>)

The within-plant distribution of soybean aphid, Aphis glycines Matsumura has been attributed to various causes, yet it is unclear which factor or combination of factors most influence aphid within-plant distribution. Understanding soybean aphid distribution within the soybean canopy is important both in developing sampling programs and in evaluating injury. Because natural enemies have the potential to alter aphid dispersion, we examined how within-plant distribution of soybean aphid was influenced by different natural enemies. Field cages of different mesh diameters were used to exclude different sizes of natural enemies from aphid infested plots. Plots were surveyed and both natural enemy density and soybean aphid density, by strata, were recorded twice weekly. Cages were found to have minimal effect on temperature and soybean plant development. Aphid densities were significantly different each year among dates, treatments, and strata. In 2004 aphid densities were concentrated on the top part of the plant and in 2005 aphid densities were concentrated in the middle of the canopy. Treatments had significant impacts on aphid density in both years of the study. The total exclusion treatment had the highest aphid densities in both years. Treatment also had a significant effect on which stratum aphids were most commonly found. The factor that was found to best explain the differences in aphid distribution was the abundance of natural enemies, primarily Orius insidosus (Say). In treatments left exposed to natural enemies the percentage of the total aphid population was found lower in the canopy. It is clear that aphid densities and distributions are being affected by the presence of natural enemies. Understanding how natural enemies affect the distribution of this economically damaging aphid species is an important component of its management, specifically the development of accurate scouting protocols and pesticide efficiency.