By John Obermeyer, Christian Krupke and Larry Bledsoe
An incredible entomological phenomenon has occurred among the whirling combines this fall, but has mostly gone unnoticed.
Massive numbers of winged soybean aphids have left maturing soybean fields in search of their overwintering host, buckthorn. In some area of the state, clouds of these gentle flying insects have rivaled the soybean dust floating throughout the countryside. The populations seen in late September on buckthorn were impressive -- now for "the rest of the story."
Common buckthorn (Rhamnus cathartica) is a shrubby tree that can form dense thickets on the edges of woodlands. Because of its hardiness, it was brought to North America from Asia decades ago for fencerows and wildlife habitat. It is now classified as an invasive species because of its ability to outcompete native forest species. Ironically, it is considered an ornamental by many, and can be found around home and business landscapes.
Buckthorn exists in scattered dense patches in Indiana, with the highest concentrations of common buckthorn occurring in the northern Midwestern States (Minnesota, Wisconsin). Buckthorn is considered the soybean aphid's primary host, because this is where eggs are laid in the fall for overwintering. For this reason, availability of buckthorn is necessary for the aphid's life cycle.
In late September, we observed local patches of buckthorn being inundated with soybean aphid, winged and nymphs. Interestingly, a lone buckthorn bush in the middle of campus, at least a mile away from the nearest soybean field, was thoroughly infested with aphids.
This and similar sightings prompted researchers Bob O'Neil and David Voegtlin, from Purdue University and the University of Illinois respectively, to investigate known buckthorn patches in the Great Lakes region. In some areas, especially northern Indiana, the density of aphids and eggs were remarkable. These observations correlate well with the unprecedented high numbers of winged soybean aphid captured in Indiana's suction traps (see graph).
While the soybean aphid migration to and egg-laying upon buckthorn this fall may sound like impending doom for next year's soybean crop, there is a glimmer of hope. In the midst of the birthing and egg-laying aphids were multitudes of predators: Asian lady beetles, syrphid fly larvae, minute pirate bugs (the tiny ones that bite), etc. The previously mentioned buckthorn bush on campus was completely aphid-free in one week's time because of the numbers of natural enemies.
Where low densities of buckthorn exist, the predators seem to have won out. However, the "good guys" were probably overwhelmed with aphids/eggs in large buckthorn thickets. This is evident by some buckthorn, observed by Bob and Dave, still heavily infested with eggs.
For 2007, as in the previous three odd years (2001, 2003, 2005), the stage seems to be set for a significant risk of soybean aphid infestations next season. Several variables need to be played out before anyone can say with certainty what will occur. For example, predators will likely continue to feed on overwintering eggs this fall and early spring, reducing next year's numbers further.
Though it may seem counter-intuitive, early aphid movement to soybean may actually prevent them from getting to yield-threatening levels throughout the summer because of a boost in natural enemy numbers. As always, timely soybean planting and ample moisture throughout next year's growing season will reduce the soybean aphid impact, even with long-term, low infestation levels.
Simply put, timely soybean scouting and treatments when necessary are the most efficacious and economic way to deal with pests such as soybean aphid. More to come as this story unfolds next spring.
SOURCE: Pest & Crop newsletter from Purdue.