Soybean aphids have been detected on V1 soybeans. This is not unusual as we know soybean aphid egg hatch and initial generations occurred on the overwintering host buckthorn earlier this spring.
Field Note: Soybean aphids detected on soybeans last week of May
Eileen Cullen, Extension Entomologist, UW-Madison Entomology Department
Soybean aphids have been detected on V1 soybeans. The first report of the season at UW-Madison came from Dr. Dave Hogg, Entomology Department, last Thursday May 24th from the West Madison Agricultural Research Station. On inspection in small plot experimental plantings he looked at 60 plants and observed 3 plants with winged adult aphids plus nymphs, and another 5 plants with nymphs only. Thus, 8 of the 60 plants inspected had aphids, ranging between 1 and 13 aphids per plant. On Wednesday, May 30th John Gaska, Sr. Outreach Specialist, UW Madison Soybean Agronomy Program, reported a similar find at the Arlington Agricultural Research Station. Likewise, UW-Extension Jefferson County Crops/Soils Agent Tim Bender related a report from Tom Novak, CCA, Total Crop Management of soybean aphids on V1+ soybeans found in a commercial field.
This is not unusual as we know soybean aphid egg hatch and initial generations occurred on the overwintering host buckthorn earlier this spring. Field detections this week simply point to the fact that soybean aphids have left buckthorn and are locating soybeans, their summer host plant. By comparison, in 2006 the first soybean aphids were reported in Wisconsin during the first week in June, and in 2005 on June 3rd (according to reports received by WI DATCP Pest Survey and UW Madison).
fall suction trap captures were very high in some Midwestern states (IL and IN) and quite low in others (WI and MI). High trap captures in states like IL and IN indicated potential for a heavy soybean aphid overwintering population and potential regional source of summer migrants to soybeans.
Soybean aphid populations in 2007 will depend on several factors. Predators, such as the Asian lady beetle, can reduce aphid populations on buckthorn in fall before aphids deposit eggs; and again in spring after eggs hatch. Eggs that are laid must survive the winter. Heavy rains or sub-freezing temperatures in early spring can kill young aphid colonies on buckthorn in spring. Once winged soybean aphids leave buckthorn and arrive on soybean (as they now have been detected), factors such as temperature will begin to affect soybean aphid population growth in the field.
What do we know about likelihood of overwintering survival on buckthorn?
During the last week of April 2007, entomologists David Voegtlin (University of Illinois Natural History Survey) and Bob O’Neil (Purdue University Entomology Department) completed their bi-annual soybean aphid survey of buckthorn. They traveled from NE Indiana (Fort Wayne) to Toledo, Ohio, across lower Michigan, and then over to the Quad Cities area of Illinois. They surveyed Rhamnus cathartica (common buckthorn) in 9 sites and found soybean aphids in all but one site. Aphid numbers in all the sites, except those in the Quad City area were relatively low. In the Quad City area, large soybean aphid colonies were easily found on buckthorn with many plants having distorted leaves. Ants were observed tending colonies and predators (multicolored Asian lady beetles were present).
Comparing the number of soybean aphids found at these overwintering locations spring 2007 to the number of soybean aphid eggs they found at the same sites fall 2006, there was evidence of significant winter kill, presumably associated with the sub-freezing temperatures in early April (e.g., after soybean aphids had begun emerging late March, near buckthorn bud-break). However, relative to other years of the buckthorn soybean aphid overwintering survey route, spring 2007 aphid numbers were relatively high. For example, in 2006 sampling the transect they found only 2 soybean aphid colonies total. In 2005, roughly 50% of the sites had colonies. In spring 2007, although soybean aphid numbers were lower than expected from very high fall 2006 egg deposition, soybean aphid were located at 8 of 9 sample locations.
Soybean aphid scouting should begin mid to late June, by late vegetative stage of soybean growth and continue on a weekly basis or near-weekly basis until R5. Regular articles in Wisconsin Crop Manager will feature soybean aphid scouting, threshold and treatment decision support information and 2007 resources.