Anticipating Aphids in 2011

As farmers anticipate another growing season, researchers have some early thoughts about what might be expected in regard to soybean aphids, but they note much will depend on the weather in spring and early summer.

University of Illinois Crop Sciences Extension Coordinator Michael Gray makes several observations based on what is known about soybean aphids and what happened in 2010.

“Soybean aphids did not reach economic levels across much of Illinois in 2010,” Gray says. “In the fall of 2009, a very large flight of soybean aphids occurred late in the season. Many winged aphids arrived on buckthorn in impressive numbers. However, not long after that large flight, a fungal disease swept through the crowded aphid colonies on buckthorn, causing populations to crash. Those few aphids that survived winter produced a small spring flight to soybeans in 2010. Then the summer of 2010 was very hot, which was not conducive to aphid establishment in many soybean fields across Illinois.”

Because overwintering numbers are quite low, Gray expects early season aphid numbers will be low.

David Voegtlin, an insect systematist with the Illinois Natural History Survey, has devoted the last several years to studying the biological lifecycle of soybean aphids. Voegtlin gives additional insight into why 2010 was not conducive to soybean aphids.

“In September and October, aphids produce the generation that will go back to buckthorn. Last year, in addition to low late season populations, the beans matured early so aphids didn’t have green soybeans on which to produce the generation of migrants that would fly back to buckthorn. The result was the lowest fall flight ever recorded since suction traps have been initiated to follow the fall migration.

“A lot can happen between fall and spring, but low fall numbers indicate relative small numbers in spring,” Voegtlin says. “Therefore, we expect aphids getting off to a little slow start in early summer. What happens later will depend on the weather.”

Iowa State University Extension Soybean Entomologist Erin Hodgson is cautious about making overall forecasts at this time of year about soybean aphids for the upcoming season. “In June we can be more confident about making predictions for the summer, once we know more about spring colonization.”

For example, Hodgson says, “In 2010 there was extreme rain and wind as aphids tried to migrate in May and June. Aphids are considered ‘weak fliers.’ They don’t survive severe weather as well as hardier insects. Therefore, they couldn’t build up numbers.”

The Biology of the Soybean Aphid

Soybean aphids, which seem to have been brought accidentally to North America from Asia, have a complex cycle through the season. In the fall, eggs are laid on buckthorn, which is considered their primary host and the only place they are known to overwinter. Buckthorn is a shrub or small tree that grows widely across northern states but it is unevenly distributed. Observations suggest most successful overwintering occurs in Minnesota, Wisconsin, Michigan, the northern part of Illinois and Iowa, and eastern South Dakota and North Dakota.

Small aphid colonies can occasionally be found in northern soybean fields from late May to mid June. As large populations develop in these areas, winged migrants can, in some years, distribute across a majority of the soybean acreage in the United States and Canada. If this occurs early enough in the season, soybean aphids can reach economically damaging levels as far south as Kentucky and Missouri. In fact, there may be 15 to 18 generations of aphids on soybeans during the summer.

Voegtlin says there is still much to learn. “When do winged aphids leave buckthorn? What happens if they fly before soybeans are up? How far do winged aphids fly from buckthorn? Is it only as far as necessary to find soybeans? How rapidly do spring colonies develop on soybeans and when do they scatter across the landscape to eventually populate much of the Midwest bean crop?” He believes understanding this biological information can lead to possibly limiting aphids’ spread.

“Why aphids move and when they move is largely unknown,” Hodgson says. “We think it because of crowding, but we don’t know what they consider crowding. If they’re happy where they are, they stay. If they are crowded and need food, they travel farther.”

Although aphids have been found as far south as Tennessee and Alabama, Voegtlin says, “It is unlikely these colonizations are by aphids that have flown from the northern tier of states; rather, colonies at the latitude of central or southern Illinois produce winged aphids that are carried south by favorable winds.”

Hodgson agrees. “On their own, aphids don’t fly far, maybe from a half mile to a mile. To move farther, they seem to go higher into the atmosphere and travel with the jet stream to move between counties and states.”

In late summer, Voegtlin says, it seems to be reduced photoperiod and temperature that produce a generation of winged females and males that migrate back to buckthorn. In late October and early November, overwintering eggs are laid that will hatch in spring to begin the cycle all over again. In some years, like 2010, when much of the soybean crop matures early, fields stop supporting further aphid development before the generations are produced which would migrate to buckthorn, thus limiting that year’s overwintering.

As Voegtlin notes, the soybean aphid continues to adapt to the climate of North America.

Scouting is Important

While the aphid situation in the summer of 2011 will depend on what happens in the spring, Hodgson says farmers must be diligent about scouting. “It depends on where you are. Early season scouting, through soybeans’ emergence and flowering, is especially important in the north. After flowering and later, we need to check for long distance movement.”

Aphid-resistant soybean varieties are a new tool to help minimize yield loss. Hodgson cautions, however, that this new technology is not the panacea for aphid control.

“Even if you are using varieties with host plant resistance to aphids, you need to use the same scouting methods and follow the same treatment thresholds with resistant varieties as with nonresistant soybeans. Because aphid-resistant varieties became commercially available just last year and it was not a bad year for aphids, we don’t have much real world experience about how resistant varieties will perform. Therefore, farmers should be in contact with Extension and be scouting to make decisions.”