February 18, 2004 | By: Laura Skillman
OWENSBORO, Ky.

Soybean aphids have only been found in the United States within the past four years and are beginning to be a problem in some northern states. Kentucky also has seen the insect, but to date has not had any major infestations.

Kentucky farmers have some reasons to be hopeful that the insect may never become a problem pest, but it is an insect they should become aware of and understand scouting and management methods associated with it, said Doug Johnson, Entomologist with the University of Kentucky Cooperative Extension Service.

The insect is a threat because it has a lot of ways it can hurt soybeans. Mold can build up as the result of honeydew from the aphids reducing photosynthesis. It can also cause premature yellowing. Stunting the soybean plant is the biggest factor with the aphid. It also has a strong ability to transmit viruses.

The aphids are bright yellow, pear-shaped and soft bodied. If a producer finds aphid colonies in his soybeans it is the soybean aphid because no other aphid in the United States colonizes soybeans, Johnson said.

Researchers throughout the north central United States are studying the insect. In Kentucky, research also is under way but the number of aphids has been too small to allow for much data to be collected.

Parts of Iowa, Minnesota and Wisconsin are having problems with the insect while other areas of infestation are near urban centers, which is good for Kentucky because most of the soybean production is not near urban centers. The aphid over winters on particular species of Buckthorn tree that is very rare in Kentucky, Johnson said. There are now three species of the tree known to host the aphid. However, none of the tree species are common in Kentucky. There are, however, at least two species of Buckthorn found in Kentucky. They primarily are in urban areas, not rural areas, and are not known to host the overwintering aphids.

“That’s good for us because we don’t want it to have the ability to over winter, if it is going to get here, we want it to have to come from somewhere else,” he said. “Migration back to the host is poor, which is also good for Kentucky producers.”

In the past growing season, there were more aphids than in previous years and the migration study from the previous fall noted a heavier migration back to the host plants. That may mean farmers in Kentucky and other areas could be given advance notice that the aphids could be heavier one year to the next.

Asian Lady Beetles eat these aphids and follow them right back to their host trees, which also bodes well for Kentucky producers.

“They are doing a world of work for us,” Johnson said.

The soybean aphid found in the United States also seems to prefer cooler climates, which may be a limiting factor for its progress into Kentucky.

“Most, not all, populations in Kentucky remain small, and I think we will continue with that,” he said. “The kicker may be that at 86 degrees there is a 60 percent reduction in the insect’s ability to reproduce and that’s pretty good because we have a lot of 86 degrees days. At 95 degrees you get 100 percent mortality. So it is definitely not a southern adapted pest. It can survive in the south, but surviving and being a big pest for us are two different things.”

Farmers should begin scouting for these insects in the first week of July. Late-planted and double-cropped soybean fields have the biggest potential for problems and should be checked first. Scouts need to look at the entire plant because the aphids move about the plant as temperatures rise.

A threshold has been developed to help producers determine when it is necessary to use chemical controls. The aphids must have an actively increasing population, so fields must be looked at more than once. If populations are increasing, the thresholds are 250 insects per plant from right before flowering to full pod. If the plant is in full pod to full seed, then the threshold is 1,000 insects per plant. After full seed, no benefit from using an insecticide is shown, he said. It is common for the insect to get near the threshold level and not go any higher.

Insecticides are available, if needed. The best yield results have been shown to occur when spraying is done between beginning bloom and beginning pod. Results go down as the plant progresses through the growing season.  

When determining whether to spray, farmers need to remember they get a return only if aphids are in the right numbers. Control varies with insecticides but it did not make any difference with yield based on early research.

Growers also might want to consider that some insecticides will move through the canopy better than others and some have better residual effect. Temperatures also can negatively affect some chemicals’ effectiveness.

Growers don’t want to spray unless it is needed because the sprays also will eliminate the aphid’s predators, and if good control is not attained with the spray this insect can quickly rebound. Doubling time of the insect is two days.

For more information on soybean aphids or soybean production, contact a county office of the UK Cooperative Extension Service.

                                   -30-

Contact: 

Writer: Laura Skillman 270-365-7541 ext. 278
Source: Doug Johnson, 270-365-7541 ext. 214