Soybean aphids are being found in a number of Iowa fields this summer. Current dry weather would favor aphid development. However, itâ€™s also very hot, which should help keep aphid populations in check.
The best way to know for sure what aphid populations are doing on your farm is to scout your bean fields at least once a week, says Paul Kassel, Iowa State University Extension crop specialist at Spencer in northwest Iowa. Are the aphid populations going up or down in your fields? Kassel says the speed sampling method is described at www.soybeans.umn.edu/crop/insects/aphid/aphid_sampling.htm
While spider mites thrive in 90-degree heat, soybean aphids do not. Aphids like it a little cooler, such as temperatures around the 80-degree mark. The very hot weather will likely keep aphid populations from rising, explains Kassel.
Aphids can increase population very quickly
This pest continues to lack predictability in 2005, with most reported infestations that have reached threshold (250 aphids per plant and increasing) being spotty within fields. "While all soybean fields should be checked weekly for this pest, the fields likely to be at a higher risk at this time are the later planted, less mature stands," says Brian Lang, ISU area crop specialist at Decorah in northeast Iowa.
The aphidsâ€™ winged-migration that occurs in early to mid-July tends to find these fields. The aphids can increase in population quickly on the younger, rapidly developing soybean plants.
"Our research plot near Decorah peaked in population last week," Lang said on July 19. "Heavy winged-aphid movement off this site, plus the possibility of continued high temperatures resulted in a large drop in aphid population at this site this week. I just received a phone call from the Cresco area where an agronomist told me of the same situation in a field he is in right now."
"Please do not take this to mean that the populations will drop in your fields. Consider that these and other winged-aphids may have moved off of sites like this one and entered your fields. So keep scouting weekly for potential problems. Seeing a drop in population in late July at this site is not entirely unexpected. We had a similar drop in 2001 and a slight drop in 2003. However, these same fields showed a resurgence in aphid populations in early August."
If scouting, keep an eye on spider mites too
When scouting your bean fields looking for aphids, also watch for spider mites. Continued dry weather and the presence of spider mites will influence your insecticide choice, says Kassel. Scout weekly through August. For supporting information on soybean aphids go to www.soybeanaphid.info
"We continue to see increased spider mite activity in northeast Iowa," says Lang. "A few fields are being treated this week," he said on July 19. "If dry weather continues, this pest could become a significant problem."
Under drought conditions, treatment is recommended if leaves in infested areas are stippled (yellow spots or freckles on the upper side of the leaf) and live mites are present on the underside of the leaf. Symptoms often appear on field edges first, but symptoms could easily occur first within a field especially on patches of soybean plants that are growing on lighter soils within a field.
Should you spot-treat spider mites?
Spot treatment with an insecticide spray can work if the infestation is localized, but its usually best to treat the entire field, says Lang. Treatment may be delayed if cooler temperatures and high humidity are expected, however, scattered thunderstorms and rain alone cannot reduce mite infestations. See the following Web site for photos and recommendations.www.ipm.iastate.edu/ipm/icm/1999/8-9-1999/spimiteburn.html
Best control of spider mites tends to be with the use of organophosphate insecticides, says Lang. While Dimethoate (also called Cygon) is one of the recommended products for spider mites, it is clearly not as effective on soybean aphid as Lorsban. Pyrethroid insecticides tend to be weak on spider mites, and may actually create a worse problem by killing off beneficial predators that feed on spider mites. Lorsban, an organophosphate, would be the best recommendation for a field infested with both of these pests.