Posted 16 August 2007. PMN Crop News.
Whiteflies Found in Soybeans Raise Alarm, But Aphids Remain Major Economic Threat
University of Missouri. cafnr.missouri.edu
Columbia, Missouri (August 9, 2007)--Soybean farmers scouting their fields for aphids are finding another more colorful insect in abundance: whiteflies.
"No need to panic," said Wayne Bailey, University of Missouri Extension entomologist. "There is no known economic damage to soybeans from these whiteflies." He does not recommend spraying whiteflies.
Some whiteflies can be a problem, but the ones in soybean fields this year are not of that species, the entomologist said.
"Soybean aphids remain a major concern," Bailey said. "In most fields, especially across central Missouri, the aphid population is being controlled by both the hot, dry weather and beneficial insects, such as ladybugs."
However, extensive spraying of aphids is underway in the northern tier of Missouri counties, Bailey said. The economic threshold for possible damage is 250 aphids per plant.
Recent rains across northern Missouri may revitalize the aphid populations there, Bailey warned. Scouting should continue in all soybean fields.
"If you find only 50 or 60 aphids, let the predator insects have first chance," he added. "But, keep an eye on those fields."
"Most of my phone calls and e-mails concern whiteflies," Bailey said. "Whiteflies, being white, are much more noticeable. Fifty whiteflies look much more impressive than 50 aphids." Aphids are so small that they must be viewed with a 10-power hand lens to be properly identified.
"There was a great a deal of confusion about whiteflies based on Internet messages," Bailey said. "A report of 25 percent crop loss from whiteflies was being circulated, but that was for a whitefly found in Australia that is a different insect.
The whitefly found in Missouri this year is the sweet potato whitefly. "The high number of whiteflies is unusual, but low levels are present most years," Bailey said. "Economic damage by sweet potato whiteflies to soybeans is rare; however, they can wreck a sweet potato patch or other horticultural crops."
Bailey suspects the population built up on weeds, "especially velvet which they love." When farmers sprayed weeds in their soybean fields, they killed the host weeds. "The whiteflies have to go someplace, so they congregate on the underside of the leaves of the nearest soybean plant."
Some producers went ahead and sprayed insecticide on the whiteflies. Bailey's advice: "Save your money, unless other more important pests are present."
If aphids are at levels of more than 250 per soybean plant, he recommends spraying.
Across most of Missouri the hot, dry weather was slowing the aphid population, Bailey said. "When temperatures get up into the 90s, aphids stop reproducing."
Recent rains across northern Missouri may revive the aphid populations there. North winds that brought the cool fronts with much needed rain also brought a fresh invasion of winged aphids from Iowa.
"We're finding both winged and wingless aphids in fields across north Missouri," Bailey said. "The ones with wings are new arrivals."
Soybean fields in the hot, dry regions could see an outbreak of two-spotted spider mites, Bailey said. "The spider mites can kill the weather-stressed soybean plants."
"Spider mites are smaller than aphids, so you have to get out into the fields to find them," Bailey said. "With the price of soybeans, they are worth watching closely."