Weather Extremes Are Learning Opportunities
Dean Malvick, University of Minnesota, found more than 20 different pythium species while sampling soybean fields.
Wet weather or dry, the pests are with us, though which ones are troublesome varies with the weather. Weather conditions in 2012 and the spring of 2013 stressed plants in extreme ways and those who study these pests and diseases. The results have raised questions about efficacy of existing controls and the pests themselves.
Last year’s heat and drought and the extremely heavy and prolonged rains in many areas this spring created textbook opportunities for studying insects and pests as corn and soybean planting stretched out into mid-June and in some cases later. Fields of newly emerging corn and soybeans sit alongside fields with seedlings inches, or in some cases, a foot and half to two feet taller.
“Last year’s heat speeded planting and the development of insects,” said Matt O’Neal, field crop entomologist, Iowa State University (ISU). “Growers were concerned by defoliation from Japanese beetles on plants in May and June, a pest that is normally seen in July and August. The early emergence of this pest was worrisome because it is not controlled by seed treatments. We also had the worst spider mite outbreak since 1988. Spider mites do very well in dry, hot conditions. This year the cold weather slowed the development of these pests, likely reducing their impact in 2013.”
When interviewed in late June, O’Neal was still waiting to see if the wet soils would increase insect mortality. He was surprised to hear reports of early and high levels of soybean aphids. Because of the extreme weather, everything is up for grabs. In a normal year, predator insects tend to emerge with prey. This year, O’Neal suggested, that may not be the case.
“The delays in planting and emergence may delay a predator response as well,” he said. “Predators need insect prey to fl ourish. How strong their response will be is the question.”
THE MYSTERY OF SOYBEAN APHIDS
In 2012, one pest that was hard to measure was aphids temperatures,” said O’Neal. “This year appears to be the opposite of 2012, with cooler spring temperatures allowing soybean aphids to establish and later colonize soybean fields. The evidence is they are out there, and we may see populations we haven’t in the last couple of years.”
He suggested that heavy populations in areas that have a history of problems with the pest might encourage farmers to consider aphid-resistant soybean varieties. Work done at his lab at ISU emphasizes the benefits of multiple or pyramided resistant genes. He noted that plants carrying a single aphidresistant gene benefi tted from a seed treatment. However, plants with two resistant genes were so aphid-clean that no benefit in aphid control could be seen with a seed treatment.