Extension entomologists around the Midwest found themselves plenty busy last week fielding reports and questions from growers about alfalfa weevil activity. Here are what several had to say.
Throughout Iowa, the cold, wet spring has delayed the emergence of many insect pests, including alfalfa weevil, says Erin Hodgson, Iowa State University Extension entomologist. Through the first of week of May, she reports, weevils had just started to emerge in the state’s southern counties.
Sample using a sweep net. If larvae are found, collect six alfalfa stems from five locations throughout the field. Take each stem and shake into a bucket to dislodge larvae. Average the number of larvae per stem and plant height, then use the economic threshold table from this ISU report to determine if a foliar insecticide treatment is warranted.
“Remember, cutting alfalfa is an effective management tool for alfalfa weevil larvae,” says Hodgson, “and an insecticide application may be avoided if harvesting within a few days.”
In Ohio, alfalfa weevil feeding has been heavy even to the northeastern border, report Ron Hammond and Andy Michel, entomologists with Ohio State University (OSU) Extension.
To determine an economic threshold for treatment, they advise checking the number of larvae per stem, the size of the larvae and the height of the alfalfa. On alfalfa 12” or less in height, use a rescue treatment if one or more large larvae/stem is found. For alfalfa 12-16” tall, the threshold increases to 2-4 larvae/stem depending on the vigor of the alfalfa growth. When alfalfa is 16” in height and there are more than four larvae/stem, early harvest is the best bet.
In central Missouri the first week of May, warm and wet weather brought out fungal pathogens that killed off 90% of the weevils in some alfalfa stands, reports Wayne Bailey, University of Missouri Extension entomologist.
Growers weren’t as fortunate in the southern part of the state. Alfalfa emerged early, there were more weevil hatches and many recommended pesticide sprays did not work there this spring, he says. “Approved pesticides work best when temperatures reach 60ºF. The weevils go to work at 48º and that leaves a huge window that favors the bugs.”
Fungal pathogens can wipe out alfalfa weevils in 72 hours. Infected weevil larvae become lethargic and the lime-green worms turn a sickly yellow. The larvae die shortly after.
“Alfalfa growers who find sick weevils should not spray. The pathogens will do their work,” Bailey says.
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As temperatures in Nebraska warm up, expect to see alfalfa weevil larvae in the southern part of the state first, then, slightly later, in northern Nebraska, report University of Nebraska Extension specialists. They also recommend scouting for the pests and their feeding damage. For scouting and threshold information, visit this CropWatch page.
In North Dakota, cold weather has kept alfalfa weevil eggs from successfully overwintering, according to Janet Knodel, entomologist with North Dakota State University.
That’s quite a difference from last spring, when weevils emerged earlier than normal after a mild winter and caused extensive damage.
Based on accumulated degree days, Knodel doesn’t expect adult weevils to start emerging in the state for another one to two weeks. Even so, she encourages growers to routinely check the insect degree-day map on the North Dakota Agricultural Weather Network website for updates. “(It) will give you a window of when to scout, when damage might occur and when control action might be necessary,” she says.
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