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Be On The Lookout For Bugs In Alfalfa

Be On The Lookout For Bugs In Alfalfa
The alfalfa weevil will be one of the main perpetrators this year.

By Jennifer Bradley

There is potential for a whole lot of pest problems in alfalfa this year, says Jim Stute, Rock County Extension crops and soils agent. He says the alfalfa weevil will be one of the main perpetrators this year. The weevil is approximately 1/4 inch in length, light brown in color with a brown V-shaped shield on its back.
Bryan Jensen, an Integrated Pest Management coordinator with University of Wisconsin-Extension in Madison, says that he doesn't believe the late spring will have an impact on the insect population, including the alfalfa weevil which overwinters in the state.

WEEVIL DAMAGE: Alfalfa leaves need to be scouted for pinhole or lacing type feeding damage from alfalfa weevils.

The current weather does, however. Jensen says that calendar days are inaccurate to use when scouting because they don't consider actual weather. He suggests using degree days, and that farmers should begin monitoring for alfalfa weevil at around 300 Weevil Degree Days. An up-to-date thermal model helps farmers calculate this for their area. (

Jensen walks through the scouting process in an online video, which can be found here

Scout early and often
Jensen suggests farmers walk their fields in "W" pattern and pick 50 stems at random. Alfalfa leaves need to be scouted for pinhole or lacing type feeding damage from weevils. He says that the management threshold is reached if 40% or more of the stems show tip feeding. A farmer should then consider harvesting early or treating the pests. Jensen recommends scouting again four to five days after harvest.

"Harvesting the first crop could take care of the weevils, but some years they get ahead and you have to watch if they are eating the regrowth," Stute adds

Jensen says that in the case of the insects that migrate to Wisconsin, the spring weather will definitely influence populations. Two examples of these pests are the potato leafhopper and variegated cutworm. "Last year we had a very warm March which ultimately brought an unprecedented migration of variegated cutworms to Wisconsin," he says.

The potato leafhopper populations were also abnormally high. While the 2012 spring weather fronts brought these pests to the state early, the hot, dry weather also helped them reproduce rapidly once in the fields. 


When the pests are here, what can a farmer do?

"I hate to sound like a broken record, but they should be scouting on a regular basis," says Jensen. This practice, he says, keeps farmers ahead of the game and not surprised, as with last year's infiltration of the cutworm. Jensen says it's rare to get a population that high and that early, and those who scouted picked them up quickly and were prepared.

With the potato leafhopper, consistent scouting can give farmers the data necessary to decide if they need to control the population. Jensen says , if pest management is necessary, applications can be timed appropriately.

The other big issue, especially in the southern half of the state lies in the alfalfa that was planted last year. It was really stressed, Stute explains, saying that the plants are now questionable in their longevity and productivity.

"The problem was they didn't develop a normal root system, and research data shows clearly that those with smaller roots systems never do as well," he says.

These plants are much more susceptible to stress, which they experienced over a dry summer in 2012 and long winter. The pests are also a concern for these weaker plants. Stute says, several farmers have approached him with questions about what to do. He says that scouting is a must, and though timing might be tight, inventory is too. Regardless of where a farm is, alfalfa pests will return and scouting will help control them.

Bradley lives in East Troy.

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