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Issue No. 2, Article 2/April 6, 2007

Alfalfa Weevil Activity Is Conspicuous in Southern Illinois

Let me begin with an overview:

  • Alfalfa weevils are causing significant damage to alfalfa fields in southern counties of Illinois. The injury and densities of weevils in some fields have justified insecticide applications.
  • Use accumulated degree-days to predict when larvae hatch and begin causing injury.
  • Scout now in alfalfa fields south of Route 50.
  • Use economic thresholds (static or dynamic) to make control decisions.

As soon as we entomologists returned from our annual meeting of the North Central Branch of the Entomological Society of America in Winnipeg, Canada (March 25-28), we began getting reports of significant alfalfa weevil activity in southern Illinois. Several reports received between March 30 and April 3 indicated noticeable injury in alfalfa fields as far north as northwestern Bond County. Some fields farther south have been damaged significantly. In the words of Kevin Black, insecticide/fungicide technical specialist with Growmark, "Alfalfa weevils are tearing apart alfalfa fields in the south." Many producers who are getting ready to plant corn have been surprised by the intensity of the injury this early in the season.

Ron Krausz, superintendent at the Southern Illinois University Belleville Center (St. Clair County), reported alfalfa weevil feeding as early as March 27. Ron Hines, now a seed agronomist with Growmark in southern Illinois, observed 25% to 30% of the tops of alfalfa plants injured by alfalfa weevil larvae in a field at the University of Illinois Dixon Springs Agricultural Center on March 30. He estimated about 20% leaf loss. Robert Bellm, crop systems educator at the University of Illinois Edwardsville Extension Center, visited an alfalfa field in Madison County on April 3 and found an average of 3 larvae per stem and heavy feeding injury. Jim Nusbaum, Rolling N Services, scouted alfalfa in northwestern Bond County on April 2 and found minor injury but weevils themselves.

In several of these instances, alfalfa weevil injury and densities were at or greater than the published static thresholds of 25% tip feeding and/or 3 larvae per stem, justifying an insecticide application. Dennis Epplin, crop systems educator at the University of Illinois Mt. Vernon Extension Center, sprayed an insecticide on an alfalfa plot in Franklin County on April 2 and suggested that the spray was probably three days late. Obviously, the relatively early warm-up in late March has "activated" alfalfa weevils in a relatively large area of Illinois.


Alfalfa weevils and pea aphids from sweep-net samples, Dixon Springs Agricultural Center, March 30, 2007. (Photo courtesy of Ron Hines, University of Illinois)

As you know, we can predict biological events in some insects' life cycles by keeping track of degree-days with specific baseline temperatures and start dates. For alfalfa weevils, we begin accumulating degree-days from January 1 with a baseline temperature of 48°F. Larvae begin to hatch from spring-deposited eggs after 300 degree-days have accumulated; larvae hatch from fall-deposited eggs (a biological event that occurs in southern and central but not northern Illinois) even earlier.

A glance at degree-day accumulations on April 3 revealed that larvae have been hatching from spring-deposited eggs anywhere south of a line from Alton (Madison County) to Robinson (Crawford County), roughly a little north of Route 50. A 1-week projection (using historic temperature data) suggests that alfalfa weevil larvae from spring-deposited eggs will begin hatching in fields south of Springfield. A 2-week projection shifts the "larval hatching front" north to about Bloomington. A link to more information about alfalfa weevils indicates that major leaf feeding by third and fourth alfalfa weevil larvae occurs between 439 and 595 degree-days. You can keep track of degree-day accumulations for alfalfa weevils, a site co-developed by scientists at the Illinois State Water Survey and entomologists in the University of Illinois Department of Crop Sciences. The site is maintained at the Water and Atmospheric Resources Monitoring Web site (www.sws.uiuc.edu/warm).

It's important to point out that two distinct peaks of larval activity usually occur in southern Illinois, one from fall-deposited eggs and one from spring-deposited eggs. Hatching of overwintering eggs usually occurs when 200 degree-days (above a base temperature of 48°F) accumulate beyond January 1, and we suggest that scouting should begin when 250 to 300 degree-days accumulate. An early peak of third-stage larvae from overwintering eggs occurs after an accumulation of 325 degree-days; a second major peak of third-stage larvae from spring-deposited eggs occurs after an accumulation of 575 degree-days.

People throughout the southern half should be looking for alfalfa weevil activity; scouting now in southern counties is crucial. When you begin to scout alfalfa fields for alfalfa weevils, look first in areas of the field that might warm up early, such as south-facing slopes and areas of the field with lighter soils. The small, yellowish, early-instar larvae with black heads feed on these terminal leaves, causing injury resembling pinholes. Pinhole injury is not economic because the larvae are too small to cause significant defoliation. It is not until alfalfa weevils grow into third instars that they begin to cause more economic damage by skeletonizing the leaves. Third instars are bright green with a distinct white stripe along the center of the back, and the head capsule is distinctly black or dark brown.


Injury to tips of alfalfa stems, Madison County, April 3, 2007. (Photo courtesy of Robert Bellm, University of Illinois)


Fourth instar alfalfa weevil. (Photo courtesy of Kelly Cook, University of Illinois)

If you prefer to use dynamic economic thresholds (which incorporate plant height and value of alfalfa hay) instead of static thresholds for alfalfa weevils, consult Table 1, developed by entomologists at Iowa State University (published in Integrated Crop Management, IC-494 [6], April 11, 2005. Insecticides suggested for alfalfa weevil control in Illinois are presented in Table 2.--Kevin Steffey

Author:
Kevin Steffey

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