Issue No. 4, Article 3/April 18, 2008
Alfalfa Weevil Development
Despite the wet and cool conditions that have prevailed through most of April, enough degree-days have accumulated in southern Illinois that feeding activity by alfalfa weevil larvae may be noticeable in some alfalfa fields, particularly on south-facing slopes or in fields that tend to warm up early in spring.
Start scouting for alfalfa weevils when 150 to 200 degree-days above a threshold of 48°F have accumulated from January 1. A quick check of the "Daily Pest Degree-Day Accumulations" on April 16 revealed that as of April 15, 200 degree-days or more had accumulated south of a line roughly corresponding to Route 50 from St. Louis to Lawrenceville. In Dixon Springs and Fairfield, 279 and 220 degree-days, respectively, had accumulated as of April 15.
Dennis Epplin, University of Illinois Extension crop systems educator, has observed alfalfa weevil larval injury in one of his field trials at Ewing (Franklin County), so it's likely that similar or worse injury could be noticeable throughout the southern quarter of the state. As you begin to monitor alfalfa fields for alfalfa weevils, look for leaf-feeding injury first near the tops of the plants. Small alfalfa weevil larvae often feed within the folded tip leaves, and pinholes are symptomatic of early-season alfalfa weevil injury. As the larvae grow and consume more leaf tissue, more dramatic skeletonization of the leaves becomes apparent.
Early-season leaf feeding injury caused by alfalfa weevil larvae (University of Illinois).
For more photographs, descriptions, and information about scouting for alfalfa weevils, refer to our fact sheet. If scouting reveals an economic level of larvae and/or leaf-feeding injury, consider a registered insecticide to prevent further damage. A list of insecticides suggested for control of alfalfa weevils in Illinois in 2008 can be found on page 15 of the 2008 Illinois Agricultural Pest Management Handbook (in the PDF file of chapter 1).
With alfalfa hay currently worth more than it has been in the recent past, the rule-of-thumb economic threshold of 3 larvae per plant and 25% to 50% of tips being skeletonized likely will be questioned. As I indicated in "Will Economic Thresholds for Making Insect Control Decisions Be Lower in 2008?" in issue No. 1 (March 21, 2008) of the Bulletin, questions about economic thresholds for alfalfa weevil management are still up for grabs. A working group of field crop extension entomologists will address this issue throughout 2008, but thus far we have no easy answers. So use your best judgment when making alfalfa weevil control decisions, and respect the potential for ecological backlash if lots of fields are sprayed when numbers of alfalfa weevils are relatively low.--Kevin Steffey