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 Prepare to Monitor Alfalfa Stands for Alfalfa Weevil Larvae Minimize
Location: BlogsWisconsin Crop ManagerInsects and Mites    
Posted by: WCM Staff 4/28/2005 11:10 AM
Alfalfa weevils overwinter as adults, becoming active with the first warm days of spring in which they migrate from plant debris, woodlots, ditchbanks, etc. into alfalfa stands to lay eggs. Most eggs are deposited in stems of the growing alfalfa. By using degree days (accumulated above a base 48F), growers and crop advisors can monitor fields for alfalfa weevil activity.
Prepare to Monitor Alfalfa Stands for Alfalfa Weevil Larvae
 
Eileen Cullen, Extension Entomologist
UW Entomology Department
 
Alfalfa weevils overwinter as adults, becoming active with the first warm days of spring in which they migrate from plant debris, woodlots, ditchbanks, etc. into alfalfa stands to lay eggs. Most eggs are deposited in stems of the growing alfalfa. By using degree days (accumulated above a base 48F), growers and crop advisors can monitor fields for alfalfa weevil activity. This is based on the knowledge that alfalfa weevil eggs will begin to hatch at 300 degree days. Degree day heat units (DD) are accumulating, bringing alfalfa weevil eggs closer to hatch. Now is the time to remember to start monitoring first crop for alfalfa weevil larvae and potential tip feeding on leaves over the next couple weeks.
 
Check the following web site for a daily updates on Degree Day accumulations in your part of the state: http://www.soils.wisc.edu/wimnext/crops.html.  Click on ALFALFA. Follow the Insect Phenology link for Alfalfa Weevil. The WI-MN Cooperative Extension Agricultural Weather Page displays updated Degree Day (DD) accumulation information for Wisconsin. At this article deadline (Apr 27), the following alfalfa weevil DD had been reached in various parts of WI: southcentral, southwest and westcentral 210-240 DD; southeast 120-180 DD; central 180-210 DD; and east central 90-150 DD. Remember, first alfalfa weevil eggs are predicted to hatch at 300 DD and continue from there. Time your alfalfa weevil larval monitoring accordingly.
 
Start monitoring alfalfa stands in fields with south-facing slopes or sandy knolls since these areas dry and warm up faster, accelerating temperature-dependent larval development relative to lower, colder spots in the field. You can use a sweep net for general detection purposes. Alfalfa weevil thresholds are NOT determined using a sweep net (as for potato leafhopper beginning mid-June). However, as we get into May and alfalfa weevil larvae are developing, if your goal is a general alfalfa acreage survey then sweeping can help you pick up larger larval stages and identify fields of concern. Look for tiny pinhole feeding. Later as larvae become bigger (2nd – 4th instars), pinhole feeding gives way to tip feeding and more extensive leaf tissue damage.
 
Scout alfalfa fields by collecting groups of 10 plants at 5 random locations throughout the field (50 random stems total). You want these plants to be representative of the field, so walking an M-shaped or similar pattern and avoiding sampling from field edges is recommended.
 
Smaller alfalfa weevil larval instars are slate-colored, but they become a bright green by the time they are full grown at 3/8 of in inch length. Later instar alfalfa weevils are clearly distinguished by their green color, a white stripe running down the back and black head capsule.
 
Count all stems that show signs of tip feeding damage and divide that number by 50 (total number of stems initially collected) to determine percentage tip feeding. Management action is recommended when 40% or more of the stems show signs of alfalfa weevil larval feeding.
 
A treatment decision at the 40% tip feeding action level follows one of two routes - cultural or chemical. If 40% tip feeding occurs more than 7-10 days out from harvest, an insecticide treatment is recommended. For insecticides labeled for alfalfa weevil control, consult University of Wisconsin-Extension bulletin number A3646 "Field Crop Pest Management in Wisconsin" which is available from your local county extension office. You can also access the bulletin on-line at: http://ipcm.wisc.edu/wcm/pdfs/2003/wcm03-7.pdf . (The 2005 edition is posted - we update this each year at the end of the previous summer to reflect insecticide new registrations, cancellations and label changes).
 
Alternatively, “early” cutting, within 7-10 days of planned harvest, will afford you alfalfa weevil larval control without the cost of an insecticide application or compromise to first crop quality. In the event that multiple fields approach the 40% tip feeding level consider early cutting in fields with relatively lighter infestations close to 40% damage, reserving insecticide applications for the more heavily infested fields at or above 40% tip feeding. You’ll also want to keep an eye on re-growth following an early first cutting to confirm that larvae are not suppressing re-growth.
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