Alfalfa Weevils Already?
Bryan Jensen, IPM Program
What a spring! I don’t recall thinking about alfalfa weevils this early. Take a look at UW Extension’s Ag Weather Site, http://www.soils.wisc.edu/uwex_agwx/ (click on Thermal Models). Southern Wisconsin may soon be at the 300 weevil degree day (wdd) scouting threshold. Not only is it the earliest I’ve seen the scouting threshold reached, but it is the shortest I’ve seen alfalfa when it was. Certainly a lot can happen between now and the time weevils could cause economic damage. However, it will soon be time to start scouting.
Weevil populations have increased in recent years. To get an early indication of damage, scout sandy knolls and/or south facing slopes. If feeding injury is found in these areas monitor the rest of the field. A concern I have is the sudden and dramatic rise in temperatures has put weevil development ahead of alfalfa. Under typical growing conditions, first crop harvest and peak weevil feeding coincide. This year could be different and weevil injury may peak earlier.
At 300 wdd, eggs start to hatch. First and second instar larvae, which are slate colored, can be found in the upper developing leaflets and injury will appear as small pinholes. This injury is not considered economic but a sign of things to come. When larvae reach third instar (504 wdd) a larger volume of foliage is consumed and is the time that most control decisions are made. To make a detailed evaluation of first crop weevil damage, walk an M-shaped pattern and collect 50 stems at random. When finished, carefully look over each stem for signs of weevil feeding. Count all stems that show signs of feeding and divide that number by 50 (total number of stems initially collected) to determine percentage tip feeding. Control is suggested when 40% or more of the stems show signs of weevil feeding and you are more than 7-10 days from harvest.
Occasionally, you may find some alfalfa “weevil-like larvae” which are significantly larger than the degree day model suggests. If they have a tan colored head they are clover leaf weevil. Clover leaf weevils overwinter as larvae and therefore develop quicker than alfalfa weevil. Don’t get alarmed. They rarely cause economic damage, just confusion. Both weevil larvae are the same color and will have a white strip down their back. However, alfalfa weevils always have a black head.