Author(s): Kelley Tilmon, Mark Sulc, Rory Lewandowski, Andy Michel
We have been getting a number of reports about alfalfa weevil damage recently. A recent C.O.R.N. newsletter article reviewed basic scouting and management advice, available here, and we also have a factsheet with more detailed information, entitled "Alfalfa Weevil on Alfalfa."
However, organic producers face a particular challenge in managing this pest due to the limited number of products available for rescue treatment. Potential options for Ohio organic producers are Pyganic (an organic pyrethrum) at 4.5 oz – 17 oz / acre and Entrust (a spinosad) at the higher rate of 1.25 oz/acre. We have not tested these products, but a University of California study found Entrust at the high rate to be moderately effective for alfalfa weevil control. There is also a Bt product called beetleGONE!® ag that has shown some efficacy in a USDA-ARS trial in Montana.
If the weevil infestation is above the action threshold, we recommend treatment even with a high cost product. Why? In the past we have known organic alfalfa producers who lost most of their entire year’s crop as a result of weevil feeding in the spring. If weevil pressure is heavy, the first crop will be lost (unless it is cut before severe defoliation occurs). The weevil can then survive on the stubble and feed on the crown buds, preventing the crop from regrowing, to the point where the stand never truly recovers during the summer.
Whether organic or not, alfalfa producers should be careful and attentive to weevil infestations because severe damage can happen quickly by this voracious pest.