A January 12, 2016 KPN article speculated on the impact of the 2015-16 winter on alfalfa weevil in Kentucky. Mild temperatures should have allowed females to be active and lay eggs on many winter days, potentially resulting in an early peak of larval feeding this spring. It appears this was the case, so this is a good year to watch alfalfa early and closely.
Sampling for Alfalfa Weevil
The 30-tip sampling method (ENTFACT 127) provides a good way to assess the critical factors in weevil infestations: current degree-day (dd) accumulation, plant height, and numbers of larvae per 30 stems. Sampling should begin when the 190 degree-day accumulation occurs in your area. This information is available on the UK Ag Weather Center Calculating Degree Days webpage. The 190 dd level was reached on March 14 in central Kentucky. This is the recommended point to begin field checks for weevil larvae that hatched from winter-laid eggs. Usually, this is a small portion of the population. The more common weevil peak following a “normal” winter is around 270 degree-days, when most of the spring-laid eggs are hatching. The current temperature forecasts predict the 270 dd accumulation to occur about April 7 in the central part of the state.
This is a year when the early field checks could be critical. Newly hatched and second-stage alfalfa weevil larvae, along with adults, were found during a March 14 field check in central Kentucky. Alfalfa plants in the sampled fields were in the 3 inch to 6 inch range. It takes a lot of weevil larvae (30 to 100 larvae per 30 stems) to justify treatment at this point, but this is a year when early sampling and assessment may prevent unfortunate surprises. With adults still in fields, some eggs are still being laid so the weevil season may have some “stretch” beyond an early start. Weevil eggs laid now may take as long as 3 weeks to hatch. Plant growth and natural insecticide breakdown can mean feeding damage, even if an early spray was applied.
If alfalfa weevil numbers exceed treatment guidelines early in the season, application of the high label rate is recommended to provide longer residual control.
By Lee Townsend, Extension Entomologist