As indicated by the growing degree day predictive system for alfalfa weevils available on the Kansas Mesonet, http://mesonet.k-state.edu/agriculture/degreedays/, eggs should be hatching and some small ‘pinprick’ sized holes will be appearing in alfalfa leaves. That is exactly what is happening in north central Kansas this week.
It is too early to apply an insecticide to control alfalfa weevils as eggs may continue to hatch for the next 2-4 weeks, depending upon the temperatures. Cooler temperatures slow development while warmer temperatures speed up development. Anytime the average daily temperature reaches 48 degrees F or above, the eggs are hatching and larvae are feeding. Many fields do not currently have much foliage, due to cool temperatures and dry conditions, so there isn’t much leaf area to accept spray and thus, residual activity will be reduced. Treatment thresholds generally are when infestation levels reach between 33-50%, or when there is one larva per 2 or 3 stems. This can occur very quickly, so monitoring should be conducted every 2-4 days. If the treatment threshold is reached, and the determination is made to treat with an insecticide labeled for alfalfa weevil control, please remember to use sufficient carrier to get good coverage throughout the entire canopy. For more information please refer to the Alfalfa Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/mf809.pdf
Jeff Whitworth, Extension Entomologist
Holly Schwarting, Entomology Research Associate