Very small alfalfa weevil larvae were first detected on April 5, 2019 in north central Kansas. Every field sampled had at least one newly hatched larva. There were not enough to sample for a treatment threshold as they are just hatching. The first indication of these small larvae are very tiny pinholes in the leaves, or a little chewing on plant terminals (Figure 1). These tiny larvae are quite difficult to dislodge from their feeding sites when they are this small. Thus, sampling at this early stage to determine an infestation level is not practical using the bucket-shake method or a sweep net.
Figure 1. Examples of alfalfa weevil larvae feeding. Photos by Extension Entomology, K-State Research and Extension.
As a reminder, producers can visit the Kansas Mesonet website and utilize the Growing Degree Day predictive system for alfalfa weevils at: http://mesonet.k-state.edu/agriculture/degreedays/. Users can select the nearest station to their field, input the date range as January 1 to current date, select “Alfalfa Weevil” in the calculation window, and then select “Submit”. This will generate a table that lists the actual growing degrees, the normal, and the departure from normal (Figure 2). For more information on the life cycle of alfalfa weevils and the influence of growing degree days, please see the KSRE publication “Alfalfa Weevils: Kansas Crop Pests” at https://www.bookstore.ksre.ksu.edu/pubs/mf2999.pdf
Figure 2. Screenshot of the Growing Degree Day predictive tool for alfalfa weevils on the Kansas Mesonet.
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 Davis, Entomology Research Associate