Alfalfa weevil populations seem to have “exploded” around north central Kansas in the past week. Tiny alfalfa weevil larvae were first detected in north central Kansas on April 5, but probably a few started hatching a day or two prior. However, the infestation levels that were detected on April 5 and 6 were all well below 10%, and mostly less than 1%. In contrast, fields sampled on April 16 all greatly exceeded 100% infested using the stem shake bucket method and large numbers of different stages of larvae were detected (Figure 1).
Figure 1. Different stages of alfalfa weevil larvae collected from a field on April 16, 2019. Photo by Extension Entomology, K-State Research and Extension.
To sample using the “shake bucket” method, randomly select individual alfalfa stems and quickly and vigorously shake them into a small white bucket. Then, count the number of dislodged larvae in the bucket and divide by the number of stems to get the infestation level. For example, 15 larvae from 10 stems = an average of 1.5 larvae/stem. Do this in several areas throughout each field to get a good indication of the alfalfa weevil infestation level and the stage of development of the weevil. One of the problems with the shake bucket method is that some stems have several larvae/stem while others have none (yet). Thus, the infestation level may appear to be higher than the actual infestation.
However, in north central Kansas, with as many larvae as there are already (with more to come probably) and as much damage as we are starting to see in spots (Figure 2), it may be prudent to treat fields as soon as possible.
Figure 2. Visual signs of damage by alfalfa weevils. Photo by Extension Entomology, K-State Research and Extension.
For information on insecticides registered for use for alfalfa weevil control, please see the K-State Alfalfa Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/mf809.pdf