Alfalfa weevil status

KSU ENTOMOLOGY NEWSLETTER – Thanks to the unusually warm winter, alfalfa weevil larvae are already present in some areas. Many more will no doubt be hatching in the next few days to weeks. Alfalfa weevils will continue to hatch and larvae continue to develop any time temperatures exceed 48°F.

Thus, it looks like larvae will be emerging, and damage progressing, relatively quickly. Whether this warm weather will compress the alfalfa weevil larval feeding so that the period of damage is not as stretched out as usual remains to be seen. There are also lady beetles active in the alfalfa fields, as well as a few pea aphids. The treatment threshold we use for alfalfa weevil insecticide applications is 30-50% infestation, i.e. 1 larva/2- 3 stems.

alfalfaweevil-larvae

Alfalfa weevils are cool-weather insects. Adults lay eggs in alfalfa fields in the fall or even the winter. Most of these eggs survive the winter. Eggs hatch and larvae emerge after accumulating enough degree days or thermal units, normally in early spring. Alfalfa weevil adults also lay eggs in the spring, but in many cases the first larvae to emerge are from eggs that were laid in the fall and overwintered.

That said, an anomaly we encountered in the fall of 2016 was a significant infestation of relatively large (2nd and 3rd instar) larvae from mid-November to mid-December. Alfalfa weevils normally overwinter as eggs or adults – not larvae. In the last week we could find none of these more mature larvae, or any pupae. So, hopefully they perished in the colder weather.

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Jeff Whitworth, Extension Specialist Field Crops phone: 785/532-5656, e-mail: jwhitwor@ksu.edu

Holly Schwarting Research Associate Phone: (785) 532-4730, e-mail: holly3@ksu.edu

Eva Zurek Insect Diagnostician Phone: (785) 532-4710, e-mail: ezurek@ksu.edu

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