David Owens, Extension Entomologist, owensd@udel.edu
The time is NOW to scout alfalfa for alfalfa weevil larvae. Overwintered eggs began hatching two weeks ago and larvae are starting to accumulate enough degree days to grow quickly. Adults are also active now, laying eggs that will lead to a sustained pressure. The good news is that if a field is over threshold, it does not look like we will have a strong or long lasting cold snap next week to worry too much about dancing around when it comes to treating. If the temperatures are in the low 50’s to low 30’s, it is advisable to wait a couple of days to have more moderate temperatures before treating. Larvae need to be active to come into contact with or feed on treated foliage. Most alfalfa is less than 6 inches tall, in short alfalfa, there’s a couple of different thresholds: 50% injured terminals or 0.7 larvae/stem.
This winter, we updated our alfalfa weevil control fact sheet. Check it out. It includes information on biological control, product selection, and, perhaps most importantly, updated threshold tables to better reflect current alfalfa value and better ballpark control cost estimations for Steward. Steward is probably he most expensive weevil insecticide, but it is the most consistent treatment in four years’ worth of testing. We can usually point to Steward plots from the road versus control plots or poor performing materials. You can find the revised fact sheet here: https://www.udel.edu/content/dam/udelImages/canr/pdfs/extension/factsheets/Alfalfa-Weevil-Control-in-Alfalfa.pdf.
While sampling for alfalfa weevil, make a mental note of aphid populations. Aphids are best sampled with a sweep net, using a threshold of 50 per sweep, but if alfalfa is really short, sweep nets are hard to use. Others recommend 10-12 aphids per stem. We were in a field earlier this week in which aphids were quite noticeable, perhaps not yet at 10-12 per stem, but with potential to get there. In such fields, it might be worth using malathion or dimethoate + pyrethroid or Lannate + pyrethroid for weevils to pick up aphids. Otherwise, there are a couple of very selective insecticides labeled including Sefina, Transform, and Sivanto Prime.