David Owens, Extension Entomologist
University of Delaware
If you have not yet scouted alfalfa for weevils, now is the time! Alfalfa weevil eggs began hatching earlier this year than usual, and we found active weevil larvae almost three weeks ago in Hebron and Greenwood (Figure 1). Alfalfa weevil adults fly into fields in the fall, mate, and begin laying eggs. In our area, fall weather is warm enough for weevils to mate and have enough time to lay a significant number of eggs before advancing cold weather causes them to seek overwintering shelter outside of the field. Weevils are not active below 48 °F. Once warmer weather returns in March, weevils will come out of overwintering habitat in woods and leaf litter and fly back to alfalfa to continue laying eggs. Egg laying scars on stems will appear as small round circular punches in the stem.
There are three implications our warm fall weather has when comparing Delmarva alfalfa weevil activity to other states. First, we can have extremely early weevil hatch, and I have heard rumor of weevil damage to alfalfa in December in some states. Second, it means that degree day models used in more northern and western regions might not be accurate enough and fields may need to be treated before the degree day targets have been reached! Third, it means we can have an extended spring activity period lasting 8 weeks or more.
To scout your field, select 30 stems by randomly walking to six locations and plucking stems from those locations. Take a few of the stems at a time and beat them vigorously against the side wall of a bucket to dislodge weevil larvae. Also measure the length of a few of those stems to get an average stem height. Penn State has a great alfalfa weevil control fact sheet with a dynamic threshold table based on stem height, value of the hay, and control costs. We incorporated the table into the UD Insect Control in Alfalfa document which can be found here: https://www.udel.edu/academics/colleges/canr/cooperative-extension/sustainable-production/pest-management/commercial-field-crop-pest-management/alfalfa/. Be advised that small weevil larvae often hide in the half folded terminal leaflets and are not easy to dislodge. You may want to visually examine the terminals after shaking stems.
When making an insecticide application, use higher water volume rates to ensure good coverage, especially into the terminals. Do not apply an insecticide a couple of days before or after a significant cold spell. Weevil larvae need to be active to come into contact with treated foliage. Keep good records of previous applications and be sure to scout fields after application. We can have new weevil hatch after a spray, justifying a second and sometimes even a third application prior to cutting. Our insecticide selection is very limited in alfalfa, and for this reason it is possible that besides environmental factors there can be some intrinsic factors in your local population that could render them less susceptible to a given insecticide if it has been used continuously for a long period of time. Adding to this concern is the chlorpyrifos tolerance revocation effective February 28, 2022. In last year’s spray trials, pyrethroids provided between 40 and 70% efficacy three weeks after application. Carbaryl initially provided a 70% reduction in alfalfa weevil but residual activity was very short and weevil counts increased afterwards. The most consistent active ingredient has been indoxacarb (Steward). It has a supplemental 2ee label for reduced rates, 4.0 to 11.3 fl oz per acre as opposed to the main label rate range of 6.7 to 11.3 fl oz. It also provided excellent residual control in last year’s trial at the 6.7 fl oz rate. FMC advises lower rates should have about 8-14 days residual, higher rates up to 21 days. When making an application to alfalfa, industry recommends including an spreader sticker type adjuvant such as an organosilicone to improve coverage. As always, make sure to read the labels of all products applied or tank-mixed to make sure there are no insecticide/adjuvant/herbicide concerns.
Please note that Besiege is labeled for alfalfa but does not appear in the 2020 control guide. A high rate of Besiege is equivalent to a high rate of WarriorII in terms of lambda cyhalothrin active ingredient. The addition of chlorantraniliprole is not effective on alfalfa weevil.
Finally, be aware that pyrethroids can flare aphids. Dimethoate, while not consistent in its ability to suppress alfalfa weevil, is excellent on aphids, and you may see some advice regarding tank mixing the two active ingredients. While aphid outbreaks are unusual, they can happen. Last year we had a spray trial on aphids following a lambda cyhalothrin application. A repeat application in the trial had no impact on aphids. There are two newer products that need to be updated into the guide but are registered and are excellent on aphids: Sivanto and Sefina (see supplemental labels). Neither has alfalfa weevil efficacy but will just about zero out aphids should they be a concern. Otherwise, if you do experience an aphid outbreak, they are usually late enough that the field can be cut early.
