Alfalfa Weevil Management BMPs
Late March saw alfalfa weevil infestation numbers at scouted sites (4) ranging from 9-14%. Those numbers almost doubled this week, with one site reaching infestation levels (33-50% – 1 larvae on each 2 or 3 stems) where treatment is typically recommended. If treatment is on your mind, consider some best management practices to maximize application efficacy:
Right Product
Efficacy data from KSU Field Crops Entomologist Dr. Jeff Whitworth is available at the bottom of our Meadowlark Extension District Crops & Soils page. The results underscore a couple of things:
- Many products work: whenever there’s a treatment failure, resistance is often blamed, and while that could be a reason, these results show we still see good efficacy from most products.
- Products differ in how long they last.
Right Rate
While insecticide resistance isn’t as likely to develop in alfalfa weevil as it might be in multiple generation pests reproducing at much higher numbers, it’s still a good idea to rotate different product classes. The Alfalfa Insect Management Guide at the link above can help you sort through those products and a short list is included here as well.
Rate ALSO applies to carrier gallonage – don’t skimp! The trial results at the link above are done at rates of water approaching 15-20 gpa. Entomologists are testing the product, not carrier gallonage and they want product to get to the pest or the pest to come in contact with the product (Whitworth notes in those trials he never has to do a second treatment because he’s applying plenty of product + carrier to achieve control). Field application conditions may prohibit those types of carrier rates, but balancing acres with appropriate carrier rates is still important. Check out Figure 4 at the bottom of this KSU eUpdate article for a visual example of an alfalfa weevil insecticide product at 8 vs. 15 gpa. It’s a stark difference!
Right Time
Treatment thresholds are designed to guide timely control actions to keep crop damage below economic levels. A larvae per every 2-3 stems might seem pretty low, but damage can increase quickly if you aren’t vigilant. When scouting, scout multiple spots. Variability across 10 spots (5 stems sampled per spot) scouted at one site last week ranged from 0 to 6 larvae. A single sampling site would have either drastically over or under estimated actual numbers. If you do approach treatment thresholds, keep scouting to confirm consistency of sampling numbers and validate the need for initiation of control efforts. Accuracy increases with more samples.
What About Freeze Injury?
While the freeze event of two weekends ago did do some damage to weevil larvae, the damage to foliage is equally concerning. Not only have we slowed growth, but reduced green material to apply insecticides to as well. In many cases, stands will have recovered by the time treatment is needed, but keep in mind: larvae are going to feed on green material. Product applied to wilting leaves – or bare ground – isn’t going to help with control.
Fun Fact:
When scouting look for lady beetle levels as well. Known aphid consumers, lady beetles also have some efficacy against early instar alfalfa weevil larvae!