Weevils are popping in alfalfa fields
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EmailConsider these integrated pest management steps for controlling alfalfa weevil.
The cropping season is in full swing and so are the pests in our fields. Alfalfa weevil larvae are already present in some fields. Older stands (or new stands planted near old ones) are a priority for scouting because weevils overwinter adjacent to fields. Take these steps to evaluate stands.
Step 1: Sweeps
This week, start using a sweep net in a dry canopy on a warmish day. Your goal is to simply check if larvae are present. As of May 2, they will be small (several could fit on Lincoln’s head on a penny), green with a black head. If no larvae are present, rescout the field once a week. If you find larvae, move to Step 2.
Step 2: Tips
If larvae are present, switch to a quick rating of percent tip-feeding. Don’t look across the canopy to make this estimate. The human eye often focuses on the worst areas. Instead, pick and evaluate individual stems. Feeding first appears as round holes then tattered or skeletonized leaves as larvae grow. Keep track of the number of total and damaged stems to calculate percent fed-on. Problematic levels of feeding vary with plant height—mark your sweep net handle to use as a ruler. Use the chart below (the Step 2 column) to determine if you should move to Step 3.
Step 3: Counts
Carefully pick individual stems and shake them vigorously into a bucket or sweep net. We suggest picking 10 stems at a time from 10 different locations. Count the total number of larvae—any size—and keep track until you’ve done 100 stems (you can use a tally meter so you don’t lose count). Calculate the number of larvae per stem and use the chart below (the Step 3 column) to determine what action to take based on crop height.
|
Plant height |
Step 1: Larvae in sweeps? If yes, check tip feeding |
Step 2: Level of concern, % of tips fed-on. Over this %, count larvae |
Step 3: Level of concern, # larvae/stem. Over this #, take action |
Recommended action |
|
Any |
No / very few |
Not available |
Not available |
Do nothing |
|
~6 inches |
Yes |
> 25% |
Under 1 per stem |
Scout again in 7 days |
|
~9 inches |
Yes |
> 50% |
> 1 per stem |
Spray |
|
~12 inches |
Yes |
> 75% |
> 2 per stem |
Spray |
|
~16 inches |
Yes |
~100% |
> 4 per stem |
Harvest early, scout regrowth |
|
Regrowth after cutting |
Not available |
Not available |
2 per crown |
Spray |
Table modified from Ron Hammond and Ohio State University/Ohioline
The reason to go through all of this is to be sure you are spraying only when necessary. For the most part, alfalfa weevil is controlled for free by natural enemies, which include specific larval and adult parasitoids plus general predators like lady bugs. Spraying kills these beneficials, and it surely takes a while to build them back into a field (especially the parasitoids, which attack only alfalfa weevil). Thus, spraying now might increase your weevil issues in the future.
Source for 15-inch sweep nets and tally meters: Great Lakes IPM in Vestaburg, Michigan.
Free, online Michigan State University insect guide for forages, with insecticide list: Insect Guide for Forages
The Michigan State University Extension field crops team has many ways of informing farmers and agriculture professionals with pest pressure updates, including the Fast Fonz Facts that Chris Difonzo emails out to subscribers. To subscribe to Fast Fonz Facts, email difonzo@msu.edu. Virtual options are watching the Field Crops Virtual Breakfast Series at 7 a.m. on Thursdays, Central Michigan updates from Monica Jean on Wednesdays, wheat watcher updates from Jenna Falor and southwest Michigan updates from Nicolle Ritchie, both published on Thursdays.
