by Chris DiFonzo

Alfalfa weevil larvae are already present in some fields. Older stands (or new stands planted near old ones) are priority for scouting because weevils overwinter adjacent to fields.

Take these three steps to evaluate stands:

1. Sweeps. Start using a sweep net in a dry canopy on a warmish day. Your goal is to simply check if larvae are present. They will be small (on a penny, several could fit on Lincoln’s head) and green, with a black head. If no larvae are present, re-scout the field once a week. If larvae is found, move to the second 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 percentage 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 the third step,

3. Counts. Carefully pick individual stems and shake them vigorously into a bucket or sweep net. I 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 (I use a tally meter, so I 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.

The reason to go through all of this is to be sure the crops are sprayed 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 such as ladybugs.

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.

DiFonzo is a Michigan State university professor and field crops entomologist.