By Blake Jackson
As spring temperatures rise, farm fields begin to awaken—but so do pests like the alfalfa weevil, a major threat to alfalfa crops. These pests overwinter and become active once temperatures rise above 48°F. Their larvae, more harmful than adults, damage plants by feeding on leaves, especially during later growth stages.
To estimate their activity, farmers can track Growing Degree Days (GDD). From January 1 to April 5, GDD ranges from 136 in northeast to 295 in south-central regions. Peak larval feeding occurs between 325–575 GDD.
Scouting is vital. Start by checking plant tips for pinhole feeding damage. Then, collect 10 stems from across the field, shake them in a bucket, and count visible larvae. Older, established fields are more likely to show signs due to overwintering populations.
Management depends on plant height and pest levels. If the alfalfa is:
- 6" tall: Recheck if more than 1 larva per stem is found
- 9" tall: Spray if over 1 larva
- 12" tall: Spray if over 2 larvae
- 16" tall: Harvest early if more than 4 larvae per stem
If spraying is needed, rotate insecticides to prevent resistance and check product labels for safe application. Some weevils have shown resistance to pyrethroids.
Photo Credit: gettyimages-gabrielabertolini
Categories: Kentucky, Crops, Alfalfa