Watch for Alfalfa Weevil Larvae; Some may be Resistant to Pyrethroids

With warmer weather recently, it should not be surprising that some insect pests are already becoming active. Alfalfa weevil adults tend to be active once temperature hit 45°F, so it is not surprising that alfalfa weevil adults are active, particularly in the southern tier of counties and on warmer, south-facing slopes. We have heard a few reports of larvae feeding already.

But be aware that for fields that have received regular applications of pyrethroid insecticides during first cutting there is potential for populations of alfalfa weevil to be resistant to pyrethroids, particularly in eastern PA in and around Berks and Lehigh Counties. Last spring from these areas, we received many reports of fields that were treated with pyrethroids, but weevil larvae did not die. To avoid this challenge from developing in your fields, I encourage you to embrace integrated pest management (IPM).

Using IPM involves scouting alfalfa fields for larvae and if their populations are above economic thresholds  managing their populations by either cutting (if plants are tall enough) or by applying an insecticide. If you choose to use an insecticide, changing up the class of insecticide regularly can help prevent resistance from developing.  For example, if you used a pyrethroid, like lambda-cyhalothrin or zeta-cypermethrin, last year, consider switching to an oxadiazine insecticide (active ingredient indoxacarb). Indoxacarb (there should be generic formulations available) is likely to be more expensive than most pyrethroids, but using it will help you avoid challenges from resistance in coming years and/or help you avoid needing to re-spraying your fields this year. 

Young alfalfa weevil larvae (Figure 1) cause pin hole-sized damage to leaves near the tips of plants, often on unfolded leaves (Figure 2). Older larvae consume leaves that are more open, typically leaving jagged edges. Much of this early season feeding does not result in economic loss, but it is good to recognize the damage and be aware of which fields have greater activity. Those fields with larger populations will, of course, be where economically significant populations are more likely to develop, but keep in mind that most fields will not require an insecticide application and you should scout to quantify your own populations, rather than just spraying blindly. Economic thresholds for alfalfa weevil are determined from the size of plants, the value of the hay, the cost of insecticidal treatment, and the number of larvae per 30 stems of alfalfa, and are organized in a table available in our fact sheet linked below.

To sample weevil larvae, systematically select 30 stems from across a field and shake them into a bucket.  If the number of larvae exceeds the threshold, a treatment might be warranted. See our recently revised fact sheet for thresholds and more details on alfalfa weevil biology. Happy scouting and I hope you do not find any!