Root Weevils

Asked September 15, 2018, 1:39 PM EDT

Root Weevils are decimating my salal, red flowering currant and Kelsey dogwood plants. I’ve tried physically removing them night after night, nematodes on a number of occasions, and that pokey clay dust that’s supposed to irritate them. Nothing has worked. I’ve never used any chemicals in my yard—ever—but I’m ready to bring in “the big guns” if it will alleviate this problem. Any suggestions on chemicals that will kill the weevils but have minimal impact on the environment?

Multnomah County Oregon insect issues

1 Response

Are you losing plants or is the damage confined to leaf notching? Does the growth of the species described seem to be less than other plants in the area? Plants can tolerate a lot of leaf-notching without their growth being affected if damage to the roots/crown themselves are modest. Some plant resist leaf feeding, either due to anti-feeding chemicals produced by the plant , difficult to notch leaf architecture, or both. This is especially visible between rhododendron cultivars. That said, beneficial nematodes will work but timing is everything. They have to be applied when the larva are present but before they pupate and emerge as adults. Complicating the timing is that the soil temperatures need to be warm enough and the soils somewhat moist. Further complicating it is that the nematodes die over most winters and need to be reapplied each year. So, for us west of the Cascades, the best time for the nematodes is around early to mid October. The larva are present and the soils usually warm and moist enough to support a nematode feeding frenzy. I would give them one more try. Follwo the label instruction on their use. Mulching the area under the plants with shredded leaves after you apply the nematodes will keep the soil warmer longer and can encourage birds like the varied thrush to search for root weevil larva, pupa, and adults. Keep the cat inside! Finally, conventional insecticides labeled for home use with a fair history of efficacy include products with acephate, bifenthrin, or cyfluthrin. For root weevils, they are generally drenched around the roots. Please read and follow all label instructions for use. If you have more questions, please feel free to call at 503 397-3462 or email me at chip.bubl@oregonstate.edu... Chip