JoAnne Skelly: What’s wrong with my lilac?

I bumped into my friend Cat at a nursery buying marigolds to solve a problem her lilacs were having. She said something was eating the leaves along the edges leaving sharp-edged cutouts. She said someone swore marigolds planted around the base of the shrubs would solve the problem.

The cause of the angular cutouts is an insect, a root or black vine weevil. They come out at night and chew leaves. As larvae, they’re legless grubs that grow over winter in the soil feeding on roots of many plants besides lilacs such as peony, hosta, euonymus, wisteria, rose, raspberry, rhubarb, strawberry and more. They pupate in the spring in the soil with adults emerging mid- to late spring.

Fortunately, these critters don’t fly, so they only travel short distances. They hide during the day under a plant and emerge around sunset, crawling up the plant to feed. If disturbed, they fall from the plant and play dead. The adults live for a couple of months, laying eggs in the soil from late spring to early summer. The eggs hatch within a few days and the larvae start feeding on the roots of plants. They can severely damage roots causing weak or stressed plants to decline.

While marigolds do contain the chemical pyrethrum, which is sometimes formulated either in its natural or manmade form as an insecticide to control root weevils, planting them around an infested plant probably won’t eliminate the pests. There are other insecticides listed for root weevil control, but damage is usually aesthetically displeasing rather than health threatening, so insecticides are rarely called for. One of the best management options is a soil drench of beneficial nematodes (tiny, often microscopic, worms) to manage root weevil larvae in soil. Good nurseries carry products that contain these nematodes. Timing the application is important for success and a drench should be applied after most eggs have hatched and when soil temperatures are warm. This is usually during the summer months, mid-July to mid-September.

Cat wondered if a product containing the chemical imidachloprid would work against root weevils. My research suggests it does, but I wouldn’t use this chemical because of its potential to kill bees. Although honeybees aren’t going to lilacs now the flowers are done, leafcutter bees are and could be killed as they chew off leaf material to build their nests. Leafcutter bees are also important pollinators, so we want to keep them healthy.

Tickets are available for the July 10 benefit concert for the Greenhouse Project at www.carsoncitygreenhouse.org. Poco, Firefall and Pure Prairie League will perform.

JoAnne Skelly is associate professor and Extension educator, Emerita at University of Nevada Cooperative Extension. She can be reached at skellyj@unce.unr.edu.