Black vine weevils cause lilac damage

My very old lilac bushes have just started to look like someone took pinking shears to the leaves, starting at the bottom and working to the top. My neighbor is experiencing the same thing. This is the first year in my yard, but it is the second for him ... any clue? I would appreciate any help you could give me.

— Karol

You’re dealing with something that a lot of people are right now. What you have is an insect called a black vine weevil. The adult of black vine weevil looks like a miniature, black boll weevil. They are very elusive, and you’ll almost never see them.

They do their feeding at night, and hide in the soil or leaf litter during the day. They feed on the foliage of plants, and their feeding is the easiest way to tell if you have a problem.

They eat out small, squarish, notches in the margin of the leaf, eventually creating a lacy effect. Like you said, it is almost like someone took some square pinking shears to the leaf.

The damage that the adults do is inconsequential. It’s the feeding of the larva that can really hurt the plant. The larva is a small, ivory-colored legless grub that lives in the soil and feeds on the roots. Their feeding can damage and even kill the plant. As with all plant insect and disease problems, their numbers will naturally be higher or lower from year to year. It seems like they’ve been on an upswing the past year or two, and we’re seeing a lot more of their damage this year.

Though black vine weevils feed on a variety of plants, they tend to have their favorites. We see them mostly on lilac, rhododendron, privet, euonymus, yew and peonies.

I think the two exceptionally cold winters we’ve had the past couple of years have made the problem worse. We’ve seen widespread damage and dieback on privets and evergreen-type euonymus especially. Obviously, this weakens the plant, and I think these little monsters take advantage of that situation to really get going.

Controlling black vine weevil will take a bit of patience and persistence.

First, you’ll want to use a two-pronged approach. Thoroughly spray the foliage and the ground around the plant with an insecticide to help control the adults. You then want to go further and drench the soil around the plant to try and kill the larvae. The best materials to use would be Bifenthrin, Permethrin or Imidacloprid.

I’d repeat the foliage and ground spray in a couple of weeks. The soil drench could be repeated in a month or so. You also should plan on doing this again next year just to clear out any stragglers and prevent new weevils from coming in.

Once you’ve given the plant a chance to recover, you should be in the clear, but it is something that you’ll want to keep your eyes open for down the road.

There are five mature cottonwoods growing along the canal behind our patio home. The cotton is flying like crazy right now, and it keeps us from enjoying our patio. Is there anything that can be applied to the cottonwoods that will “sterilize” them so they will not produce the cotton?

— Roxie

I’m afraid there’s nothing that will sterilize them short of a chain saw.

There is a spray called Florel that helps to cut down on nuisance fruit drop (including cottonwood cotton) that is sprayed over the whole tree during its bloom early in the spring, then repeated a couple weeks later.

I don’t think this is much of an option for you since a mature cottonwood is huge, probably even too large for a commercial spray company to tackle. Also, the spray only helps for the current year and must be reapplied every spring.

Dennis Hill is the nursery manager at Bookcliff Gardens, Send questions to Bookcliff Gardens, 755 26 Road, Grand Junction 81506; or email .(JavaScript must be enabled to view this email address).


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