What are some effective treatments for weevils?
Last season, leaves on some of my rhododendrons show notching that may be due to root weevils. What stage of growth are weevils at now and what is effective treatment for this stage?
Katherine Garland, Horticulturist
The weevils hide in the soil by day, then climb onto the plants at night, feeding almost exclusively at night and almost always on the undersides of the leaves. If you were to go outside just after dark with a flashlight and place something large and light-colored underneath the leaves (ex: old sheet or large box) and then flick hard on the leaves with your finger (the way you’d try to flick away a dead fly) then one or two weevils would be likely to drop right into whatever you’re using for collection.
The first four images on our photo gallery page here, shows them: https://extension.umaine.edu/home-and-garden-ipm/critter-id/photo-gallery/lawn-tree-ornamental/ This is a good shot here (the 4th one on that page): https://extension.umaine.edu/home-and-garden-ipm/wp-content/uploads/sites/43/2021/05/Raspberry-Weevil-051621.jpg. Here’s another very helpful fact sheet: https://extension.umaine.edu/home-and-garden-ipm/fact-sheets/common-name-listing/black-vine-weevil/
I think one of the most effective solutions is to just go out every night for a week or so, as soon as you start finding the feeding damage, and literally knock them (shake them) from the leaves, making sure you have something underneath to catch them. They have a defensive ‘dropping’ response, at the first sign of disturbance, so it doesn’t take very much to startle them and cause them to ‘drop.’ They are also experts at ‘playing dead’ and blend into the dirt so amazingly well that if you let them drop onto the ground, it’s extremely difficult to find them afterwards, even if you’re watching pretty well and see them drop. They’re uncanny at escaping that way!!! You can also grab individual leaves where you see the feeding injury, and run your hand along the entire bottom side, in order to ‘slide’ the weevil(s) right off — like bulldozing it off, essentially, using your hand, so you don’t even have to look but just slide your hand underneath and across the whole bottom of the leaf.
They are robust beetles, so the systemic insecticide probably just isn’t strong enough to kill them. There might only be a total of 30 weevils doing all of that damage, which is why I think just the physical removal, done often enough, is the best way to go.