Can I use a paint brush to coat the unfinished surfaces of am oak desk with CYPERMETHRIN to kill powderpost beetles?

You could but it wouldn't be the best treatment option for several reasons. If you read our POWDER POST BEETLE CONTROL ARTICLE, you'll note we don't list Cypermethrin as a recommended product to use. This is because it won't penetrate the wood being treated nor will it last long. As our article explains, the larvae of the beetles are what's eating it. To get control of the problem, you need to use a product that will both penetrate through and through as well as last a good long time. Though Cypermethrin can be used for shallow living wood boring pests, it's not designed to get deep into the wood grain which is where most powder post beetle larvae will be living.

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