Garden Q&A: Do beetles in old wood harm trees?

Thank you for supporting our journalism. This article is available exclusively for our subscribers, who help fund our work at The Baltimore Sun.

Q: I found several of these beetles in an old decaying stump and am concerned for my healthy trees. Will they attack live trees?

A: No, these beetles feed on rotting wood and the fungi decaying it, and they pose no threat to other trees. Several common names are given to them: patent-leather beetle, bess bug, and horned passalus. These insects have a rare life history in that they live in groups and provide parental care for their larvae, feeding them pre-chewed rotting wood, likely for over a year while the young mature slowly.


The feature I find the most entertaining about them is their ability to squeak. Both adults and larvae can stridulate, which means they use one body part to rasp against another to create noise. The purpose of this is probably for communication with each other. Cricket chirping and katydid calling are forms of stridulation, but in the case of these beetles, it produces more of a high-pitched sound akin to a person making kissing noises at a pet.

Interestingly, Iowa State University’s BugGuide web page for bess bugs speculates that the “bess” part of its name might derive from baiser, French for “to kiss.” (Or it’s derived from the fact that their forward-facing jaws can pinch, though I’ve never been bothered and I pick up these beetles every time I see them because they’re fun to find. Petting them sometimes makes them stridulate, which is always endearing.)


Wood-recycling insects like these are great to have around and rarely if ever pose a risk to healthy plants. Not only do they get those old stumps and logs out of the way for free (even though it can take a while), but both they and the fungi they work with are a means to make the old tree’s nutrients available again to the rest of the ecosystem.

Patent-leather beetle on a rotting log.

Weekend Watch


Plan your weekend with our picks for the best events, restaurant and movie reviews, TV shows and more. Delivered every Thursday.

Q: It’s been suggested to me that I use a mulching mower when I mow the lawn, but I’m not sure what this means or what good it’ll do compared to my regular mower. Should I use one?

A: Although the term “mulch” in their name is a little misleading since it makes us think of bark-type mulches on flower beds, mowers that mulch their clippings do essentially turn this material into a light, temporary mulch for the lawn’s soil surface, with similar benefits. Grasscycling is another name used to describe this recycling of turfgrass resources.

A mulching mower uses a special design to keep the cut grass pieces airborne inside the machine for longer, allowing them to be cut into finer bits before letting them fall back down into the turf. Picture this a bit like a food processor pulsing a few times to mince vegetable slices. Since the resulting falling pieces are small, they don’t clump on the top of the lawn. Microbes and invertebrates consume the grass clippings relatively quickly, returning key nutrients to the turf roots and beneficial organic matter to the soil.

Since keeping the grass clippings on the lawn instead of removing them nourishes the grass and soil life, this reduces the need for fertilization to support good turf growth, and eliminates the hassle and expense of bagging or disposing of clippings. You don’t need a mulching mower to take advantage of retaining clippings for the benefit of the lawn, but it does save you the laborious step of raking-out clumps so they break apart and settle down onto the soil surface.

Some gardeners confuse this debris layer with thatch, but it isn’t the same thing, and the most common type of lawn grown in Maryland, tall fescue, creates thatch very slowly and usually not extensively enough to bother with removing. (Zoysia and Kentucky bluegrass do, though. While mulch-mowing doesn’t contribute directly to thatch formation, having a thick thatch layer already present can exacerbate its buildup.)

A final tip: keep those mower blades sharp (on any style of mower) so the machine does its job well. Aim to sharpen blades about every six months; more if you mow often or have a very large lawn to maintain. Sharp mower blades reduce the damage done to the lawn itself, and less grass blade tearing from dull blades makes them less prone to infection by turf pathogens. Neither blade sharpness nor the mulching feature of these mowers will allow you to mow and mulch successfully in wet weather, though (an occasional misconception), so do still wait for the turf to dry off after rains or heavy dew.

University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at Click “Ask Extension” to send questions and photos.