Boxelder & seed bugs
Boisea trivittata & Leptoglossus occidentalis
also called Western conifer seed bug
They do almost nothing to the garden and then try to spend the winter in your house. A nuisance entry rather than a pest one, and worth saying so.
How to know it
The boxelder bug is a flat black bug 12 mm long with three red lines on the back of the thorax and red veins on the folded wings; the nymphs are bright red. The western conifer seed bug is a longer brown bug 18 mm with flattened, leaf-like flares on the hind legs. Both mass on warm sunlit walls in autumn.
What you'll see
- Hundreds of bugs sunning themselves on a warm south or west wall on still October afternoons.
- Bugs indoors at windows through the winter, appearing on mild days.
- Clusters on the trunk and seeds of a female boxelder maple in late summer.
- Very light stippling on maple leaves and slightly shrivelled seed, which is the whole extent of the plant damage.
- A sharp piny smell and a small stain if one is squashed.
Where it comes from
Adults overwinter in shelter — under bark, in wall cavities, in attics — and emerge in spring to feed and lay on boxelder, maple and ash seed, or on developing conifer cones. One or two generations a year. Everything about the autumn aggregation is about finding a warm dry place to spend the winter, not about your garden.
Warm dry autumns after a hot summer produce the biggest numbers. A female boxelder maple in or near the garden is the strongest predictor, and so is a house with a warm sunlit wall and gaps around the soffits.
Dealing with it
- Vacuum them up indoors with a bag you can throw away outside, and do not squash them on soft furnishings.
- Hose them off the wall with soapy water outdoors, which kills on contact and needs no insecticide.
- Do not spray inside the wall cavity; the dead bugs then feed carpet beetles, which is a worse problem.
- Sweep them out rather than sealing them in once they are already in the wall.
- Leave the garden alone entirely — there is nothing to treat out there.
Keeping it out
- Seal gaps around windows, doors, soffits, vents and utility entries in late summer, before the aggregation.
- Fit or repair window screens and chimney caps.
- Remove a female boxelder maple if the numbers are intolerable, though the bugs travel and this is no guarantee.
- Accept them. This is a nuisance rather than a pest, and no insecticide program is warranted for it.
- Keep firewood and stored items away from the sunny wall they gather on.
What it goes for
Boxelder and other maples, and the developing cones of pines and firs. Garden plants are essentially unaffected.
Who hunts it for you
- Garden birdsPasseriformes Both things are true: birds strip your currants and a brood of blue tits takes several thousand caterpillars out of one fruit tree in a fortnight. Net what you must and welcome them everywhere else — a nest box near the apples is worth more than a spray. Grow to recruit it: Echinacea,Elderberry,English holly,Goldenrod,Rudbeckia,Sunflower
- SpidersAraneae Webs, ambushes, and night patrols make spiders the garden’s steadiest generalist predators, taking whatever flies or crawls past. Mulch, ground cover, and undisturbed corners keep the population dense. Grow to recruit it: Lavender,Thyme
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com