A crowd of red and black bugs on a wall, a step or the base of a tree looks alarming the first time you see it. Almost always, it is one of three seed-eating true bugs doing what its species does every year: the firebug, card #113, the boxelder bug, card #111, or the large milkweed bug, card #112. All three are in the true bugs section of the Field Guide, which holds 14 of our 181 cards. None of them is dangerous, and each one has a giveaway.
Which red and black bug is it?
| Bug | What to look for | Where you see it |
|---|---|---|
| Firebug | Red wings with two pairs of black spots, wings often short | Europe; Utah and southern Idaho; at the foot of lime trees and mallows |
| Boxelder bug | Mostly black, three red lines behind the head, red wing edges | North America; on boxelder and maple trees, sunny walls in autumn |
| Large milkweed bug | Orange-red with a broad black band across the middle | North America; on milkweed seed pods |
| Cinnabar moth | Black forewings with red streaks and spots, red hindwings | Europe; meadows with ragwort, by day as well as night |
| Seven-spot ladybird | Round, domed, hard red shell with black spots | Gardens almost everywhere |
The last two are not true bugs, but they get photographed as “red and black bugs” all the time. A ladybird is a beetle: rounder, shinier and hard-shelled. A cinnabar is a moth, with soft, scaly wings that it opens to fly. If the insect is flat, shield- or oval-shaped and walks in a crowd, it is probably one of the first three.
Firebug: red with black spots
The firebug is a European insect. Utah State University Extension describes its forewings as red with two pairs of black spots, lying flat over the back, and puts adults at about a quarter to half an inch long. The British Bugs recording site gives 8.5 to 10 mm. Most firebugs have short wings and cannot fly, although Utah State notes that fully winged adults also occur.
It is new to North America. Utah State records the first find in 2008, in south-eastern Salt Lake City, and says it is now established across Utah and southern Idaho. The University of Idaho reported that populations were first found near Twin Falls and Burley in 2021. In Britain, British Bugs says it was long known from a single colony in Devon, with newer colonies in several southern and eastern counties that were probably introduced by accident.
Firebugs feed on seeds, mainly of the mallow family. Utah State lists common mallow, hollyhock, hibiscus and linden (lime) among their hosts, and says they do not damage plants enough to need control. British Bugs notes their dense groups in spring before mating. iNaturalist research-grade records on card #113 show the same thing: records peak in April, when there are about 16 times as many as in December, and are also high in May and July.
Boxelder bug: black with red lines
The boxelder bug is the red and black bug most North Americans find on the house. Ohio State University describes adults as about half an inch long, dark brownish-black, with three red stripes behind the head, red edges on the front wings and a bright red underside. The young are bright red and wingless and get darker as they grow.
They feed on the seeds of female boxelder trees and seed-bearing silver maples. In autumn, Ohio State notes, they gather on tree bark and on the sides of buildings, then slip into cracks around windows, doors and foundations to spend the winter. Our data shows this clearly: on card #111, iNaturalist research-grade records peak in October, at about nine times the January level, with smaller peaks in April and September. Our guide to bugs that come into the house in autumn covers the boxelder bug alongside ladybirds and stink bugs.
Milkweed bug: orange-red with a black band
NC State Extension describes the large milkweed bug as half to three-quarters of an inch long, reddish-orange, with a black band across the middle and a diamond-shaped black patch at each end. It lives on milkweed and eats its seeds. Mississippi State University Extension notes that it takes in toxins from the plant, which make it unpleasant to predators, and that even large numbers do little real damage. A smaller look-alike, the small milkweed bug, shares the same plants. iNaturalist records on card #112 peak in August and September, when the seed pods ripen.
Front desk note: if you planted milkweed for monarchs and it is now covered in orange and black bugs, that is part of the same garden working as intended. Mississippi State points out that sprays strong enough to kill milkweed bugs also kill monarch caterpillars.
Are they harmful?
Not to people. Ohio State says boxelder bugs do not bite and do not damage buildings, clothing, food or people. Utah State says the same of firebugs: they cannot bite or sting people or pets. Neither causes significant harm to garden plants.
What they can do is smell. Both release an unpleasant odour when crushed or roughly handled, and Utah State warns that crushed firebugs can permanently stain carpets and fabrics. Ohio State adds that boxelder bugs can leave small droppings on curtains and walls.
The bright colours have a purpose. Mississippi State explains that the milkweed bug’s orange and black pattern warns predators it tastes bad, and Butterfly Conservation says the same of the cinnabar moth’s caterpillars, which store poisons from ragwort. A bug that is safe to sit in plain view has every reason to sit there in a crowd.
How to move them on
- Do not crush them indoors. Vacuum them with a nylon stocking held in the hose by a rubber band, then empty the stocking outdoors, away from the house. Utah State recommends this for firebugs.
- Outdoors, leave them or wash them off. Utah State suggests hosing clusters down, or spraying them directly with soapy water (about half a cup of washing-up liquid to a gallon of water). Ohio State suggests vacuuming or picking them up by hand.
- Close the gaps before autumn. Both universities put exclusion first: seal cracks around windows, doors, vents and pipes, add weather stripping, and keep screens in good repair. Utah State notes that no insecticides are registered for firebugs in Utah, and that they tolerate many sprays anyway. Ohio State calls indoor insecticide treatment for boxelder bugs impractical.
If the bugs on your wall do not match any of these, a photo in the identifier will give you a name and a confidence level, and the Field Guide has a card for each. For smaller, all-red specks, try our guide to tiny red bugs.
Sources
- Utah State University Extension: Red Firebug (Pyrrhocoris apterus)
- University of Idaho Extension: Red Fire Bug – A New Idaho Invader? (BUL 1019)
- British Bugs: Pyrrhocoris apterus, Firebug
- Ohio State University CFAES: Boxelder Bugs and Leaf-footed Bugs
- NC State Extension: Milkweed Bug
- Mississippi State University Extension: Large Milkweed Bug (Bug’s Eye View, Vol. 7, No. 11)
- Butterfly Conservation: Species of the Month: Cinnabar
- Seasonal pattern: iNaturalist research-grade observations, as shown on card #113, card #111 and card #112
Questions
What are the red and black bugs on my house?
In North America they are most often boxelder bugs: dark brownish-black, about half an inch long, with three red lines behind the head and red edges on the wings. In Europe, and now in Utah and southern Idaho, they may be firebugs, which are mostly red with two pairs of black spots on the wings. Both gather on sunny walls, especially in autumn.
Are red and black bugs harmful?
The common ones are not. Ohio State University notes that boxelder bugs do not bite and do not damage buildings, clothing or food, and Utah State University Extension says red firebugs cannot bite or sting people or pets. The real nuisance is that both smell and can stain fabric if they are crushed.
What is the difference between a firebug and a boxelder bug?
A firebug's wings are red with two pairs of black spots, and in most populations the wings are short. A boxelder bug is mostly black with thin red lines and red wing edges, and it has full wings that it flies with. Their young are both red and easy to confuse, but Utah State University Extension notes that every stage of the young firebug has a line of three black dots down its back.
How do I get rid of red and black bugs without the smell?
Do not crush them. Vacuum indoor ones with a nylon stocking held inside the hose so they collect in the stocking, then empty it outdoors, away from the house. Outdoors, Utah State suggests hosing clusters off or spraying them directly with soapy water, and both Utah State and Ohio State say sealing gaps around windows, doors and vents works better than insecticide.
Why are there so many red and black bugs together?
These bugs are gregarious. Firebugs gather in dense groups in spring before mating and again in late autumn, and boxelder bugs cluster on tree bark and sunny walls in autumn before looking for cracks to spend the winter in. Their bright colours are thought to warn birds that they taste bad, so sitting together in plain sight costs them little.
Boxelder Bug#111
Brown Marmorated Stink Bug#109
Green Stink Bug#108
Green Peach Aphid#120
