Millipedes

This is one of the few household arthropod problems whose published remedy is to do nothing for a few days. Colorado State University Extension: “Because millipedes require high moisture, they usually die in a home within a day or two.” What follows is why they arrived, why the arrival stops on its own, and the one thing the sources disagree about.

What This Page Carries

Are They Dangerous? Two Answers, and a Caveat

Two sources say no, without qualification. Colorado State: “Millipedes do not bite or pose any danger to humans.” NC State Extension rules out more in one sentence: “Millipedes do not bite, sting, or transmit diseases, nor do they infest food, clothing or dry, structurally sound wood.”

University of Georgia agrees on the first point and then adds something the other two do not carry at all. It states that “Millipedes are not poisonous, but many species have glands capable of producing irritating fluids that may cause allergic reactions in some individuals.” And then, more concretely: “The defensive sprays of some millipedes contain hydrochloric acid that can chemically burn the skin and cause long-term skin discoloration.” followed by “The fluid can also be dangerous to the eyes.”

Its practical instruction is one line, and it is the reason this section exists: “It is not advisable to handle millipedes with your bare hands.”

The disagreement is narrower than it first looks, and this page will not collapse it in either direction. Colorado State and NC State are describing a millipede you leave alone, and on that they are almost certainly right — nothing in the Georgia text describes an animal that approaches or attacks. Georgia is describing a millipede you pick up or crush. Both are published; only one of them is about a choice you make. Use a dustpan or a vacuum rather than fingers and the two positions stop competing.

Telling One From a Centipede

Count the legs per segment. It is the whole test. University of Georgia describes the millipede: “They are wormlike, with rounded body segments that each bear two pairs of legs.” A centipede has one pair per segment, is flattened rather than round, and moves fast.

Behavior separates them just as reliably. Colorado State: “When dead or disturbed, they tend to curl into a tight coil.” A millipede curls; a centipede runs. Our house centipedes page covers the fast one, including the bite question its own eight sources disagree about — and note that a centipede in the same basement may be there because there is something to eat.

Colorado State also records the description most people actually arrive with, which is worth printing because it is exactly what a reader types into a search box: a common description is little black worms crawling in the basement windows.

And if it is gray, oval and rolls into a ball, it is neither. Colorado State publishes a third animal in the same fact sheet as these two — the sowbug and the pillbug, which are not insects at all but crustaceans that breathe through gills. Our sowbugs and pillbugs page covers the test that separates those two from each other, and why the same source says control is generally unnecessary for them.

Why They Came, and Why It Will Stop

Rain moves them, and the calendar is published. Colorado State: “Most millipede movement takes place in September and October and again in midspring.” Purdue University Extension names the trigger: “Large amounts of rainfall can trigger mass migrations of millipedes from the soil.”

Alabama Cooperative Extension describes it from the animal’s side: “However, at certain times during summer and fall, millipedes become restless and may migrate in large numbers from their normal living places to residents’ living spaces.”

And the shape of the event is the diagnostic. Colorado State observes that invasions, usually into cellars, often take place shortly after a period of wet weather and end as suddenly as they start. An influx that began within a week will end within a week.

University of Georgia explains the route in, and it is why a ground-floor door is the usual point of failure: “Millipedes can climb walls easily and will often enter homes around ground level doors and through foundation cracks above ground level.”

The Remedy Is to Wait, Then Vacuum

Colorado State states the reason the animals cannot persist indoors, and then states the remedy that follows from it. The reason: “Because millipedes require high moisture, they usually die in a home within a day or two.” The remedy: “Because millipedes cause no damage in homes other than a minor annoyance, the best way to handle infestations is to wait a few days for the problem to subside, then vacuum the bodies that remain.”

One observation from the same source explains a common false alarm: “The hard body of the millipede, however, remains intact for a considerable time after it is dead.” A dry millipede on a basement floor is not evidence that anything is still arriving. It may have died a fortnight ago and simply not decayed.

Given Georgia’s warning about the defensive fluid, a vacuum or a dustpan is the sensible tool here rather than a hand, and that follows from the sources rather than from us.

What Stops It Happening Again

Debris first, then the gaps. Colorado State: “Remove debris and other favorable habitats from around building foundations to help reduce problems.” and “Seal or caulk openings around the foundation to reduce future millipede problems.”

Purdue gives the same instruction for the openings above ground level: any cracks around doors, basement windows or similar openings should be sealed with caulking compound. University of Georgia: “Make sure doors and windows fit tightly and caulk cracks and crevices that permit their entry to the inside.”

NC State names the specific weak point, and it is worth knowing before you start: door thresholds, especially at the base of sliding glass doors and garage doors. That is where a millipede walking along a foundation finds its way in.

And University of Georgia draws the inference that outlasts the millipedes: “This fact should not be overlooked; excessive moisture will provide habitat for a wide range of household pests and is indicative of a structural or functional problem in a home.” Our earwigs page and our camel crickets page describe the same damp perimeter producing two entirely different animals, and neither page repeats this one.

Why No Product Is Named Here

Colorado State names four pyrethroid active ingredients, and in the same passage explains why they are not a shopping criterion. It gives bifenthrin, cyfluthrin, lambda-cyhalothrin and permethrin for a perimeter treatment, and says insecticides with these active ingredients are widely available at nurseries, sold under various trade names. A list of chemistries that every competing product carries does not distinguish one product from another, so there is nothing here to rank.

This estate has already answered this exact question, from the same publisher. Our black widow page was given essentially the same list of pyrethroids by Colorado State and named no product for the same reason. Two pages disagreeing about that would be the defect, not the consistency.

The source’s own weight also runs against a purchase. Its Quick Facts say control is rarely necessary, and that if control is required it should primarily involve moisture control. In the insecticide passage itself it says usually only shaded sides of the home need treatment and that whole yard treatments are unnecessary and not recommended. A product card at the bottom of a ladder whose first rung is wait would misreport the page it sits on.

Sealing has the opposite problem: five sources support it and none specifies it. Caulk, door sweeps, tight-fitting doors and screened openings are all named; no gap width, no material and no dimension is published for any of them. That is a supported practice with no product criterion attached, which is the position our camel crickets page already reached on weather-stripping.

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