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Editor’s Note: This article was reprinted with permission from Techletter/Pinto & Associates.

Peridomestic cockroaches species are those that are capable of living and reproducing both outdoors and indoors. In warmer regions, these cockroaches usually live outside but tend to migrate indoors or into sewers in the fall when the weather cools a bit.

The cockroach species pictured at right are peridomestic species, at least in some areas of the United States, but the American and Oriental cockroaches are known as occasional indoor pests throughout the U.S., especially in basements and lower levels of buildings. Adults are capable of flying or gliding, with the exception of the Oriental cockroach, the Florida woods cockroach and females of Parcoblatta wood cockroaches. The mature nymphs of most species are hard to tell apart. There are a few other outdoor cockroaches that will enter structures but have limited distribution. (See box below.)

When outdoor cockroaches move indoors, whether or not they can survive and then reproduce depends, for most species, primarily on whether moisture levels are high enough. They will gravitate towards the damper indoor areas of a structure: crawlspaces, basements, wall voids, damp attic soffits, laundry rooms, steam tunnels, garbage rooms, boiler rooms, etc. A few species are serious pests in greenhouses and indoor plantings. Because the various woods cockroaches don’t survive long or reproduce when they find themselves indoors, they can’t be considered true peridomestic species.

OUTDOOR HABITATS. These cockroaches like moist and shady areas that have lots of leaf litter or mulch. Outdoor cockroaches are active at night so schedule your inspection accordingly. Look for them in and on trees (some species live in palms or palmettos), in tree holes, under bark, in rotting logs, wood piles, in planters and pots, under wood decks, and of course in and near garbage containers, compost piles and municipal dumps. Outdoor cockroaches often can be found hiding on buildings in cracks and crevices, under shingles and siding, in roof gutters with debris, in brick weep holes and in similar sites.

FALL MIGRATIONS. Fall is the time of year when outdoor cockroach populations peak, along with complaints, and there is a scientific reason why this is the case. Most peridomestic cockroaches are large species with slower life cycles than smaller domestic cockroaches. Unlike German cockroaches that reproduce year-round, Oriental and American cockroaches, for example, have just one generation a year. All American cockroach nymphs hatch from egg cases in June in most areas. By fall, these late stage, large nymphs are looking for food and overwintering sites, along with the existing adults, many of which will die during the winter. At this time, they often enter substructures.

Outdoor cockroaches also may move indoors when the weather outside becomes too hot, wet, dry or cold, or when the population gets so large that the outside harborage sites can no longer contain or feed them. Parcoblatta wood roaches end up inside accidentally when males fly to lights or follow a female inside during their late spring mating season.

SEWER DWELLERS. The American cockroach is the number one cockroach inhabitant of municipal sewer systems and storm drains. In some regions, the Oriental cockroach or the smokybrown cockroach are found in sewers. Nymphs often enter the warmer sewer system during fall migration. Sewer roaches can then invade homes, offices or retail businesses directly through sewer pipes and floor drains or during sewer line breaks or backups. Check sewer vaults (with the help of the local sewer authority) and unused floor drains.

CONTROL MEASURES. The first control step for outdoor cockroaches that are entering buildings is to find and eliminate, or treat, their outside harboring sites. Pest-proof doors, openings around conduits (and other entry points) and reduce outside lighting. Have customers move firewood, stacks of stones and debris piles away from the foundation, reduce heavy mulch or piles of leaves, clean roof gutters and correct moisture issues, both inside and outside. A perimeter treatment or perimeter baiting can knock down populations. Cockroach baits can be very effective in late summer to early fall while nymphs are still actively feeding and before they slow down and move into winter sites.

The authors are well-known industry consultants and co-owners of Pinto & Associates.