Why roaches show up after rain (and which kind you're seeing)

Big roaches that turn up after a storm are usually outdoor species, such as American, oriental or Turkestan cockroaches, pushed out of soaked mulch, drains and sewers into drier shelter. Small, light-brown roaches with two dark stripes behind the head are a different story: those are German cockroaches, which live indoors in Texas and point to an infestation inside, rain or no rain.

Why do roaches come inside when it rains?

Several large Texas cockroaches live mostly outdoors. According to Texas A&M AgriLife Extension's cockroach biology and management guide, American, oriental and smokybrown cockroaches "live mostly outdoors but may move indoors in search of food or water." Their outdoor homes include compost piles, ground cover, hollow trees, mulch, old stumps, woodpiles, sewer manholes and underground water meters.

When a storm floods those spots, the roaches move. An AgriLife Extension article from Bexar County notes that American cockroaches are among the large outdoor roaches that head indoors during and shortly after rain. The same article points out that these roaches normally live outside in damp areas like flowerbeds and under trees, and that they tend not to survive long indoors.

So a single big roach in the laundry room the morning after a downpour is usually a displaced outdoor visitor. Where it got in matters more than the fact that it is there.

How do outdoor roaches get in?

AgriLife's guide names the usual routes:

• Poorly sealed doors. Check the threshold seal under each exterior door.
• Roof soffits. The guide recommends making sure soffits are screened.
• Floor drains. American and oriental cockroaches use floor drains as common entry points. Keeping P-traps filled with water creates a barrier between the sewer and the house.

If a floor drain is rarely used, running water through it now and then helps keep that trap filled.

Know the suspects: which roach are you seeing?

The illustrations on this site are drawings, not identification photographs. Use the field marks below and compare them with the linked profiles.

American cockroach

• Size: The largest cockroach in Texas, 1½ to 2 inches long, per AgriLife.
• Look: Reddish brown, with tan to light-yellow bands outlining the shield (pronotum) behind the head.
• Habits: Both males and females can fly. Also called waterbugs or palmetto bugs, they are among the most common cockroaches in sewer systems.
• Profile: American cockroaches

Oriental cockroach

• Size: About 1¼ inches long.
• Look: Dark brown to nearly jet black. They do not fly. The female's wings are tiny, and the male's wings reach only a little more than halfway down the body.
• Habits: AgriLife notes they prefer cooler, ground-level sites such as basements, crawl spaces and wet areas.
• Profile: Oriental cockroaches

Turkestan cockroach

• Look: Males and females look very different. Males are slender and brownish orange to yellowish, with long wings, and can fly or glide. Females are dark brown to black, broad, with short wings and cream-colored markings at the edges of the pronotum.
• Size: AgriLife lists adult males at ½ to ⅞ inch.
• Habits: AgriLife's Turkestan cockroach overview calls it an emerging pest in Texas, living outdoors in water meter boxes, compost, leaf litter, potted plants, sewers and cracks in concrete. Males show up indoors more often than females because they fly and are drawn to lights.
• Easy to confuse: Females are often mistaken for oriental cockroaches. Look for the pale cream edging on the pronotum and wing bases.
• Profile: Turkestan cockroaches

Smokybrown cockroach

AgriLife also lists the smokybrown cockroach among the most common Texas species. Adults are 1¼ to 1½ inches long, dark brown to black, with wings longer than the abdomen. It favors protected, moist, warm sites such as ground cover, mulch, soffits, attic eaves and tree holes.

German cockroach

• Size: Small, about ½ to ⅝ inch long.
• Look: Light brown with two dark stripes on the pronotum. Adults have wings but do not fly.
• Habits: AgriLife is direct about this one: "German cockroaches do not live outdoors in Texas." They spend their lives indoors, usually in warm cracks and crevices near food and water.
• Profile: German cockroaches

Is it a storm visitor or an infestation?

Read the tracks. These signs help separate a one-off visitor from a problem that lives in the house.

Signs of an outdoor roach passing through:

• One or two large roaches, especially right after heavy rain
• Sightings near exterior doors, garages, laundry rooms or floor drains
• Nothing found in cabinets, around the stove or under the sink

Signs of a German cockroach infestation:

• Small, striped roaches in the kitchen or bathroom
• Roaches seen in daylight. AgriLife notes that seeing cockroaches during the day usually means there are a lot of them.
• Droppings that look like crushed pepper stuck to surfaces near hiding spots
• Egg cases: an adult female may carry a capsule that produces 30 to 40 nymphs

German cockroaches can produce a generation in about 100 days, according to AgriLife. AgriLife notes they can be brought in with groceries or boxes rather than walking in from the yard, and in apartments they can move from unit to unit. Because they are linked to allergies and asthma, finding them is worth acting on quickly.

How do you keep outdoor roaches outside after a storm?

AgriLife's approach to outdoor species is inspection, sealing and reducing shelter close to the house.

1. Find the entry point. Place sticky card monitors near exterior doors, behind laundry appliances and in utility rooms. AgriLife notes they are most useful for monitoring and give some control of low numbers of large roaches.
2. Seal the gaps. Repair door threshold seals, screen soffits, and caulk gaps around plumbing.
3. Keep drains wet. Keep P-traps in floor drains filled with water.
4. Pull mulch and clutter back. Outdoor species shelter in mulch, ground cover, woodpiles and compost, so keep those away from doors and foundation edges where you can.
5. Cut the indoor draw. Fix leaks, keep mops and wet items off floors, and empty pet bowls at night.

If you consider an insecticide, AgriLife describes granular and containerized baits for large roaches outdoors, and perimeter sprays at entry points. Whatever you use, the label is the law: the EPA notes it is a violation of federal law to use a product in a manner inconsistent with its labeling. For a walkthrough of what a label tells you, see how to read a pesticide label. AgriLife also warns against leaving visible piles of granular bait where dogs or cats might eat it.

What works for German cockroaches indoors?

German cockroaches need a different plan, because they breed indoors. AgriLife's guide recommends an integrated approach of inspection, sanitation, exclusion and low-toxicity insecticides:

• Sanitation first. Clean floors and counters, fix leaks and remove food and water overnight. Less food makes baits work better.
• Seal hiding spots. Caulk gaps around sinks, plumbing and kitchen splash guards.
• Baits over bombs. AgriLife calls baits among the most effective options for homes and says total-release aerosols (bug bombs) generally do not eliminate cockroaches because they do not reach the cracks where most hide.
• Rotate if needed. If a bait does not control the population within 2 to 3 weeks, AgriLife suggests switching to one with a different active ingredient.

An ingredient you may encounter in AgriLife's guide is boric acid, used as a dust in wall voids and under appliances. AgriLife cautions against applying dusts in the open or on counters and shelves. Always follow the label.

When to get help

A few big roaches after a storm usually call for sealing, not panic. Small striped roaches in the kitchen, roaches in daylight or roaches that keep coming back after you seal up are signs of a larger problem. In Texas, the Structural Pest Control Service of the Texas Department of Agriculture licenses applicators who treat in and around structures, so a licensed pest professional is the right call for an established German cockroach infestation. Renters should notify their landlord or property manager, since German cockroaches can spread between units.

If someone swallows or is exposed to a cockroach bait or insecticide, call Poison Control at 1-800-222-1222.

This field guide is educational and does not offer treatment bookings.