Q. What are those brown spots that you find in corners and in places that have lots of cockroaches? Is that cockroach poop? It’s hard to wash off of walls, I’ve discovered.

A. It sort of is cockroach “poop.” Cockroaches leave behind dark, dry feces that looks like black pepper, but they also leave liquid fecal spots, especially in areas where they all congregate together. The more cockroaches, the more fecal spots…and the more cockroach odor. In a heavily infested home, you can see the characteristic spotting along the edges of kitchen cabinets and in corners, on the top of door frames, and in other out-of-the-way places where cockroaches hide. Many of us in the pest control business have a photo of a wall where a picture frame has been removed. The missing frame is perfectly outlined on the wall in fecal spots left by cockroaches that hid behind the picture.

The fecal spots work like fecal trails to attract other cockroaches to a location. The spots contain an aggregation pheromone which other cockroaches “sniff” out using their antennae. The pheromone tells newly arriving cockroaches that this is the spot to find food, water, hiding places, and others of their kind.

Spotting serves pest control technicians, too. Technicians know to concentrate their baiting or trapping near accumulations of cockroach spotting because that’s where the roaches are. Spotting serves as a pest control roadmap to cockroach hideouts. If you do your own cockroach control, you can use the spotting as a guide for where to locate your baits or where to spray.

Researchers figured if cockroaches follow the aggregation pheromones in fecal trails, why not use the chemicals in these trails as bait to lure roaches into traps? A good idea, except, as you might expect, the bait material was just too foul-smelling to be of practical use! But scientists at the University of Florida found that if they dissolved the material and transferred the chemicals to a water solution, they could produce an odorless material that the roaches still followed. Maybe someday we will have a pheromone trap for cockroaches that is based on their own fecal spotting.