If you understand cockroach biology, this common complaint makes sense and it doesn’t mean a botched treatment. When you have a professional treatment for cockroaches, if the technician does a good job, most of the roaches will be killed. But, short of fumigating your house, it’s almost impossible to kill them all immediately . There may be survivors. Believe it or not, the survivors are most likely to be females carrying egg cases full of baby cockroaches.

When a female cockroach is developing and carrying her egg case, she usually remains in hiding in cracks or crevices. This is to protect her offspring from predators, dehydration, and pesticides. Even though pest control technicians use crack and crevice application (see A Crack & Crevice Tip Improves Cockroach Control) when applying cockroach bait or residual insecticides, some cracks will be missed. Because the female is not out foraging like the other cockroaches, she is less likely to come in contact with the insecticide. So she survives and her egg case survives, at least temporarily.

Even if all the cockroaches are killed during treatment, an egg case can still hatch out afterwards. The German cockroach carries her egg case until just before it’s ready to hatch. Then she places it in a hidden, protected place. Even if the egg case was sprayed with insecticide, the sealed, leathery covering protects the cockroach nymphs inside. So, yes, you can expect more than 30 little cockroaches to hatch from that hidden egg case. And if you’re only seeing baby roaches, that’s a good thing.

Even when there are surviving cockroaches or newly hatched nymphs after treatment, it’s not a great cause for concern. These cockroaches will die as soon as they come in contact with the insecticide left during treatment. If the technician applied residual insecticide spray, the nymphs will contact that when they walk across surfaces or enter treated cracks—and they will soon die. If the treatment was gel bait, as soon as the newly hatched cockroaches find and eat the bait, they will die, too.

You should know, though, that if you are doing your own pest control and using total release aerosols (also called “bombing” or “fogging”), you are not leaving any insecticide in place to kill those newly hatched cockroaches. Foggers give immediate but very short term control. Foggers may kill all the cockroaches present at the time of treatment but they will not kill any new cockroaches or cockroach nymphs that show up even the next day. Best to have a professional do the job right!

Photo credit: Mark Dumont / Foter.com / CC BY