German Cockroaches: 10 Key Facts to Remember

Editor’s Note: The following article was adapted from Techletter, with permission from Pinto & Associates.

The German cockroach was once the No. 1 pest in residential and commercial buildings. That changed with the introduction of cockroach baits, which resulted in one of the great success stories in insecticide development. Cockroaches became almost a second tier pest, and remained so for more than a decade. But have you noticed? They’re back! It’s time for pest management service technicians to review some of the critical aspects of German cockroach biology and habits that can make it one of our more challenging pests.

1. German cockroaches have a phenomenal rate of reproduction. One female cockroach and her offspring can theoretically produce hundreds of thousands of cockroaches in just one year! To prevent a population increase, either the service has to be frequent enough to stay ahead of the growth curve, or the controls (baits or sprays, chemical or nonchemical) have to remain effective from one service visit to the next.

2. German cockroaches love heat and humidity. Cockroaches prefer a temperature of 85° to 95° F with a humidity at 90 to 95% (although they do quite well at normal room temperatures and humidities, too). Within a given room, cockroaches will be concentrated where the temperature or humidity is highest, such as above refrigerators and other heat-generating equipment, under the sink, and around water pipes (hot water pipes provide warmth, cold water pipes provide water from condensation).

3. German cockroaches spend most of their lives hidden and protected. For every cockroach in view in daytime, hundreds can be hidden inside wall and ceiling voids, behind cabinets, inside appliances, and the like. A German cockroach’s favorite location is inside a narrow crevice with its antennae extended out. Typical “prime” locations are the cracks between cabinets and walls, the flange under the kitchen sink, and the space between door corners and jambs.

4. German cockroaches aggregate (crowd together). Miss an aggregation site and you may leave your client with a mess of cockroaches. On the other hand, the fact that German cockroaches aggregate can help in their control. The brownish “spotting” left behind by cockroaches marks sites of aggregation, which are also excellent sites for application of residual insecticides or baits. Use spotting as a roadmap to direct your treatment.

5. German cockroaches protect their egg cases.The female German cockroach carries her egg case for as long as a month, dropping it just before it hatches. While she is carrying the egg case, the female German cockroach is less active and tends to stay hidden away in cracks and crevices and protected voids. For this reason, when applying residual insecticides, it is especially important to (1) treat deep inside cracks, crevices, and voids, and (2) schedule follow-up treatments from a few weeks to a month later in problem locations to control newly-emerged nymphs.

6. German cockroach egg capsules are not susceptible to insecticides. Most insecticides do not have the ability to penetrate the egg case, even if directly applied to the case. The unaffected egg cases will hatch days after treatment. This is another reason to schedule follow-up treatments in heavy infestations.

7. German cockroaches regularly disperse to new areas. Older, larger nymphs are most likely to disperse, followed by adult males and then adult females without egg cases. Small nymphs are least mobile. During an outbreak of German cockroaches, technicians need to expand their search for cockroaches into adjacent rooms above and below, even into sites that are not typically infested with German cockroaches.

8. German cockroaches detect and avoid many insecticides. Most insecticide sprays and dusts are repellent to a greater or lesser degree. If cockroaches are repelled by a treated surface, they are not likely to contact the insecticide long enough to pick up a toxic dose. Be sure to treat deep inside cracks and crevices and voids. If there is no untreated harborage to escape to, cockroaches will be forced to contact insecticide residues even if those residues are repellent.

Janet Hurley, Bugwood.org

9. German cockroaches can develop resistance to insecticides. When insects are exposed to an insecticide, the least susceptible may survive to pass on their “resistance” to their offspring. This resistance may be physiological or behavioral, as we learned recently with cockroach aversion to certain baits. Eventually, the insect cannot be controlled with that insecticide, sometimes not even with related insecticides they have never been exposed to before (cross resistance). To prevent control failures periodically change insecticides or use combinations of insecticides.

10. German cockroaches can change their normal behavior. You may find a residence infested with cockroaches in the living room but not the kitchen. Or German cockroaches will be seen moving out in the open in bright light, and not responding to activity in the area. Or you may find high populations outdoors in the lawn and shrubbery. Stay on your toes!

If your service fails to control German cockroaches in an apartment building it is probably because of one or more of the following mistakes:

• Not identifying and treating “focus apartments.” These apartment units have high cockroach populations that keep infesting the units next door, above, and below. Focus apartments typically are cluttered, have poor sanitation, and refuse regular service.
• Not getting into every apartment. Missed apartments often turn into focus apartments.
• Not rescheduling problem apartments. When cockroach populations become large, service needs to be repeated every few weeks to succeed.
• Not rescheduling missed service. Apartments can’t simply be skipped.
• Poor cooperation from residents. Cooperation means allowing access, preparing for service as directed, upgrading sanitation, and reducing clutter.
• Poor cooperation from management. Property management has to cooperate by enforcing resident compliance, providing access (keys and cooperating), reporting problems, and correcting structural deficiencies.
• Inadequate trash management. Trash chutes, compactors, and trash rooms can generate large numbers of cockroaches if not managed properly. Consequently, pay particular attention to these areas.
• Not applying insecticides effectively. Whether baits, liquids, dusts, or other formulations, cockroach insecticides need to be applied in cracks, crevices, voids, and other sites where German cockroaches are aggregating.
• Not keeping good records. Good recordkeeping allows you to identify problem areas and forecast trends for better control, and to support further efforts to ensure resident compliance.
• Not providing alternative treatments for ill residents. Often, apartments with ill or infirm residents are simply skipped by service technicians. These units can turn into focus units without some level of service.

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