It is not uncommon for commercial and residential buildings in urban areas to contain both German cockroaches and house mice. For the cockroach, such co-habitation is risky business. Inside buildings, domestic cockroach species, such as German and American cockroaches, have long been preyed upon by domestic house mice and Norway rats. In fact, several technical publications by entomologists over the years have stated that the three primary enemies to the German and American cockroach are (1) themselves via cannibalism, (2) humans and (3) mice.
This has been verified both in the field and in the lab. In the field, pest professionals the world over have noticed cockroaches captured on sticky monitoring traps are sometimes eaten off of the traps by rodents. Usually, the only part of the cockroaches left in the glue trap are the legs, head and antennae. Amazingly, the rodent can eat a roach out of a glue trap without itself becoming entangled. Often, the only evidence that a rodent had been there is the presence of a few of the rodent’s guard hairs that become stuck in the glue while the rodent dined. Presumably, these hairs are from the chest area of the rodent as a result of when it gingerly bent over to eat the cockroach.
Occasionally, professionals report seeing mice or rats directly eating roaches or other insects. Prior to the advent of cockroach baits, I watched a mouse emerge from beneath a refrigerator and feed on immobilized German cockroaches that I had just flushed out of cracks and crevices with a pyrethrin aerosol. I remember thinking it was like watching someone quickly shucking an ear of corn. It also seemed to me that the mouse was experienced in this food-handling technique.
Researchers have also observed this opportunistic predatory behavior. Purdue University graduate students Brian Schneider and Judy Bertholf both recorded house mice eating their "research cockroaches" out of their live traps and sticky monitors within multi-family housing research projects during the 1980s.
At Boston University in 1980, Mark Wourms demonstrated that house mice would kill and eat mobile cockroaches — at least those roaches it could first capture, which apparently was not an easy task. For instance, Wourms recorded that out of a total of a 170 attempts by 26 mice, only 22 cockroaches were caught — a meager 13 percent predation success rate by this group of mice. Most times the cockroaches escaped.
THE KILL. Wourms recorded a blow-by-blow description to the predation event as follows: upon the first encounter, the mouse would approach a cockroach within a half of an inch, partially close both eyes and sniff the cockroach. For the first few encounters, the mouse usually withdrew from the insect after sniffing it. Thereafter however, the mouse would begin by sniffing at the cockroach and then strike at it with one or both front paws. The striking behavior caused the cockroach to run rapidly and erratically. This excited behavior by the cockroach caused the mouse to rapidly paw and repeatedly lunge at the cockroach. After several seconds, the cockroach either escaped or was captured.
If the mouse captured a roach, it would position it with the front paws until the mouse placed the head of the roach inside its mouth. The cockroach was then killed when the mouse bit through the cervical region or removed the cockroaches’ entire head. Then the mouse would proceed to bite off the wing covers, wings and each of the legs — the same pattern that I had observed my mouse doing during my cockroach cleanout. Sometimes only the soft viscera of the cockroaches abdomens are consumed; other times, the mouse eats the entire remaining parts of the cockroach — exoskeleton and all.
Apparently, cockroaches provide excellent nutrition for rodents. Research by Francis Marks at Fairleigh Dickinson University in 1978 demonstrated that the nutritional content of a German cockroach was an unbelievable 62 percent protein and 25 percent fat. Other studies have shown many insects are also rich in minerals. Fat provides a mouse with energy and body insulation against the cold. Protein provides for building tissues, daily physiological activity and reproduction. Minerals are necessary for facilitating many bodily functions. Finally, the chitin within the exoskeleton may provide certain benefits relative to digestive processes.
To make a human comparison, think of a double cheeseburger with bun, tomato and lettuce at your favorite fast food joint. The cheeseburger contains 19 percent protein, 48 percent fat and 33 percent carbohydrates — not a good return in nutrition. Perhaps the only parallel here is that considering how quickly most roaches move and escape the rodent, the rodent must consider a roach "fast food."
Does any of this suggest that mice provide biological control of cockroaches? Well, if the Wourms experiment is reflective of the actual capture rate in real-world accounts, its not likely. It’s probable that domestic rodents in buildings prey upon roaches to supplement their daily diet.
What percentage of the mouse’s diet is comprised of cockroaches within an average urban structure containing human foods, spilled garbage, and so forth is unknown. But it is likely to vary from none to perhaps a significant amount (if a severe cockroach infestation is present). Other factors are likely to be at play as well. For example, it may be that a mouse in an unkempt restaurant is less predaceous on the resident cockroaches because other foods are so readily available.
ON-THE-JOB USES. Besides the fascinating aspects of this predator-prey relationship, is there any way you can put this information to work for you on the job? Yes. Servicing professionals should always look closely at sticky monitors that contain only fragments of cockroaches, especially if the fragments are of legs, heads and little else. Sometimes, technicians hastily assume these insects have "disintegrated" on the trap. But cockroaches embedded in glue do not disintegrate as readily as they do when they die on floors and their internal organs and viscera are consumed by ants or other insects.
Glue traps and sticky pest monitors containing only pieces are not only good indicators that mice are about, but these situations also provide a valuable piece of information that can help minimize mouse callbacks. Mice that can successfully pluck cockroaches off of glue traps may also be quite adept at avoiding these dangerous surfaces altogether. Therefore, either rodenticide baits or snap traps may be necessary to achieve control for these mice.
So why should a cockroach spend energy to attack and eat a cockroach? For one, a nutritional snack. For two? Well, I can’t think of a two. Maybe for sport?
The author is president of RMC Pest Management Consulting and can be reached at 765/939-2829.