Among American cockroaches (Periplaneta americana), different individuals have distinct “personalities” that relate to their fleeing behavior and preference for venturing into open spaces or remaining close to walls or other objects. A study by researchers at the Université libre de Bruxelles in Belgium suggests these differences “could be an evolutionary benefit for the collective fleeing response to disturbances.” (Photo credit: Alex Wild, Ph.D., Insects Unlocked)

By Andrew Porterfield

Most non-entomologists do not readily discern personality traits among cockroaches, lice, or ants. But a team of entomologists from the Université libre de Bruxelles (ULB) in Belgium, have not only found that members of the family Blattidae (in this case, American cockroaches) have distinct “personalities”; they found that those personalities affect how the insects flee to avoid predators and other hazards.

Andrew Porterfield

Michel-Olivier Laurent Salazar, Ph.D., previously a doctoral student at ULB and now a postdoctoral researcher at the University of Ryukyus in Japan, and a team of colleagues at ULB examined the phenomenon of thigmotaxis, the trend of any animal to seek contact with other objects. Positive thigmotaxis involves an animal seeking contact with a wall, crevice, or other object, providing close quarters and therefore protection. The researchers discovered that, at least among the American cockroach (Periplaneta americana), thigmotactic tendencies varied, indicating personality differences. (“Personality” carries a complex meaning in humans; in animals, biologists measure personality as consistent individual behavioral traits.) The researchers’ work was published February 1 in of the open-access Journal of Insect Science.

Since cockroaches generally flee to darker, closer quarters when confronted with light, Laurent Salazar’s team—who have been studying cockroach and other insect behavior for more than 15 years—examined how their responses to light differ among individuals.

The researchers set up a circular arena, with an electric fence on the perimeter to prevent escape, and covered the floor of the experimental arena with white paper, which they changed after each experiment to prevent the possibility of the roaches leaving chemical markings. Plastic rings with two openings constituted shelters. The room was darkened except for one single light bulb that exclusively lit the shelter area.

To investigate differences in cockroach “personalities,” specifically the degree to which individual roaches prefer to stay close to a wall or venture into open areas, researchers at Université libre de Bruxelles in Belgium conducted an experiment in which adult male American cockroaches (Periplaneta americana) were placed in a small arena under a light. The researchers observed and measured the position, orientation, and wall contact of each roach before and after turning on the light. Their results showed that cockroaches that stayed close to walls were slower to react to light than those that preferred the center of the arena. (Image originally published in Laurent Salazar et al 2018, Journal of Insect Science)

Over three days, adult male roaches were tested and kept in darkness until their trials (each had three trials). During the trial, the roach was placed in the shelter for a waiting period. Then, shelter openings were opened and light was shown. Individual behavior was recorded for five minutes or until the cockroach fled the shelter. The researchers measured the position, orientation, and wall contact of each roach before and after turning on the light. They also measured individual reaction times to the light.

The researchers discovered personality differences among roaches even before the experimental light disturbance. Individuals showed preferences toward standing in the center or on the side of the shelter, with some touching the walls with their antennae and others never touching the wall. Over the trial period, 28 roaches expressed “low” thigmotaxis (tendency toward the shelter center) and 44 showed higher thigmotaxis. In addition, higher thigmotactic individuals tended to orient themselves toward the shelter wall.

After the light was switched on, reaction times between “high” and “low” thigmotactic roaches differed greatly. Highly thigmotactic roaches had slower reaction times, of about 27 seconds, while low thigmotaxis roach reaction times was much faster, at about 7 seconds. And, in contradiction with earlier studies, reaction times for an individual were not consistent across the three-day period, indicating that roaches might be reacting to light intensity or that other thigmotactic behavioral reactions are occurring.

The study showed that “thigmotaxis is a quantifiable personality trait in P. americana,” the authors wrote, and will help entomologists and others understand the remarkable ability of the cockroach to withstand inhospitable conditions and even resist pest control attempts. “The presence of personalities within a group has been seen to have an important impact on the performance of the group during collective behaviors. These personalities could be an evolutionary benefit for the collective fleeing response to disturbances.”

Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He writes frequently about agriculture issues for the Genetic Literacy Project. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.