The Walking Behavior Of German Cockroaches

Control strategies for household cockroach pests are based primarily on the application of liquid insecticides to infested harborages. Successful control depends on the amount of insecticide that reaches the target surface and the target pest contacting the residue on that surface.

Modern insecticides, such as organophosphates (Dursban, diazinon), carbamates (Baygon, Ficam) and pyrethroids (permthrin, cypermethrin) function as nerve poisons. Mortality occurs hen cockroaches are exposed to insecticide residues by walking on treated surfaces when they leave harborages to search for food and water. Cockroach control depends on efficient and effective placement of insecticides so that cockroaches will contact them. Understanding the structure of cockroach legs and their walking behavior can help the professional PCO appreciate the importance of proper insecticide application methods.

Structure of cockroach legs. Although there are many different kinds of cockroaches and they are distributed all over he world, their body structure is basically the same. Like other insects, cockroaches have ahead with associated antennae and mouthparts, a thorax to which the legs and wings are attached, and an abdomen.

There are six segments to a cockroach leg: coxa, trochanter, femur, tibia, tarsus and pretarsus.

Coxa and trochanter. The coxa is a large segment at the base of the leg close, where it attaches to the thorax. The trochanter is a small segment a the end of the coxa.

Femur. This is the longest and strongest segment of the cockroach leg. It contains large muscles that move the legs. There are usually some large spines located on this segment of the leg.

Tibia. This segment is very slender, but usually about the same length as the femur. There are also spines located on this leg segment, especially at the end.

Tarsus. This segment is small and is the part of the leg that contacts the surface. This portion of the leg is composed of five segments, and on the underside of some of the segments is a small pad. These small pads contact the surface a cockroach walks upon; they probably provide the traction necessary to walk and climb on a variety of surfaces. In the German cockroach the pads have small spines on the surface.

Pretarsus. At the end of the tarsus are a pair of large movable claws. There is a small pad located between these claws this portion of the leg also contacts the surface and probably assists the roach in climbing.

The small pads on the tarsi of the legs are the major part of the cockroach that touches a surface. The large spines on the underside of the leg may also touch the surface when a cockroach walks. Roth and Willis (1952) studied the tarsi of five species of household roaches and reported that when a cockroach walks, the only parts of the tarsi that always contact the surface are the tips of the claws an the pads on the third and fourth tarsal segments. Cornewll (1968) reported that the front legs of a cockroach are used for gripping, the midlegs raise the thorax and abdomen off the surface and the hind legs proved the thrust for walking. The three legs and different parts of the legs have different roles, but they are all involved in the process of walking.

Cockroach walking behavior. Although a cockroach has six legs - three pairs on each side of the thorax - only three legs at one time are used for walking. During walking, the body of a cockroach is supported by two legs on one side of the body and one leg on the other side - a tripod arrangement. At any time during walking there are just three legs in contact with the surface; the other three legs are moving forward to a new position.

Stage 1. The front leg and the hind leg on the left side and the middle leg on the right side of the cockroach are on the surface. There three legs form the tripod that supports the body. The other legs are in motion.

Stage 2. The three legs not contacting the surface are moved forward a sort distance before they are placed on the surface.

Stage 3. A new tripod is formed as the legs in motion contact the surface and the other legs move forward to a new position.

Walking and chemical control. There is a connection between the structure of cockroach legs - specifically the small pads on the tarsi - the fact that only three of the legs touch the surface at one time and chemical control. Liquid insecticides (insecticide and water mixed) are applied to various household surfaces to control cockroaches. Soon after the liquid is applied, the water evaporates, leaving a thin residue of insecticide on the surface. When a cockroach walks across this treated surface, a small or large amount of insecticide penetrates the small pads and enters the body. If enough insecticide enters the body, the cockroach will die.

The structure of the legs indicates that only a very small part (pads of the tarsi) of the entire cockroach actually comes n contact with the insecticide. During normal walking only three legs at a time may be on a treated surface. Effective cockroach control depends on treating a surface adjacent to a harborage so that a cockroach must walk across a treated surface for enough time to pick up a lethal dose of insecticide.

-- Jing Zhai and William H. Robinson

The authors are graduate research assistant and professor, respectively, at the Urban Pest Control Research Center at Virginia Polytechnic Institute and State University in Blacksburg, Va.