Factors that influence insecticide transfer from a surface
If you’ve ever handled a cockroach, you know it has prickly legs. Look at the legs under magnification and you can see the rows of spines on the roach’s tarsi, the last segments of the leg. These spines, along with tarsal pads on the legs, dislodge and pick up insecticide residue as the roach walks on treated surfaces. Insecticide dust, powders, or microcapsule particles picked up on the legs are then swallowed when the cockroach grooms itself.
The importance of Tarsal Pads- The tarsus of an insect’s leg is made up of five segments and functions much like an ankle. It connects to the pretarsus which functions like a foot. Each tarsal segment has a tarsal pad (arolium) beneath. Their purpose is to create friction with surfaces much like a suction cup, allowing cockroaches to traverse slippery and vertical surfaces. The downside to tarsal pads for a cockroach is that they are the main pickup point of insecticide residue from a treated surface. The oriental cockroach has poorly developed tarsal pads and consequently has trouble climbing vertical surfaces (this explains why they are often found stranded in sinks or bathtubs). Larger tarsal pads collect more insecticide residue than smaller pads. In the Brownbanded cockroach, only the last tarsal segment has a large pad. Less insecticide is transferred to its legs which is one reason why it is more difficult to control the Brownbanded cockroach with liquid and dust insecticides. It’s also thought that the smaller tarsal pads allow Brownbanded cockroaches to pull away from sticky traps more easily.
When a German cockroach walks on a smooth, flat or inclined surface, at least three of the five tarsal pads on each leg are in contact with the surface, but the large hooked claws and the pad on the pretarsus at the end of the leg are not in use. However, when a cockroach moves on a rough or vertical surface, the last pretarsal segment with two claws and a large pad in the middle is bent down so that the claws and pad can provide extra traction.
Body weight matters, too- It follows that the smaller and lighter the insecticide particle, the more likely it is to stick to a cockroach’s leg. The weight of the cockroach and the size of its tarsal pads also determine how much insecticide is picked up by the legs. Larger, heavier cockroaches with larger tarsal pads can more easily pick up a toxic dose. Adult German cockroaches are heavy enough to crust the capsules in microencapsulate formulations when they step on them with their front legs. Large German cockroach nymphs may be able to crush the capsules, too, but small nymphs may not weigh enough.
Contact with the treated surface- When crawling insects like cockroaches are walking or running, only the tarsal pads are in contact with the surface. Since most of our domestic cockroaches don’t readily fly, whenever they move they are in touch with the surface and can pick up a dose of the insecticide. When a cockroach is standing or walking, most of the legs (and tarsal pads) are touching the surface. But when running, only the tarsi of three legs are in contact at any one time.
Although cockroaches have tarsal pads on each of their six legs, the pads on each pair of legs function differently. The hind legs are used to push forward and have large tarsal pads, although they carry the least amount of body weight. The middle pair of legs move only slightly back and forth and have moderately-sized tarsal pads. The front pair of legs move the greatest distance, have different nerves, and carry the most weight. These are the legs that will first contact and crust insecticide microcapsules and are the first to pick up insecticide residues from a treated surface.