In last month’s column, I gave some reasons why callbacks for cockroach control are so common. Recently, I learned more about how and why cockroach communities survive pesticide treatments from Robert Kopanic Jr., a doctoral candidate at North Carolina State University. Kopanic is studying cockroach behavior to understand how current baiting systems may be improved. In his studies he has demonstrated why some cockroach life stages do not forage as often as others do, and spend more time in protected sites. The youngest nymphs, known as first instars, are the focus of Kopanic’s research. These neonates do not forage as frequently as other life stages but will readily consume the feces of other colony members, especially in the absence of other food sources. As a result of this behavior, called “coprophagy,” the foraging activity of immature stages can be substantially reduced.
COCKROACH DEVELOPMENT. Under laboratory conditions where food and water are plentiful, the German cockroach requires about six days to pass through each of its four instars. The fifth instar requires roughly twice the amount of time. These periods could be longer in the field where food and water may be a limiting factor on development. In his studies, Kopanic found that the smallest nymphs can survive for extended periods on fecal material alone.
Adult female cockroaches, however, can remain immobile for the longest period of time of any member of the cockroach community. As soon as she sprouts wings, the adult goes on a feeding frenzy for five to six days. Gorged with food, she seeks a male for mating, then spends up to a full month gestating in solitude to produce another generation. The male cockroach, however, forages and cavorts much more than his mate. The pregnant adult females and small nymphs are two life stages that could potentially escape a residual insecticide application.
When neonate cockroaches do emerge, we would hope that they walk over a surface treated with a long residual. But this does not always work. Various surfaces can absorb the toxicant so that the amount available to be contacted is substantially reduced. Worse yet, the deteriorating effect of water, heat and time can reduce whatever toxicant is available to levels so low that only the weakest insects will be affected. Resistant strains usually follow.
Kopanic states that one way to reach the hidden members is with baits, and he targets the most numerous members of the colony, the first instar nymphs, because their food is essentially feces from older siblings or adults. Kopanic is also currently testing different baits to see how well each performs in horizontal transfer studies and to understand how baits may be improved from the standpoint of horizontal transfer.
THE FOOD CHAIN. A myth conception exists concerning the reasons immature cockroaches consume fecal liquids. I thought the early instars did this to get an initial stock of protozoa and other microorganisms to help digest their food. After all, cockroaches are closely related to termites, which solicit anal fluid for microscopic symbionts that are vital in termite digestion.
But Kopanic has demonstrated that German cockroaches also use feces as a food source and can survive on fecal material alone for extended periods when other food resources are scarce. Regardless of whether cockroaches engage in coprophagy for food or for symbiont transfer, the behavior can still be exploited to enhance the activity of baits. Kopanic’s research on fecal pellets and bait preferences of various stages of the cockroach colony, as well as other current research, is sure to provide material for future discussion. I never thought I’d see the day when cockroach droppings would become so valuable.
Harry Katz is a contributing editor to PCT.