Roach Battlers Turn to Biological Weapons : Hormone Used to Lock Pest Into an Adolescent, Non-Reproductive Stage

Ever since civilized man first turned on the kitchen light and spied a cockroach running for cover, relations between humans and the most loathed of household insects have been strained.

Cockroaches have been paralyzed, asphyxiated, doused with every conceivable poison and clubbed endlessly with blunt objects--all in an often fruitless battle to rid the world, or at least the kitchen, of roaches.

But now, in what is best described as a new spirit of roach-man rapprochement, entomologists and pest-control experts have begun to shift their strategy, moving away from extermination and toward managing roach populations at minimal levels and modifying their biology.

Behind the new roach detente are several compounds known as insect growth regulators (IGRs), which offer an environmentally friendly alternative to conventional insecticides that are widely felt to be too toxic, and in some cases too ineffective, for widespread use.

Adolescence Locked In

Unlike most existing poisons, IGRs don’t try to kill cockroaches outright. In fact, the leading IGRs, known as juvenile hormone analogues, are not poisons in the traditional sense. Instead, they are chemical copies of hormones that the roach uses to govern its own larval development.

Given to the insect in large enough quantities, the juvenile hormones trap the roach in the insect equivalent of pre-adolescence, leaving it as healthy and vigorous as any other juvenile cockroach, but without the sexual maturity to reproduce.

“We call it the Peter Pan syndrome,� said Roger Meola, a roach expert at Texas A&M; University. “They’re forever young.�

The appeal of insect growth regulators is their specificity. While many insecticides are broad-acting toxins poisonous not just to cockroaches, but in some cases to any other organism they touch, juvenile hormones affect roaches and nothing else.

One hormone known as hydroprene, in fact, is so selective that it is effective only during a 5- to 18-day period just before the roach’s final metamorphosis from adolescence to adulthood.

During that critical period, the IGR keeps the level of juvenile hormone in the roach artificially high, just when it is supposed to drop and be replaced by adult hormones that drive the insect’s transformation to maturity.

The result is that the cockroach is crippled as its moves into adulthood, with deformed wings and inadequately formed genitalia.

Because these sterile roaches live just as long--in fact longer--than the typical four months of their untreated counterparts, IGR administration can take up to eight months, or two roach generations, to have an effect.

Other IGRs offer more immediate results. One set of compounds, for example, blocks the roach’s ability to manufacture the material known as chitin, which makes up the insect’s hard outer shell, or exoskeleton.

At each of the cockroach’s five molting stages during development, the chitin inhibitor is potentially deadly, since once the roach sheds one skin, it cannot make another.

Unlike many other growth regulators, this one is not specific to roaches. In fact, it works on any organism that produces chitin, which includes most other arthropods such as insects and crustaceans.

For this reason, chitin inhibitors are considered suitable only for agricultural uses that will not allow them into streams or lakes to affect shrimp and other crustaceans.

Safer for Humans

“Some of the more conventional insecticides will affect the nervous system of a cockroach and will also affect the nervous system of a mammal,� said Donald Cochran, an entomologist at Virginia Polytechnic Institute and State University. “But these (juvenile hormones and chitin inhibitors) both attack systems that don’t have any parallel in a human.�

The big question surrounding IGRs, however, involves their effectiveness. In field trials at the University of Florida, hydroprene was found to be 95% as effective as conventional pest-control methods after seven months. The chemical companies currently promoting wider use of IGRs against cockroaches claim high levels of effectiveness for their products.

The catch is that to bring down roach populations significantly, juvenile hormones have to sterilize an unusually high number of insects--more than 80%--because otherwise the method simply improves the survival rate and fertility of those unaffected by taking away other roaches with which they would have had to compete.

“We haven’t seen the same population crashes that other researchers have,� said Michael Rust, an entomologist at the UC Riverside. “Juvenile hormones are a potential alternative, but they are not a panacea.�

Still, several manufacturers are selling products based on IGRs. Hydroprene, for example, is sold as Gencor by Zoecon Corp. Three companies make versions of chitin inhibitors--Larvadex by Ciba-Geigy, Baysir by Mobay Chemical, and Dimilin by Roussel Bio.

Immunity Developed

There is also some doubt about how durable IGRs will be as a form of pest control. Because of the large size of most domestic cockroach populations and their reproductive speed, some mutant roaches always will have the unusual ability to break down any toxin before it can take effect.

Those select few survive the initial chemical onslaught and pass on their unique genetic characteristics to their offspring, creating a new and hardier generation.

Even the most potent of chemicals have, over time, seen their effectiveness reduced through this process of natural selection. At first, researchers thought it was unlikely this same scenario would be repeated for juvenile hormones.

While it was understandable, after all, for an insect to have resistance to a foreign chemical, how could there be a roach that breaks down a copy of one of its own hormones?

Resistance Possible

However, recent research with fleas subjected to the same juvenile hormone techniques as cockroaches shows that resistance is possible, and that in critical stages of development some insects have a higher tolerance for juvenile hormones, an unusually effective method for breaking the chemical down or a particularly adept way of avoiding sources of it in the environment.

“Say the insect picks up the hormone through its feet,� said Ted Shapas, manger of insecticides research at American Cyanamid, maker of Combat roach poison. “Then it’s not unlikely for it to develop thicker feet. That may sound facetious. But it’s the type of thing insects do to protect themselves.�

“I don’t think anything is out of the question with insects,� said George Rambo, director of research at the National Pest Control Assn. “Especially with cockroaches.�