Genetic sleuths explain insects' resistance.

Coffee drinkers, take note! The coffee bean's number one enemy, a bug that practices incest as a way of life and goes by the name of Hypothenemus hampei, or the coffee berry borer, can now outwit its archenemy, the pesticide endosulfan. Although investigators believe they have figured out the insect's secret genetic weapon, they have yet to defuse it.

In 1989, scientists discovered that many coffee berry borers on the South Pacific Island of New Caledonia tolerate endosulfan, the most common insecticide used against them. Unlike many other chemicals, this fumigant reaches inside the beans, where the insects lay their eggs. Whether coffee berry borers elsewhere have also become resistant isn't clear.

The secret of the bugs' success lies in their unusual genetic makeup and breeding habits, assert Luc O. Brun of the Institut Franaais de Recherche Scientifique pour le Developpement en Cooperation in Noumea, New Caledonia, and his colleagues in the Oct. 10 Proceedings of the National Academy of Sciences.

In a previous study, the researchers described a genetic mutation that enables the pests to withstand the insecticide. The mutation has a good chance of spreading through the population because the few males born in each brood mate only with their sisters.

However, there's more to the mutation's success, the team argues.

Most mammals and insects are diploids--they inherit a set of chromosomes from each parent. Haploids, including ants, wasps, and bees, inherit only their mother's chromosomes. Haplodiploids have diploid females and haploid males.

The coffee borers are functional haplodiploids. Although each sex inherits both sets of chromosomes, at some point in male development the father's set appears to shut down. Males function from then on using only their mother's genes, and their sperm contain only maternal genes, the team contends.

So if a male coffee berry borer inherits a resistance gene from his mother, all of his offspring will have a copy of it, explains coauthor Jeff Stuart of Purdue University in West Lafayette, Indiana.

If a diploid female inherits a resistance gene from her father, a susceptible gene from her mother, and then mates with a diploid male with two resistance genes, half of her progeny will have both resistant and susceptible genes and the other half will have only the resistance genes, says Stuart.

Other studies have not examined the genetics of resistance in a functional haplodiploid, says Richard W. Beeman of the U.S. Department of Agriculture's Agricultural Research Service in Manhattan, Kansas. "The resistance gene provided an easy way to follow the chromosomes," he says. Functional haplodiploidy may serve as a step in the evolution of true haplodiploidy, Beeman and the authors assert.

The flightless male borers are tough. A male with only one resistance gene and a female with two survive equally well.

"In view of the past global dispersal of H. hampei . . . the appearance of endosulfan-resistant lines in New Caledonia may represent a serious threat to the international coffee industry," the authors warn.