Infective stages (spores) of a microsporidian pathogen that infects the beneficial weevils Neochetina bruchi and N. eichhorniae. Click the image for more information about it.

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Study Reveals Cause of Disease in Beneficial Insects

The mystery behind the decline of beneficial biocontrol weevils that help control the invasive weed water hyacinth in Florida has been solved by Agricultural Research Service scientists. They've found that a microorganism is responsible for killing the weevils and reducing their reproductive capacity.

The two closely related South American weevils, Neochetina eichhorniae Warner and N. bruchi Hustache, have been used since the 1970s to control water hyacinth, an aquatic weed that clogs waterways, displaces native vegetation and degrades wildlife habitats in Florida. The weevils are mass-reared and, once released, feed on the weed and help to keep it from spreading. But recently, the effectiveness of the weevils has been declining for unknown reasons.

James J. Becnel, a research entomologist at the ARS Center for Medical, Agricultural and Veterinary Entomology in Gainesville, Fla., and Theresa Rebelo, a postdoctoral researcher from Portugal, worked with the ARS Invasive Plant Research Laboratory in Fort Lauderdale, Fla., to determine what was wrong with the weevils.

The scientists suspected that a disease caused by a microorganism, called a microsporidium, was responsible. Tests determined that a recently discovered, still unnamed microsporidium was decreasing weevil survival rates by 30 percent and was reducing the weevils' reproductive capacity--one species by 72 percent, the other by 62 percent.

According to Becnel, the study's findings demonstrate the importance of selecting disease-free organisms for biological control programs. If it's determined to be economically practical to treat weevil colonies, researchers will work at developing a method to cure them of the disease.

Read more about the research in the April issue of Agricultural Research magazine.

ARS is the U.S. Department of Agriculture's chief scientific research agency.