Agricultural Research Service (ARS) scientists and their collaborators have uncovered a potential weakness in the armor of fungus-farming ambrosia beetles that could lead to new possibilities for control, according to findings just published in the Proceedings of the National Academy of Sciences.
Exotic ambrosia beetles, which entered the United States in 1974, are multimillion-dollar pests of fruit and ornamental trees including pecan, peach, plum, cherry, persimmon, oak, elm and magnolia.
Entomologist Christopher Ranger, with the ARS Applications Technology Research Unit in Wooster, Ohio, led a team of researchers who demonstrated for the first time that ambrosia beetles specifically attack trees emitting ethanol but not for the reason most people suppose.
Ambrosia beetles carry a fungus with them in a symbiotic relationship. Female beetles tunnel into trees, deposit the fungus and then grow fungal gardens on which the larvae and adult beetles must feed for nourishment. Opportunistic ambrosia beetles can attack many different species of trees, but they prefer to attack trees emitting ethanol from the bark tissues.
“Trees produce and emit ethanol when they are stressed from any number of conditions, like drought or flood stress, or frost injury,” says Ranger. “Even though the trees may look healthy, ambrosia beetles quickly find and attack trees emitting ethanol.”
Researchers have long thought that ethanol attracts ambrosia beetles because it indicates a weakened tree. But results from this study demonstrate that ambrosia beetles have a strong affinity for ethanol because it actually benefits their fungus farming. Ethanol’s antimicrobial properties benefit the growth of the beetles’ fungal gardens and suppress the growth of “weedy” fungal competitors.
“Normally, ethanol acts as a potent anti-fungal agent, but that isn’t the case with the fungus that ambrosia beetles carry with them” says Ranger.
By understanding what factors promote and inhibit the growth of the fungal gardens, researchers might now be able to develop novel control strategies for ambrosia beetles. Notably, female ambrosia beetles only lay eggs after their fungal gardens are growing. “This relationship represents a weakness in their life cycle—female beetles will lay eggs only after the fungal garden is established. If the fungal garden doesn’t start growing, the female beetle dies or leaves to search for another suitable host,” says Ranger.
Collaborators are from the Julius Maximilian University-Wurzburg (Wurzburg, Germany), Bowling Green State University (Bowling Green, Ohio), the Technical University of Munich (Freising, Germany), Virginia Polytechnic Institute and State University (Virginia Beach, Virginia), Western Sydney University (Penrith, Australia), The Ohio State University (Wooster, Ohio), and the USDA-Agricultural Research Service.
The Agricultural Research Service is the U.S. Department of Agriculture's chief scientific in-house research agency.
Photo courtesy of the U.S. Department of Agriculture.