Chemicals from Rhododendrons Act as Weevil Repellents
Dr. Robert P. Doss
USDA-ARS Plants Research Unit, WWREC, Puyallup, WA
Chemical compounds secreted by rhododendron varieties resistant to weevils have been identified and could play a role in future weevil control programs for all plants. The compounds, called terpenes, act as weevil repellents and one of these terpenes, germacrone, is powerful enough to override even as strong a weevil feeding stimulant as table sugar. Dr. Robert P. Doss, a plant physiologist with USDA's Agricultural Research Service, Puyallup, working with researchers at the University of Washington in Seattle, identified the compounds.
Adult obscure root weevils feed on rhododendron leaves, causing unsightly damage. Females lay their eggs on the plant and the resulting root-eating larvae can kill pot-grown plants in nurseries. Investigations into rhododendron resistance to weevils began after growers in the Pacific Northwest claimed that the insects fed heavily on some varieties while leaving other varieties in the same field virtually untouched. Studies established that rhododendrons featuring glandular scales on their leaves, called lepidotes, do indeed exhibit a general resistance to weevil feeding, while those without scales, called elepidotes, do not.
To determine the source of this resistance, Doss and the other researchers tested leaf extracts from resistant and susceptible rhododendron varieties on the obscure root weevil (Sciopithes obscurus, Horn), which despite its name, is the most common and worst rhododendron pest in the Northwest United States. Leaf extracts were tested by placing them on small discs of specially prepared weevil-feeding material treated with table sugar to stimulate weevil appetites. In nearly every case, extracts from resistant scaled rhododendrons prevented weevils from feeding on the discs, while extracts from non-scaled rhododendrons increased feeding even more than when no extracts were placed on the discs.
"The results suggest that the lepidote rhododendrons owe their resistance to the production and secretion of volatile repellents such as germacrone, probably from their glandular leaf scales," says Doss. The possibility exists, he says, that slow-release formulations of germacrone or another terpene could be developed. These formulations, acting as biological repellents, could be sprayed on any plant that weevils feed on. According to Doss, germacrone is active as a repellent against black vine weevil also.
As for rhododendron varieties themselves, while resistant scaled rhododendrons can't be cross bred with susceptible non-scaled varieties, Doss says that the technique used to identify germacrone and other terpenes should make it easier for breeders in the future to screen new selections for weevil resistance.