The fungus Raffaelea lauricola modifies behavior of its symbiont and vector, the redbay ambrosia beetle (Xyleborus glabratus), by altering host plant volatile production

Authors: MARTINI, XAVIER - University Of Florida; HUGHES, MARC - University Of Florida; KILLINY, NABIL - University Of Florida; George, Justin; Lapointe, Stephen; SMITH, JASON - University Of Florida; STELINSKI, LUKASZ - University Of Florida

Submitted to: Journal of Chemical Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/10/2017
Publication Date: 6/6/2017
Citation: Martini, X., Hughes, M.A., Killiny, N., George, J., Lapointe, S.L., Smith, J.A., Stelinski, L.L. 2017. The fungus Raffaelea lauricola modifies behavior of its symbiont and vector, the redbay ambrosia beetle (Xyleborus glabratus), by altering host plant volatile production. Journal of Chemical Ecology. 43:519-531

Interpretive Summary: The redbay ambrosia beetle is a recently introduced pest of trees in the family Lauraceae such as the swamp bay tree. The beetle, native to Asia, was discovered in the southeastern United States in the early 2000s. The adult beetle inoculates trees with a fungus that causes wilting and eventual tree death. During the period when the tree is declining, additional beetles are attracted to the diseased tree. We collected and studied the odors produced by diseased trees. At three days after infection (3 DAI), beetles were repelled by the odors produced by swamp bay tree leaves. However, by 10 and 20 DAI, beetles were attracted to the infected leaves. This corresponded to an increase in a chemical, methyl salicylate, known to be involved in plant response to infections. Our findings provide a better understanding of the ecology of the redbay ambrosia beetle and chemical interactions with its symbiotic fungus. Our work also demonstrates how the laurel wilt pathogen alters host defenses and influences vector behavior to attract more vectors.

Technical Abstract: The redbay ambrosia beetle, Xyleborus glabratus, is the vector of a symbiotic fungus, Raffaelea lauricola that causes laurel wilt, a highly lethal disease to members of the Lauraceae. Pioneer Xyleborus glabratus beetles infect live trees with Raffaelea lauricola, and only when trees are declining because of disease, sick trees attract more Xyleborus glabratus. Until now this sequence of events was not well understood. In this study, we investigated the temporal patterns of host volatile and phytohormone production and vector attraction in relation to laurel wilt symptomology. Following inoculations with Raffaelea lauricola, volatile collections and behavioral tests were performed at different time points. Three days after infection (DAI), we found significant repellency of Xyleborus glabratus by leaf odors of infected swamp bay as compared with controls. However, at 10 and 20 DAI, Xyleborus glabratus were more attracted to infected than non-infected leaf odors. GC-MS analysis revealed an increase in methyl salicylate (MeSA) 3 DAI; whereas, an increase of sesquiterpenes and leaf aldehydes was observed 10 and 20 DAI in leaf volatiles. MeSA was the only repellent of Xyleborus glabratus in laboratory bioassays. Xyleborus glabratus response to MeSA and to Raffaelea lauricola-infected leaves was confirmed by antennal response as measured with electroantenography. In contrast, Xyleborus glabratus did not prefer infected wood over healthy wood, and there was no associated significant difference in their volatile profiles. Analyses of phytohormones revealed an initial increase of salicylic acid (SA) following fungal infection, suggesting that the SA pathway was activated first by R. lauricola infection and that this activation caused increased release of MeSA. Overall, our findings provide a better understanding of Xyleborus glabratus ecology and underline chemical interactions with its symbiotic fungus. Our work also demonstrates how the laurel wilt pathosystem alters host defenses to impact vector behavior and suggest manipulation of host odor by the fungus to attract more vectors.