Why shading cedar (Cedrela fissilis) reduces damage caused by mahogany shoot borer, Hypsipyla grandella (Zeller)?

Abstract

Cedar (Cedrela fissilis) is a major timber species native to South America that has suffered frequent impacts from illegal harvesting. Cultivation efforts have experienced recurrent failures due to the action of the mahogany shoot borer Hypsipyla grandella, which, when attacking the plants in the first years of growth, inflicts negative effects on the commercial use of this timber. One of the methods used to reduce attacks by this insect is planting cedar in shaded areas, but the factor that makes shaded plants to be less attacked is still unknown. Considering that variation in photosynthetically active radiation can alter the emission of volatile organic compounds (VOCs) from plants, our hypothesis is that shading cedar plants alters their production of volatile compounds, including those that affect host location by H. grandella adults. To check this hypothesis, the authors used an air-entrainment system to collect VOCs emitted by cedar under different levels of artificial shading (50%, 30%, 15%) and full sunlight. The influence of the volatiles emitted by cedar plants on H. grandella was then assessed through electroantennographic/GC assays. One volatile compound, methyl salicylate, which was found to experience enhanced emission in shaded cedar plants, was then subjected to a field experiment to evaluate its influence on host location by H. grandella moths. From thirty VOCs emitted by the tested cedar plants, nine exhibited enhanced emission when plants were shaded: methyl salicylate, methyl-5-methylhexanoate, the monoterpenes α-pinene and eucalyptol, the homoterpenes (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT) and (E-E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene (TMTT) the sesquiterpenes, β-caryophyllene, β-cubebene and an unidentified sesquiterpene. Antennal response from H. grandella female and male antennae was observed to four volatile compounds, nonanal, decanal, methyl salicylate, and β-caryophyllene, when stimulated with the air-entrainment cedar plant extracts. A synthetic blend comprising these four compounds also elicited antennal responses from H. grandella. Field studies showed that plants treated with methyl salicylate exhibited a lower rate of damage by H. grandella caterpillars. These results indicate the potential of this semiochemical as a management tool for H. grandella in cedar cultivations.