Olfactory preferences of popillia japonica, vanessa cardui, and aphis glycines for glycine max grown under elevated CO ₂

Levels of atmospheric CO ₂ have been increasing steadily over the last century and are projected to increase even more dramatically in the future. Soybeans (Glycine max L.) grown under elevated levels of CO ₂ have larger herbivore populations than soybeans grown under ambient levels of CO ₂. Increased abundance could reflect the fact that these herbivores are drawn in by increased amounts of volatiles or changes in the composition of volatiles released by plants grown under elevated CO ₂ conditions. To determine impacts of elevated CO ₂ on olfactory preferences, Japanese beetles (Popillia japonica Newman) and soybean aphids (Aphis glycines Matsumura) were placed in Y-tube olfactometers with a choice between ambient levels of CO ₂ gas versus elevated levels of CO ₂ gas or damaged and undamaged leaves and plants grown under ambient levels of CO ₂ versus damaged and undamaged plants grown under elevated levels of CO ₂. All plants had been grown from seeds under ambient or elevated levels of CO ₂. Painted lady butterflies (Vanessa cardui L.) were placed in an oviposition chamber with a choice between plants grown under ambient and elevated levels of CO ₂. A. glycines and V. cardui showed no significant preference for plants in either treatment. P. japonica showed no significant preference between ambient levels and elevated levels of CO ₂ gas. There was a significant P. japonica preference for damaged plants grown under ambient CO ₂ versus undamaged plants but no preference for damaged plants grown under elevated CO ₂ versus undamaged plants. P. japonica also preferred damaged plants grown under elevated levels of CO ₂ versus damaged plants grown under ambient levels of CO ₂. This lack of preference for damaged plants grown under elevated CO ₂ versus undamaged plants could be the result of the identical elevated levels of a green leaf volatile (2-hexenal) present in all foliage grown under elevated CO ₂ regardless of damage status. Green leaf volatiles are typically released from damaged leaves and are used as kairomones by many herbivorous insects for host plant location. An increase in production of volatiles in soybeans grown under elevated CO ₂ conditions may lead to larger herbivore outbreaks in the future.