Behavioral responses of Aphis citricola to non-host plant volatiles

Plant volatiles can act as chemical signals that influence the behavior and distribution of insects. Although considerable information has been acquired on the effects of plant volatiles emitted from plants on herbivorous insects and their natural enemies, practical implementation of this knowledge is still lacking. We investigated 3 aromatic plant species, French marigold, Tagetes patula L. (Asteraceae), ageratum, Ageratum houstonianum Mill. (Asteraceae), and catnip, Nepeta cataria L. (Lamiaceae), to test their effectiveness in repelling or attracting spirea aphid, Aphis citricola van der Goot (Hemiptera: Aphididae), and its natural enemy, the multicolored Asian lady beetle, Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae), in the field and the laboratory. We found that intercropping apple trees Malus spp. (Rosaceae) with aromatic plants in an orchard significantly reduced the number of A. citricola aphids present, but had the opposite effect on H. axyridis. In addition, the association between H. axyridis and A. citricola numbers was strengthened when the intercropping included French marigold. Using an H-tube olfactometer, we found that A. citricola was repelled by French marigold and catnip, whereas H. axyridis was attracted most by French marigold. Volatile analysis revealed that the sesquiterpenes Dlimonene and terpinolene and the alcohol 2-ethyl-1-hexanol were the most abundant volatile compounds released by French marigold and catnip. Harmonia axyridis was significantly attracted by 12.5 [micro]L/L D-limonene, 50 [micro]L/L terpinolene, and 25 [micro]L/L of a 1:1 mixture of the 2 compounds, but was repelled by higher concentrations of D-limonene. The results suggest that aromatic plants increase the resistance of apple trees to A. citricola both directly, by reducing the population of A. citricola through chemical repulsion, and indirectly, by increasing the H. axyridis population through chemical attraction.