Host finding and oviposition behavior in a chrysomelid specialist--the importance of host plant surface waxes

Although several studies have elucidated the role of plant epicuticular waxes in host recognition and oviposition by herbivorous insects, there is little known about this subject in Chrysomelidae. In the present study, chemical cues for host recognition behavior and oviposition by the monophagous chrysomelid species Cassida stigmatica were investigated with special regard to surface waxes of the host plant. Tanacetum vulgare (Asteraceae). After hibernation in the soil, adults of this species must climb the plant's petioles, which emerge from the ground in spring. The response of adult C. stigmatica to contact and volatile cues from petioles and leaves of T. vulgare was investigated in a "stem arena," in which differently treated petioles and petiole dummies were offered. Volatile and contact cues of T. vulgare petioles served as well for host recognition. The contact cues were isolated from the petioles and leaves by hexane extraction and by cellulose acetate treatment, which removed cuticular waxes. The attractive volatiles were not extracted sufficiently by hexane. To examine the role of cuticular waxes of the host leaf surface in oviposition, female C. stigmatica were offered intact leaflets and leaflets from which cuticular waxes had been stripped by cellulose acetate treatment. Females did not discriminate between intact and stripped leaflets when only the upper leaf surfaces were offered. However. when the lower leaf surfaces that are generally used as oviposition sites were offered, C. stigmatica preferred to lay eggs on intact leaflets. We conclude that waxes of the lower leaf surface contain crucial information for oviposition in C. stigmatica.