Robert BAUR-1, Meena HARIBAL-2, J. Alan A. RENWICK-2,
and Erich STÄDLER-1
1- Zoology, Swiss Federal Research Station, CH-8820 Widenswil,
Switzerland
2- Boyce Thompson Institute at Cornell University, Tower Road,
Ithaca, NY14853, USA
Behavioral events during host selection by ovipositing monarch butterflies (Danaus plexippus) include tapping the leaf surface with foretarsi and touching this surface with mid tarsi ('drumming') and antennae. Occurence of these events depends on host (Asclepias) species but is also highly variable among individual females. Flavonoids (Quercetin-glycosides) identified from host-plant extracts are known to stimulate oviposition. Scanning electron microscopy revealed the presence of contact-chemoreceptor sensilla on all appendages that contact the leaf surface. This electrophysiological study was aimed to identify the contact chemoreceptors sensitive to the known oviposition stimuli and thus probably involved in host recognition.
Receptor cells of conspicuous sensilla grouped in clusters on female fore tarsi were found to be sensitive to the behaviorally active butanol fraction of hostplant (Asclepias curassavica) extract. However, these receptors generally had low sensitivity to the oviposition stimulating flavonoids identified from this fraction. The receptors on the fore tarsi were remarkably sensitive to the butanol fraction of a nonhost (Brassica oleracea). Chemoreceptors on tarsomers 2 - 4 of the mid legs also responded to the behaviorally active fraction of host-plant extract and showed some sensitivity to one of the flavonoids stimulating oviposition. Similar results were obtained from some receptor cells in sensilla on the tip of the antennae. Most sensilla responded to the butanol fraction of Asctapias extract but only 25 % of these sensilla were also sensitive to one of the behaviorally active flavonoids isolated from this fraction.
Electrophysiological results, in combination with behavioral observations suggest that host selection in monarch butterflies is based on a complex pattern of decisions integrating peripheral sensory infommathn from several types of tarsal and antennal contact chemoreceptors.