Lindy MESSCHENDORP, G.J.Z. GOLS, J.J.A. van LOON
Department of Entomology, Wageningen Agricultural University,
P.O. Box 8031. 6700 EH Wageningen, The Netherlands
In earlier work, several drimanes were found to be feeding deterrents to Pueris brassicae larvae (5th instar) when applied to leaf discs of the host plant Brassica oleracea. It was shown that, when tested in a dual choice situation, the antifeedant index correlated significantly with the sensory response of the deterrent cell in the medial sensillum styloconicum-1.
In the work presented here, the same drimanes were tested in a no choice situation. We found that some of the drimanes that had a feeding inhibiting effect in a dual choice situation dit not inhibit feeding in the no choice test. Other drimanes also inhibited feeding in the no choice test.
To study the behaviour of the larvae in a no choice situation more precisely, interval observations (one observation per minute) were carried out. From these observations starting time and duration of the 'meals' taken by the larvae could be determined. lt was shown that, in general, duration of meals taken from drimanes that dld not inhibit feeding in a no choice situation, increased in time. On the other hand, duration of meals taken from drimanes that did inhibit feeding in a no choice situation decreased.
We conclude that P. brassicae larvae can show habituation or sensitization when exposed to different drimane antifeedants in a no choice situation. Probably, drimanes to which sensitization occurs have, after long-term exposure (e.g. in a no choice situation), besides stimulation of the deterrent cell, also other effects on P. brassicae larvae. At present, we are examining possible effects of drimanes that could explain the occurrence of sensitization or habituation, e.g.: