Synergism in the oviposition behavior of Plutella xylostella: Sinigrin and wax compounds

Diamondback moth, Plutella xylostella, host examining and oviposition behaviors were measured in response to sinigrin dosages (10^-3, 10^-4, and 10^-5 M) and controls with and without the addition of n-alkanes. Individual females were presented with a treatment and videotaped while an observer documented specific behaviors during 5-min observation periods. Behavior in response to sinigrin alone was not significantly different from that in response to controls. Alkane alone significantly reduced movement rate during treatment contact, but did not significantly affect other behaviors. Sinigrin concentrations combined with alkane significantly slowed the rate of insect movement, increased turning, and led to significantly longer treatment encounter durations. Behavior changes in response to sinigrin + alkane increased insect exposure to the sinigrin concentrations and led to greater oviposition compared to that in response to sinigrin treatments alone. The synergistic effect that mixing sinigrin and alkane has on P. xylostella behavior arises because the additional time females spend in contact with the treatment increases the rate at which they experience the available stimuli. Involvement of the antennae during examining of a treatment, referred to as 'swabbing,' was usually associated with oviposition on alkane-coated sinigrin treatments. The presence of alkane may alter the way sinigrin is perceived. Oviposition in response to the treatment combinations was also tested in overnight bioassays. The pattern of oviposition in response to treatments during bioassays differed from that established during observations. The value of direct observations and the mechanistic interpretations they allow are emphasized.