Binary mixtures of feeding deterrents mitigate the decrease in feedingdeterrent response to antifeedants following prolonged exposure in thecabbage looper, Trichoplusia ni (Lepidoptera: Noctuidae)

Summary. The effect of rearing larvae of Trichoplusia ni onindividual feeding deterrents or on binary mixtures of deterrentson their subsequent gustatory sensitivity was measuredin paired choice leaf disc bioassays. Our working hypothesiswas that mixtures of antifeedants (pure allelochemicals)would mitigate decreased feeding deterrent response followingprolonged exposure in this generalist herbivore. Neonatelarvae were reared on cabbage leaves treated with individualfeeding deterrents (digitoxin, thymol, toosendanin or xanthotoxin),or with binary mixtures of these until the third instar.Feeding deterrent responses to each antifeedant or mixturewas then determined in a leaf disc choice bioassay. All of themixtures produced additive deterrence when presented tonaïve larvae. Larvae reared on individual antifeedantsshowed a significantly decreased feeding deterrent response(except to digitoxin), whereas larvae reared on binary mixturesof antifeedants did not show a decreased feeding deterrentresponse to any of them. Such mixtures were synergisticin terms of their feeding deterrence to experienced larvae.Our experiment supports the hypothesis (Jermy 1986) thatmixtures of deterrents can prevent decreased feeding deterrentresponse following prolonged exposure, and provides oneexplanation for the multiplicity of chemical defenses foundin many plants.