Evaluation of synergism in the feeding deterrence of some furanocoumarins on Spodoptera littoralis.

The phagodepression activity of five furanocoumarins (FC), bergapten, xanthotoxin, psoralen, imperatorin, and angelicin, has been studied against larvae of Egyptian cotton leafworm Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae) using a leaf-disk choice bioassay. The dose range used was 0-10 microg/cm2 for linear furocoumarins and 0-30 microg/cm2 for angelicin, the angular furanocoumarin. Dose-feeding deterrency activity correlations were governed by various sigmoidal functions, except in the case of imperatorin and bergapten, which had dose-response curves showing irregular traces with two maxima. All five FCs induced various degrees of phagodepression in S. littoralis; at 3 microg/cm2, the relative feeding deterrence was bergapten = xanthotoxin > psoralen = imperatorin = angelicin. Structure-activity correlations indicated that a methoxy group on C-5 or C-8 enhanced the activity. Comparison of experimental feeding deterrence of binary mixtures of imperatorin with xanthotoxin, bergapten, or psoralen at various relative concentrations with the calculated additive activity of each combination indicated that a proportion of 40% or more of imperatorin may exert a greater antifeedant effect on S. littoralis than the sum of individual compounds. The effect of angelicin also was examined in combination with psoralen, both of which are present in Psoralea plants. Their mixtures yielded a clear synergistic effect in the 20-80% angelicin range (coactivity coefficient = 124-133). By contrast, the effect of angelicin on xanthotoxin was only additive. In the dose-response curves of imperatorin/bergapten combinations, synergism was found at >60% imperatorin relative concentration and 1 microg/cm2, whereas lower proportions led to antagonism. The threshold between the two opposing effects was found to depend on the total FC concentration employed. Some natural systems contain FCs in the range of synergistic proportions recorded here and, thus, may have been produced by the host to increase its defensive effect at a lower metabolic cost.