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Insect antifeedant and growth regulating activity of phytochemicals and extracts from the plant family Meliaceae Champagne, Donald Edmond


This thesis represents studies on aspects of the defenses against insect herbivores in species of the plant family Meliaceae, particularly with regard to phytochemicals. Methanolic extracts of foliage from thirty species in twenty-two genera were bioassayed for toxicity and growth inhibitory activity against the variegated cutworm, Peridroma sauciaf and for feeding inhibition against the migratory grasshopper, Melanoplus sanguinipes. All but three species were inhibitory to P. saucia, members of the tribe Melieae being most inhibitory. Members of the subfamily Melioideae were more active than members of the Swieteniodeae. Newly identified species with activity comparable to neem (Azadirachta indica) foliage extracts included Aglaia odorata and Turreae holstii. Deciduous species produced extracts which were significantly more active than evergreen species, indicating a greater reliance on phytochemical-based defenses. Evidence is also presented to suggest that the leaves of evergreen species are tougher than deciduous species, and that there is a negative correlation between leaf toughness factors (physical defenses) and phytochemical-based defenses. These results are in agreement with predictions of the resource availability hypothesis. The phytochemistry of Aglaia odorata, A. odoratissima. and A. argentia was examined in detail. Compounds identified included the dammaranes, aglaiondiol and aglaitriol, and the bis-amides (S,S)-odorine, (S,R)-odorine (a new natural product), (S,S)-odorinol, and (S,R)-odorincT. Three dihydroflavanones were identified from the Meliaceae for the first time: 3-hydroxy-5,7,4'-trimethoxyflavanone (a new natural product), 5,7,4'-trimethoxyflavanone, and 5-hydroxy-7,4'-dimethoxyflavanone. All compounds were inactive against P. saucia. The inhibitory activity of A. odorata appeared to be due to a compound, tentatively identified as a limonoid, which may be acting in conjunction with a synergist. This compound inhibits P. saucia larval growth in the absence of antifeedant activity. The toxicology of limonoids, representing the major biosynthetic classes, was examined against P. saucia and the large milkweed bug, Oncopeltus fasciatus. Cedrelone and anthothecol inhibited P. saucia growth by 90%, but not feeding, when applied in diet at 0.5 μmol/g fwt. Cedrelone also inhibited O. fasciatus molting, with an MD₅₀ of 12.2 μg/nymph. In contrast, anthothecol, with an acetoxy function at C-11, was inactive against O. fasciatus. The D-seco compound gedunin, and the A,D-seco limonoids obacunone, nomilin, and pedonin were inactive in these assays; harrisonin initially inhibited feeding by neonate P. saucia but produced no long-term effects on growth rate. Bussein inhibited growth by 35% but entandrophragmin had no effect. Azadirachtin was the most toxic compound examined in this study. Peridroma saucia growth (EC₅₀ =0.4 nmol/g diet fwt), survivorship (LC₅₀ =5.2 nmol/g), pupation, pupal weight, and adult emergence were decreased in a dose-dependent manner. Chemosensory antifeedant activity was implicated in neonates but was much less marked with third instar larvae. Azadirachtin decreased relative growth and consumption rates at doses lower than those affecting nutritional efficiency, or feeding in the choice tests. This suggests an action directly on the gut or on the neural regulation of feeding. Bioactivity of other limonoids did not correlate with measures of skeletal oxidation or rearrangement, although these are dominant themes in the evolution of the limonoids. Melanoplus sanauinipes lacked an antifeedant response to azadirachtin, up to concentrations of 500 ppm. However, subsequent molting was markedly effected. Application of azadirachtin orally, topically, or by injection, allowed determination of the role of the gut and integument in limiting the bioavailability of this compound to putative target site(s) within the insect. The oral MD50, 10.8 μg/g insect fwt, was significantly higher than the injected MD₅₀, 3.01 μg/g, indicating a barrier to bioavailability in the gut. The oral activity of azadirachtin was synergised by coadministration of piperonyl butoxide, indicating that the barrier is due largely to oxidative metabolism. There was no significant difference between topical (3.8 μg/g) and injected activity, indicating that the integument does not pose a barrier to bioavailability. Azadirachtin decreased growth and consumption at doses which did not affect nutritional efficiency, again indicating an effect on the gut or neural regulation of feeding. No difference was seen in nutritional indices of nymphs treated with azadirachtin at 10 and 15 μq/q, although these doses produced markedly different effects on molting. This observation suggested that effects on endocrine events are not directly related to nutritional effects. The effects of azadirachtin treatment were not alleviated by dietary supplementation with cholesterol, and azadirachtin did not affect the hemolymph transport or metabolism of ¹⁴C-β-sitosterol, indicating that sterol metabolism is not the target for azadirachtin activity. Azadirachtin also did not form adducts with cysteine, suggesting that non-specific binding to sulfhydryl-rich protein is also unlikely as a mechanism of action.

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