Résumé :
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1. Datura wrightii Regel (Solanaceae) is polymorphic with regard to trichome type. Some plants are densely covered with short, non-glandular trichomes, whereas other plants in the same populations possess glandular trichomes that excrete a sticky exudate. The hypothesis that glandular trichomes enhance resistance to all small insect herbivores is evaluated. 2. Field censuses in four southern Californian D. wrightii plant populations revealed that glandular plants are indeed resistant to whitefly spp. (Homoptera: Aleyrodidae). Whiteflies are almost exclusively found on non-glandular plants. In contrast, Tupiocoris notatus (Distant) (Heteroptera: Miridae), another sap-sucking herbivore of similar body size, is found predominantly on plants with glandular trichomes. 3. Laboratory experiments showed that whiteflies are unable to colonize glandular D. wrightii phenotypes. After the whitefly adults had landed on the leaves of these plants, they were trapped in the exudate and died. 4. Tupiocoris notatus adults, on the other hand, laid significantly more eggs on glandular plants. The presence of the exudate was shown to be the cue that determined their choice of,glandular plants. 5. In no-choice experiments, T.: notatus nymphs reared on glandular plants had significantly higher survival rates and had shorter developmental periods than those raised on non-glandular plants. This, combined with the higher oviposition rates, resulted in higher T.: notatus population growth rates on glandular plants than on non-glandular plants. 6. Glandular trichomes are not therefore a universal protection against small herbivores. Differences in distribution over the two plant types within the natural herbivore guild on D. wrightii may, among other selection pressures, contribute to the maintenance of the observed trichome polymorphism.
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