DIFFERENCES IN DISTRIBUTION AND PERFORMANCE OF 2 SAP-SUCKING HERBIVORES ON GLANDULAR AND NON-GLANDULAR DATURA-WRIGHTII

1. Datura wrightii Regel (Solanaceae) is polymorphic with regard to tr ichome type. Some plants are densely covered with short, non-glandular trichomes, whereas other plants in the same populations possess gland ular trichomes that excrete a sticky exudate. The hypothesis that glan dular trichomes enhance resistance to all small insect herbivores is e valuated. 2. Field censuses in four southern Californian D. wrightii p lant populations revealed that glandular plants are indeed resistant t o whitefly spp. (Homoptera: Aleyrodidae). Whiteflies are almost exclus ively found on non-glandular plants. In contrast, Tupiocoris notatus ( Distant) (Heteroptera: Miridae), another sap-sucking herbivore of simi lar body size, is found predominantly on plants with glandular trichom es. 3. Laboratory experiments showed that whiteflies are unable to col onize 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 signi ficantly more eggs on glandular plants. The presence of the exudate wa s 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 develop mental periods than those raised on non-glandular plants. This, combin ed with the higher oviposition rates, resulted in higher T.: notatus p opulation growth rates on glandular plants than on non-glandular plant s. 6. Glandular trichomes are not therefore a universal protection aga inst small herbivores. Differences in distribution over the two plant types within the natural herbivore guild on D. wrightii may, among oth er selection pressures, contribute to the maintenance of the observed trichome polymorphism.