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More wasps of Encarsia formosa Gahan (Hymenoptera: Aphelinidae) were found on fertilized poinsettias, Euphorbia pulcherrima (Willd.) (Euphorbiaceae), than on non‐fertilized plants. Parasitization of Bemisia argentifolii Bellows & Perring (Homoptera: Aleyrodidae) by E. formosa was higher on plants treated with calcium nitrate than with ammonium nitrate or on control plants. In a no‐choice test, host feeding by E. formosa was higher when hosts were on fertilized plants than when hosts were on control plants. The nitrogen content of whitefly pupae reared on plants treated with ammonium nitrate was higher than those on calcium nitrate‐treated plants. Variability in the parasitization of B. argentifolii by E. formosa appears to be due to host plant‐mediated differences in the whiteflies. E. formosa may be influenced by the nutritional suitability of the host, which influences whether wasps continue to oviposit, feed, or disperse.

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for assistance. Wealso thank staff members of the Potomac Electric Power Company for cooperation and assistance. Portion of a thesis submitted by the senior author to the Graduate School of the University of Maryland in partial fulfillment of the requirements for the M.S. degree.

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