Journal of Chemical Ecology, Vol. 25, No. 8, 1999 PLANT LATEX AND FIRST-INSTAR MONARCH LARVAL GROWTH AND SURVIVAL ON THREE NORTH AMERICAN MILKWEED SPECIES MYRON P. ZALUCKI1 and STEPHEN B. MALCOLM2'* 1 Department of Zoology and Entomology The University of Queensland Brisbane, Queensland, 4072, Australia 2 Department of Biological Sciences Western Michigan University Kalamazoo, Michigan, 49008 (Received January 9, 1997; accepted March 24, 1999) Abstract—First-instar larvae of the monarch butterfly, Danaus plexippus, a milkweed specialist, generally grew faster and survived better on leaves when latex flow was reduced by partial severance of the leaf petiole. The outcome depended on milkweed species and was related to the amount of latex produced. The outcome also may be related to the amount of cardenolide produced by the plants as a potential chemical defense against herbivory. Growth was more rapid, but survival was similar on partially severed compared with intact leaves of the high-latex/low-cardenolide milkweed, Asclepias syriaca, whereas both growth and survival were unaffected on the low-latex/low-cardenolide milkweed A. incarnata. On the low-latex/low-cardenolide milkweed A. tuberosa, both growth and survival of larvae were only marginally affected. These results contrast sharply to previous results with the milkweed, A. humistrata, in Florida, which has both high latex and high cardenolide. Larval growth and survival on A. humistrata were both increased by partially severing leaf petioles. Larval growth rates among all four milkweed species on leaves with partially severed petioles were identical, suggesting that latex and possibly the included cardenolides are important in first-instar monarch larval growth, development, and survivorship. Key Words—Asclepias, cardenolide, Danaus plexippus, growth rate, latex, laticifer, milkweed, neonate larvae, plant defense, survival. *To whom correspondence should be addressed. 1827 0098-0331/99/0800-1827/S16.00/0 © 1999 Plenum Publishing Corporation