A comparison of mixed host-plant diets on the sequestration of iridoid glycosides by larvae of three lepidopteran species

Sequestration of plant secondary compounds by herbivorous insects, which can render them unacceptable to natural enemies, may be influenced by the insects’ diet. In this study we compared the sequestration of iridoid glycosides by larvae of three lepidopteran species. The buckeye, Junonia coenia, is a specialist on plants which produce iridoid glycosides (IGs), whereas the host ranges of the generalist arctiids Spilosoma congrua and Estigmene acrea include IG-producing plants along with other species. Larvae of each species were reared on leaves of Plantago lanceolata, which produces the IGs aucubin and catalpol, or P. major, which produces only aucubin. Four diet combinations were used: half of the larvae were fed each plant species until reaching the fourth instar, whereupon half were fed the same host plant and half received the other. Each larva was analyzed for sequestration upon reaching the fifth instar. All species sequestered IGs; J. coenia sequestered approximately twice the amount of IGs as the two generalist species. J. coenia contained similar IG concentrations when fed P. lanceolata only or P. lanceolata following P. major; these concentrations were greater than those fed P. major only or P. major following P. lanceolata. S. congrua larvae fed P. lanceolata throughout development contained twice the IGs compared to those fed any other diet, while E. acrea contained similar IG amounts regardless of diet. When fed diets containing P. lanceolata, S. congrua sequestered roughly equal proportions of aucubin and catalpol, whereas the other species both sequestered aucubin at much greater concentrations than catalpol.