Food Plant Quality of Cynoglossum Officinale and Herbivory By Ethmia Bipunctella (Lepidoptera, Ethmiidae)

FOOD PLANT QUALITY OF CYNOGLOSSUM OFFICINALE AND HERBIVORY BY ETHMIA BIPUNCTELLA (LEPIDOPTERA, ETHMIIDAE) by ADRIANA H. PRINS1, RONALD M. LAAN, JANA VERBOOM1 and BEN VERBOOM (Department of Population Biology, University of Leiden, P.O. Box 9516, 2300 RA Leiden, The Netherlands) ABSTRACT The oligophagous lepidoptcran Ethmia bipunctella F. (Lepidoptera: Ethmiidae) occurs in low numbers in Meijendel, the Netherlands, in spite of the great abundance of its host plant Cynoglossum officinale L. In this study, we examine the importance of food plant quality on individual performance of E. bipunctella, and discuss the impact on its population density. Grazing by E. bipunctella reduced the growth of plants in a growth room. In the field, flowering plants rather than rosettes were chosen for oviposition. In a choice ex- periment, larvae preferred undamaged rather than damaged leaves, suggesting that herbivory causes a rapid fall in plant acceptability. However, the larvae grew equally well on damaged and undamaged leaves. Our observations imply a role for alkaloids: flowering plants have a much lower alkaloid content than rosettes; and plants with eggs in the field had a lower alkaloid content than plants without eggs. KEY WORDS: alkaloids, Cynoglossum officinale, Ethmia bipunctella, food quality, her- bivory, Lepidoptera. INTRODUCTION For decades, regulation of population density in the field has been a central question in ecology. Predators and parasites have sometimes been implicated (LAWTON & McNEILL, 1979; STRONG et al., 1984), but in other studies abiotic factors such as climatic conditions and the number of overwintering sites have been given more attention. Also, competition for limiting resources may be important in determining population size (STRONC et al., 1984). A sea of food may seem to be available for herbivores, but its unsuitability for phytophagous insects may limit their growth and survival. Thus, these herbivores fre- quently suffer from food shortage even when their host plant is abun- dant (MURDOCH, 1966; WHITE, 1974; SMITH & WHITTAKER, 1980). Also, it has been suggested recently that sometimes plant material may