FOOD QUALITY OF CYNOGLOSSUM OFFICINALE AND HERBIVORY BY ETHMIA BIPUNCTELLA (, ETHMIIDAE)

by

ADRIANA H. PRINS1, RONALD M. LAAN, JANA VERBOOM1 and BEN VERBOOM (Departmentof PopulationBiology, Universityof Leiden, P.O. Box 9516, 2300 RA Leiden, The Netherlands)

ABSTRACT The oligophagous lepidoptcran Ethmia bipunctellaF. (Lepidoptera: Ethmiidae) occurs in low numbers in Meijendel, the Netherlands, in spite of the great abundance of its host plant Cynoglossumofficinale 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. bipunctellareduced the growth of 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 , 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, Cynoglossumofficinale, 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 185 be repellent or even poisonous for herbivores (LAWTON & McNEILL, 1979). In the present study, we describe a field situation in the dune area of Meijendel, near The Hague in the Netherlands, in which the oligophagous lepidopteran Ethmia bipunctella F. occurs in low numbers, in spite of the abundance of its food plant Cynoglossum officinale L. (PRINS & NELL, 1990a). We investigated food quality as a factor deter- mining population size of E. bipunctella in the study area. We expected herbivores to have a negative effect on plants, although overcompensation of plants after attack by herbivores has been claimed (OWEN, 1980; McNAUGHTON, 1983; PAIGE & WHITHAM, 1987). Normally, herbivory will result in smaller plants, fewer , smaller seeds or delayed flowering (see reviews by BELSKY, 1986 and VERKAAR, 1988; PRINS & NELL, 1990b). Plants have evolved deterrents (FEENY, 1976; RHOADES & CATES, 1976) to counter the negative effect of herbivory on their growth and survival. These may be constitutive, thus present at a constant level, such as hairs, tough leaves or secondary plant substances, or defensive chemicals produced after herbivore attack to reduce plant palatability (HAUKIOJA, 1982; KARBAN & CAREY, 1984; HAUKIOJA & HANHIMAKI, 1985; TALLAMY, 1985; KARBAN, 1986). The plants of C. officinale con- tain pyrrolizidine alkaloids (PEDERSEN, 1975) and we suspected these alkaloids to be involved in its inductive defence. To examine the possible relationship between plant quality and the low number of E. bipunctella, we determined which characteristics of C. officinale had an effect on oviposition of E. bipunctella. Moreover, we collected indirect evidence of an induced plant defence after damage.

MATERIAL AND METHODS

The study was done in the calcareous sand dune area Meijendel, where C. officinale is abundant. Populations of C. officinale occur in open sites with a low humus content, as well as in more fertile sites near shrubs and trees. C. officinale is a 'biennial' that, as most biennials, delays flowering to the third or even later year (KLINKHAMER et al., 1987). C. officinale flowers in May, and seed sets and ripens in June and July. The larvae of E. bipunctella are the main folivores of C. officinale (PRINS & NELL, 1990a). This species is oligophagous (SATTLER, 1967), feeding only on species of belonging to the genera Echium, Cynoglossum, Anchusa and . E. bipunctella is bivoltine, with two generations a year from May until October/November. Eggs arc mostly laid singly, sometimes more than one egg is found on a plant.