Quinolizidine alkaloids obtained byPedicularis semibarbata (Scrophulariaceae) fromLupinus fulcratus (Leguminosae) fail to influence the specialist herbivoreEuphydryas editha (Lepidoptera)

Abstract

Pedicularis semibarbata is apparently an obligate hemiparasite of coniferous trees. It is also a facultative parasite ofLupinus fulcratus from which we find that it obtains quinolizidine alkaloids, principally α-isolupanine. As a result, a single population ofP. semibarbata contains both alkaloidrich and alkaloid-free plants. The butterflyEuphydryas editha naturally oviposits on both plant types. This butterfly population, which is the principal herbivore attackingP. semibarbata at this site, is known to contain two morphs. Individuals of a specialist morph discriminate when ovipositing among individualP. semibarbata plants and produce offspring that survive better on accepted than on rejected plants. Those of a generalist morph accept allP. semibarbata plants and produce offspring that survive equally well on plants accepted or rejected by the discriminating morph. Because of the existence of this complex variation among the butterflies, the presence of naturally laid eggs on alkaloid-containing plants still leaves the possibility that the alkaloids may defend the plants against the specialist morph. In experiments on both oviposition preference and larval performance in early instars, we failed to detect any correlation between alkaloid content of a plant and either its acceptability to or suitability for the discriminating morph of the insect. Alkaloid presence in the host-plant population, achieved through root parasitism, is currently neither subject to strong insect-mediated selection nor a major cause of selection on the insects.