We examined how quantitative variation in the expression of iridoid glycosides in two lines of the ribwort plantain, Plantago lanceolata, affected the development of specialist and generalist herbivores and their respective endoparasitoids. Plants had been clonally grown from two lines containing high or low levels of iridoid glycosides. High lines contained a four-fold higher level of iridoid glycosides as a function of leaf dry weight, than did low lines. Development time in the adapted specialist herbivore, Melitaea cinxia, and its solitary endoparasitoid, Hyposoter horticola, proceeded most rapidly when reared on the high iridoid line, whereas pupal mass in M. cinxia and adult mass in H. horticola were unaffected by plant line. Cotesia melitaearum, a gregarious endoparasitoid of M. cinxia, performed equally well on both lines of P. lanceolata. In contrast, the pupal mass of the generalist herbivore, Spodoptera exigua, and emerging adult mass of its solitary endoparasitoid Cotesia marginiventris were significantly lower when reared on the high line, although development time was unaffected. The results of this investigation are discussed with regards to (1) differences between specialist and generalist herbivores and their natural enemies to quantitative variation in plant secondary chemistry, and (2) the context of potentially differing selection pressures on plant defence as these affect secondary plant chemistry.
Species 1: Lepidoptera Nymphalidae Melitea cinxia (Glanville Fritillary)
Species 2: Hymenoptera Ichneumonidae Hyposoter horticola
Species 3: Lepidoptera Noctuidae Spodoptera exigua (beet armyworm)
Keywords: iridoid, parasitoid
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