Insect-plant interactions between magpie moth (Nyctemera annulata) and four species of Senecio.

The ecological interactions between the endemic magpie moth (Nyctemera annulata) and four weedy Senecio spp. (two indigenous spp. , S. glomeratus and S. quadridentatus and two adventive spp., S. sylvaticus and S. vulgaris), were examined in relation to insect preference and development, and counterdefences or developmental responses of the:hosts. Larval development, adult fecundity and host preference as a result of larval food plant conditioning were studied. Potential depredation of the insect on the hosts was determined and their physical defences and responses to defoliation were assessed. Imago ovipositing was not species specific but directly correlated with dry weight and leaf number of host Senecio spp. Under laboratory conditions fifth instar larvae selected the plant species supporting the most successful larval development, independent of larval conditioning. Food plant preference was in the order: Senecio vulgaris> S. glomeratus = S. quadridentatus > S. sylvaticus. Larvae selected S. vulgaris in field conditions before anthesis but this preference decreased at anthesis. Larvae reared on S. vulgaris had the highest pIc efficiency and E.C.I. value, resulting in rapid larval and pupal development, with larger and heavier instars when compared with other Senecio spp. Larvae reared on S. vulgaris always selected this species in petri dishes. However larval development on different specific hosts did not alter the correlation of oviposition with dry weight and leaf number. The order of imago fecundity for larvae raised on three Senecio spp. was: S. glomeratus > S.vulgaris > S. quadridentatus. Fifth instar larvae acquired the major proportion of the larval food and defoliated the four species to ground level under glasshouse conditions. However the expected overcrowding and complete defoliation in nature has not been observed, possibly due to populations being checked by a high juvenile larval mortality. All Senecio spp. were cold-tolerant and established during winter when larvae showed minimal activity. S. glomeratus and S. quadridentatus~ the two indigenous species, responded to defoliation by vegetative regeneration from root stock. S. vulgaris responded by producing mature fruit more rapidly when attacked. N. annulata may influence the altitudinal distribution of S. sylvaticus as this species is more prolific at higher altitudes where the insect is probably univoltine