Interaction among two biological control agents and the developmental stage of their target weed,Dalmatian toadflax,Linaria dalmatica (l.) Mill. (Scrophulariaceae)

An experiment with potted Linaria dalmatica (L.) Mill. was set up to test the effects of root‐feeding by Eteobalea (syn. Stagmatophora) intermediella Riedl (Lep. Cosmopterigidae) and stem‐mining by Mecinus janthinus Germar (Col. Curculionidae), as well as the interaction among the insects and the developmental stage of the host plant. During the summer season, mining by M. janthinus decreased the stem biomass substantially, while E. intermediella did not produce statistically significant effects on the performance of host plant individuals (the experimental conditions represented the effect of the univoltine M. janthinus adequately, but the potentially important effect of mining by the second generation of E. intermediella, at the beginning of the growing season, was not assessed). M. janthinus survived almost exclusively on plants in the flowering stage, although adult females readily used vegetative (younger) plants for oviposition. E. intermediella had a higher chance of survival on vegetative plants. Both species have recently been released as biological control agents on L. dalmatica in North America. For this purpose the complementary survival rates on the two developmental stages of the host plant is advantageous.