Molecular taxonomy, bionomics and host specificity of Longitarsus jacobaeae (Waterhouse) (Coleoptera: Chrysomelidae) : the Swiss population revisited

Abstract

The ragwort flea beetle, Longitarsus jacobaeae (Waterhouse) (Coleoptera: Chrysomelidae) is considered to be the most important biological control agent for the suppression of tansy ragwort, Senecio jacobaea L. (Asteraceae) in the Pacific Northwest. A recent infestation of tansy ragwort in northwest Montana has rekindled the search for a cold adapted strain of the ragwort flea beetle. This study endeavored to determine the molecular taxonomy, host specificity, bionomics and life history of the Swiss strain. I found that populations of L jacobaeae from Switzerland are phenologically adapted to cold continental climates. Molecular techniques of species determination were applied to L. jacobaeae and three other species in the genus Longitarsus. Application of these techniques were able to discriminate between L. jacobaeae and its cryptic sister species L. flavicornis (Stephens). Five Swiss flea beetle populations and three Oregon populations were determined to be clustered together in the L. jacobaeae species. This is the first report of life history observations for naturally occurring populations of L. jacobaeae in Switzerland. Adult flea beetles emerge in early spring and immediately start oviposition by mid-July. Oviposition continued into November for captive beetles. Eggs enter a diapause phase and hatch in the spring after exposure to cold temperatures. Larvae initially feed in the leaves and then move to the root crowns in their second instar to complete their development. Pupation occurs in the soil after the third instar leaves the plant. Twelve plant species closely related to S. jacobaea were exposed to ragwort flea beetles in three host tests. In all three host tests, larval development was completed only in the usual host plant, S. jacobaea. Limited larval feeding was observed in the cut foliage host test on three non-target species, S. eremophilus, S. flaccidus, and S. triangularis, however all the larvae tested died during development. Very slight damage to no-target plants was observed in host tests utilizing whole potted plants in the greenhouse. An. open field host test in Switzerland revealed no substantial attack on non-target plants and no larval development. Eight previously untested North American plant species were found unacceptable hosts to L jacobaeae.