Host Specificity of Longitarsus quadriguttatus Pont., a Below-Ground Herbivore for the Biological Control of Houndstongue

Abstract

Abstract Cynoglossum officinale is a Eurasian plant that has become an invasive weed of rangelands in the United States and Canada. It is a herbaceous, monocarpic, short-lived perennial. Since there are no satisfactory means of control of houndstongue, a biological control project was initiated in 1988. The first species introduced for the control of houndstongue is a root-mining weevil, Mogulones cruciger, which was first released in Canada in 1997. The flea beetle Longitarsus quadriguttatus is another below-ground herbivore species considered for the biological control of C. officinale. Previous host-specificity investigations with 49 European test-plant species demonstrated that larval development of the flea beetle is limited largely to plant species within the tribe Cynoglosseae in Boraginaceae. However, investigations neither included any native North American Boraginaceae species nor examined results of earlier tests for plant species that were susceptible to attack by the flea beetle. Therefore, additional host-range investigations were conducted between 1994 and 1996. Different no-choice and choice tests were conducted with 7 plant species that showed susceptibility to attack in previous tests and 5 native North American plant species. Experiments were carried out under laboratory and open-field conditions. L. quadriguttatus was host specific in these tests. Two European test-plant species outside the genus Cynoglossum and 2 native North American Cynoglossum species supported complete development of the flea beetle to some extent, but in all cases the test plants were significantly less suitable hosts than houndstongue. The remaining 5 European and 3 native North American test-plant species did not support development and/or survival of L. quadriguttatus. The introduction of L. quadriguttatus into Canada was approved in 1998.