Giving dyer's woad the blues: encouraging first results for biological control.

Abstract

AbstractDyer's woad, Isatis tinctoria L. (Brassicaceae), has been cultivated since Roman times throughout Europe for the blue indigo dye extracted from its leaves and was introduced by early colonists into North America. Today, it is a declared noxious weed in ten western US states. A literature survey for insects, mites and pathogens associated with dyer's woad revealed several biological control candidates. Three were found in 2004 during preliminary field surveys in Switzerland and Germany: Ceutorhynchus rusticus Gyllenhal and Aulacobaris fallax H. Brisout, both root-mining weevils, and Psylliodes isatidis Heikertinger, a shoot-mining flea beetle. Results of host-specificity tests conducted at CABI Europe-Switzerland are particularly promising for C. rusticus, a very damaging species able to kill overwintering rosettes. Results of additional host-specificity tests with P. isatidis are necessary to decide whether it is worth continuing with this species, while A. fallax is not specialized enough to be further considered. In 2006, additional field surveys were conducted in Turkey, Bulgaria, Romania and Kazakhstan to find new candidates. Based on the material identified thus far, two species are of interest, a flea beetle preliminarily identified as Psylliodes sophiae var. tricolor Weise and a root-mining weevil preliminarily identified as Aulacobaris near fallax. For both species, rearing colonies were established in Switzerland, and methods for host-specificity tests were developed. A literature survey revealed 62 species to be associated with dyer's woad in Europe. Of the ten species only described from dyer's woad (I. tinctoria) or closely related Isatis species, four are of particular interest, viz. the rootmining weevil Aulacobaris licens Reitter, an as-yet undescribed Lixus sp. and the two seed-feeding weevils Bruchela exigua Motschulsky and Ceutorhynchus peyerimhoffi Hustache. Surveys will be conducted in 2007 to find at least two of these four species, and investigations on already available agents will continue.