Giving dyer’s woad the blues: encouraging first results for biological control G. Cortat,1 H.L. Hinz,1 E. Gerber,1 M. Cristofaro,2,3 C. Tronci,3 B.A. Korotyaev4 and L. Gültekin5 Summary Dyer’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 can- didates. 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 withP. 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 in- terest, a flea beetle preliminarily identified asPsylliodes sophiae var. tricolor Weise and a root-mining weevil preliminarily identified asAulacobaris near fallax. For both species, rearing colonies were es- tablished in Switzerland, and methods for host-specificity tests were developed. A literature survey re- vealed 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 root- mining 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. Keywords: Isatis tinctoria, weed biological control, host range testing. Introduction as a textile dye crop and then accidentally spread as a contaminant of crop seed (Hegi, 1986; McConnell et Dyer’s woad, Isatis tinctoria L. (Brassicaceae), is a al., 1999). Today, dyer’s woad is a declared noxious winter annual, biennial or short-lived perennial mus- weed in ten western US states (USDA-NRCS Plants tard of Eurasian origin. The original distribution of National Database, http://plants.usda.gov; Invaders dyer’s woad in Europe and Eurasia is difficult to re- Database, http://invader.dbs.umt.edu). It was estimated construct because it has been widely distributed and that dyer’s woad reduced crop and rangeland produc- grown by humans as a dye crop since Roman times. tion in Utah by $2 million in 1981, and its infestation It was introduced to North America by early colonists doubled there within 10 years (Evans and Chase, 1981, in McConnell et al., 1999). Unlike many other mustard weeds, dyer’s woad does not depend on disturbance to 1 CABI Europe-Switzerland, Rue des Grillons 1, 2800 Delémont, establish and can readily invade and dominate well- Switzerland. vegetated rangeland sites. 2 ENEA-Casaccia, Rome, Italy. 3 BBCA, Via del Bosco 10, 00060 Sacrofano (Rome), Italy. In spring 2004, an initiative was started by a consor- 4 Russian Academy of Sciences, Zoological Institute, 199034 St. Peters- tium of southeastern Idaho and Utah counties, admin- burg, Russia. istered through the Hoary Cress Consortium (Dr Mark 5 Atatürk University, Department of Plant Protection, 25240 Erzurum, Schwarzlaender, University of Idaho) to investigate Turkey. Corresponding author: G. Cortat <[email protected]>. the potential for classical biological control of dyer’s © CAB International 2008 woad. During preliminary surveys conducted by CABI 133 XII International Symposium on Biological Control of Weeds Europe-Switzerland in 2004, three species were pri- During no-choice oviposition and development oritized as biological control candidates, and investiga- tests, 45 test species were exposed, including 24 native tions started. to North America. Of the 43 species for which results Here we present data on the biology and host-speci- (adult emergence) are available, adults thus far have ficity testing of the three species under study, as well as only emerged from three test species, viz. Arabis hol- a list of additional candidates resulting from extensive boellii Hornemann, Schoenocrambe linifolia Nutt., and literature and field surveys conducted in 2006. Stanleya viridiflora Nutt. However, while an average of 8.1 ± 1.5 adults emerged from dyer’s woad (control) Biological control candidates plants, only an average of 0.5 ± 0.5 to 1.5 ± 0.6 adults emerged from these non-target test plants. In subse- studied since 2004 quent choice tests with two of the species using five One flea beetle and two weevil species were found replicates each, no attack occurred on A. holboellii, and during preliminary surveys in southern Germany and S. linifolia sustained attack at a lesser degree than dy- southern Switzerland in 2004, and host-specificity tests er’s woad (Hinz et al., 2005, 2006). Hoffmann (1954) were started in 2005. The current test plant list for dy- and Freude et al. (1983) each listed I. tinctoria as the er’s woad comprises about 100 plant species and va- only host plant for C. rusticus. Although C. rusticus ap- rieties, nearly two thirds of which are native to North pears to have been collected previously from Rorippa America (Wilson and Littlefield, 2006). palustris (L.) Besser (B. Korotyaev, unpublished data), we did not find attack on three other Rorippa spe- cies (Rorippa amphibia L., Rorippa sinuata Nutt. and Aulacobaris fallax H. Brisout Rorippa sylvestris (L.) Bess.) that we tested (Hinz et (Coleoptera: Curculionidae) al., 2006). Females of this univoltine weevil lay eggs from the In conclusion, C. rusticus is regarded as a very prom- end of March until early summer into the petioles of ising biological control candidate for dyer’s woad. We dyer’s woad or directly into the root. Larvae mine in will continue host-specificity tests in 2007 and are also the petioles, root crown and root of dyer’s woad, in planning to conduct an experiment to quantify the im- which they also pupate. Heavily infested plants are vis- pact of C. rusticus on dyer’s woad. ibly stunted and may die (Hinz et al., 2004). In no-choice oviposition and development tests con- Psylliodes isatidis Heikertinger ducted with 39 test plant species, mining and/or larvae (Coleoptera: Chrysomelidae) were detected in 16 plant species, 12 of which are in- digenous to North America. Subsequently, these plants In autumn, females of this flea beetle lay eggs in the were exposed in multiple-choice field cage tests. Apart soil close to the root crown of the host plant. Larvae from dyer’s woad, mines or larvae were found in eight hatch in early spring and feed in the developing shoots test plant species, all native to North America (Hinz et of dyer’s woad. Mature larvae leave the plants to pu- al., 2005, 2006). Although in five cases only, one or pate in the soil, and adults of the subsequent generation two of the plants offered were attacked, results indi- emerge during June. After a brief feeding period, the cate that the range of plants A. fallax females accept beetles aestivate. This species shows good potential for for oviposition and on which larvae are able to develop rapid population increase and at high densities can kill is broader than expected and includes several North shoots or whole plants (Hinz et al., 2004). American species. In conclusion, A. fallax is not regar- During no-choice larval transfer and development ded as specific enough for consideration as a biological tests with nine test species, adult flea beetles emerged control agent for dyer’s woad. from four. However, a lower proportion of larvae suc- cessfully developed into adults on test plants (3–12%) Ceutorhynchus rusticus Gyllenhal compared to control (dyer’s woad) plants (28%). In a subsequent multiple-choice field-cage test exposing (Coleoptera: Curculionidae) 14 test species together with dyer’s woad, adult bee- Females of this weevil lay eggs from September tles emerged from nine test species. However, while throughout winter until early spring. Eggs are mostly an average of 46 adults (maximum 130) emerged from laid directly into the leaf surface under the epidermis. individual dyer’s woad plants, only 0.3 to 3.6 emerged Larvae feed in the petioles and later in the root crown. from test species (Fig. 1). We assume that the density Mature larvae leave the plants to pupate in the soil, and of adults released into the field cage was artificially adults of the next generation start to emerge from the high (considering the high number of adults that emer- end of May onwards. Newly emerged weevils aesti- ged from controls), and plants may have been placed vate during July and August and resume feeding at the too close to each other (Hinz et al., 2005). A second beginning of September (Hinz et al., 2004). The main multiple-choice field cage test will be established shoot of heavily attacked plants often dies, which eas- in autumn 2006 releasing fewer beetles and expos- ily distinguishes attacked plants in the field. ing fewer test plants (Hinz et al., 2006). Depending 134 Giving dyer’s woad the blues: encouraging first results for biological control 60 s 50 40 d 30 20 emerge 10 Mean number of adult 0 e a a a a a Reseda lutea Isatis tinctori Rorippa sinuata Peltaria alliace Rorippa amphibia Rorippa sylvestris Lepidium latifolium Erysimum asperum Armoracia rustican Lepidium campestr Physaria chambersii Lesquerella ludoviciana Schoenocrambe linifoli Erysimum inconspiccum Brassica oleracea sabaud Figure 1.