So, what's the problem
Dyer's woad (Isatis tinctoria) is a plant of Eurasian origin in the mustard family. It was introduced to North America by early colonists as a textile dye crop and then accidentally spread as a contaminant of crop seed. Today, it is officially recognized as a serious weed in 10 western US states. Unlike most other invasive plants, dyer’s woad can invade undisturbed sites. This makes it a particular problem in well-vegetated sites, such as rangelands, because it can readily invade and dominate them.
Control is difficult. Chemical control is limited in rangeland and forests by often inaccessible terrain, the risk of undesirable environmental impacts and questionable economic returns. Cultivation can be effective against the weed in crop areas, but it must be repeated 2–3 times a year for several years to be successful.
What is this project doing?
In 2004, Mark Schwarzländer (University of Idaho, USA) and Jim Hull (Weed Superintendent, Idaho) invited CABI to participate in an initiative to investigate the potential for biological control of dyer's woad. We identified several potential biological control agents, which we subsequently studied to varying degrees. The aim was to identify species that are both specific to the target plant and cause sufficient damage to limit the weed’s impact and spread in the USA.
Current work concentrates on two weevils: the seed feeder Ceutorhynchus peyerimhoffi and the root-crown miner C. rusticus. We tested them on over one hundred plant species, two-thirds of which are native to North America and include species of conservation concern.
C. peyerimhoffi laid eggs and developed in only four non-target species in no-choice tests. Unfortunately one of them, Boechera hoffmannii, is a federally listed endangered species. However, additional tests suggest very limited risk of non-target attack on B. hoffmannii and other test species in the field.
C. rusticus developed to adult in 11 species, nine native to North America. But dyer’s woad was highly preferred for egg-laying in field tests, so we believe C. rusticus will not have significant impact on non-target species.
In impact experiments, up to 100% of seeds were destroyed by C. peyerimhoffi, while C. rusticus reduced seed production by up to 72% and plant biomass by 46%. A combination of both agents would be ideal to control dyer’s woad in North America.
We have begun – with Massimo Cristofaro (Biotechnology and Biological Control Agency, Rome, Italy) and Radmila Petanovic (University of Belgrade, Serbia) – working on an eriophyid mite species in the genus Metaculus, which is probably new to science.
Technical Research Assistant