So, what's the problem
Whitetops or hoary cresses (Lepidium draba, L. chalepense and L. appelianum) are deep-rooted, creeping, perennial mustard plants. Since their introduction to the USA in the late 19th century they have spread throughout western and northeastern states. They are difficult to control and are officially recognized as serious weeds in 14 US states and three Canadian provinces.
In rangeland whitetops displace forage species and are toxic to livestock. In crops they pose a dual problem: as well as being weeds, they can boost populations of some insect pests, such as the cabbage seedpod weevil (Ceutorhynchus obstrictus) – a major pest of canola and oilseed rape – by providing abundant alternative host plants.
What is this project doing?
In 2001, Mark Schwarzländer (University of Idaho, USA) established a consortium to investigate the scope for classical biological control of these weeds: introducing natural enemies from the weed’s area of origin. Our work at CABI is coordinated with that of USDA-ARS-EBCL (Agricultural Research Service – European Biological Control Laboratory) in Montpellier, France, and Montana State University, USA (Dr Jeff Littlefield). We are focusing on L. draba because it is the most prevalent whitetop species in North America.
Based on published information and field surveys in Europe, we prioritized insect species that seem likely to attack a limited number of plants including L. draba. We are testing them against a selected range of non-target test plants to provide evidence on whether they would be safe to introduce in North America.
We are currently focusing on two species, both weevils. On the basis of our test results, we applied for permission to introduce the gall-former Ceutorhynchus cardariae to the USA, but were asked to conduct some additional testing. This has now been done, ready for a new application to be submitted in spring 2018.
The seed feeder C. turbatus is the most specific agent we have worked with. Larvae did not develop on any native North American species, but only on our main target, L. draba, and to a lesser extent on another invasive whitetop (L. chalepense) and one other European species (L. campestre). While established whitetops reproduce vegetatively, long-distance plant dispersal relies on seeds, which could be reduced by this weevil.
The root-galler C. assimilis, has unfortunately turned out to be less specific than we initially thought. In addition, L. draba is mostly a problem in the continental USA, where a much harsher climate than the weevil’s home range in southern France potentially limits its establishment. Given these two factors, we decided to stop working on this species.
Country Director and Head Weed Biological Control
Technical Research Assistant