Controlling hoary cress in the United States
Trade in seed brought crops to new regions, but many weeds were spread by this route too. Whitetops, also known as hoary cresses, arrived in the USA as contaminants of seed from Eurasia. They are now aggressive invaders of crops, rangeland and riverbanks. One reason for this is the absence of the natural enemies that keep them in check in their area of origin. CABI staff in Switzerland are looking into the prospects for biological control of these invasive plants.
So, what’s the problem
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 the 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 rangelands, hoary cresses 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, 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, (Dr Jeff Littlefield). We are focusing on L. draba because it is the most prevalent hoary cress 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. We have submitted the petition for field release to the USDA-APHIS (Animal and Plant Health Inspection Service) Technical Advisory Group (TAG) in January 2020.
The seed feeder, C. turbatus, is the most specific agent we have worked with. To date, under no-choice conditions, larvae have only developed on a single native North American species, Lepidium huberi, and one other European non-target species (L. campestre). Our main target, L. draba, remains the most preferred under natural conditions with no recorded non-target attack in the multiple-choice tests with any other species. While established whitetops reproduce vegetatively, long-distance plant dispersal relies on seeds, which could be reduced by this weevil.