So what's the problem?
Whitetops or hoary cresses, Lepidium draba (= Cardaria draba) and L. appelianum, are deep-rooted, perennial mustards that are aggressive invaders of cropland, rangeland, and riverbanks. They are declared noxious weeds in 14 US states and three Canadian provinces.
What is this project doing?
In spring 2001, Prof. Mark Schwarzländer (University of Idaho, USA) established a consortium to investigate the scope for classical biological control of these weeds.
Results so far
In 2012, work at CABI concentrated on the gall-forming weevil Ceutorhynchus cardariae and the seed-feeding weevil C. turbatus. In addition, we started working with a new species, the root-gall forming weevil Ceutorhynchus assimilis, which is widely distributed in Eurasia and northern Africa. In the literature, this weevil is recorded as a pest of several Brassicaceae crops. However, preliminary host range tests and molecular analyses conducted by the USDA-ARS, European Biological Control Laboratory (EBCL) indicate that a type host-specific to L. draba exists in southern France. Work on this species will be shared between USDA-ARS and CABI.
During a field trip in spring 2012, in southern France, galls of C. assimilis were collected and brought back to CABI’s centre in Switzerland. During June, over 200 adults emerged. After a short feeding period, weevils became more or less inactive over summer and only re-started feeding during September. On 1 October 2012 the first eggs were found and host specificity tests have been started.
In 2012, additional no-choice oviposition (egg laying) tests were conducted with C. turbatus (the seed-feeding weevil). Since 2004, we have now exposed 47 test plant species to this seed-feeding weevil, 21 of which are native to North America. Apart from L. draba (hoary cress), eggs were found in seven test species, three native to North America. When exposed to the weevil, only the closely related European L. campestre supported the development of larvae. When offered L. campestre and L. draba simultaneously, C. turbatus clearly preferred L. draba. Because adult feeding can also damage seeds, we conducted single-choice adult feeding tests in 2009. Again, C. turbatus clearly preferred L. draba: usually less than 10% of adult feeding holes were found on test species (range: 0.2–16.7%). So, C. turbatus is the most specific agent on L. draba we have found so far.
Collection of additional data on the phenology of C. cardariae (the gall-forming weevil) during 2010, 2011 and 2012 showed that its life cycle is more flexible than previously thought. Under favourable conditions, egg laying and larval development can occur even during winter, potentially affecting the growth and vigour of hoary cress (Lepidium draba). Rearing the gall-forming weevil on different hoary cress populations from Eurasia and the USA again showed a high variability among populations in their suitability for gall and adult development but, on average, populations from the area of origin and the introduced ranges did not differ.
Between 2003 and 2012, we have exposed 114 test plant species and varieties under no-choice conditions to C. cardariae, over half of which are native to North America and including four threatened or endangered species. Galls (to a certain degree) developed on 22 species, while adults emerged from 15, ten of which are native to North America. C. cardariae females normally produced far fewer offspring on test species than on the target weeds – and none of the threatened or endangered species or any of the Brassica crops (e.g. canola, cabbage) supported adult development. In four cases only one adult emerged, and in one case (with Lobularia maritima (Sweet Alyssum)), an adult emerged but no galls had formed. Under single- and/or multiple-choice conditions, i.e. in the presence of the target weeds, there was only very limited gall formation on non-target plants. Lepidium latipes (San Diego pepperweed) was the only native North American species that was readily attacked under multiple-choice cage conditions. However, a combined impact and survival experiment with this plant (L. latipes) showed that attack by C. cardariae does not negatively affect it, even under no-choice conditions, and the weevils not able to sustain a population on this plant alone. In addition, an open-field test showed that L. latipes was only attacked when exposed in close proximity to hoary cress (L. draba).
In 2010 and 2011 we collected extensive data on the field host range of C. cardariae in part of its native range in eastern Romania. A total of 3,074 plants of 15 different Brassicaceae species and L. draba were sampled. A total of 794 weevil larvae were found and 246 adult weevils emerged. Morphological identification and molecular analyses revealed that only the closely related European Lepidium campestre (field pepperweed) appears to act as an alternative field host of this gall-forming weevil - C. cardariae.
Based on the results above, we think the likelihood of C. cardariae attacking non-target plants is very low and most likely to occur in situations where weevils are present in high numbers, such as after successful control of hoary cress, and where non-target plants occur in close proximity to the target weeds. Any potential non-target effect that might occur under these conditions is expected to be temporary and without negative consequences for the fitness of the non-target species. We therefore submitted a petition for field release of C. cardariae to the USDA, APHIS Technical Advisory Group for review.
Between 2002 and 2006, we worked with the stem-mining weevil C.merkli, but then postponed investigations because of inconsistent attack on L. draba control plants under lab and common garden conditions and questions concerning its host specificity. Since only a few potential agents remain to be studied, we decided to revive work on this species. A multiple-choice field cage and/or open-field test with a selected number of critical test plant species planned in southern Russia in 2012, unfortunately had to be postponed to 2013.
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by A Bailey, D Chandler, W Grant, J Greaves, G Prince, M Tatchell
28 October 2010
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