So what's the problem?
European hawkweeds (Pilosella spp.) have been introduced into New Zealand and North America where several species have become troublesome weeds. A number of Pilosella species are invasive in the North-western USA and in British Columbia in Canada: Pilosella officinarum (Hieracium pilosella, mouse-ear hawkweed), P. aurantiaca (H. aurantiacum, orange hawkweed), P. caespitosa (H. caespitosum, meadow hawkweed), P. flagellaris (H. flagellare, whiplash hawkweed) P. piloselloides (H. piloselloides, tall hawkweed), P. glomerata (H.glomeratum, yellowdevil hawkweed) and P. floribunda (H. floribundum, kingdevil hawkweed). They invade roadsides, pastures, clear-cut areas and nature reserves and reproduce by seeds, stolons (creeping horizontal stems, except P. piloselloides) and, in the case of P. caespitosa, suckers (shoots developing from adventitious root buds).
What is this project doing?
The project originally started in the early 90s on behalf of the Hieracium Control Trust, a consortium of sponsors in New Zealand, investigating the prospects for biological control of P. officinarum. Five insect species associated with this hawkweed in central Europe were studied: Oxyptilus pilosellae, a plume moth feeding on the above-ground plant parts; Aulacidea subterminalis, a wasp causing galls on stolons; Macrolabis pilosellae, a gall midge attacking the stolon tips and rosette centres; and Cheilosia urbana and Cheilosia psilophthalma, two hoverfly species which feed externally on the roots and on the above-ground plant parts, respectively. All five insect species were released in New Zealand and the two gall-forming insects established in the field.
Since 2000, CABI has been looking for natural enemies that could be used to control invasive alien hawkweeds in North America. In contrast to the situation in New Zealand, where all existing hawkweeds are naturalized, native Hieracium spp. occur in North America.
Results so far
Investigations on the gall wasp Aulacidea hieracii revealed that it only rarely forms galls on the target species and investigations have therefore been discontinued.
A joint petition for field release of Aulacidea subterminalis in the USA and Canada was submitted by Dr Jeffrey Littlefield (Montana State University, Bozeman), Dr Linda Wilson (British Columbia Ministry of Agriculture and Lands), and Dr Gitta Grosskopf-Lachat (CABI) in 2009 and approved in 2010. The first North American releases took place in British Columbia and Montana in summer 2011. We continue to maintain a rearing colony at CABI’s centre in Switzerland to complement North American lab colonies.
A petition for field release of the root-feeding hoverfly Cheilosia urbana will be submitted in 2013, while work with C. psilophthalma has been discontinued due to difficulties in obtaining conclusive field data regarding its specificity.
Aulacidea pilosellae causes small galls on the midrib of leaves, stolons and flower stalks of Pilosella spp., and appears to have a restricted host range. Two forms showing different host preferences are being investigated. Molecular analyses were conducted by Chandra Moffat, who successfully completed her MSc at the University of British Columbia in collaboration with CABI in 2012, and Dr Kevin Floate (AAFC, Canada) to determine the level of genetic differentiation between the two forms. Although preliminary results provide insufficient evidence to consider the two populations as separate species, it appears that A. pilosellae collected from P. officinarum (A. pilosellae ex P. officinarum), regardless of the area of collection, are of a different lineage or biotype to the wasps collected from P. caespitosa, P. glomerata and P. piloselloides (A. pilosellae ex Pilosella spp.). Individuals collected from these three hawkweed species are of high genetic similarity, which essentially reflects their close geographical origin, rather than the plant species they were collected from. The molecular study shows no evidence for further differentiation in host preferences within this group. However, field studies suggest that the host choice of A. pilosellae ex Pilosella spp. is a combination of selection of the most abundant Pilosella spp. at each site and a host preference for P. caespitosa and P. glomerata. So far, lab studies have been confirming the preference of A. pilosellae ex Pilosella spp. for these two species.
Tests conducted between 2003 and 2012, with A. pilosellae ex Pilosella spp. on 38 species, subspecies and populations and with A. pilosellae ex P. officinarum on 27, revealed that while the two wasp populations differ in their host preferences, they both appear to have a restricted host range. Of the three native North American Hieracium species so far attacked by A. pilosellae ex Pilosella spp. under no-choice conditions (exposing one plant to the insect), only H. scouleri, was attacked to a very limited extent under multiple-choice conditions (exposing several plant species). Choice tests still need to be conducted with the Eurasian H. robustum. So far, A. pilosellae ex P. officinarum has only attacked the native North American H. scouleri and H. umbellatum outside the genus Pilosella under no-choice conditions. The latter was not attacked in single-choice tests (exposing one test plant and the target simultaneously). No-choice and multiple-choice tests will be continued in 2013. Studies on this insect are also being conducted by Dr Rosemarie DeClerck-Floate (AAFC, Lethbridge, Canada) and Dr Jeffrey Littlefield (Montana State University, Bozeman). Part of the field collected galls and insects reared at CABI in Switzerland are regularly sent to North America to complement their rearing colonies.
Currently, it is unlikely that further arthropods will be found in Europe that would be both effective and safe to be used as biological control agents against invasive hawkweeds in North America. However, fungal pathogens, especially rusts, can be much more host specific than insects, and rusts of the genus Puccinia are of particular interest since they have already been studied as potential agents for invasive hawkweeds, like Puccinia hieracii var. piloselloidarum, which was first evaluated for controlling Pilosella officinarum in New Zealand. This fungal pathogen occurs in Southern, Central and Northern Europe, including the British Isles. It has been recorded on both P. caespitosa (meadow hawkweed) and P. aurantiaca (orange hawkweed). In 2012, we conducted surveys for this rust on hawkweeds and found it on P. officinarum in the Swiss Jura and collected samples from P. aurantiaca and P. caespitosa in Eastern Germany and the Czech Republic. Samples were sent to Dr Rosemarie DeClerck-Floate (AAFC), where the identity of the rust will be confirmed and studies in quarantine initiated. Preliminary inoculation trials at CABI failed, but will be continued in 2013.
Hariet L. Hinz
Address: Rue des Grillons 1, CH-2800 Delemont, Switzerland
Tel: +41 (0)32 4214872
Tel: +44 (0) 1491 829053
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