Mite We Have the Answer for Old Man’s Beard?
Three agents have already been released to control old man’s beard (Clematis vitalba) (OMB) but none have been able to do the job for different reasons.
However, a new line of enquiry involving an eriophyid mite sourced from Serbia and a fresh attempt to test a bark beetle in the UK offer some new hope. OMB has proved to be a very tough biocontrol target because surveys in the native range did not yield a long list of potential biocontrol agents and New Zealand has native Clematis species (e.g. C. marata, C. forsteri, C. foetida, C. quadribracteolata, C. paniculata) so any agents needed to be highly host specific.
The first agent, released in 1996, was a leaf-mining fly (Phytomyza vitalbae) that showed early promise, establishing readily and dispersing throughout New Zealand within a couple of years. But it didn’t take long for its own natural enemies to catch up and it now regularly gets “hammered” by six native and two exotic parasitoids, which are usually hosted by other leaf-mining species. Parasitism significantly reduces the effectiveness of the OMB leaf miner as a biocontrol agent, but damaging outbreaks are still occasionally seen.
The second agent, released around the same time, was believed to be a superior strain of a fungus (Phoma clematidina) already known to be in New Zealand that generally caused only cosmetic damage to OMB. Initially heavy damage to OMB was observed at release sites, but did not persist. Subsequent studies could find no trace of the released strain, and it is possible that other strains already present, often as symptomless endophytes, were able to outcompete the introduced strain.
The OMB sawfly (Monophadnus spinolae) was released a few years later. It proved to be a very difficult insect to mass rear and then disappointingly failed to establish at any of the 16 sites where it was released, and so it was back to the drawing board.
A bark beetle (Xylocleptes bispinus), which was known to regularly kill vines in Europe, was investigated by colleagues at CABI in Switzerland early on, but had proved difficult to test. The beetle’s behaviour ruled out tests in confined spaces, and New Zealand native Clematis plants failed to reach stems of sufficient diameter for any attack to occur before funding for the work ran out. Tests attempted with cut stems, shipped from New Zealand, also proved unsatisfactory. However, with a solution for OMB still desperately needed we revisited the bark beetle about 5 years ago. “Attempts were made to set up a field trial in the UK using mature New Zealand native Clematis shipped over from New Zealand,” explained Hugh Gourlay. Unfortunately it proved to be too cold for our native Clematis to thrive and we were thwarted once again. However, a new collaboration with a small botanical garden in the Isle of Wight has provided a new ray of hope. The Isle of Wight has a more benign climate and the beetles occur there naturally. “We are establishing a new field trial there with the help of CABI Europe-UK, but the results may still be years away,” warned Hugh.
We are also exploring the potential of a European leaf and bud galling mite (Aceria vitalbae) that we only became aware of recently. The mite stunts the new growth and is likely to be highly host specific. Several attempts have been made to establish a mite colony in containment at Lincoln but again this has not proved to be straightforward. “Initial shipments of the mites arrived in bad shape after being delayed in transit or were heavily diseased,” explained Lindsay Smith. Fortunately, mite expert Dr Biljana Vidovic from the University of Belgrade, Serbia, agreed to hand-deliver a colony of mites, which she painstakingly transferred onto potted OMB plants in containment in April. We are now waiting to see if they produce a colony that we can use to undertake host specificity testing. If we are unsuccessful we will explore the possibility of the host-testing being undertaken in Serbia.
This project is funded by the National Biocontrol Collective.