What’s Happening with Old Man’s Beard?
Lynley finds a sawfly in the field. Image - L. Grueber
Old man’s beard (Clematis vitalba) has proven to be a challenging biocontrol target. Since we have many valued native Clematis species in New Zealand biocontrol agents need to be highly host-specific, and surveys in the native range did not yield a long list of candidates worthy of further study. Three agents have been released in New Zealand to date.
The first agent released, back 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 unfortunately it did not take long for its own natural enemies to catch up. While mining of leaves is now common on old man’s beard, six native and two exotic parasitoids, generally keep the leaf miner populations lower than the threshold needed to impact on the growth of the plant. However, damaging outbreaks do sometimes occur. “We saw heavy mining this autumn at Ashburton and on Banks Peninsula,” reports Hugh Gourlay.
The second agent, released around the same time as the leaf miner, was believed to be a superior strain of a fungus (Phoma clematidina). This fungus was already present New Zealand but only causing cosmetic damage to old man’s beard in the autumn. Initially heavy damage was observed at release sites, but this did not persist. Subsequent studies could find no trace of the released strain, and it is possible that it was outcompeted by other fungi on old man’s beard, some of which studies have shown occur as symptomless endophytes that may confer disease resistance to the plant.
The third agent, a sawfly (Monophadnus spinolae) was released with high hopes in 1998. Although a rare insect in its native range in Europe, each larva can consume several leaves, and it was hoped that it might be able to complete three generations a year here. Other sawfly species, such as the willow sawfly (Nematus oligospilus), which self-introduced to New Zealand in 1997, can be damaging pests. However, the old man’s beard sawfly proved to be a difficult insect to mass rear. With much perseverance enough were produced to make 16 releases at 14 sites from the Bay of Plenty to Otago before the rearing colony died out. It is thought that the rearing colony probably became too male-dominated and too inbred. If conditions are not good for mating the females produce unfertilised male eggs. Six sites quickly succumbed to floods or human disturbance and, since no sign of the sawfly was ever seen in the field, it was thought to have failed to establish.
Recently the merits of having another attempt at establishing the sawfly have been considered, given the ongoing seriousness of the old man’s beard problem in many regions. Before investing in such an undertaking, it was agreed that surviving release sites should be checked once more at the optimal time of the year. Lynley Hayes, along with Robin Van Zoelen and Lindsay Grueber (Tasman District Council) checked a site near Nelson in January. Nearly 3000 sawflies were released here in 2002, by far the largest release made, and the larvae were initially covered with mesh to protect them from being eaten by birds. The site has since been highly modified to provide better flood protection, and much of the old man’s beard had been removed to make space for native plantings, so the trio’s expectations were low. However, clumps of old man’s beard remain at the site and Lynley discovered one of the distinctive white caterpillar-like larvae on the second clump examined. “However, 2 hours of searching only yielded another two larvae and one adult, so the sawflies remain rare at this site,” confirmed Lynley. Reasons for the low population may be that it is inbred or taking a hit each year from wasps. “We hoped that we might find sawflies elsewhere, so we could have a go at creating a new rearing colony with increased genetic diversity, and then release the progeny in a wasp-free area, to see if that could yield better results. However, that hope was dashed when no sawflies were found at any of the other release sites. Given that wasps are likely to be a limiting factor for sawflies in New Zealand, we recommend focussing instead on two remaining options that are not likely to be affected in this way,” said Lynley.
One of these is a bark beetle (Xylocleptes bispinus), known to regularly kill vines in Europe, which was investigated by colleagues at CABI in Switzerland early on, but proved difficult to test. Tests in confined spaces, and attempts to use cut stems of native Clematis, shipped from New Zealand, proved unsatisfactory, and the work was discontinued without resolution due to a lack of funding. When it became clear that the three agents released in New Zealand were unlikely to do the job required, the bark beetle option was revisited and a field trial in the native range deemed the way to go. Frustratingly, several attempts to set up such a field trial in the UK were unsuccessful as New Zealand native Clematis plants failed to thrive due to the cooler climate, severe storms and even rabbit browsing. So arrangements were made with a botanical garden on the Isle of Wight, which offers a milder climate and has native populations of the bark beetles. Even then, some plants were lost before they were planted out, due to a severe winter storm damaging the glasshouse they were being stored in! But finally in the last couple of years New Zealand native Clematis plants have been successfully established at the garden, allowing the field trial to get underway. More plants, recently shipped over to boost numbers, will be added this spring. “Results are still several years away,” warned Hugh. In breaking news, some funds have been secured from councils seriously affected by old man’s beard that will allow us to also import some bark beetles into containment in 2017 and undertake studies that will complement the field trials.
The other option is a leaf and bud galling mite (Aceria vitalbae) not found during the original surveys that only came to light more recently. The mite stunts the new growth and is likely to be highly host-specific. However, again a range of unexpected setbacks to studying this agent occurred, which have only recently been overcome. Several attempts to establish a mite colony in containment at Lincoln were unsuccessful after shipments were delayed in transit or heavily diseased,” explained Lindsay Smith. At that point, mite expert Dr Biljana Vidovic from the University of Belgrade, Serbia, hand-delivered a colony of mites but plant quality issues then emerged that could not be treated without harming the mites and the colony was lost. The narrow window in which mites can be collected in the field further complicated matters. At that point it seemed more feasible to attempt the host-testing in Europe. “The logistics of shipping all the test plants successfully to Serbia took some time to overcome,” said Lindsay. However, testing is now well underway and is expected to be completed soon. With a bit of well overdue luck, an application to the Environmental Protection Authority to release a potentially useful new agent for old man’s beard might well be possible later this year.
This project is funded by the National Biocontrol Collective.