Darwin's barberry: playing the numbers game
Darwin's barberry fruit infested with rust
Recently there have been some important breakthroughs in the Darwin’s barberry (Berberis darwinii) project, involving both the seed-feeding weevil (Berberidicola exaratus) and the rust fungus (Puccinia berberidis-darwinii) in what is really proving to be a numbers game.
Only a handful of releases of the seed-feeding weevil have been made since the first release in Southland in February 2015. It is not possible to mass-rear the weevils, because Darwin’s barberry does not grow well enough in pots, especially under artificial conditions, to produce sufficient flowers or fruits. So we developed a direct field release technique instead and proved this could work in 2015/16, when we were able to make seven releases of the weevil in Southland, Wellington and Manawatū−Wanganui.
With the concept proven, we put extra effort into collecting an even larger quantity of infested fruits in Chile in November 2016 with the hopes of having even more weevils to release last summer. It was therefore disappointing and perplexing to have only a handful of weevils emerge from pupation and not the thousands expected. A subsequent review found that in order to accommodate the much larger quantity of infested material, bigger containers were used for pupation, which probably did not provide the conditions the weevils needed. Dehydration, in particular, may have been an issue, but possibly also overcrowding, requiring a rethink about how best to rear through future shipments.
At that point we also reflected on the optimal release and establishment strategy for this species. Often the best way to establish biocontrol agents is to put out the minimum number that experience tells us should generate a viable population, and do this at many sites. This strategy reduces the risk of an unforeseen event such as extreme weather wiping out a fledgling population, and increases the chances they will find conditions to their liking (which can be hard to accurately predict in advance). “However, with an insect like this, which has to survive for many months post-release (including winter) and then find a mate before it can reproduce, it can be more effective to ‘put all your eggs in one basket’, so to speak, and just load up one or two sites with as many as possible,” explained Lindsay Smith. If establishment can be achieved, then the agent can be harvested, once sufficiently abundant and released more widely. The National Biocontrol Collective, which funds the Darwin’s barberry project, agreed to try this approach with the next lot of available weevils and load up existing sites in Southland and Wellington.
Lindsay Smith had a very successful trip collecting infested fruits in Chile in November 2017, and managed to rear through from these several thousand adults, which were released at the Southland and Wellington sites in early February. As a bonus, Randall Milne (Environment Southland) found two adults at the site of the first ever release in early November. This is no small feat given the extensive Darwin’s barberry infestation at the site, the small number of weevils released, their tiny size, plus the relatively short time that has elapsed. So it looks very promising that the weevil is already well on the way to successfully establishing at that site. “The extra weevils added recently should ensure establishment and allow collection and redistribution from this site before too long,” said Lindsay.
We have also finally managed to work out how to infect Darwin’s barberry plants with the rust fungus, considered to be one of the most promising potential agents for this weed. We have imported several shipments of infected leaves and whole plants from Chile into our pathogen containment facility in Auckland, initially for identification purposes and then for further study. No-one has worked with this rust before, and some experimentation has been required to work out the infection process.
A number of techniques tried initially were unsuccessful. The germination rate of the spores was found to be low, suggesting that a large amount of inoculum may be needed for successful infection, as well as ideal humidity and temperature. Lynley Hayes was able to collect a large quantity of rustinfected material in Chile in November 2016 since it was easy to find while collecting fruits infested with the seed weevil. However, it was some relatively rare infected fruits, packed with spores, which provided the breakthrough needed to get infection. “Some Darwin’s barberry plants developed a few pustules two and a half months after inoculation, and the technique that worked involved rubbing the leaves directly with rust spores,” explained Chantal Probst.
Since then we have been gradually bulking up the rust on live plants to provide sufficient material for host testing trials to begin. The life cycle of the barberry rust is relatively long (around 3 months), so this has been a slow process. However, when Lindsay was collecting more weevils in Chile last November he was able to find lots of the rust-infected fruits to bring back, probably thanks to a wet spring there. This material has allowed some host testing to get underway immediately. The rust fungus is expected to be highly host specific, and with some luck we should be able to confirm that before too long.
CONTACT
Lindsay Smith – smithl@landcareresearch.co.nz
Chantal Probst – probstc@landcareresearch.co.nz