Landcare Research - Manaaki Whenua

Landcare-Research -Manaaki Whenua

Darwin’s Barberry Weevil Released

Randall Milne making the first ever release of the Darwin’s barberry seed weevil.

Randall Milne making the first ever release of the Darwin’s barberry seed weevil.

There is finally some relief in sight for farmers and other land managers who have been struggling to control a Chilean plant, Darwin’s barberry (Berberis darwinii).

The thorny evergreen weed generates a huge number of fruit and seeds that are fed on by birds, allowing the plant to quickly spread, reducing pasture quality and native plant regeneration. Over the past few years, Lindsay Smith has been working to develop insect biocontrol agents that could limit the plant’s reproductive ability. The Environment Protection Authority approved the release of two insect agents in 2012, a seed-feeding weevil (Berberidicola exaratus) and a flower-feeding weevil (Anthonomus kuscheli), but rearing them for release has proved unexpectedly difficult.

“We have discovered that barberry plants do not thrive in pots, particularly inside our containment facility under artificial conditions, and they often abort their flowers and fruit and produce little new growth, all of which the weevils need,” explained Lindsay. When it became apparent that these plant issues could not easily be overcome to allow mass-rearing, direct field release options were explored. Our Chilean collaborator, Hernan Norambuena, sent a large shipment of both weevils in spring 2013 when they had overwintered and were ready to begin egg-laying. “At this point we discovered that both species were infected with microsporidia parasitic fungi, something we had not encountered with earlier shipments, ruling out any possibility of direct field releases. So we explored whether we could identify disease-free populations of the weevils in Chile,” explained Lindsay. Material collected by Chantal Probst during pathogen surveys in Chile a couple of months later confirmed some disease-free sites, so last spring Hernan collected weevils from these sites and shipped them to New Zealand. It was therefore a blow to discover infection in these shipments too.

We then hypothesised that the microsporidian diseases might become prevalent during winter, when the weevils hibernate closely together, but not be passed on via the eggs to offspring (which become infected later on). To test this we asked Hernan to send us seed weevils from Chile as larvae inside infested fruits proved to be disease-free. We originally planned to hold the subsequent new adults in cages over the winter and field release them this coming spring. However, because the new adults were not doing well, perhaps due to the lack of new growth on our sulky potted plants, we decided to release them instead into a field cage where they had a better chance of finding what they need.” So in April, with the help of Randall Milne (Environment Southland) we released 70 seed-feeding weevils onto Darwin’s barberry plants in Southland, where the plant is a particularly bad problem. To prevent immediate dispersal the weevils were initially caged for a couple of weeks. More weevils will be released at this site when they are available. “This is the first time anywhere in the world this species has been released as a biocontrol agent, and it is great to have finally overcome the challenges to get to this point,” confirmed Lindsay.

Now that we have a suitable method we are planning to import an even larger quantity of infested fruit next spring, with the aim of making more direct field releases. The priority is to get the seed weevil established and then to develop direct release methods for the flower weevil also, if subsequent monitoring shows that it is needed. Meanwhile efforts to find a pathogen that could attack the barberry bushes themselves are continuing. Previously two rusts and an unidentified pathogen were identified in Chile as potential biocontrol agents. Further surveys in 2013 by Chantal confirmed rust as the most promising prospect, and last spring we imported some samples for identification. “Molecular studies confirmed that we had found the species of greatest interest for further study (Puccinia berberidis-darwinii),” said Chantal. “This rust is likely to be highly host-specific and, now that the identity of the rust has been confirmed, we have been able to get a permit that will allow us to import the rust into containment for life-cycle studies and host-specificity testing, as soon as we are able to get a new shipment from Chile.

This project is funded by the National Biocontrol Collective.