Barberry Weevil Breakthrough
Widespread releases of a seed-feeding weevil (Berberidicola exaratus) finally got underway last summer with batches of weevils sent off to new homes in Southland, Wellington and Manawatu-Wanganui. While Darwin’s barberry (Berberis darwinii) is problematic currently in only a few regions of New Zealand, it has strong potential to become another ‘gorse’ of the future.
A key challenge in managing this prickly plant is that birds readily feed on the abundantly-produced berries, which helps it to spread and re-infest cleared areas. Clearing infested areas is no small task. It has been estimated that 37 person days were required to cut and stump treat a 3.3 ha Darwin’s barberry infestation in Wellington.
The female weevils lay eggs inside the developing fruits, and the resulting larvae damage or entirely consume the developing seeds. There is one larva per fruit. Host-testing has suggested that the weevil will also attack the seeds of the next weediest barberry species in New Zealand (Berberis glaucocarpa), and possibly some closely-related ornamental species (such as Berberis thunbergii) to a lesser extent. The latter might have the added bonus of helping to prevent more barberry species from becoming future weeds while not detracting from their appearance. Although the weevil was approved for release by the Environmental Protection Authority in 2012, some unexpected challenges cropped up that we have needed to overcome before widespread releases could begin. A flower-feeding weevil (Anthonomus kuscheli) was also approved for release, but efforts are being made to establish the seed weevil first. Neither weevil has been used as a biocontrol agent anywhere in the world before.
“The first challenge we ran into was that Darwin’s barberry does not like being grown in pots, and especially not under artificial conditions, such as in our containment facility,” said Hugh Gourlay. The plants would abort most of their flowers and fruit and produce little new growth, all of which we needed to rear the weevils. With mass rearing ruled out, we then explored direct field release options. Our Chilean collaborator, Hernan Norambuena, sent us a large shipment of over-wintered adult weevils in spring 2013, but hopes of direct field releases were dashed when we found they were infected with microsporidia and parasitic fungi, something we had not encountered with earlier shipments. Further research that spring showed that not all populations of the weevils in Chile were diseased, so material collected from ‘disease-frees sites’ was shipped to New Zealand in early spring 2014. “It was therefore a blow to discover infection in these shipments too,” said Hugh.
We then hypothesised that perhaps the disease became prevalent during winter when the weevils hibernate closely together, but was not passed on via the eggs to offspring (which become infected later on). So we asked Hernan to send us larvae inside infested fruits later in spring 2014 and were relieved when they proved to be disease-free. “We originally planned to hold the subsequent new adults in cages over the winter and release them in spring. However, the new adults were not doing well, likely due to the lack of new growth on our sulky potted plants, so we decided to release them instead into a field cage where they had a better chance of finding what they need,” said Hugh. So in April 2015, with the help of Randall Milne (Environment Southland), the first release of around 100 weevils was made in Southland.
We had proved that direct release of a small number of weevils was possible, but could we successfully bulk this up? We asked Hernan if he could collect a much larger shipment of infested fruit, which he did, and, to be on the safe side, hand carried them to New Zealand. This shipment produced 1260 healthy weevils. The plan now is to repeat the exercise again this spring to allow a similar number of releases to be made. “We will repeat this process until no further releases are needed,” said Hugh.
As well as these efforts to shut down the reproductive ability of the plant, we are also seeking an agent that could damage the plants themselves. Surveys revealed the best likely candidate to be a rust fungus (Puccinia berberidis-darwinii). We have imported rust-infected plants from Chile into our pathogen containment facility in Auckland for further study. No-one has worked with this rust before, so there is a bit to learn, including some experimentation to work out infection processes so host-range testing can get underway.
This project is funded by the National Biocontrol Collective