First Major Green Thistle Beetle Outbreak
Thistles defoliated by the green thistle beetle near Masterton (L). Close up of a defoliated thistle (R). Image - Harvey Phillips
Some of the people who released the green thistle beetle (Cassida rubiginosa) a few years ago to control Californian thistle (Cirsium arvense) on their properties, have been surprised by the levels of damage to this ‘prickly customer’ this summer.
The green thistle beetle was brought to New Zealand in 2006 by Landcare Research, on behalf of the Californian Thistle Action Group (CTAG), to try to combat a range of thistles, but in particular Californian thistles. Californian thistle is a perennial weed that has been here for over 100 years, during which time it has become widespread throughout New Zealand. This weed is notoriously difficult to control because of its persistent root system. Using herbicide on thistles is problematic because of impacts on other desirable pasture plants and because of the sheer extent of infestations in both lowland and high-country areas.
Many of the earliest releases of green thistle beetles were made in the Southland/Otago region where CTAG is based, but they have now been released widely throughout New Zealand. As with most insects, it is the larval stage that does most of the damage to plants. The younger larvae begin feeding on the lower sides of the leaves and then when they are a bit bigger they graduate to feeding on the top side of the leaf. “The larvae have a trick to deter predators,” said Simon Fowler. “They make a protective canopy out of discarded skin and excrement to carry on their back which seems to deter predators such as birds.” The adult beetles live for about 80 weeks if conditions are good, which is quite a long life in beetle terms. During this time the females will lay up to 1000 eggs, making them excellent candidates for biocontrol.
Early reports suggested 90% establishment at the original release sites, with sites with shelterbelts or plantations nearby (which act as an overwintering refuge for the beetles) seeming to be favoured.
Previously, painstaking research by Graeme Bourdot’s team at AgResearch has revealed that Californian thistle roots don’t persist for more than one year. Instead, new roots grow each year, surviving the winter and then sending up new shoots the following season to form the above ground foliage. The roots that have overwintered then die, and new roots are formed to live through the next winter. The AgResearch team found that the amount of above ground foliage is directly proportional to the size of the new root material produced each season. Thus if the amount of green foliage is reduced in one season, the root formation will also be reduced and there will be proportionally fewer resources for the plant the next spring. “This information gives us a clue to use biocontrol agents that are foliage feeders rather root feeders,” says Simon Fowler. “Previous attempts to find root feeders have failed and we now realise that this is because the roots completely replace themselves each year, preventing an insect from completing its lifecycle on them,” he added. “So we are now concentrating on agents that limit the amount of foliage on the plants, so that there are less vigorous roots and vegetation produced the following year,” Simon said. Mowing can be used successfully to the same end, but is not feasible on all terrain.
Some of the earliest release sites are now showing the great potential of the green thistle beetle. Harvey Phillips from the Greater Wellington Regional Council reported a huge outbreak this summer at a release site near Masterton, where 65 beetles were released in 2008. “There was a visible feeding front, behind which you could easily see where the beetle larvae had demolished the thistle plants,” Harvey said. Harvey estimates some stems had as many as 300 beetles on them and he has collected and distributed around 30,000 beetles already from this site.
Similar reports come from Neil Gallagher from Horizons Regional Council, who has been involved with the project since 2008, when he released 120 adult beetles on a farm in the Pohangina Valley. Since then the beetles have dispersed right across the 278-acre farm and are so plentiful that some have already been harvested and redistributed to other farms in the district. “An article in our local newspaper reporting the success of the project resulted in a flood of enquires from other landowners wanting to know where to get some of these little green guys,” said Neil. “As a result, a further 16 releases were made this summer in Manawatu, Tararua, Palmerston North, Wanganui hill country, Taihape, and Taumarunui,” Neil explained.
Meanwhile, AgResearch scientist Mike Cripps is leading a Sustainable Farming Fund project to monitor the spread of the green thistle beetle and assess their impact on thistle populations. Mike is very encouraged by the heavy defoliation of thistles in some regions, but said that the next step is to get some quantitative data to demonstrate their true impact. “The key will be to determine how much the above ground defoliation by the green thistle beetle limits the growth of thistles in the following season,” Mike explained. The project will also enable Mike to promote the use of the beetles to control thistles at field days held at release sites. Mike said that the long-term benefits of having self-sustaining green thistle beetle populations included increased farm productivity, a reduction in the reliance on costly herbicides, and improved engagement with landowners on the benefits of using biocontrol technology to manage weeds at a landscape scale.
If you have a green thistle beetle site that is also doing well we would like to hear from you.