200 years ago, there were no thistles present in New Zealand. Fast-forward to 2020 and they are just about everywhere.

The worst species is Californian thistle (Cirsium arvense), which is estimated to cost almost $700 million in productivity losses per year.  A beetle, recently introduced to New Zealand to help control Californian thistle, is showing great promise as a useful and cost-efficient tool in the fight against this noxious weed.

Californian thistle is reported to occur on 89% of all pastoral farms in New Zealand, with sheep farms being the worst affected (97% of farms). On sheep farms, the thistle covers around 1/8th of the total grazing area of most farms. This is where the problem lies for farmers, as their grazing livestock avoid eating pasture around the spiky thistles, leading to losses in productivity from their land.

Each year, over winter, the above-ground part of the thistle dies, but the roots survive. From these roots, new clonal thistle shoots emerge that grow to replace the patch from the previous year. The clonal spread of new shoots from its root system is the main method that this thistle reproduces and allows the weed to form large patches that continue to grow in size if they are not controlled. Windblown seeds contribute to spreading the thistle over longer distances.

Farmers battle the thistle by spraying herbicides and mowing them, but this only kills the above-ground part of the plant. The roots that survive can use the energy they store to grow new shoots, replacing the ones that were killed. Farmers must consistently repeat mowing or spraying the weed, which will cause the root systems to shrink over time as they use up their energy-producing new shoots.

The control of this weed can be very labour intensive and costly, and often difficult considering that many of our sheep farms reside on steep hill country. Having personally been involved in the control of this weed, hauling heavy knapsacks of herbicides up steep rocky hills, I can assure you it is not easy work!

This is where another method of control comes into play – biocontrol agents. Biocontrol agents are ‘natural enemies’ that eat or cause damage to a pest in its native range. They are introduced to a location where the pest has become invasive. If a biocontrol agent is able to establish and is successful, they can be extremely cost-effective control tools. Once introduced they can spread and multiply passively, working around the clock, assisting in the control of a pest without any human intervention.

In 2007, the most recent biocontrol agent to combat Californian thistle arrived on New Zealand’s shores. The biocontrol agent is the green thistle tortoise beetle, also known as Cassida from its scientific name Cassida rubiginosa. The beetle lives its full lifecycle on plants in the thistle family, with Californian thistle being its favourite host. The larvae and adults consume the leaves of the thistles, stripping them bare, leaving the thistles with less ability to photosynthesize. The beetle is now widespread throughout the country but is not yet common.

Around Lincoln, the beetles are relatively common due to populations that have entered the wild after being part of studies at the Lincoln Research Centre – the forefront of global research on the Cassida biocontrol beetle.

The opportunity to release Cassida as a biocontrol species is unique to New Zealand as we do not have a single species of native thistle – all thistles in New Zealand are weeds. At the time of its introduction, it was recognized that the beetle could potentially be able to reduce the populations of multiple thistle species in New Zealand.

Cassida is showing great results so far in its ability to help control its primary target – Californian thistle. In the first post-release study on the impacts of the biocontrol beetle led by Mike Cripps at AgResearch, the beetle was shown to reduce established patches of Californian thistle over a two-year period on a farm, under normal grazing management. The feeding damage caused by the beetles reduces the amount of root mass the plants can produce so that next season the root system isn’t able to put up as many shoots. Over time, the beetle can cause populations of the thistle to shrink – opening more pasture space for grazing, reducing the losses caused by this weed.

Another paper by the same research group published with the Bio-Protection Research Centre at Lincoln University investigated whether the beetle might be able to act as a biocontrol agent on other thistle species, secondary targets that are also weeds in New Zealand. Unlike Californian thistle, which reproduces mainly through its root system, all other thistles reproduce by seed. To be an effective biocontrol the beetle would need to reduce the seed output of these other thistle species.

The scientists tested the beetle’s impact on Marsh thistle; a species evolutionarily closely related to Californian thistle .  Unfortunately, it was found that the beetle had little impact on the number of viable seeds that the Marsh thistle plants were able to produce. It is unlikely that this biocontrol agent will be able to lead to population declines of this secondary target weed.

Future research on Cassida will investigate which other factors could be used in combination with biocontrol to create more pressure on populations of Californian thistle. For example, grazing routines that improve the competitive ability of desirable pasture plants, or other stresses applied to the thistle plants, may have an additive effect with the beetle that could lead to improved population control of Californian thistle.

Cassida may not be the multitargeting thistle destroyer that we had hoped for but the great news remains it has a significant impact on reducing established patches of Californian thistle – the primary thistle the beetle was introduced to control.  Cassida has the potential to reduce populations of this thistle around New Zealand, which in the long term will result in reduced costs and reliance on herbicides for the control of this weed.

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Jonty Mills is a postgraduate student completing a Master of Pest Management. He wrote this article as part of his assessment for ECOL 608 Research Methods in Ecology.