A beetle to beat the thistle

RESEARCH WRAP HERBICIDE ALTERNATIVES

GREEN THISTLE BEETLE: LIFE CYCLE

Words by: Elaine Fisher

AgResearch is combining empirical and modelling work to understand and optimise the regulatory effects of nonchemical control methods on weed populations, including manual removal and natural enemies, under current and future climates.

AgResearch’s Mike Cripps and Graeme Bourdôt are key researchers focusing on biocontrol of naturalised species of thistles (tribe Cardueae). This tribe contributes more species to New Zealand exotic pasture weed flora than any other taxonomic group. It contains some of the most economically damaging weeds and many ‘sleepers’.

The tribe is uniquely amenable to biocontrol using agents imported from the plants’ native ranges overseas because there are no native NZ thistles and few economically valued species in the tribe, the researchers say. An example from this element of the research programme is the green thistle beetle, imported and released in NZ in 2007 for control of Californian thistle. The impact of the beetle on thistle shoot population density and spread was assessed over two years on a North Canterbury sheep and beef farm. Where there were high beetle densities of 10 or 20 beetle larvae per thistle shoot, the population of Californian thistle declined. At these high beetle densities, the spread of the weed in the paddock was also reduced. In comparison, where there were no beetle larvae, or only low densities of five larvae per shoot, the thistle population increased in density. The green thistle beetle can also attack other thistle weeds, however, its impact on other thistles that reproduce via seeds appears limited.

• (Source: Cripps MG, Jackman SD, van Koten C 2019. Folivory impact of the biocontrol beetle, Cassida rubiginosa, on population growth of Cirsium arvense. BioControl 64: 91-101).

That yellow pest

Words by: Elaine Fisher

An app to assist farmers to control the pasture weed species giant buttercup has been developed by AgResearch and is available free to use from the Dairy NZ

website.

Dr Graeme Bourdȏt, AgResearch principal scientist, Weeds, Pests and Biosecurity Team, says the app enables the user to determine if the giant buttercup infestation in a paddock is economically worthwhile controlling (do the benefits outweigh the costs?).

“It accounts for the loss of clovers from herbicide damage, enables herbicide options to be compared and herbicide resistance to be managed.”

The tool requires estimates for: • % of the pasture covered by the buttercup prior to the intended control operation • pasture drymatter eaten or grown currently in the paddock • conversion rate for pasture drymatter to milksolids • pasture utilisation rate • clover content • milksolids payout • herbicide cost • N-fertiliser cost and % N content • herd lactation length • stocking rate

Default values which can be edited are provided as examples for some of these input variables. The calculations used in the app utilise a unique data set comparing the efficacies of all herbicides available in New Zealand with a label claim for giant buttercup obtained from a three-year experiment on nine dairy farms in the Golden Bay district. The model used in the App is described in the peer-reviewed journal paper: https://www.dairynz.co.nz/feed/pasture/pestsand-weeds/giant-buttercup/.

• (Source: Bourdôt GW, Lamoureaux SL, Jackman

S, Noble A, Chapman DF 2021. Net economic benefit of Ranunculus acris control in dairy pasture - accounting for herbicide damage to clovers and evolved resistance. New Zealand Journal of Agricultural

Research https://doi.org/10.1080/00288233.2021.19 81954)