Western Australian horticulture growers have been urged to take steps to reduce the risk of insecticide resistance developing in the widespread pest, Cluster caterpillar.
Western Australian horticulture growers have been urged to take steps to reduce the risk of insecticide resistance developing in the widespread pest, Cluster caterpillar.
Cluster caterpillar (Spodoptera litura) is an endemic armyworm species in WA, particularly in the State's north, which affects a wide range of host plants, including table grapes, cotton, capsicums, egg plant, cow peas, mung beans, maize and asparagus.
The Department of Primary Industries and Regional Development recently commissioned testing of caterpillar and moth samples for insecticide resistance in response to field reports some commonly used insecticides no longer provided adequate control.
Resistance develops when a pest is repeatedly exposed to the same insecticide, which can happen in the same crop or when multiple commodities are grown successively in an area.
Samples from Broome, Kununurra, Carnarvon and Perth were tested, which confirmed concerning levels of resistance to several chemical groups.
Extremely high resistance to Group 28 insecticides, containing the active chlorantraniliprole, were detected in cluster caterpillar populations from Broome and Carnarvon, while the Kununurra population displayed lower levels of resistance.
DPIRD research scientist Kiran Mahat said the research findings highlighted the need for growers to employ strong pesticide management practices to prevent resistance developing.
"Incorporating integrated pest management strategies will help mitigate the risk of resistance developing and support long term, sustainable horticulture production," Dr Mahat said.
"The results showed Group 28 insecticides were likely to be ineffective in the management of cluster caterpillars in Broome and Carnarvon.
"DPIRD recommends growers do not use Group 28 products in areas where high levels of resistance were detected and to rotate selective insecticides, instead of using broad-spectrum insecticides.
"Working out insect pressure thresholds to determine when it's economically necessary to treat crops will further support sustainable chemical use."
Resistance mutations were also detected to older broad-spectrum insecticides, such as Group 1A carbamates, Group 1B organophosphates and Group 2B phenyl pyrazoles, across all regions.
No resistance was detected to Group 22A oxadiazines, Group 30 meta-diamides, Group 18 diacylhydrazines, Group 4A neonicotinoids and Group 11, known as Bt.
"While these actives may provide good insecticide options for rotation, they should be applied judiciously as overuse can potentially accelerate resistance development," Dr Mahat said.
"It is important to remember to always use registered chemicals according to label directions."
Group 6 avermectins showed low resistance in the Kununurra and Carnarvon cluster caterpillar populations, while moderate resistance was already observed in Broome.
In Perth and Carnarvon, collected cluster caterpillars showed resistance to Group 3A pyrethroids - insecticides commonly used in residential gardens and vegetable growing areas.
Dr Mahat said the study provided an important reminder to home gardeners and growers with small holdings to be mindful when applying chemicals that are found to be resistant.
"In general, any chemical can cause resistance development so it is essential to pay attention to the active ingredients in insecticides and not to exceed the number of applications permitted," he said
"Collectively, the study results suggest broad spectrum insecticides should be avoided, where possible, as their long history of intensive use has increased resistance risk and they are highly disruptive to beneficial insects."
The laboratory testing for the study was undertaken in association with the New South Wales Department of Primary Industries and Regional Development, with support from the Cotton Research Development Corporation.
As part of the project, a novel molecular testing technique was developed to detect insecticide resistance mutations in cluster caterpillar.
This tool will help in rapid detection and monitoring of insecticide resistance in cluster caterpillar populations and support sustainable use of available insecticides.
For more detail on the study results and advice see the Insecticide resistance testing in cluster caterpillar (Spodoptera litura) from population across WA on the DPIRD website. </strong>
A DPIRD webinar on ' Managing Insecticide Resistance in Cluster Caterpillar' will be held on Wednesday, 11 February 2026.
