February - March 2025 Vol 46 No 1
Increasing insecticide resistance in northern populations of cluster caterpillar
The cluster caterpillar Spodoptera litura is very abundant throughout the year in Australia’s tropical north, and cotton is a host. While it is most common in tropical regions, it is occasionally found in sub-tropical regions, such as Central Queensland. Although S. litura primarily feeds on the leaves of cotton plants, larvae can also attack fruiting structures.
While Bollgard 3 provides incidental control of S. litura, the technology is not registered for this pest and provides less robust control than it does for the primary target pest Helicoverpa spp. This is because, like other Spodopteran species, S. litura has a naturally higher tolerance to Cry1Ac produced by Bollgard 3, with Vip3A being primarily responsible for the plant’s lethal toxicity, with a lesser contribution from Cry2Ab.
S. litura are largely kept in check in cotton crops by pesticides that target Helicoverpa spp but, we’re seeing a surge in resistance in nearby horticultural crops. Growers should consider S. litura control as part of a strong IPM strategy to protect their crops.
Recent research by PhD student Sharna Holman (Qld DPI/CottonInfo) showed that when Bollgard 3 crops were exposed to prolonged periods of shade, S. litura larval survival increased. Importantly, research has also shown that control efficacy is rapidly restored with a return of sunny conditions.
A potential reduction in Bt expression under certain environmental conditions such as cloudy weather, and movement of larvae from nearby non-cultivated host plants may explain the occasional build-up of S. litura in Bollgard 3 crops, which may require treatment with applications of synthetic insecticide.
In the past, larvae of S. litura have been incidentally controlled by selective insecticides used to target Helicoverpa and fall armyworm including.
However, recent findings from the NSW DPIRD insecticide resistance surveillance program supported by CRDC, show that levels of sensitivity to two of these chemical classes has declined in S. litura in northern regions of Queensland and Western Australia.
Testing has shown that field populations of S. litura from northwest WA previously exposed to extremely high selection pressures from Group 28 insecticides has resulted in over 1000-fold resistance to chlorantraniliprole and these populations are now unable to be controlled by this insecticide. Research has also shown that resistance to chlorantraniliprole is controlled by a recessive gene.
This means that although resistant field populations are largely homozygous (with two copies of the resistance gene), it may be possible for resistance to be diluted by mixing with susceptible insects from unsprayed refuges, effectively increasing the proportion of susceptible heterozygotes (one copy of resistance gene) in the population.
Testing has also shown reduced sensitivity to the Group 6 insecticide emamectin benzoate in some northern regions and preliminary findings suggest this this resistance gene is strongly dominant.
Regions currently impacted by escalating resistance to Group 6 and 28 are horticultural areas in northern Queensland and north-western WA. The most recent testing results from cotton growing areas in the Kununurra region show that these populations remain largely susceptible to both mode-of-action groups.
It will be important to preserve natural enemies in northern farming systems by avoiding the use of broad-spectrum insecticides. Testing indicates there are reduced levels of sensitivity to Group 1A (carbamates) and Group 3A (pyrethroids) in Australian S. litura.
Although biopesticide options such as registered formulations of NPV for Helicoverpa will have no impact on S. litura, growers should be aware that naturally occurring entomopathogens of S. litura have been observed in some fields at levels which are likely provide additional population suppression which may negate the need for additional insecticide applications.
Top tips for management of Spodoptera litura
- Know if this pest is in your area by conducting regular in-crop monitoring for larvae and signs of damage. If in doubt about which species of larvae is in your crop, contact your local entomologist.
- Optimise control costs by timely application of insecticides on above threshold populations. If sprays are warranted, act fast with full rates of insecticide to target small larvae.
- Approach control with an IPM focus and take advantage of natural enemies present in crops. Make spray decisions that will conserve beneficial insects which help suppress S. litura populations in your crop. Presence of naturally occurring entomopathogens may negate the need for additional insecticides.
- Know which insecticides are compatible with IPM. Selective insecticides such emamectin benzoate (Group 6 e.g., Affirm, Proclaim), indoxacarb (Group 22A e.g., Steward) and chlorantraniliprole (Group 28 e.g., Altacor, Coragen) will help to preserve populations of beneficial insects in your crop.
- Know which insecticides are likely to be less effective on S. litura. Preliminary testing has shown that S. litura may have developed resistance to carbamates (e.g., methomyl) and synthetic pyrethroids (e.g., alpha-cypermethrin) in some regions. Growers are urged to avoid these products because they may not control of S. litura, and they will destroy beneficial insects in your crop.
- Practice good resistance management by using chemical rotation by using a planned approach to insecticide selection and chemical rotation to minimise resistance developing further and spreading to other regions. Do not respray a field failure with products from the same (mode of action) MoA group.
- Minimise selection pressure on any one MoA group. There is higher than expected resistance to Group 28 insecticides in some northern S. litura populations. To reduce the risk of resistance increasing further growers are urged not overuse Group 28 products.
- Be mindful of the effect your insecticide sprays could be having on other pests in the crop. Overuse of selective insecticides will increase resistance levels in other important pests such as fall armyworm (Spodoptera frugiperda) and corn earworm (Helicoverpa armigera).
- Comply with label instructions. Use recommended thresholds and use only the recommended number of sprays. Avoid prophylactic sprays and remember, THE LABEL IS THE LAW.
- From CottonInfo e-news with content developed by Dr Lisa Bird, Senior Research Scientist, NSW DPI, with support from CRDC Innovation Brokers Dr Nicola Cottee and Susan Maas.
For more information please contact Dr Bird directly via lisa.bird@dpi.nsw.gov.au
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