Although it has been several years since a major outbreak of silverleaf whitefly (SLW), the ongoing risk of honeydew contamination means they remain a pest of concern for growers in almost all cotton production valleys. Control of SLW still relies heavily on the use of insecticides, although biological control, including conserving natural populations and augmentation via parasitoid wasp releases, has been adopted by some growers.

Silverleaf whitefly resistance to pyriproxyfen emerged as a significant issue during the 2016/17 season. The industry responded by introducing a 30-day application ‘spray’ window to restrict pyriproxyfen use, aiming to reduce resistance selection pressure. From 2018 to 2020 this approach showed promise with a steady decline in the number of populations carrying resistance. However, in 2021 and 2022 close to half of the tested populations contained some resistant individuals. While the severity of resistance is lower than that observed in 2016/17, it is still a concerning reversal of the earlier trajectory.

Within the cotton industry, detection of SLW resistance to spirotetramat was first documented in Emerald in 2019. During the last three seasons, spirotetramat resistance was detected in populations collected from cotton in Emerald, Theodore, Darling Downs, Mungindi, and Macintyre and Namoi valleys. DNA sequencing of populations collected between 2019 and 2021 found the frequency of the mutation in resistant populations was low (1.2-4.1%). While current resistance levels are unlikely to cause management issues, SLW resistance to spirotetramat has the potential to increase (as already observed in other industries). A proactive change to the Insecticide resistance management strategy (IRMS) was made in 2019, reducing spirotetramat use to a single spray per field for SLW control.

A change in registration that occurred in 2020 now means buprofezin can be used in cotton for SLW control. Testing by bioassay has found no evidence of SLW resistance to buprofezin. Testing of laboratory strains with known resistance to pyriproxyfen and spirotetramat showed no signs of cross resistance to buprofezin. At the time of its inclusion in the IRMS, buprofezin was restricted to a single application when targeting SLW.

Widespread SLW resistance to imidacloprid and pymetrozine was found during testing in 2020. These results from 2020 and testing of laboratory neonicotinoid-resistant strain against a range of neonicotinoids show there is low-level cross resistance between imidacloprid and acetamiprid but no cross resistance to dinotefuran. Testing from 2020 to 2022 showed no definitive evidence of resistance to either acetamiprid or dinotefuran.

Resistance to the pyrethroid bifenthrin has been detected in many of the tested SLW populations over the past three years. This data, along with DNA sequencing data was recently published (Pest management Science, 78 – issue 8, August 2022,). Resistance to bifenthrin is widespread in cotton production regions, but within a population the mutation’s frequency is generally low (1-7%). In the same study, we documented that Bemisia tabaci Middle East-Asia Minor 1 (formerly B-biotype) is the dominant species found in cotton grown in NSW and QLD. The potentially invasive B. tabaci Mediterranean (formerly Q-biotype) was not detected in any of the samples.

Extension of project findings was circulated primarily via annual updates in the Australian cottongrower, via CottonInfo factsheets and by CottonInfo’s regional extension officers (REOs). Extension of SLW parasitism was achieved by engaging REOs in an in-field assessment of parasitism levels. The REOs gained firsthand experience and increased recognition of the value of biological control from parasitism during the exercise, which has been shared at a local level with agronomists.