A timing guide for the management of red scale

Close up of red scale

Figure 1: Red scale.

A close up of the wasp.

Figure 2: An adult Aphytis wasp.

The California red scale (CRS), Aonidiella aurantii, is pest of Australian citrus. It feeds on fruit, leaves, and young twigs. Infested fruit can be covered with the scale causing commercial rejection and/or downgrade.

More importantly, it is a declared quarantine pest for citrus export to Korea limiting access to that market. A number of chemical and biological options are available for CRS control in Australia including the soft chemical option of petroleum spray oil (PSO) and releases of Aphytis wasps, a natural enemy of CRS. Timing is critical for CRS control.

In Australia, two annual sprays are currently recommended for PSO, first during November–December and second during February–March. Aphytis releases are recommended for October–March.

The recommended windows have not taken into consideration the likely variations of CRS phenology between locations, years, and citrus varieties, and are too wide to target the vulnerable stages of CRS. PSO and other foliar chemical sprays are most effective against the crawler stage and the following whitecap stage when the insect's wax cover is not fully developed.

Aphytis wasps parasitise late stage scales with a preference for 3rd instar females. Consequently, it is critical to time controls when the vulnerable stages are most abundant.

To find the optimal application windows, we need to know the relative abundance of individual stages of CRS over the season. Stage-specific abundance of CRS at a particular time can be estimated by monitoring, or predicted with degree-day models.

Monitoring by visual inspection is straightforward but labour intensive and requires skills. Pheromone traps can substantially reduce monitoring costs and are routinely used to monitor CRS populations in many countries. In California, peak 3rd instar females occur around the same time as the peak of the first male flight.

First generation crawlers appeared about 550 degree days (DD) after the first male flight. A similar timing was observed in Spain. Our preliminary pheromone trapping data from a citrus orchard in the Riverina in south-western NSW suggests the presence of three annual peaks of male flights with the spring peak being the most prominent.

However, there is, as yet, no published study from Australia relating male flights to a stage-specific abundance of CRS.

The project seeks to improve the control of CRS in the southern NSW citrus regions by developing DD-based phenology models to guide PSO applications and Aphytis releases. Testing started in October 2015 and will continue into the new year.

Early results will be reported in the first half of 2016 and an easy-to-use online DD calculator will be published by December 2018 to enable growers and pest scouts to predict the optimal timing for CRS control.

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