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Building up natural enemy populations for controlling Citrus Gall Wasp

Citrus Gall Wasp (CGW) is a pest of major concern in the southern citrus growing regions of the Riverina, Sunraysia, and Riverland. Except for the adult stage, CGW lives inside woody galls and is thus shielded from predators during development. Parasitic wasps are the only known natural enemies of CGW. They are similar-sized to CGW but are honey-coloured, in contrast to the black-coloured CGW. Parasitic wasps develop inside CGW larvae and eventually kill them. In Queensland, up to 90 per cent of CGW larvae can be parasitised.

Parasitic wasps have also established in the southern regions, however, parasitism is less than 2 per cent in most orchards, which is insufficient to reduce CGW infestations. Frequent use of insecticides has likely contributed to the suppression of parasitic wasp populations.

In an effort to establish a local repository of parasitic wasps, New South Wales Department of Primary Industries (NSW DPI) has been releasing parasitic wasps annually in a citrus orchard in the Riverina. This work started in November 2019.

A single row in a block of Valencia oranges with a moderately high level of CGW infestation was used for building up the parasitic wasp population. This row was left untreated from insecticides.

Each year from 2019 to 2023, CGW galls were sourced from citrus orchards known to have high parasitism levels. These galls were taken from farms in the Riverina, Queensland and South Australia just before the adult wasps had emerged.

In the first two years after the first release (2020 and 2021), the parasitism rate stayed below 1 per cent. In the fourth year (2023), the parasitism rate suddenly jumped to 19 per cent. A more intense release of parasitic wasps in November 2022 is likely to have contributed to this rapid increase in the parasitism level.

Dissection of one of the galls in mid-November 2023 showed that an estimated 50 per cent of the un-emerged wasps were the parasitic wasps (Figure 3). This is encouraging. It highlights the potential to build-up local populations of the parasitic wasps and biological control of the CGW in the future.

NSW DPI, in partnership with citrus grower Ray Durkin, plan to continue these parasitic wasp releases in future seasons. The release area will be expanded by leaving adjacent rows to the current release row untreated from insecticides. Parasitic wasps will be released in these rows. This will further build up the parasitic wasp population within the orchard.


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