Jianhua Mo, a research entomologist with Australia’s New South Wales Department of Primary Industries (NSW DPI) authored an article about the release of parasitic wasps for the control of citrus gall wasps (CGW). Edited excerpts follow:
CGW is a pest of major concern in the southern citrus-growing regions of the Riverina, Sunraysia and Riverland. With the exception of 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 develop inside CGW larvae and eventually kill them.
In Queensland, up to 90% of CGW larvae can be parasitized. Parasitic wasps are also established in the southern regions. However, parasitism is less than 2% 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, NSW DPI has been releasing parasitic wasps annually in a citrus orchard in the Riverina since 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.
The galls were stored in containers while the adult wasps emerged from the galls. In that time, most adult gall wasps had died. Surviving adult wasps were predominantly parasitic wasps.
The parasitic wasps were taken to the Valencia block and released to build up predatory wasp numbers. To allow more parasitic wasps to emerge, the galls were transferred to mesh bags and hung inside the canopy of the trees. The total number of parasitic wasps released each year is estimated to vary from 500 to over 1,000.
In the first two years after the first release (2020 and 2021), the parasitism rate stayed below 1%.
In the fourth year (2023), the parasitism rate jumped to 19%. 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% of the un-emerged wasps were the parasitic wasps. 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, plans 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.
Source: Citrus Australia
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