Lucerne (also known as alfalfa) interplanting improved citrus gall wasp (CGW) control in Australia’s New South Wales (NSW). The interplanting is practiced by some citrus growers in NSW to suppress weeds, reduce soil compaction and improve soil fertility.
Jianhua Mo of the NSW Department of Primary Industries addressed the matter; a summary of Mo’s comments follows.
One possible explanation is that lucerne enhanced the biological control of CGW. Lucerne seeds are attacked by a gall-forming wasp, the lucerne seed wasp (LSW), which is closely related to CGW. Adult LSW looks very similar to adult CGW and, like CGW, LSW is attacked by a suite of parasitic wasps, at least two of which are present in Australia. The assumption was that parasitic wasps of LSW also attacked CGW. Does such cross-species parasitism really occur?
To answer the question, CGW parasitism was investigated on a farm in central west NSW, where the benefit of lucerne interplanting for CGW control was reportedly observed. Despite being in a historically high CGW infestation region, this farm had not seen any citrus galls until the summer of 2019, 10 years after its establishment. The farm has Hamlin orange on Trifoliata rootstock and Pineapple orange on Citrange rootstock. Lucerne was planted in two out of every three rows throughout the farm.
A total of 844 galls were randomly collected from citrus foliage in a 14-hectare Pineapple block, a 5-hectare Pineapple block and a 13-hectare Hamlin block on the farm in late October 2020. A total of 28,612 CGW adults and 3,754 parasitic wasps emerged from these galls. Random dissection of the galls revealed 11,755 unemerged CGW adults and 466 unemerged parasitic wasps, bringing the total number of CGW adults to 33,617 and that of the parasitic wasps to 4,220. The overall parasitism level was about 11%. The parasitic wasps were exclusively Megastigmus brevivalvus, the primary parasitic wasp species of CGW in Australia. No known parasitic wasps of LSW were recovered.
In addition to parasitic wasps, CGW infestation was checked in the 14-hectare Pineapple block. Similar CGW infestation levels were found between citrus rows next to lucerne plantings and citrus rows that were not next to lucerne plantings. A sweep-net sampling was done in the same block when the seed pods were available; no known parasitic wasps of either the LSW or CGW were found. In addition, over 1,500 lucerne seed pods were collected and reared for wasp emergence. No known parasitic wasps of LSW emerged from the seed pods.
In summary, no parasitic wasps of LSW from CGW galls were reared, and reduced CGW infestation in citrus rows next to lucerne plantings were not seen. The possibility of parasitic wasps of LSW parasitising CGW has not been ruled out. However, the chances of finding such cross-species parasitism are low. Most parasitic wasps only attack their own hosts. In addition, parasitic wasps of LSW and CGW emerge at different times of the year, further reducing the chances of cross-species parasitism.
Source: New South Wales Department of Primary Industries
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