This is a final research report from Hort Innovation’s historical archives. Please note that as these reports may date back as far as the 1990s, the content and recommendations within them may be superseded by more recent research.

What was it all about?

South African citrus thrips (SACT), Scirtothrips aurantii, was a major pest of citrus in South Africa, banana in Yemen and grapes in Réunion; it also attacked mango, macadamia and tea. Control relied largely on pesticides, to which it rapidly developed resistance. First detected in Australia in Brisbane in March 2002 on mother of millions, weedy Madagascan succulents, SACT posed a serious threat to Queensland horticulture, including the $100M citrus crop.

If SACT behaved in Australia as it did in South African citrus, costs of production would increase, fruit pack-outs fall, and the longstanding IPM system, which relied heavily on a suite of biological control agents likely to be disrupted by increased insecticide usage, would be jeopardised. Climate matching indicted that the threat also extended to southern Australia.

SACT was reported from a very broad range of plants, however, in extensive surveillance in Brisbane it was found breeding only on mother of millions, and did not attack citrus, mango or native plants such as Acacia or Grevillea. This aroused speculation that SACT in Australia could be a host-restricted cryptic species, actually not S. aurantii but another species indistinguishable from it, that lives on Bryophyllum but did not attack citrus or other crops.

The researcher tested the capacity of Australian S. aurantii to reproduce on 16 crop, 7 ornamental, 8 native and 5 weed species. To develop strategies to manage it in our horticultural crops they tested its susceptibility to insecticides registered in citrus, determined the capacity of the native predatory mite Euseius victoriensis to kill and survive on this thrips, conducted surveillance in the Sunshine Coast area and communicated our findings to Industry.

The researcher found that Australian S. aurantii – 1) reproduced at very high levels on Bryophyllum, the traded succulent Kalanchoe blossfeldiana, macadamia and mango - 2) reproduced at moderate to good levels on Navelina orange, Tahitian lime, Eureka lemon, grapefruit, peach, grape, tea and the natives Acacia sophorae, A. longifolia and Syzygium moorei - 3) performed poorly or very poorly, but produced some offspring on many on the other plants tested.

The researcher also found that Australian S. aurantii adults were highly susceptible to very low rates of all insecticides tested, and that the predatory mite Euseius victoriensis killed up to six first and small second instar larvae per day, and survived as well on a diet of SACT as on Typha pollen, the food used to mass rear the predator.

These findings suggested S. aurantii in Australia could attack important crops, and that natives such as Acacia may have acted as bridging hosts, allowing it to survive in the absence of its apparently preferred hosts, mother of millions. They also suggest, however, that if this thrips ever attacked Australian crops it would be easily controlled, as it was highly susceptibility to very low rates of a range of insecticides. The predatory mite E. victoriensis killed up to 6 larvae per day and should have contributed to biological control if this thrips ever attacked citrus.

In the most recent surveys at the time, S. aurantii was found at numerous sites on the Darling Downs, the most northerly near Taroom, only 150 km west of Mundubbera in the Central Burnett, Queensland’s largest citrus producing area. These detections indicated a rapid expansion in range of the thrips, previously known only from suburban Brisbane and Laidley, but again it was found only on mother of millions, to which it was causing significant damage.

If S. aurantii continued to prefer the widespread and weedy mother of millions, for which it would provide a degree of biological control, and refrained from attacking the valuable crops that overseas experience and our research had shown it could utilise as hosts, this may have been one of those rare occasions when a potentially devastating exotic pest incursion turned out to be a good thing, and the Australian form of SACT could be renamed the ‘Bryophyllum thrips’.