Aspects influencing the efficacy of Liothrips tractabilis Mound and Pereyra (Thysanoptera: Phlaeothripidae) : a biological control agent for the invasive weed Campuloclinium macrocephalum (Less.) DC. (Asteraceae) in South Africa.
Pompom weed, Campuloclinium macrocephalum (Less.) DC. (Asteraceae), an unpalatable, perennial, erect invasive herb from South America has become naturalized in South Africa, invading grasslands, savannas and wetlands, where it has a significant impact on biodiversity. In order to sustainably curb the spread and negative impact of the weed, Liothrips tractabilis Mound and Pereyra (Thysanoptera: Phlaeothripidae) was imported from South America (Argentina) as a candidate biological control agent. Quarantine tests demonstrated that the thrips was suitably host specific and damaging to the target weed and permission for its release in South Africa was granted in 2013. However, numerous biocontrol agents worldwide have displayed exceptional potential while in quarantine but have had little to no success following their release in the field. This study incorporated both laboratory and field trials to determine the likelihood of success with the thrips. Liothrips tractabilis developmental threshold trials were conducted at seven constant temperatures (15, 17.5, 20, 25, 27.5, 30, 32°C) and the data, excluding the uppermost and lowermost temperatures (as the trips did not survive at these temperatures), were ultimately used to develop a degree-day model. The findings of the model were then validated under outdoor conditions. Furthermore, the impact of the thrips was assessed on seedlings and root crown regrowth shoots under outdoor conditions, and the results were compared to those of the laboratory impact trials that were conducted while the agent was still under investigation in quarantine. The thrips completed development at all five temperatures, with the number of days taken to develop from egg to adult decreasing with increasing temperature. Lethal temperatures were recorded at 15°C and 32.5°C where no development beyond the egg stage was observed. The lower developmental threshold (t) was estimated at 9.6°C with 546.9 degree-days (°D) required by the thrips to complete its development. The degree-day model predicted that in Gauteng, parts of Limpopo, North West and Mpumalanga provinces, where C. macrocephalum is invasive, the thrips is likely to complete 3-9 generations per year. The outdoor developmental trials did validate the model and although temperatures recorded in the laboratory and field trials were not equal, the field data largely supported the predictions of the laboratory trials. Furthermore, the thrips developed significantly faster at the Pietermaritzburg site in comparison to Cedara, which was largely a consequence of low altitude and higher ambient temperatures. A significant difference was also obtained across the three seasons, where the thrips developed fastest during summer, and slowest during winter at Pietermaritzburg. The same was true at Cedara, although no development occurred during the winter trials. The impact trials showed that the thrips significantly reduced the height, number of leaves and both wet and dry masses of C. macrocephalum seedlings, which was largely in agreement with the original laboratory study. However, this was not the case with the regrowth trials, where only relative growth rates in terms of wet tuber mass were significantly reduced by thrips feeding. These results were largely a consequence of varying tuber wet masses used at the start of the trials. Liothrips tractabilis appears to be climatically compatible with conditions in South Africa, since this study has shown that the establishment and persistence of L. tractabilis is unlikely to be limited by climatic conditions in areas that are currently invaded by the target weed. Furthermore, the agent should be able to inflict appreciable damage and hence have an impact on C. macrocephalum populations in the field. Thus, prospects for the biological control of C. macrocephalum in South Africa appear promising.