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Pompom weed: potential distribution of the plant and its potential biological control agents examined

Pompom weed (Campuloclinium macrocephalum) is an herbaceous plant of South American origin that is currently invading the grasslands and savanna regions of South Africa. It spreads by means of wind-dispersed seeds. It has a fleshy perennial root system that allows it to survive winter frost and fires. It is thought to have a negative impact on biodiversity and can invade disturbed and pristine vegetation. Considerable effort has been invested in preventing it from spreading. Herbicides are used to control the species but they are expensive and labour intensive. As with many invasive plant species, biological control is expected to be a more sustainable option. Two prospective biological control agents have been identified and are currently undergoing host specificity testing in South Africa. These include a flower feeding lepidopteran, Cochylis campuloclinium (Tortricidae) and a stem-galling thrips, Liothrips tractabilis (Thripidae).

Although pompom weed has invaded several provinces already, it is thought to be in an early stage of invasion. It is not known which areas are at risk of invasion from this species. Ecological niche models are useful for predicting the potential distributions of species and have been useful for identifying regions that are climatically suitable for invasive species. These models can also be used to identify regions that are considered to be climatically most suitable for the biological control agents and where control is likely to be most successful.

The aim of the study was to predict the potential distribution of pompom weed and its two biological control agents in southern Africa and to identify areas that are likely to be at risk of invasion by pompom weed in future. The study was conducted by Mark Robertson (C·I·B associate and senior lecturer in the Department of Zoology and Entomology, University of Pretoria), Paul Trethowan (a BSc Hons student at the University of Pretoria) and Andrew McConnachie (Agricultural Research Council — Plant Protection Research Institute).

The models revealed that regions of highest suitability for pompom weed include the Grassland and Savannah Biomes in the Gauteng, Mpumalanga, Limpopo, Free State, KwaZulu-Natal and North West Provinces of South Africa and also parts of Lesotho, Swaziland, Mozambique and Zimbabwe. A simple model indicated that there are many areas that are at risk of invasion that have not been invaded. Very poor overlap was found between the potential distributions of the biological control agents and the potential distribution of pompom weed. This is probably attributable to the quality of the datasets available for calibrating the models for the biological control agents rather than a climatic mismatch. The sample sizes of these datasets are small and it is likely that these occurrence records underestimate the native ranges of these species. This work suggests that pompom weed can spread over a much larger part of southern Africa than it currently occupies.

Grassland invaded by pompom weed (Campuloclinium macrocephalum)

Grassland invaded by pompom weed (Campuloclinium macrocephalum)

Read the paper: Trethowan, P.D., Robertson, M.P., McConnachie, A.J. 2011. Ecological niche modelling of an invasive alien plant and its potential biological control agents. South African Journal of Botany. 77: 137-146.

For further details, e-mail Mark Robertson.