Thrips released to destroy Gauteng’s pink weed
Biocontrol scientists have released a tiny insect to wage war on the dreaded invasive Argentinian pompom weed and save our Highveld grasslands. A terrestrial version of the dreaded water hyacinth weed, pompom weed (Campuloclinium macrocephalum) is a Category 1 invasive alien species that spreads rampantly through productive and biodiversity-rich grasslands. Outcompeting local grasses, it turns valuable agricultural land into pink-flowering wastelands.
Pompom is a huge threat to farmers as it devastates grazing capacity by taking over areas populated with up to 73 species of farming-rich grassland grazing. Environmentalists fear the weed as it destroys grassland biodiversity.
Until recently, pompom weed has had only a minor rust as a natural enemy. Recently, biocontrol scientists celebrated the release of a tiny insect known as the pompom thrips (Liothrips tractabilis) by releasing them into fields of pink-flowering pompom weed at Rietvlei Nature Reserve, south east of Pretoria.
Recruited from Argentina, where pompom weed is indigenous, the newly-released pompom Liothrips causes significant damage to the stems and leaf tissue at the growing tips. This causes deformities in plant growth, reducing the height, biomass and flower production of this unwanted weed.
Measuring only a few millimetres in length, much is riding on the success of the tiny new biocontrol agent. Scientists also warn that it will take several seasons for entomologists to build up large enough populations of Liothrips to halt the spread of pompom weed.
“Scientists have been working with Liothrips since 2005, when we collected the parent stock in Argentina”, says Dr Andrew McConnachie, a biocontrol scientist from the Agricultural Research Council – Plant Protection Research Institute (ARC-PPRI) based at Cedara, KZN.
Funded by Environmental Programmes, (which includes the Working for Water programme), biocontrol scientists place potential biocontrol agents such as the Liothrips into quarantine as soon as they arrive in the country.
Potential biocontrol insects undergo rigorous testing in the quarantine laboratories to ensure they will only feed on the host plant and are not threaten indigenous plants or agricultural crops. The larvae feed on pompom flowers. South Africa celebrates 100 years of biocontrol this year and is regarded as a world leader in biocontrol research.
Liothrips was cleared for release by the Department of Agriculture, Forestry and Fisheries (DAFF) on 18 June 2013. Shortly thereafter, scientists from the ARC-PPRI began releasing the insect in selected sites around Gauteng, Mpumalanga, Limpopo and North West Provinces.
What does this new secret weapon against pom pom weed look like? The Liothrips tractabilis is black when mature and orange in the nymphal stages. The adults are strong and ready fliers, so can disperse to surrounding stands of pompom.
The nymphs have been shown to overwinter underground on rootstock when the plants die back and re-emerge at the onset of summer when regrowth and seedlings appear. Egg to adult duration is approximately 28 days.
Hay bale immigrant
Pompom weed was first recorded in the early 1960s when it began invading disturbed areas along roadsides and unused fields. It is thought to have come to South Africa from Argentina in bales of imported hay.
Under invasive species (NEM: BA) legislation, pompom weed is listed as a declared Category 1b invader in Gauteng, North West, Limpopo and Mpumalanga and Category 1a in the rest of South Africa, meaning that every effort must be made to remove and destroy this weed.
The distribution of pompom has almost doubled in the past five years. It threatens grasslands, open savanna and wetlands by decreasing the carrying capacity of the land. The plant is unpalatable to wildlife and agricultural livestock.
“People’s perceptions of this weed need to be changed”, says McConnachie. “It might be beautiful, but it is a huge danger to our economy and society. The sea of pink flowers threatens prime agricultural grazing grasslands and causes skin irritations to animals”.
It also threatens indigenous herbs and grasses. It is almost impossible to control mechanically due to its robust root system and copious seed production. Herbicide application is expensive and labour intensive and therefore restricted to roadsides and smaller, manageable areas, so biocontrol is deemed to best long-term solution to control pompom weed.
A second promising biocontrol agent for pompom weed is the flower-feeding moth (Cochylis campuloclinium) which is still undergoing testing by scientists in quarantine facilities around the country.
Friends of Colbyn Valley, in association with the ARC, WRC, South African Wetland Society and WETREST, invites you to take part in a 'POMPOM HACK' on Saturday, 15 February in celebration of World Wetlands Day 2014.
See details of the programme here .