The red sunflower,Tithonia rotundifolia (Mill.) S.F. Blake, and Mexican sunflower, Tithonia diversifolia (Hemsl.) A. Gray (Asteraceae), both natives of Mexico, have become invasive throughout the humid and sub-humid tropics of South America, South East Asia and tropical and subtropical Africa, including South Africa. In South Africa, the two Tithonia species were introduced in the early 1900s as ornamental plants and have now been declared alien invasive weeds classified as category 1 weeds.
Red sunflower is a large, robust, annual plant that grows up to 3 m. Its leaves are opposite, each with three broad, round-toothed lobes, and it produces reddish-orange flowers that are borne singly on long stems. Mexican sunflower is a large, evergreen, perennial shrub that grows up to 5 m tall, and has deeply five-lobed, opposite leaves and orange-yellow flowers on erect, hollow, woody stems. In South Africa, red sunflower flowers from late summer throughout autumn while Mexican sunflower flowers throughout autumn and winter. Red sunflower produces fewer, larger and heavier seeds, ensuring vigorous seedling growth and longer survival in a nutrient-poor environment. In contrast, Mexican sunflower produces numerous, smaller and lighter seeds which enable wide dispersal and rapid spread in colonized areas. However, segments of Mexican sunflower that are buried horizontally in the soil also often sprout. Both Tithonia species are aggressive colonizers of disturbed, sun-exposed ecosystems with a high water table, including plantations, abandoned sites, and along railways and roads. Tithonia rotundifolia is particularly invasive in the inland provinces of Gauteng, Limpopo, Mpumalanga and the North West while Tithonia diversifolia is invasive in the Lowveld regions of Mpumalanga, Limpopo and along the coastal regions of KwaZulu-Natal province. The escalating invasion by the two weedy sunflowers is increasingly threatening the biodiversity and ecological integrity of natural systems as well as agricultural and forestry systems in South Africa.
Currently, there are no herbicides registered to control either of the two Tithonia species. Mechanical control is often inefficient due to rapid re-invasion from the seed bank and the ability of T. diversifolia to grow from cut stems.
Biological control programmes against the two Tithonia species were initiated by ARC-PPRI in 2007 through funding from the Natural Resource Management Program (NRMP) of the Department of Environmental Affairs (DEA), formerly Working for Water (WfW) programme.
Since 2007, twoleaf feeding beetles, Zygrogramma signatipennis Stål and Z. piceicollis Stål (Coleoptera: Chrysomelidae), were screened and found to be safe for release against T. rotundifolia in South Africa. The two Zygogramma beetles have very similar life histories and both larvae and adults are highly damaging to the leaves, often skeletonising them completely, and leaving only the leaf veins. Both Zygogramma species were released during 2015 in Gauteng, Mpumalanga and KZN provinces. To increase the chances of establishment, more effort is being invested in mass-rearing and releases of the two beetles.
For T. diversifolia, a highly damaging leaf-feeding beetle, Physonota maculiventris (Coleoptera: Chrysomelidae: Cassidinae), was screened and found to be adequately safe for release against this weed in South Africa. An application for permission to release this beetle was submitted to the authorities in May 2015, and the approval is still pending.