A decade of research on biocontrol of the weedy Mexican sunflower Tithonia diversifolia (Asteraceae: Heliantheae) has finally paid dividends, following the release of the first biocontrol agent against this invader. The tortoise beetle, Physonota maculiventris (Coleoptera: Chrysomelidae: Cassidinae) originally from Mexico, was finally approved for release in July this year, and the researchers are currently hard at work mass-rearing the beetle for release and distribution throughout the country.

An invasive weed species

Mexican sunflower is an aggressive invasive weed that is naturalised in South East Asia, South America and tropical Africa, including South Africa. Initially introduced in South Africa during the 1930s as an ornamental, Mexican sunflower has become invasive in the tropical and subtropical provinces including Limpopo, Mpumalanga and KwaZulu-Natal.

It is rapidly invading farms, forest margins, disturbed lands, rail and roadsides and, as a result, it is out-competing native flora. Its ability to reproduce through both seeds and vegetatively, makes it a very aggressive weed. Thus cut branches discarded during mechanical control may regenerate and produce new plants, while wind-dispersed seeds enable it to colonise remote, new sites. Hence, Mexican sunflower is classified as a category 1b and 1 weed by the National Environmental Management and Biodiversity Act (NEMBA) and the Conservation of Agricultural Resources Act (CARA) of South Africa, respectively.

Biological control programme

In an attempt to curb the invasiveness of this weed, a biological control programme was initiated in 2007 which resulted in development of a biological control agent, the leaf defoliating tortoise beetle P. maculiventris. The female beetle P. maculiventris lays batches of about 30 eggs each on the lower leaf surface. Each female may produce between five- and six-egg batches during her lifetime.

As the eggs hatch, early instar larvae feed gregariously and this makes the larval stage the most damaging stage of this beetle. Thereafter, late instar larvae feed solitarily and pupate mostly on dry, damaged leaves that are still attached to the plant.

The damage by both larvae and adults of P. maculiventris often results in complete skeletonisation of the leaves, leaving only the leaf veins. This damage leads to substantial loss of photosynthetic area, resulting in loss of biomass and reproductive capacity of the plant.

Since August 2018, the beetle has been released at several sites in KwaZulu-Natal, Mpumalanga and Limpopo. We anticipate that, having been reunited with its host plant in the wild, the tortoise beetle will multiply and build up populations that will be sufficient to reduce the fitness and invasiveness of Mexican sunflower in South Africa. – ARC Plant Protection Newsletter

Contact Khethani Mawela at MawelaK@arc.agric.za and David Simelane at Simelaned@arc.agric.za for more information.