​​​Crofton weed / Mexican devil (Ageratina adenophora) (Asteraceae)

The weed

Crofton weed, Ageratina adenophora,  is a perennial, multi-stemmed, soft shrub, native to Mexico, which grows up to 3m tall in moist conditions, such as on the edges of slow flowing streams, in waterlogged soaks, on steep slopes and in high rainfall areas. It produces terminal clusters of small white flowers during late winter to spring, yielding tens of thousands of wind- and water-dispersed seeds. Stems root when in contact with the soil, forming dense stands.

Ageratina at pool.jpgCroften Weed.jpgAgeratina flowers.jpg

In South Africa it invades wet areas of the Magalies Mountain in the North-West Province, parts of the Low Veld in Mpumalanga and Limpopo Provinces, the mist belt region of KwaZulu-Natal and the foot of Table Mountain in the Western Cape. Furthermore, it has the potential to expand its invaded range. Crofton weed supresses biodiversity in particularly sensitive biomes, reduces the carrying capacity of pastures, is poisonous to horses and has the potential to reduce run-off in streams in catchment areas, because of dense infestations in soaks and stream-sides.

In terms of the Alien and Invasive Species Regulations (AIS), National Environmental Management: Biodiversity Act (Act No 10 of 2004), Crofton weed was declared a category 1b species, and land owners are obligated to eradicate or at least control it. No trade or planting is allowed.​

Biological control

The shoot-galling fly Procecidochares utilis was released in 1984 in South Africa. Although it is contributing to retarding the vigour of the weed country-wide, the level of control is insufficient, and it is likely to be limited by high levels of parasitism, amongst other factors.

In 1987 the leaf-spot fungus, at that time identified as a Phaeoramularia species, was introduced into South Africa. It has now been re-described as Passalora ageratinae. It became established and proliferated in KwaZulu-Natal, and is currently present in all areas where the weed has been found. However, its impact remains limited.

Surveys for new potential biocontrol candidates were conducted in Mexico during 2007, 2009, 2011 and 2013. From these collections, more than 150 insect and two pathogen species have been reared in the PPRI quarantine laboratories. Some of these were successfully cultured, and three insects and one pathogen were selected as the most promising biological control candidates. Studies are being conducted on these to determine their host specificity and their potential to inflict damage to the weed.

More details can be found in a recent (2011) review of this project -pdf

Contact person: Fritz Heystek, E-mail: heystekf@arc.agric.za

>>>Specific IAP Species and their control according to botanical names