Memories…… something that last year’s uMngeni River Walk made plenty of!
Having been away from Howick for almost six months, I (Penny Rees) returned home to find my own personal and embarrassing Bugweed plants in my garden, which made me remember the forests of invasive Bugweed that we walked around, and under, and through, all the way from the upper reaches of the uMngeni River to Durban. The furry leaves that made us itch like hell. The views of those large grey green leaves forming unbroken tracts along the river and her tributaries, as well as inland, choking out everything else. The bare earth under the large tracts of Bugweed, shaded out so that nothing grew, the lack of biodiversity in those clumps, plenty of plants to see – so long as its Bugweed.
I was thus extremely excited when I received information on my new hero – Anthonomus santacruzi, the Bugweed Weevil. This wee bio control beastie was apparently only released 2 years ago and is moving quickly throughout the coastal region of KZN. It has been seen in the Durban area: south to Park Rennie and north to Hazelmere.
I contacted Bart, our DUCT bio-control boffin based in Durban and he added the following information:
“wherever I see Bugweed I am compelled to stop and I usually find the bugs – Inanda seminary (where SASRI is collecting the bugs for redistribution), NWWTW Johanna Road Durban, Isipingo, Prospecton The Avenue East, Kwadabeka New Road Bridge.”
Bart says: “Pick a flower and have a look at the base of the flowers for the bugs – little black bugs as per the pictures on the leaflet. I find that if you handle the flowers a bit the bugs emerge from the flowers too. Get a toothpick and pry a bud open”.
Apparently these tiny creatures fly 20km to new flowers. Howzat?!
And then Bart gave me my ray of hope: “I think that you might only have weevil activity up your way in the Midlands once the big freeze is over so maybe October”.
Please read the descriptive leaflet below, and next time you are out there in the countryside (or wherever) and you see a Bugweed – please have a close look and see if you can spot a weevil or six. I would love to know if they have reached the Midlands yet.
If you do spot this bio control wonder, could you please forward the grid reference or location to me on : email@example.com, and I will pass on your information to SASRI as they are trying to map the spread of the weevil.
As Bart says “We are not alone in the war against weeds – The creepy crawlies are helping too”.
The adult insect is a small (2-3 mm) black snout-beetle or weevil (photo 1) with a characteristically elongated snout. Adults move around actively on the leaves and flowers (photos 3 and 4), and are able to fly and disperse well.
Adults feed on stamens of open flowers or chew through the petals of buds to feed on the stamens. They will also feed on apical leaflets and shoot tips of plants in the absence of floral material, but the larvae are by far more damaging.
Female weevils chew into the buds and lay their minute eggs (± 0.3 mm) in small cavities hollowed out in the sides of the anthers of both immature and mature buds. Small larvae develop within the anthers at first and then as they develop consume the entire contents (petals and anthers) of the flowerbuds (photo 2).
Feeding inhibits the opening of buds. Larvae usually develop singly, but up to 3 can be found per single flowerbud. Larvae complete development within 10 to 18 days and pupation within the cavity of the bud takes a further 4 to 10 days.
The emerging adults chew their way out of the buds in which they have developed (photo 5).
The entire life cycle takes 15 to 25 days from egg to adult. Adult weevils can live for over 3 months and females will actively lay eggs over this period.
Impact on Bugweed (Solanum mauritianum)
Bugweed is a major weed in the eastern higher rainfall areas of eastern South Africa. Seed set is extremely high and dispersal is mainly by fruit-feeding birds. Mechanical and chemical methods of eradication are successful but fast-growing seedlings generally follow these clearing operations . Anthonomus is a very promising biological control agent which prevents fruit set and reduces the enormous potential for seed dispersal. In addition, feeding on the flowers causes the abortion and abscission of buds and flowers, while feeding on apical leaf shoots in the absence of flowers also causes considerable damage.
Olckers, T., 2003. Assessing the risks associated with the release of a flowerbud weevil, Anthonomus santacruzi, against the invasive tree Solanum mauritianum in South Africa. Biological Control 28: 302-312.
Henderson, L. 2001. Alien Weeds and Invasive Plants. Plant Protection Research Institute Handbook No. 12. Agricultural Research Council, Pretoria.