Bracken and bugs

BRACKEN AND BUGS

by Dr. Bob Mesibov, Research Associate,
Queen Victoria Museum and Art Gallery,
Launceston, Tasmania

Bracken fern is a common and very widespread understorey plant in temperate Australia. The same or an extremely similar fern is widespread and common outside Australia, from the subarctic to the montane tropics to far southern South America. (We call it Pteridium esculentum, they call it Pteridium aquilinum.) In most countries Pteridium spp. are considered weeds. Bracken blocks the growth of other vegetation, including tree seedlings. It's typically the first plant to re-establish after a 'weed- clearing' fire. It causes cancer and other diseases in sheep and cattle, and blindness in goats. In Central and South America, bracken is suspected of causing stomach cancer in people who drink milk from cattle grazed on bracken-infested pasture.

There are positives for bracken. Many of them have to do with invertebrates. Green, standing bracken offers shade and a relatively moist microclimate for a wide range of flying and crawling species, not to mention a place to hide from predators. The slow-rotting fronds and stems (stipes) of dead bracken lie slightly off the ground under green standing bracken for several years; this sub-sub-sub-canopy provides an even shadier, moister microenvironment for litter invertebrates. On burned ground, bracken cover ameliorates the erosive and other effects of weather, encouraging the redevelopment of a healthy soil fauna.

Finally, believe it or not, there are bugs that eat bracken. A famous entomological study (Lawton l982) compared bracken-feeding insects in Britain and in montane New Mexico and Arizona in the USA. In Britain, seven sawflies (primitive wasps), six moths and a springtail are regular bracken chewers. Eight flies mine the bracken tissues or make galls, and four true bugs are bracken suckers. Much the same specialisation was found in the USA, but with only seven insect species in total, including a bracken - sucking thrip. The Lawton study was interesting for two other results: only a single beetle was found to eat bracken (a rare UK species), and at individual sites only a portion of the available 'regional pool' of bracken feeders was observed.

Bracken-eating insects have also been inventoried in Brazil, Papua New Guinea and Australia. Two species of Australian fruit flies enjoy bracken (Thomson et al. 1982), and bracken near Sydney is dinner for 15 insects (again, no beetles) and two mites (Shuter & Westoby 1982).

With such a short list of potential biocontrol agents, and with so little damage done by each, the prospects for controlling weed bracken around the world with insects are pretty gloomy. In Tasmania, bracken is a vigorous understorey plant which offers a home and (sometimes) something to eat for a substantial number of invertebrates. Don't burn it if you can slash it, and don't slash it unless you have to. The bugs will be grateful. (More about bracken)

(This article first appeared in the Understorey Network newsletter no. 22; reproduced with permission)

Referemces:
Lawton, J.H. (1982) Vacant niches and unsaturated communities: a comparison of
bracken herbivores on two continents. Journal of Animal Ecology 51: 573-595.
Shuter, E. and Westoby, M. (1982) Herbivorous arthropods on bracken (Pteridium
aquilinum (L. Khun) jn Australia compared with elsewhere. Australian Journal of
Ecology 17: 329-339.
Thomson, J.A., Jackson, M.J. and Bock, I.R. (1982) Contrasting resource utilisation in
two Australian species of Drosophila Fallen (Diptera) feeding on the bracken fern
Pteridium scopoli. Journal of the Australian Entomological Society 21: 29-30.