Previous natural (biological) control successes

Rubbervine infestation  in Australia

Natural (biological) control of rubber vine in Australia The rubber vine weed was introduced to Australia from Madagascar in the 19th century as an ornamental garden plant and as a source of latex. An aggressive climber, capable of smothering trees up to 40 metres high, it was described as the single biggest threat to natural ecosystems in tropical Australia and had a huge impact on pastoral areas. By the late 1980s the rubber vine infestations were vast, covering 40,000 km2 (that’s twice the size of Wales) across often remote areas. Wide-scale chemical control was considered impractical, uneconomic and environmentally undesirable. As the weed advanced towards the prestigious national parks of the Northern Territory, urgent calls were made for its widespread control.

Exploratory surveys by CABI scientists in Madagascar revealed a rust fungus, Maravalia cryptostegiae, to be a highly promising biological control agent. After extensive trials to assess its safety, aerial releases were made in Australia in 1995 and it is proving to be one of the most successful biological control programmes ever carried out against an alien invasive weed.

Originally predicted to take 10 years, the rust delighted farming communities by bringing the weed under control earlier than expected, defoliating and killing the weed and allowing regeneration of native forests, providing a net value of AUD $232.5 million – a figure which grows year on year.
 

Parasitoid wasp

Natural (biological) control of the cassava mealybug in Africa Cassava, yucca or manioc was introduced from South America into Africa by the Portuguese in the 16th century and is today the staple root crop for more than 200 million people in Africa alone. This major source of carbohydrates came under threat from a devastating pest, the cassava mealybug (CMB). CMB was first recorded in Congo and Zaire (now Democratic Republic of the Congo) in the early 1970s and quickly spread over the whole of the cassava growing area of Africa, since there were no natural enemies to control it in its new habitat.

In Africa, it led to the complete collapse of cassava production, depriving several hundred million people of one of their main carbohydrate sources not to mention livelihoods. In a combined effort involving IITA, CABI, IAPSC and other agencies, biological control agents were found in South America following extensive fieldwork. A parasitoid wasp Anagyrus lopezi was quarantined in the UK, then shipped to Africa, mass reared, and finally release was authorized for field trials. 

The cooperation was so successful that in the whole of sub-Saharan Africa CMB is now under complete control and poses no threat anymore.

The programme cost, according to Swindale (1997) about US$ 27 million, while the benefits are estimated at US$ 4.5 billion (108)! The real beneficiaries are the millions of cassava growing smallholders who – often unaware of the programme or the parasitoid wasp – enjoy the fruits of this work. Food security is increased through improved harvests and health through reduced pesticide use, both of which come at no cost for the smallholders who receive the full benefits.

Galerucella leaf beetles

Natural (biological) control of purple loosestrife in North America The popular European native purple loosestrife (Lythrum salicaria) was introduced to the US and Canada as an ornamental plant, however it has since invaded the natural environment forming large stands and degrading many prime wetlands. Purple loosestrife is an herbaceous perennial which grows up to about 1.5 metres in height. In its native range in Europe, the plant can be found on the margins of lakes and swamps and rarely dominates these environments.

The costs associated with the control of purple loosestrife are huge – in the late 1980s around $45.9 million was spent each year to control this weed across 19 US states. 

The weed is now found in 48 states of the US where it crowds out nearly 50 native plant species, endangers rare marsh wildlife, and restricts access to open water. A single plant can produce up to three million tiny seeds annually which are easily carried by wind and water, and germinate in moist soils after overwintering. The plant can also sprout anew from pieces of root left in the soil or water. Once established, loosestrife stands are difficult and costly to remove by mechanical and chemical means.

To bring this weed under control, a biological control programme began in 1987. CABI scientists evaluated more than 100 insect species that feed on purple loosestrife in its native range in Europe.

After extensive testing, six agents proved to be safe for release in North America. Of these, two leaf-eating beetles – Galerucella calmariensis and G. Pusilla – proved to be particularly effective at controlling the invasive weed. 

As a result of this work, this aggressive invader is now being successfully controlled in the western and mid-western USA – 95% of purple loosestrife is destroyed within two to five years of release – allowing native wetland plants to flourish once more.