ÐÓ°ÉÔ­´´

AFRICA IS fighting a quiet battle to save its cassava from devastation
by two exotic insects that were accidentally imported from Latin America.
The cassava mealybug (Phenacoccus manihoti) and the green spidermite (Mononychellus
tanajoa) have reduced yields by an average of one-third across Africa’s
cassava belt, an area one-and-a-half times the size of the US. In some places,
the pests wiped out entire fields, until entomologists recruited natural
enemies to fight the invaders.

The African-Wide Biological Control Program (ABCP) of the International
Institute of Tropical Agriculture at Ibadan, Nigeria, in partnership with
national research projects throughout Africa, is probably the world’s most
extensive effort at biological control currently underway. The programme
has so far released natural enemies of the pests in 20 of the countries
in the cassava belt. A network of institutions around the world also collaborates
in the campaign, which cost $4.5 million in 1987. The project with the mealy
bug has reduced losses of cassava by more than half in some cases.

Cassava was brought from South America to Africa 300 years ago, escaping
its pests until recently. ÐÓ°ÉÔ­´´s first reported the mealybug in 1973,
when it was infesting cassava in the Congo and Zaire. With no natural enemy
in Africa to curb the mealybug’s advance, it soon became the principal pest
for cassava, cutting yields by as much as 84 per cent. The mite appeared
in Uganda in 1971, and now attacks cassava in 27 countries.

The mealybug damages cassava by sucking sap from buds, terminal shoots
and leaves which stunts and defoliates the plants. The leafless tips of
the plant after attack by mites look like spindly candlesticks. Weakened
cassava plants encourage soil erosion because their root systems are feeble
and the sparse canopy of leaves fails to protect the soil’s surface from
rain. Fewer leaves also means that cassava can no longer shade out the weeds
that compete with it.

Subsistence farmers who grow cassava cannot afford chemicals to control
pests. Many of them live in remote areas beyond the reach of roads, and
commonly intersperse cassava with other crops in fields, making chemical
control impractical.

Biological control, however, promised an environmentally sound and sustainable
solution. Natural enemies do not completely wipe out the pests, but rather
keep their populations in check. Cassava, grown over several seasons, could
provide a stable home for pest and predator to maintain their natural equilibrium.
The ability of beneficial insects to spread on their own after release was
another advantage, eliminating the need to assign overburdened services
to support agriculture with yet another technology to disseminate.

The mealybug was the first target, because it appeared to be the more
serious pest. Because entomologists did not know exactly where it had come
from, nor the identity of its natural enemies, scientific expeditions sought
the mealybug in the southern US and Central and South America from 1977
to 1981. The pest was finally discovered in Paraguay. Researchers also identified
its natural enemies there and in neighbouring countries, where they generally
keep the insect from becoming a serious pest.

The ABCP hoped to replicate this natural balance in Africa. Entomologists
eventually sent more than 60 species of both pests’ natural enemies to the
Commonwealth Institute of Biological Control, London, for quarantine. Rigorous
scrutiny by the research ers at the institute over several generations of
insects ensured, as far as possible, that they posed no threat to Africa’s
ecology.

The wasp Epidinocarsis lopezi, which is tiny, appeared to be the likeliest
candidate to control the mealybug. The female wasp is a parasitoid. She
injects her egg into a live mealybug. The growing larva slowly consumes
its host from the inside out, eventually emerging as an adult wasp from
within the mealybug mummy.

Researchers at the ABCP first released the wasp experimentally in cassava
fields in 1981 and 1982 around the headquarters in Ibadan, Nigeria. Mealybug
numbers soon plummeted, first in the release area and then in nearby fields
which the parasite had reached on its own. Three years after the initial
release, the wasp had spread to more than 200 000 kilometres in southwestern
Nigeria, Benin, and parts of Togo.

By August 1988, the ABCP and local of both pests in 20 countries, all
by request from national governments. In many cases, local researchers help,
or carry out releases themselves, and survey fields.

The wasp has made the most striking impact. Peter Neuenschwander, an
entomologist with the ABCP, describes its ability to establish itself as
astonishing. In every area where researchers could carry out follow-up studies,
which included several ecological zones in 19 countries of sub-Saharan Africa,
the wasp was thriving. Releases in 1985 of E. lopezi in a cassava growing
region of Malawi have virtually purged the areas that were sampled of the
cassava mealybug. In several countries, the wasp cut lossesof cassava by
more than half. Researchers estimate that the programme has a cost-to-benefit
ratio of 1:149.

To produce sufficient numbers of E. lopezi for release, researchers
replicate its natural food chain in captivity at the ABCP’s insect-rearing
facility, or insectary, which entails raising cassava, mealybugs and wasps.
Researchers counter the green mites’ destructive potential by rearing five
of its predators, which are species of mite from the Phytoseiidae family,
and releasing them for evaluation. So far, control of the green mite appears
to be a more complex problem than that of the mealybug.

The ABCP’s new technologies are passed on for use in national research
programmes. They include what is probably the world’s only application of
hydroponic techniques in an insectary, allowing more plants and insects
to be reared in a smaller space.

In a typical phase in the campaign to establish E. lopezi, researchers
release between 10 000 and 20 000 insects weekly into fields, either on
the ground or from an aeroplane. In one case, the aerial release allowed
scientists to set the wasps free in remote fields in Zaire less than 24
hours after the insects were packed up in Ibadan. Follow-up studies confirmed
that the wasp populations had taken hold.

The International Institute of Tropical Agriculture and African scientists
hope to build the success with mealybug into a comprehensive programme of
biological control for Africa’s crops. Because indigenous specialists in
biological control are in short supply, nearly 200 African scientists and
technicians from most countries of the cassava zone have undertaken short-term
training in the subject, while a further 35 candidates have had advanced
training at African and international universities. After such intensive
studies the researchers return home to initiate national biological control
projects that become part of the continent-wide research network.

The ABCP has just shifted to a larger and more flexible facility at
Cotonou, Benin, that may become the African centre for biological control.
The new headquarters provides much more room to rear insects as well as
facilities to develop new research methodologies and expertise, providing
a setting for research into other crops, pests, and perhaps even weeds.

Lynn Teo Simarski is an agricultural writer, based in Washington DC,
who has worked for the International Institute of Tropical Agriculture and
at the International Centre for Agricultural Research in Dry Areas.