Cassava, a mealybug, and a wasp

Cassava (Manihot esculenta) came to Africa from South America in the 1500s. It became a staple for hundreds of millions: carbohydrates in the roots, protein and vitamins in the leaves, and a crop that stays in the ground when the rains fail. What did not come with it, for centuries, was the neighbourhood of insects that keep cassava pests quiet in the Amazon.

In the early 1970s — first reports around 1973 — an unknown mealybug began devastating fields in Congo (Brazzaville) and Zaire (now DR Congo). It was formally described in 1977 as Phenacoccus manihoti. Because cassava travels as stem cuttings, the pest almost certainly hitchhiked on plant material. Soft-bodied, sap-sucking, and able to reproduce without males (parthenogenesis — an all-female army), it exploded where nothing was waiting to eat it.

Farmers and officials first talked about a “candlestick” affliction: leaves gone, bare stalks standing like candles. Damage was insect work, not a pathogen. Documented tuber-yield losses reached about 80% in the worst places; peak infestations could put more than a thousand mealybugs on a single shoot tip. Chemical spraying was tried. Across smallholder mixed-cropping it was expensive, impractical, and temporary. Someone needed a plan that could travel as far as the pest had — and keep working after the project vehicles left.

The idea is older than the programme and younger than the crop: find the pest’s natural enemies where pest and crop evolved together; screen them carefully; introduce them so the invader meets the neighbour it left behind. Cassava is Amazonian. So is P. manihoti. In its homeland the mealybug is usually rare — kept that way by parasitoids and predators. In Africa that neighbourhood was missing. Search South America.

Early hunts hit a false start: a look-alike mealybug in the Caribbean and northern South America turned out to be a different species (later P. herreni), and its parasitoids would not attack the African pest. Frustrating, but useful — biology does not care about our deadlines.

The breakthrough came in 1981. Scientists of CIAT found P. manihoti at low density in Paraguay. Among natural enemies collected via CIBC and later exploration teams was a tiny wasp then called Epidinocarsis lopezi (later Apoanagyrus lopezi; today often Anagyrus lopezi). A female lays a single egg inside a mealybug; the larva eats the host from within and leaves a “mummy.” Host-specific. Precision labour the size of a comma.

Hans R. Herren arrived at IITA in 1979 — Swiss entomologist, early thirties, initially the sole staff of what became the Africa-wide Biological Control Programme. He led and built the campaign — not as a lone collector with a butterfly net, but with a wide team. CIAT located the pest in Paraguay; explorers including CIBC’s M. Yaseen and later GTZ/IITA teams (among them B. Löhr and A. M. Varela) gathered enemies; quarantine ran through Silwood Park in England. Shipments were screened so wasps would not harm bees or silkworms, and so hyperparasitoids (parasites of the parasite — the office politics of the insect world) did not sneak in.

First releases of E. lopezi at IITA/Ibadan began in November 1981 (start of the dry season). By late 1982, after a second release near Abeokuta and a rainy season survived, permanent establishment was confirmed. Within a few years the wasp had raced across southwestern Nigeria — one of the fastest dispersals ever recorded for such a tiny wasp.

Mass rearing was a living Russian doll: grow cassava in pots, grow mealybugs on the plants, grow wasps on the mealybugs. Quality mattered more than raw headcount; for inoculative releases, sometimes fewer than a hundred wasps established a new population. Still, the insectaries became industrial — dozens of rearing units, mechanized cages, schedules that never quite slept, and a smell veterans remember as honeydew and damp green.

Releases went out only on request from national programmes, always with local collaborators. Ground crews carried parcels to chosen fields. Then came the flying chapter: IITA’s Automatic Insect Release System (AIRS), built with Austrian and Swiss partners, installed in a twin turbo-prop, to drop wasps over hard-to-reach stretches of the cassava belt. Herren later described wasps “packaged in a way that they could be shot out of an airplane travelling at 300 kilometres per hour or more across Africa, from Senegal on one side to Mozambique on the other” (swissinfo interview, 2011).

The World Food Prize Foundation has described aerial release as “almost 2,000 wasps per second” — a vivid picture of the method. According to Right Livelihood, some 1.6 million wasps were released between 1982 and 1993 across 24 countries. By around 2001 the wasp was established in about 26 African countries. Madagascar notably stayed free of the pest through the mid-1990s.

When classical biological control works, daily life can look oddly ordinary again. Fields stop whitening. Yields recover. Markets stay open. The wasp does not send a press release.

Peer-reviewed work documents sharp drops in damage after establishment, control in roughly 95% of fields by the end of the 1980s, and exclusion experiments showing the wasp — not weather alone — was the key factor. Equilibrium has held for decades. Economic analysis (Zeddies and colleagues, 2001) found benefit–cost ratios on the order of 200 (world cassava price) up to several hundred at intra-African prices — for a programme that, spread over the belt and decades, cost on the order of cents per hectare per year.

The most-repeated human figure — “saved upward of 20 million lives” — comes from World Food Prize / biography accounts as an estimate of famine averted. Cassava fed on the order of 200 million people; protecting it mattered whether or not anyone can put a single number on every meal saved. This site is not here to restate that number as a victory lap — it is here for the people who lived the work.

In 1990, IITA and CIAT jointly received the CGIAR King Baudouin Award. In 1995, Herren received the World Food Prize (presented 16 October 1995 in Washington). He later received the Right Livelihood Award (2013). Last African releases wound down around 1994. When the mealybug later hit Thailand, IITA’s Georg Goergen hand-carried a starter colony of about 500 wasps from Benin to Bangkok. Biology has a long passport stamp.

People who lived the programme are growing older. The papers are on shelves and in DOIs. The texture is still in kitchens, photo albums, compound jokes, and the odd story that only makes sense if you have ever counted wasps before breakfast — or waited for a parent who was still in the insectary.

biologicalcontrol.org is modelled on a folklore archive: find someone you remember, share a story from the compound, caption a photograph. We will keep the primer accurate. We need you for the rest — the night the generator failed, the collaborator who drove all night, the field that finally looked green again, the name misspelled on a shipping label, the kids racing between houses after dark.

If you were in quarantine, in an insectary, on a release flight, in a national programme office, on a farm that stopped looking like a candlestick factory, or living on station beside someone who was: write it down. Families welcome. Two paragraphs count. A name on a photograph counts. Folklore survives on specifics.