The tsetse fly gives live birth to a larva that can outweigh its mother, feeds it internally with a substance entomologists call fly milk, and shrugs off a fly swatter. It also carries trypanosomes, single celled parasites that cause sleeping sickness in people and nagana in livestock. This episode explains how an insect the size of a fingernail came to rule roughly 10 million square kilometers of sub-Saharan Africa, turning fertile country into a green desert where farmers cannot keep the draft animals they need.

The story is stranger than a pest profile. The parasite rewires the fly’s brain chemistry so it feels constantly starved and bites more often. A cattle virus brought to Eritrea in 1887 killed the herds, let thorn bush spread, and handed the fly vast new territory that colonial powers mistook for untouched wilderness and turned into famous reserves. Then came a century of countermeasures, from bulldozers dragging chains to blue and black traps and radiation sterilized males.

• Both sexes feed only on vertebrate blood and can drink their own body weight in a single meal to fuel the mother’s milk production.
• Trypanosoma brucei shuts down an enzyme in the fly’s glucose metabolism and disrupts its dopamine and serotonin, driving it to feed more.
• Rinderpest reached South Africa by 1897, killing some 5.5 million cattle there, and an estimated two thirds of the Maasai died in 1891 alone.
• Early conservationists called the tsetse the best game warden in Africa because its diseases kept farmers and cattle out of places like the Serengeti.
• Because a female usually mates only once, releasing sterile males wiped out the fly on Unguja in Zanzibar between 1994 and 1997.