The African armyworm, a migratory pest that strips maize, sorghum, millet and sugarcane and destroys grazing, may have met a natural enemy in South Africa. Entomologist Letodi Luki Mathulwe, writing for The Conversation in a piece carried by Food for Mzansi, reports the first observation and successful isolation in the country of a soil fungus, Metarhizium rileyi, that infects and kills the caterpillar stage.

The last major outbreak ran from February to April 2025 across KwaZulu-Natal, Limpopo, Gauteng and Mpumalanga and neighbouring Zimbabwe. Control has relied on pyrethroid insecticides, which work only on larvae about 1–5 mm long; larger caterpillars, the most damaging stage, are far less susceptible. Dense 'armies' of larvae move across the ground together and can strip crops to soil level.

Damage to kikuyu pastures is doubly costly: the grass releases a cyanic compound in defence as the larvae feed, and the cyanide poisons livestock, mostly cattle, with signs including abdominal pain, dehydration, drooling, 'sham drinking', loss of coordination and cardiac and respiratory distress.

Inspecting infested kikuyu pastures in KwaZulu-Natal, Mathulwe found dead larvae covering large areas, attached to grass blades and on the soil surface, coated in the white and green conidia of Metarhizium rileyi. The fungus is one of many entomopathogens used in agriculture, with several fungus-based insecticides already on the market; it had previously been reported effective against the fall armyworm in countries such as the Philippines, but never against the African armyworm.

The discovery opens the way to develop the organism as a biological control product for outbreak seasons in South Africa and beyond; mass production on a commercial scale and registration as a biopesticide would be the next step, adding to the tools available when the armies march.

Source: Food for Mzansi