South African entomologist Letodi Luki Matule has discovered the natural soil fungus Metarhizium rileyi, which is capable of killing African armyworm larvae, paving the way for the creation of safe biopesticides. He provided detailed information about his research findings.
The African armyworm is an extremely destructive migratory pest that damages crops such as maize, sorghum, millet, and sugarcane. Furthermore, these caterpillars cause indirect harm to livestock by destroying pastures and meadows.
The last major outbreak of armyworm in South Africa occurred from February to April 2025. It affected the provinces of KwaZulu-Natal, Limpopo, Gauteng, and Mpumalanga in South Africa, as well as neighboring Zimbabwe.
Controlling the armyworm has proven to be a serious challenge. The most common method has been the application of synthetic chemical insecticides based on pyrethroids. However, this method has shown insufficient effectiveness for complete control, as the insecticide only acts against larval stages (caterpillars) measuring about 1–5 mm, since they are more susceptible to the poison than larger larvae.
The insect goes through a complete metamorphosis cycle, including egg, larva (caterpillar), pupa, and imago (moth) stages. The caterpillar stage is the most damaging.
As an entomologist, his work is primarily focused on integrated management of agricultural pests. Specifically, he studies the use of pathogenic (entomopathogenic) fungi and nematodes as biological agents for insect control in agroecosystems.
In a recent scientific paper, he presented his findings on a fungus that naturally infects and kills the larval stages of the African armyworm. This is the first report of the presence and successful isolation of this fungus in South Africa.
Outbreaks of the African armyworm are typically characterized by a high density of insect larval stages: they form dense armies moving across the ground together. The worm feeds on plants and is capable of completely cutting off crop leaves down to ground level. Damage to kikuyu grass pastures also results in less feed for livestock, and animals can become poisoned.
This occurs because when eating kikuyu grass, the larvae release a cyanide chemical compound as a defense mechanism against insects. This compound is toxic to livestock, mainly cattle. Clinical signs of kikuyu grass poisoning in affected livestock during outbreaks include abdominal pain, dehydration, excessive salivation, inability to drink even if the animal brings its mouth to water, coordination impairment (loss of muscle control leading to unsteady and involuntary movements), and cardiopulmonary distress.
While inspecting kikuyu grass pastures infected with this insect in KwaZulu-Natal, he noticed something killing the African armyworm larvae. It turned out to be an isolate of the fungus Metarhizium rileyi. This fungus lives in the soil and is one of many entomopathogens that infect and kill various harmful insects. They are often used in agriculture to combat pests that cause serious damage to crops. Several commercially available fungal insecticides have proven effective.
He found that dead African armyworm larvae covered large areas of fields, mostly attached to grass leaves, and some were on the soil surface. They were infected with Metarhizium rileyi. The infection was visible as white and green conidia, which are asexual spores on the surface of the insects' cuticle.
This was the first observation and report on the effectiveness of this fungus against the African armyworm. Previously, this fungus had been mentioned as an effective biological control agent for the autumn armyworm in countries such as the Philippines.
The next step will be the mass production of this specific fungal species on a commercial scale and the registration of the fungus as a biopesticide for this insect. This could complement management strategies that can be effectively used during outbreaks of this pest.
The article was first published in The Conversation and written by Letodi Luki Matule, a scientific entomologist at the Department of Conservation Ecology and Entomology at Stellenbosch University.