The African armyworm (Spodoptera exempta), also called okalombo, kommandowurm, or nutgrass armyworm, is a species of moth of the family Noctuidae. The larvae often exhibit marching behavior when traveling to feeding sites, leading to the common name "armyworm". The caterpillars exhibit density-dependent polyphenism where larvae raised in isolation are green, while those raised in groups are black. These phases are termed solitaria and gregaria, respectively. Gregaria caterpillars are considered very deleterious pests, capable of destroying entire crops in a matter of weeks. The larvae feed on all types of grasses, early stages of cereal crops (e.g., corn, rice, wheat, millet, sorghum), sugarcane, and occasionally on coconut. The solitaria caterpillars are less active and undergo much slower development. The species is commonly found in Africa, but can also be seen in Yemen, some Pacific islands, and parts of Australia. African armyworm outbreaks tend to be devastating for farmland and pasture in these areas, with the highest-density outbreaks occurring during the rainy season after periods of prolonged drought. During the long dry seasons ("off-season"), the population densities are very low and no outbreaks are seen.
Taxonomy and phylogeny
Spodoptera exempta was first described by Francis Walker in 1856. The species of Spodoptera are distributed around the world, mostly inhabiting tropical and subtropical areas. There are 30 known species in the genus, and roughly half are considered agricultural pests. There are two final larval stages based on the mandible structure of the species: serrate-like mandibles and chisel-like mandibles. Along with S. exempta, the following species and subspecies have chisel-like mandibles that were evolved to consume silica-rich leaves: S. triturata, S. m. mauritia, S. m. acronyctoides, S. umbraculata, S. cilium, , and S. pecten. Like S. exempta, a few species are migratory as adults and travel downwind for hundreds of kilometers, namely S. exigua, S. frugiperda, and S. litura. Due to the broad distribution of the species, the origin of the genus is unknown. However, dating of phylogenic trees reveal that the common ancestor for the genus lived between 22 and 30.7 million years ago and began diversification during the Miocene Epoch.
Geographic range
The African armyworm is commonly found in the grasslands of Africa and Asia. Within Africa, it is mostly seen near the Sahara in the following countries: Tanzania, Kenya, Uganda, Ethiopia, Somalia, Malawi, Zimbabwe, Zambia, and South Africa. Outside of Africa, the species also inhabits southwest Saudi Arabia, Southeast Asia, Australia, and New Zealand.
Habitat
Akin to other species of the genus Spodoptera, S. exempta lives in tropical and subtropical environments near cereal crops and dense vegetation where the larvae of the species can easily feed. Because these larvae thrive in high-density populations, African armyworm outbreaks occur in which fields become overrun by caterpillars. The surrounding vegetation is then consumed in large amounts, often with devastating effects. While seasonal winds and rains allow adult S. exempta to migrate across countries and continents, they are often confined to coastal areas and marshes during dry seasons, where the occasional rainfall supports enough vegetation growth for the moth's survival.
Food resources
S. exempta larvae feed almost exclusively on Poaceae, also called Gramineae. Poaceae is a family of flowering grasses which includes cereal grasses and the grasses of grasslands and pastures. The species targets most cereal crops, including maize, sorghum, rice, millet, and other grasses. Young caterpillars are also known to feed on wheat and oat seedlings. The larvae eat the upper and lower surfaces of the plant tissue first, preferring to feed on younger and recently germinated crops. Studies have shown that two larvae can completely consume a 10-day-old maize plant with 6–7 leaves, indicating the devastating potential of a high-density outbreak.
Life cycle
Eggs
The female can lay a maximum of about 1,000 eggs in her lifetime. She can lay 100–400 eggs per night, with an overall average of 150. The number of eggs females can lay is positively correlated with their pupal weight. African armyworms primarily lay their eggs in clusters on the lower side of leaves. Their eggs are relatively small at 0.5 mm in diameter. They are white when first laid, but will turn black prior to hatching. The eggs hatch into larvae within two to five days.