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.

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, Spodoptera depravata [sv], and S. pecten.

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.

[12] 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.

[1] Generally, African armyworms are not noticed by farmers until the caterpillars are ten days old and start exhibiting the gregaria phase.

[13] Despite how sensitive the developing larvae are to larval contact, high densities of solitaria caterpillars have been recorded due to a vegetation-dense environment limiting interaction.

[1] The adult moth wingspan is between 20 and 37 mm, with dull gray-brown forewings and off-white hindwings with visible veins.

[19] Therefore, migration is based on winds brought by the Intertropical Convergence Zone, which move northwards or southwards depending on the season.

[5][9] These larvae display highly active feeding behavior, leading to excessive consumption of the nearby vegetation, especially the leaves of cereal crops and grasses.

[9] The high-density gregaria larvae may then pupate and emerge as adults, leading to a large migration event of moths that travel downwind together.

[5] The intensity of outbreaks also increases during the start of the wet season after a drought, which allows moths to lay eggs in areas of dense vegetation which supports the larval stage growth.

[11] Natural enemies have limited effects on African armyworms due to the species' unpredictable migration paths and variable population densities.

[9] Although parasitoids do not accumulate rapidly enough to kill a S. exempta population before migration occurs, if the moths happen to return to the same spot, high levels of parasitism are experienced.

[citation needed] SpexNPV, abbreviated from S. exempta nucleopolyhedrovirus, is a baculovirus that can be transmitted vertically, from adults to offspring.

[20] S. exempta attempt to reduce the risk of contracting SpexNPV via density-dependent prophylaxis, which allows the species to invest more in resisting disease and stimulate migration to less dense locations.

[21] While dark coloration in species usually indicates a thermoregulatory function, this does not appear to be true under normal conditions in S. exempta.

The black gregaria larvae do not exhibit basking behavior which is usually present in dark-colored species that want to increase their body temperature via exposure to the sun.

[13] The gregaria phase of the S. exempta species are considered agricultural pests due to their high densities and feeding behavior.

[9] Now, newer insecticides such as azadirachtin and aqueous neem (Azadirachta indica) seed extracts are often applied, but these methods are dose dependent and have adverse side effects on both human health and crops.

For example, the S. exempta nucleopolyhedrovirus virus (SpexNPV), a naturally present disease that preys on the species, is now being investigated as a method of biological pest control.

[11] In mid-April 1999, an African armyworm infestation started in southern Ethiopia, spreading into the north the following month and into the Jubba Valley of Somalia in early May.

[22] STAR radio in Liberia reported in January 2009 that Zota District in Bong County had been invaded by African armyworms, which had consumed vegetation, polluted creeks and running water,[23] and were moving toward Guinea and Sierra Leone.

[24] On January 28, 2009, the president of Liberia declared a state of emergency to deal with the infestation of army worms in the country.

[27][28] In 2016, the fall armyworm (Spodoptera frugiperda, a species of American origin that behaves similarly) invaded maize crops in Zambia.

[29] By January 3, 2017, about 90,000 hectares were affected according to reports released by the Zambian government's Disaster Management and Mitigation Unit.