Spodoptera exempta

Definition

A noctuid moth species, the African armyworm, whose larvae undergo density-dependent polyphenism: isolated larvae develop into green, sluggish 'solitaria' forms, while crowded larvae become melanized, active 'gregaria' forms that march en masse to feeding sites. The gregaria phase is a devastating crop pest across sub-Saharan Africa, Yemen, and parts of Australia and the Pacific, capable of destroying cereal crops, pasture grasses, and sugarcane during rainy-season outbreaks following drought.

Etymology

Example

During the 2009 Kenyan outbreak, gregaria-phase Spodoptera exempta larvae migrated across maize fields at densities exceeding 1000 per square meter, leaving stripped stems within 48 hours.

Synonyms

• African armyworm
• okalombo
• nutgrass armyworm

Related Terms

• Polyphenism
• solitaria
• gregaria
• Armyworm
• Noctuidae
• Spodoptera
• density-dependent phase change
• agricultural pest outbreak

Usage Notes

The common name 'African armyworm' properly refers to this species alone, though 'armyworm' broadly applies to several Spodoptera species and other noctuid larvae with similar marching behavior. The solitaria and gregaria phases are sometimes confused with locust phase polyphenism, but in S. exempta the morphological and behavioral differences are less extreme and reversible. Outbreak forecasting relies on rainfall patterns and pheromone trap catches during the dry-season 'off-season' when populations persist at low density.