Assessing the relationship between outbreaks of the African Armyworm and Climatic Factors in the Forest Transition Zone of Ghana

Abstract

The African armyworm Spodoptera exempta (Walker) is an important migratory pest of cereal crops and grasslands in sub-Saharan Africa. It demonstrates great variability in the extent and severity of infestation of its host crops. The African armyworm is known to cause extensive damage to maize crops and rangeland in the transition zone of Ghana. The work reported here was an investigation of the relationship between the Normalized Difference Vegetation Index (NDVI), rainfall and temperature and how they influence the outbreak of this moth species in the Ejura-Sekyeredumase district of Ghana. The temporal patterns of the variables and their interrelationships were evaluated through graphical, logistic and standardization z-score transformations. A strong similarity between temporal patterns of vegetation index and rainfall was established. On the other hand, the temporal pattern of temperature runs opposite to NDVI and rainfall patterns. Standardized NDVI anomaly revealed periods of low vegetation index with corresponding high wetness denoting damage to vegetation due to the activities of the insects during outbreaks. These revelations confirm reports gathered from local famers. NDVI therefore appears to be a good predictor of armyworm outbreaks. Indeed a relationship was established between the occurrences of the moth species and multi-temporal 10-day NDVI signals. The study confirmed that rainfall and temperature influence the occurrence of armyworms.