Brown locust outbreaks and climate variability in southern Africa.
Abstract
The brown locust Locustana pardalina is a major agricultural pest in southern Africa, with populations periodically reaching plague proportions. Management and control would benefit from a predictive capacity at seasonal time scales, as yet unavailable. The results of a study into the dynamics and potential predictability of locust populations in southern Africa are presented here. The number of districts reporting locust control measures was used as a proxy for swarming brown locust populations. Spectral analysis of the annual number of brown locust infestations over southern Africa revealed dominant periodicity at 17.3 years. The data were low-pass filtered and the low- and high-frequency components were retained. The low-frequency component led the observed 18-year cycle in southern African rainfall by about 3 years, and was therefore likely to reflect endogenous controls on populations. Variability in the interannual high frequency component of brown locust infestations was strongly related to rainfall over the Karoo and Eastern Cape regions of South Africa. The highest correlations were with rainfall over the 12 months prior to the locust season (r = 0.64) and in particular with rainfall during December (r = 0.55). Evidence is presented that the high-frequency component is related to the Pacific El Niño/Southern Oscillation (ENSO) and that high-frequency locust activity is abnormally high (low) during La Niña (El Niño) events. The high-frequency component of locust activity correlates positively and negatively, respectively, with sea surface temperatures over the tropical western and eastern Pacific Ocean many months in advance of the locust season. Activity also correlates positively (negatively) with sea surface temperatures over the southwest Indian Ocean and the Southern Ocean (west and northwest Indian Ocean). These relationships occur later than those in the Pacific, developing in the austral winter and peaking in early summer. This pattern of correlations and the associated atmospheric circulation anomalies is consistent with ENSO-related and non-ENSO related patterns of climate variability. The results suggest that there may be considerable scope for future development of models for the seasonal prediction of brown locust activity in which high frequency variability is related to climatic indices.