Phase polymorphism in migratory locust

The Migratory locust is highly sensitive and able of switching from solitarious to gregarious phase once the critical density threshold is surpassed (estimated at 2 000 adults/hectare in subtropical zones). Phase polymorphism is shown by morphological, anatomical, physiological, ecological and behavioural differences. Solitary adults are characterized by an arched non saddle-shaped pronotum, they are green or brown depending on the seasonal atmospheric humidity levels and males are substantially larger than females. Gregarious males and females are almost the same size with very dark markings. There are 5 to 7 instars in the solitarious phase and only 5 in the gregarious phase. Gregarious forms develop more slowly and produce fewer generations than solitarious forms.

Gregarious Migratory locusts are excellent flyers. Swarms migrate diurnally and are able to reach further and fly longer than solitary locusts which migrate just after nightfall using different wind systems. Phase transformation takes place in outbreak centres which often have more suitable and longer-lasting breeding and densation conditions than elsewhere. In south-Saharan Africa, these outbreak centres are located in moist zones with high residual humidity in the dry season (central Niger River delta in Mali, around Lake Chad and the Blue Nile Region in Sudan). These gregarisation sites are located in enclosed bush clearings in the southwestern part of the island. In Africa, the last serious widespread plague of Locusta migratoria migratorioides took place from 1928 to 1942. Since then, the spatiotemporal rainfall patterns and anthropogenic modifications in the largest and most important West African outbreak zone (central Niger River delta in Mali) have considerably altered the environment to the detriment of this locust. Construction of a dam to control rising and falling floodwaters of the river, development of farming with animal traction and sedentarisation of livestock herds (due to an increased number of wells) have destroyed many Locusta biotopes. Only temporary regional upsurges have occurred which were easily controlled by the national crop protection services.