Pests locusts

Schistocerca gregaria (Forskål, 1775)

Family : Acrididae

Sub-family : Cyrtacanthacridinae

Common name : the Desert locust

The Desert locust is a large and highly feared acridian that invades many countries worldwide. In invasion periods, swarms sometimes reach southern Europe, Africa north of the equator, the Arabian pensinsula and the Indo-Pakistan region.

In recession periods, solitary locusts take refuge in the following outbreak zones:
  • the Indo-Pakistani border, where wind currents help to concentrate high numbers of locusts;
  • coastal regions along the Red Sea and the Gulf of Aden where the precipitation regime can provide suitable year-round locust breeding conditions;
  • around some mountain massifs where runoff can create suitable locust sites (central and southern Saharan massifs, southern edge of the Atlas mountains, western slope of the Oman mountains, valleys of the Mekran in Pakistan and in Iran).

Yearly rainfall in all of these zones ranges from 50 to 300 mm.

Solitarious Desert locusts are harmless to crops whereas in the gregarious phase their voracity, massive population movements, vast invasion area and swarm densities can be redoutable. Phase transformation occurs when the density threshold is above 250-500 adults/ha and from 0.5 to 5 hoppers/square meter.

The size disparity between adult males and females is much greater in solitarious than in gregarious locusts. Solitary adults are generally brown, grey or beige but never green. Only males become slightly yellow as they reach sexual maturity. Conversely, immature gregarious adults are pink and sexually mature adults are bright yellow when ready to breed. Solitary hoppers are green or brown with 5 or 6 instars, while gregarious hoppers only develop through 5 instars and are solid yellow with heavy black maculation (spots). Desert locusts can produce 2 to 3 generations yearly with a facultative developmental arrest in the adult state in dry periods during which time the locusts remain immature and breeding activity is sometimes stalled for several months. Females lay eggs 2-3 times in their lives, at most about 100 eggs for solitary females and 60 for gregarious females. The incubation period is around 10 days to 2 months depending on temperature and soil humidity. Overall, one solitary Desert locust generation lasts 2-6 months.

Solitarious locusts only breed in desert zones whereas gregarious forms are more flexible in their ecological requirements and able to colonize less xeric habitats thus covering a much greater surface area.

This species mainly gathers in dry environments with open steppe vegetation, including Panicum turgidum and Acacia ehrenbergiana or ephemeral formations of Tribulus mollis and Shouwia thebaica. The optimal rainfall range for the Desert locust is 25-50 mm per month. Solitary locusts fly at night during calm periods when temperatures are above 25°C to outbreak zones reactivated by the rains. They can be captured with light traps at such times. In contrast, swarms migrate during the day which enables them to take advantage of updraughts to cross orographic barriers and colonize areas thousands of kilometres away. The seasonal migration patterns of gregarious locusts are quite well established.

Preventive control could be considered for the Desert locust due to their well defined outbreak areas and invasion-recession cycle. The goal of this approach is to prevent upsurges that are very difficult to stop once triggered. Indeed, even intensive curative control operations are often ineffective and the environmental risks are much higher when acridicides are used to treat vast surface areas. There are three main steps in the preventive control strategy against Desert locusts:
  • surveys of ecological conditions in potential breeding and outbreak areas (meteorological, satellite remote sensing and bio-ecological modelling data) ;
  • organization of aerial and ground surveys in areas that become potentially suitable after heavy rainfall ;
  • control of Desert locust populations above a specific threshold limit, mainly in known outbreak centres.

Accurate identification of Desert locust imagos should be based on the following criteria :

General morphology

Size : males are 60-75 mm long, females are 70-90 mm long.

Overall body colour

In the solitarious phase, adults are pale yellow, grey or beige. In the older forms, only mature males show a slight yellowing of the hindwings. Solitary hoppers are usually green or brown, without any black.

In the gregarious phase, adults undergo considerable colour changes as they age. Immature juveniles are an overall pinkish colour. When immaturity is prolonged (imaginal quiescence) due to unsuitable breeding conditions (shortage of rainfall and/or inadequately low temperatures), imagos become progressively dark red or brown. This phenomenon is noted in quiescent populations of the Mediterranean region under cold weather conditions.

Sexual maturity begins when ecological conditions become suitable for breeding. Locusts then gradually turn a highly characteristic bright yellow. For a certain period of time, locust populations can be composed of a mixture of red and yellow locusts.

Gregarious hoppers are typically yellow and black. The extent of blackness (maculation) depends on the degree of gregarisation of the locust. Transiens forms, midway between solitarious and gregarious, show little maculation, whereas type gregarious populations are heavily maculated

Wing colour

Transparent, pale yellow, pinkish or reddish. No brown or blackish spots or crescents.

Shape of male cerci

Square, which is highly characteristic and distinguishes them from Anacridium spp., whose cerci are cone-shaped and pointed at the ends.

Prosternal tubercle

Visible, located on the ventral side between bases of the forelegs ; this feature is noted in the Desert locust, the Red locust and the Tree locust  it is missing in the Migratory locust and the Senegalese grasshopper.

Desert locust hoppers can be easily confused with those of Anacridium spp. which have many highly characteristic small white tubercles on the hind part of the pronotum.

Solitarious hoppers are generally green without any black spots in all stages of their development. Brown hoppers are observed but not as commonly. Gregarious hoppers are yellow with black maculation.