Pest locusts

Oedaleus senegalensis (Krauss, 1877)

Family : Acrididae

Sub-family : Oedipodinae

Common name : the Senegalese grasshopper

Oedaleus senegalensis is widely distributed throughout Africa in the Sahel and northern Sudan regions and from Cape Verde to the Horn of Africa. It has been reported also in Arabia, India, Pakistan and the Middle East. When serious outbreaks occur this non-gregarious acridian (it is a grasshopper) forms hopper bands and swarms which can cause considerable damage to grain crops in dry tropical zones.

It is an average-sized green or brown grasshopper that is easily identified by the presence of a cross-shape on top of its pronotum, a dark crescent on the yellow hindwings and a yellow inner side of the hind femur and tibia. In mesotrophic environments, it can be mistaken for Oedaleus nigeriensis. The distinguishing criteria are that in the latter species the back edge of the pronotum is angled rather than rounded, the wings are bright yellow with a distinct dark crescent and the inner surfaces of the hind femur and tibia are reddish-orange. Oedaleus senegalensis is xeromesophilic. It is drawn to sandy and sandy-clayey steppes and savannahs with bare areas.

It is a typical graminivore which is able to seasonally migrate hundreds of kilometres to reach suitable conditions for its develop- ment. These flights occur at nightfall when temperatures are adequately high. At such times, they are easily captured in great numbers with light traps, thus indicating the amplitude of this migratory flow.

The main environmental factors for Oedaleus senegalensis have been classified and the most critical biological thresholds were defined :

•  photoperiod : shortened daylength (less than 12 h) occurring at the onset of the dry season stimulates egg laying in females, the eggs then remain in diapause. In rainy season, development progresses steadily but is dependent on other ecological factors.

• temperature : this parameter affects the development rate of eggs, hoppers and imagos :
  – under 22,5°C : no development,
  – from 22,5 to 27,5°C : development rate is temperature-dependent,
  – over 27,5°C : temperature is no longer a limiting factor.

• Moisture factor : expressed as mean monthly rainfall, this factor has four biologically significant thresholds :
  – under 25 mm : too dry,
  – from 25 to 50 mm : optimal,
  – from 50 to 100 mm : acceptable,
  – over 100 mm : too wet.

In West Africa, during the rainy season there is a balanced south-to-north and north-to-south movement of the intertropical convergence front and moisture conditions are only temporarily suitable in given zones. Migration patterns of Oedaleus senegalensis populations are also regular and well established.

In the rainy season, this grasshopper produces 3 generations G1-G2-G3 which develop in different ecological zones. Under ideal conditions, one generation can be achieved in about 2 months with 2-3 weeks egg incubation, 3-4 weeks hopper development and 2 weeks ovocyte maturation. In the Sahel, between 10° and 18° latitude North, the Senegalese grasshopper's range is divided into three ecologically complementary areas which are utilized successively. From South to North, these are :
  – first breeding area FBA, with 750-1000 mm annual rainfall ;
  – intermediate breeding area IBA, with 500-750 mm annual rainfall ;
  – northern breeding area NBA, with 250-500 mm annual rainfall.

The increased temperatures and first rains signal the development of G3 diapaused eggs. In crop zones, G1 hatching coincides with germination and sprouting of the cereals. In outbreak years, hopper bands can destroy millet and sorghum seedlings and farmers often have to reseed several times. Hoppers develop through 5 instars to become G1 imagos. Imagos then leave this area when it becomes too wet and generally migrate to IBA to lay. Attacks on millet in foliation generally have little effect on the emergence of heads since this plant recuperates easily at this phenological state. G2 imagos continue to move northward to the NBA, shifting from the Sahelian crop region into pasturelands. Eggs laid give rise to G3 hoppers. There could be competition between grasshoppers and livestock for pasture but this is difficult to assess because the region is sparsely populated and cultivated. In the NBA, habitats rapidly dry up at the end of the rainy season and G3 adults, assisted by the harmattan winds, are carried within a few days to the IBA and FBA. The eggs laid enter a diapause period in September. Depending on how quickly the intertropical convergence front retreats, some breeding females may lay their eggs in NBA or IBA but most are laid in FBA. At this time there is a considerable risk of damage to ripened millet heads.

Senegalese grasshopper outbreaks are prompted when their development and the rain distribution are synchronized. Complementary ecological areas may be found in two or three different countries. It is therefore in these countries' mutual interest to carry out coordinated joint survey and control operations. Control operations take place at two key periods during the year : at the beginning of the rainy season against hopper bands that threaten seedlings, and at the onset of the dry season against populations of G3 adults that threaten to ravage millet heads.

Destroying the egg pods of this acridian during the dry season is not worthwhile since their concentrations are not high enough. However, stratified sampling of egg pods and evaluations of egg pod parasitism levels are useful for estimating the size of the next hopper populations. These egg pod scouting campaigns can help to determine zones where the maximum number of hatching will potentially occur during the next rainy season. Control equipment can thus be pre-positioned at strategic sites.

General morphology

Size : males are 23-35 mm long, females are 30-48 mm long.

Overall body colour

Two different colour forms are noted, green and brown. Both hoppers and imagos are green or brown. Oedaleus senegalensis does not undergo phase transformation but upsurges do occur and they form hopper bands and loose swarms. At very high densities, there is a steady change from green or brown to black, especially around the wing buds and on the external side of the hind femurs.

Wing colour

Yellowish at the base with a brown crescent.

No prosternal tubercle