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close this book Locust handbook
close this folder 2. Desert Locust-Schistocerca gregaria
View the document Anatomy of a locust
View the document Life cycle
View the document Behaviour in relation to habitat
View the document Seasonal movements and breeding areas of desert locust during plagues and recessions
View the document Recession periods, outbreaks and the origin of plagues

Seasonal movements and breeding areas of desert locust during plagues and recessions

This section uses maps to show the seasonal changes in distribution, frequency of infestation and breeding of the Desert Locust in plagues and recessions. The Desert Locust lives in a generally dry, arid environment where rainfall is sporadic and seasonal. As this locust needs moist soil for egg laying and egg development and the hoppers need fresh vegetation on which to feed, they are only able to breed after periods of rainfall.

Desert Locust plagues do not start within permanent outbreak areas. Both solitary and gregarious adult locusts move downwind between seasonal breeding areas. The areas within countries where breeding occurs differs during plagues and recessions.

During plagues, the breeding areas associated with the spring rainfall are found in North Africa, the Middle East, southern Iran and Pakistan (Fig. 35). The resulting swarms move southwards as the area dries (thus no longer providing food or suitable laying sites) to the belt where summer rains occur. This is in Mauritania, Mali, Niger, Chad, Sudan, Ethiopia and southern Arabia. Swarms also move eastwards to the monsoon rainfall areas of Pakistan and India (Fig. 36). There is also a winter breeding season around the Red Sea coasts. In East Africa (Somalia, southeast Ethiopia, Kenya and northeastern Tanzania) breeding occurs between October and December on the short rains and from February to June on the long rains (Fig. 37). In contrast to plague populations, recession populations are restricted to the central, drier parts of the distribution area where the average rainfall is less than 200 mm. As a consequence breeding does not occur in North Africa, the Middle East and East Africa. The area of recorded seasonal breeding during all recessions since 1920 is shown in Figs 38-40. In any one season, however, rainfall and consequently, breeding will be more limited.

All areas are not equally liable to infestation and Figs 41-64 show for each month, the number of years between 1939 and 1963 in which swarms and hopper bands were recorded. This was a period of almost continual plagues. The maps show the frequency but not the severity of infestation in the degree squares. A comparison of the maps for each month shows the area most likely to have swarms or hopper bands and their changing distribution throughout the year.

In contrast, Figs 65-88 represent frequencies during a period of almost continuous recession. All reports of gregarious or non-gregarious adults or hoppers, were summarised for the period 1964-1985; 1968, which was a plague year, was omitted.

Figs 41-64. The following maps show, for each calender month, the number of years in which swarms and hopper bands were recorded during the 25-year period 1939-1963, i.e., they represent frequencies during a period of almost continual plagues. Records are grouped into 'squares' of one degree latitude and one degree longitude.

 


Fig. 41. Swarms in January.


Fig. 42. Swarms in February.


Fig. 43. Swarms in March.


Fig. 44. Swarms in April.


Fig. 45. Swarms in May.


Fig. 46. Swarms in June.


Fig. 47. Swarms in July.


Fig. 48. Swarms in August.


Fig. 49. Swarms in September.


Fig. 50. Swarms in October.


Fig. 51. Swarms in November.


Fig. 52. Swarms in December.


Fig. 53. Hopper bands in January.


Fig. 54. Hopper bands in February.


Fig. 55. Hopper bands in March.


Fig. 56. Hopper bands in April.


Fig. 57. Hopper bands in May.


Fig. 58. Hopper bands in June.


Fig. 59. Hopper bands in July.


Fig. 60. Hopper bands in August.


Fig. 61. Hopper bands in September.


Fig. 62. Hopper bands in October.


Fig. 63. Hopper bands in November.


Fig. 64. Hopper bands in December.

Figs 65-88. The following maps show, for each calender month, the number of years for which adults or hoppers were recorded during the period 1964-1985 (excluding 1968 which was a plague year), i.e., a period of almost continual recession.


Fig. 65. Adults in January.


Fig. 66. Adults in February.


Fig. 67. Adults in March.


Fig. 68. Adults in April.


Fig. 69. Adults in May.


Fig. 70. Adults in June.


Fig. 71. Adults in July.


Fig. 72. Adults in August.


Fig. 73. Adults in September.


Fig. 74. Adults in October.


Fig. 75. Adults in November.


Fig. 76. Adults in December.


Fig. 77. Hoppers in January.


Fig. 78. Hoppers in February.


Fig. 79. Hoppers in March.


Fig. 80. Hoppers in April.


Fig. 81. Hoppers in May


Fig. 82. Hoppers in June.


Fig. 83. Hoppers in July.


Fig. 84. Hoppers in August.


Fig. 85. Hoppers in September.


Fig. 86. Hoppers in October.


Fig. 87. Hoppers in November.


Fig. 88. Hoppers in December.

Incubation period and hopper development

Research has shown that about 20 mm of rainfall in a short period is sufficient moisture to allow eggs to complete their development. Rain does not need to fall over the breeding site as areas can become sufficiently moist from run-off from nearby hills and mountains. If eggs do not absorb enough moisture in the first few days, however, they can remain dormant and continue their development when rewetted. Dormant periods of up to 60 days have been recorded in the field.

The speed of egg development varies with the soil temperature; the warmer the soil the faster the eggs develop. Tables 7-10 show the ranges recorded in the field for each breeding area.

Hoppers, like the eggs, develop faster in warmer temperatures. Laboratory experiments showed that at 24ºC hoppers developed at 1.5% per day but at 38ºC this rose to 5%. Thus the total time taken for hopper development would be 66 and 20 days respectively. Table 11 shows the range of hopper development periods recorded in the field. Hoppers generally spend a similar period in each of the first four instars, e.g. 6-7 days, and a longer period in the fifth instar before fledging, e.g. 10 days.

To estimate the total time from laying to fledging the egg incubation period should be added to the hopper development period, e.g. laying in Niger on 1 June

egg incubation hopper development total

+ = ..fledging from 14 July

12 days 32 days 44

When fledging is complete the immature adults will begin to fly together in swarms. If the area in which they bred is beginning to dry out they will move to another area of green vegetation. If rain falls the swarms may mature and lay to produce a second generation. At the end of the breeding season when further rain is unlikely, swarms will move away on the prevailing winds to new breeding areas. These movements are summarised in Figs 35-37. It is at these periods that the spectacular long distance migrations of the Desert Locust take place. For example, swarms produced from summer breeding in Sudan can move westwards and northwards to invade Morocco in October; swarms from monsoon breeding in India can migrate via Oman and southern Arabia across the Gulf of Aden and invade northern Somalia in November; and spring swarms from Iraq can fly eastwards to India in June. Field officers should be aware of the likely sources of swarms which could invade their country. The Desert Locust Forecasting Manual (D. Pedgley, ed. COPR 1981) discusses these in far more detail than is possible here.

TABLE 7. Egg incubation periods (days) in the summer and monsoon breeding areas

 

June

July

August

September

October

India

12

10-21

10-20

9-17

13-36

Pakistan

 

10-16

9-17

9-13

 

Arabia

15-16

 

10-12

10-15

9-12

Ethiopia, Somalia, Kenya (below 1500 m)

15-17

10-23

10-20

10-12

 

Ethiopia, Somalia, Kenya (above 1500 m)

 

13-23

14-20

   

Sudan, Chad, Mali, Niger, Mauritania, Senegal

9-16

9-14

9-25

9-23

17

 

TABLE 8. Egg incubation periods (days) in the long rains and short rains breeding areas in East Africa

 

February

March

April

May

October

November December

Somalia, south-east Ethiopia

           

0-900 m

10-13

10-19

10-17

9-18

9-17

11-15

Kenya 900-1500 m

15-19

15-22

16-17

11-20

9-20

12-17

TABLE 9. Egg incubation periods (days) recorded in the coastal areas around the Red Sea and Gulf of Aden

January February March April May June July August September October November December

12-29

11-22

13-15

10-15

15 15

12-16

10-16

9-17 1-18

9-25

 


Table 10. Egg incubation period (days) in the winter-spring breeding areas in Asia, Arabia and the Middle East

 

TABLE 11. Hopper development periods (days) recorded in the field

Summer and monsoon breeding areas

30-39

Short rains

31-45 (63% of records 35-391

Long rains

27-47

Red Sea coast/Gulf of Aden

24-48 (71% of records 30-39)

Winter-spring

25 - 57