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Changes in the Outbreak Region of the Brown Locust in Southern Africa

 

Margaret Kieser1, Anneke Thackrah2 & Jane Rosenberg3

1ARC-Plant Protection Research Institute, P/Bag X134, Pretoria 0001, South Africa.  (e-mail: Margaret Kieser).

2ARC-Institute for Soil, Climate and Water, P/Bag X79, Pretoria, 0001, South Africa. (e-mail: Anneke Thackrah). 3Natural Resources Institute, University of Greenwich, Medway Campus, Central Avenue, Chatham Maritime, Kent ME4

  4TB, United Kingdom (e-mail: Jane Rosenberg)

 


Introduction

Adult brown locust

The brown locust, Locustana pardalina (Walker), is endemic to the semi-arid Karoo regions of South Africa and southern Namibia and poses a threat to food security as far north as Zambia (Fig.1).  When good breeding conditions occur there is a dramatic increase in the density of solitary locust populations over a vast area of the Karoo. During the past 50 years, chemical control has mainly confined swarming populations within the outbreak area, with only short term invasions of neighbouring countries reported, such as in 1986 (Price and Brown, 1999)

As part of a project to develop a Brown Locust Early Warning System (BLEWS), the influence of environmental conditions on locust breeding success and migration patterns is being investigated.  This research includes the study of the frequency and distribution of outbreaks using historical records of brown locust control operations collected since the start of the 20th Century.

Fig 1. Limits of brown locust outbreak region and invasion areas

 

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Delimitation of the outbreak area from 1937 to 2001 

 

 
The first attempt at delimiting the brown locust outbreak area was undertaken by Faure and Marais in 1937 (Fig.2) and was based on the frequency of gregarious swarm occurrence per Magisterial District.  Within this general outbreak region, centres of high outbreak frequency were identified that later broadly coincided with Lea’s 1958 definition of the outbreak region (Fig.2).  Lea’s analysis was based on expenditure on locust control in each district, and boundaries were set according to the number of districts with >9 years of outbreaks from 1933 - 1958.  The current analysis (Kieser, Fig.2) is based on the distribution and frequency of gregarious first generation hatching per Magisterial District from 1984 to 2001.

Fig.2 Limits of brown locust outbreak regions

Discussion

A comparison of the outbreak area as defined by Lea (1958) and Kieser (2001) reveals a marked westward shift from Lea’s outline, especially along the eastern boundary. At the same time, the south-western limits appear to have expanded while elsewhere boundary changes are more irregular.

During the early years of organised locust control in South Africa, three locust poison depots were established at Kimberley, De Aar, and Middelburg to assist with the management of control campaigns. However, records of locust control operations from the South African National Department of Agriculture dating from 1984 to the present, indicate a decrease in locust activity in the eastern Karoo region and an increase in the west near Kenhardt. The Middelburg depot was subsequently relocated to Upington in 1997.

The reasons for these shifts still need to be investigated fully, but preliminary work suggests that changes in the rainfall and associated vegetation cover may play a role.

Rainfall

There was a general increase in annual rainfall across the Karoo from 1900 to date and Fig. 3 shows the increase and seasonal fluctuation of annual rainfall at Steynsburg from 1900 to 2000.

Fig. 3.  Annual rainfall at Steynsburg

There has also been a shift in the early summer rainfall (July – December) pattern.  The westward shift of the 150 mm isohyte (Fig. 4 and 5) corresponds almost exactly with the shift in the eastern boundary of the locust outbreak area between the 1958 and 2001 analyses, and suggests that mean early summer rainfall above 150mm is unsuitable for brown locust breeding  (eg. increased vegetation cover reduces the availability of suitable oviposition sites).  In contrast, the expansion of the brown locust outbreak area in the drier southwest (100-150 mm) may reflect enhanced breeding success due to improved soil moisture conditions for egg hatching. 

 

Fig. 4. Mean early summer rainfall (July to December) for the period 1933-1958

 

Fig. 5. Mean early summer rainfall (July to December) for the period 1984-2000

 

Vegetation

During the last century, fluctuations in the grass cover of the Karoo have occurred in response to seasonal changes in rainfall. Palmer, Hobson & Hoffman (1990) and Hoffman & Cowling (1990) agreed that increased summer rainfall results in increased grass cover and possible longer grasses (in the higher rainfall regions). These fluctuations may have influenced brown locust breeding success as brown locusts prefer bare soil with a scattered short grass cover for oviposition sites (Smit, 1939).

 

Conclusion

The distribution of the brown locust outbreak region appears to be a dynamic response to fluctuation in rainfall patterns and the associated vegetation cover, however the reasons for these shifts still require further investigation.

 

ACKNOWLEDGEMENTS

The authors wish to thank Roger Price and Dick Brown (ARC-PPRI) for their input on the brown locust, and Tim Hoffman (University of Cape Town) and Tony Palmer (ARC-RFI) for their assistance with vegetation information.

 

REFERENCES

Faure, J.C. & Marais, S.J.S. (1937). The control of Locustana pardalina in its outbreak centres. 4th Int.Locust conference, Cairo

Hoffman, M.T. & Cowling, R.M., 1990. Vegetation change in the semi-arid eastern Karoo over the last 200 years: and expanding Karoo – fact or fiction? South African Journal of Science. 86:286 – 294.

Kieser, M.E. (2001) Annual Report on Forecasting Outbreaks of Brown Locust in southern Africa. 1 July 2000 - 31 March 2001.  ARC- Plant Protection Research Institute, Pretoria.  5pp.

Lea, A. (1958) Recent outbreaks of the brown locust, Locustana pardalina (Walk), with special reference to the influence of rainfall. Journal of the Entomological Society of Southern Africa. 21(1):162-213.

Milton, S.J. & Dean, W.R. 1996. Karoo Veld Ecology and Management. Ed. Roger Ellis. ARC-Range and Forage Institute, Pretoria, South Africa. 94pp.

Palmer, A.R., Hobson, C.G. & Hoffman, M.T., 1990. Vegetation change in a semi-arid succulent dwarf shrubland in the eastern Cape, South Africa. South African Journal of Science. 86:392 – 395.

Price, R.E. & Brown, H.D. (1999). A century of locust control in South Africa. Cheke,RA, Rosenberg,LJ and Kieser,ME (eds)(2000). Workshop on Research Priorities for Migrant Pests of Agriculture ub Southern Africa, Plant Protection Research Institute, Pretoria, South Africa, 24-26 March 1999. Chatham, UK: Natural Resources Institute. Pp.37-49.

Smit, C.J.B., 1939. Field observations on the brown locust in an outbreak centre.  Un. of S. Afr. Dep. Agr. And For. Sc. Bull. No.190.

This publication is an output from a research project funded by the United Kingdom Department for International Development (DFID) for the benefit of developing countries.  The views expressed are not necessarily those of DFID.  [DFID project code R7779 Crop Protection Research Programme]

 

Published

Karoo Agric Vol 4 (1)