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When locust migration goes wrong
Long distance migration is an important strategy for surviving in an environment where resources (i.e. food) are highly unpredictable in time and space. Migration may also be important in helping locusts to escape natural enemies such as parasitoids, nematodes and pathogens.
However, long distance migration is a risky business and many locusts die in the process of trying to find suitable areas for breeding. When swarms undergo long distance migration the direction of displacement is largely at the mercy of strong winds associated with passing fronts or low-pressure systems. Ideally, these winds will take them into areas of suitable habitat where rain has or is about to fall. Unfortunately, for the locusts, this is not always the case and many swarms get blown into habitats that are totally unsuitable or too dry for breeding. When this occurs they will continue to migrate until they reach suitable habitat or until their fat reserves are depleted.
During major plagues it is not unusual to receive reports of locusts being found washed up along the beaches of southern Australia or in the guts of fish after swarms were blown out to sea. In fact, offshore migration was considered to be the major factor leading to the collapse of the 1973-74 Australian plague locust outbreak.
A spectacular example of how locust migration can go wrong was discovered on Lake Frome, South Australia, in January 2001 by scientists from the CSIRO Earth Observation Centre. The flat, bright white surface of this normally dry salt lake was being used to test a new NASA satellite. When satellite pictures of the lake were analysed by CSIRO, mysterious dark lines were detected near a group of islands...
Ground crews were sent out to investigate these dark lines and what they found was a massive locust graveyard....
Although these dead locusts were found in January 2001 they were almost certainly part of the large outbreak that occurred in eastern Australia in autumn 2000. At this time the lake was full of water after record summer rains. It is speculated that on clear nights when a bright moon reflected off the surface of the water, the lake may have acted like a giant light trap, attracting masses of migrating locusts to a watery death. As the water evaporated, the dead locusts would have been left behind by the receding edge of the lake to form a shoreline or tidemark of bodies. The action of the wind would have helped to further concentrate the dead locusts into dense bands of bodies so large that they were visible from space.
The APLC wishes to thank Ms Janelle Kennard (CSIRO Mathematical and Information Sciences) and Dr Dean Graetz (CSIRO Earth Observation Centre) for permission to use the above photographs. See www.eoc.csiro.au for details of their research and for more photos from the Lake Frome project.