Swarming Locusts (Schistocerca)
Grasshoppers in the genus Schistocerca, commonly called bird grasshoppers, are also known around the world as the swarming locusts.
One Schistocerca species, the Desert Locust (Schistocerca gregaria), is responsible for many historical and contemporary agriculture disasters across Africa, the Middle-East and Asia, created when they form billion member swarms.
Recent research suggests that Schistocerca species found in the United States descended from desert locusts that crossed the Atlantic Ocean some three to five million years ago.
|Additional Insect Information
Types of Insects
North America hosts ten Schistocerca species.
With the exception of the wide-ranging Spotted Bird Grasshopper, all inhabit a more limited geographical range.
Generally, though not always, bird grasshoppers can be identified by the presence of a dorsal stripe. The composite picture of three bird grasshopper species provides a comparative view of their physical similarities.
The American Bird Grasshopper (Schistocerca americana), second image, ranges from the central plains to the East Coast.
Large in size, they often fly into trees when startled. Within their range, they are known as voracious eaters, consuming a variety of crop plants and the leaves of crop trees.
The picture highlights the adult's speckled wings.
Another Southeastern species, the Obscure Bird Grasshopper (Schistocerca obscura), grows up to three inches in length.
The brown wings against a green body makes it stand out when startled and flying.
Scientists are also currently investigating comparative swarming behaviors between species. Most agree that swarm behavior is not a dominant characteristic of any native species.
The article To be or not to be a locust? A comparative analysis of behavioral phase change in nymphs of Schistocerca americana and S. gregaria (currently off line) concludes,
"Comparison with S. gregaria revealed that the magnitude of density-dependent behavioral change, particularly among final instar nymphs, was much reduced in S. americana."
Despite their comparatively less pronounced swarming behavior, American bird grasshoppers (Schistocerca americana) do exhibit density-dependent phase polyphenism. Their range is usually described as limited to the Southeast.
Research examining the relationship between drought and swarming locust populations continue, with a recent study (Periodic temperature-associated drought/flood drives locust plagues in China), concluding:
"We find consistent in-phase coherencies between locusts and drought/flood frequencies, and out-of-phase coherencies between locusts and temperature and between drought/flood and temperature at period components of 160-170 years."
Additional research on the historical locust swarms that visited the western United States suggests, "Locusts love drought. The dry conditions increases the nutritional value of vegetation due to concentration of sugars and nutrients and reduces plant defenses."
Thousands of years of extreme swarming locust outbreaks partially explains current locust management practices. Because of their consistent and episodic swarming patterns, most locust management strategies now rely heavily on a reactive, chemical based management strategy. That is, when conditions reach sufficiently dangerous levels, scientists and policy makers have agreed that chemical defenses become necessary.
According to the Food and Agricultural Organization (FAO), which monitors swarming locusts,
"At present the primary method of controlling Desert Locust swarms and hopper bands is with mainly organphosphate chemicals applied in small concentrated doses (referred to as ultra low volume (ULV) formulation) by vehicle-mounted and aerial sprayers and to a lesser extent by knapsack and hand-held sprayers."
The growing evidence of the relationship between drought and swarming locusts offers hope that the long and storied history regarding the deleterious effects swarming locusts create for agricultural activities need not continue for another thousand or more years.
© 2006-2012 Patricia A. Michaels