Any farmer worth their salt knows overgrazing is bad for the land and ultimately for the cattle. Now there's another reason to avoid this unsustainable agricultural practice: it may encourage some types of locusts to flourish.
Strange as it may seem, certain locusts shun nutritious grasses in favour of nutrient-deficient ones that result from overgrazing and poor soil management, Australian scientists have found.
"Some locusts thrive on vegetation grown in degraded soils," says Sydney University biologist Arianne Cease. "For farmers, our research fits well with long-term sustainability goals for managing land and, in particular, the production of sheep."
Sheep grow faster and bigger when fed vegetation grown in more nutrient-rich soils, she said.
"So limiting land overuse and overgrazing can have additional far-reaching benefits by decreasing the potential for locust plagues," Dr Cease said.
Her research, part of the university's rangeland grazing project, follows up on her doctoral research at Arizona State University which concentrated on the Mongolian locust in north-east China. "We found that heavy livestock grazing promotes locust outbreaks by lowering plant nitrogen content," she said.
Dr Cease is now working with biologist Stephen Simpson, director of Sydney University's new $500 million Charles Perkins Centre, to investigate how land use influences the composition and quality of vegetation and how this, in turn, affects locust outbreaks in Australia.
With collaborators at Arizona State, Colorado State, McGill and Yale universities, Dr Cease plans to add a social and economic dimension to her work. The interdisciplinary team, including bio-economists and social scientists, wants to help understand interactions between government agencies, farmers, locusts and regional economics.
"We'll compare closely related locusts in three countries with unique government systems in China, Australia and Senegal," Dr Cease said.
The team is joining forces with the Australian Plague Locust Commission, the Chinese Academy of Sciences and Agricultural University of Inner Mongolia, the Direction de la Protection des Vegetaux, in Senegal and US AID in Senegal.
Much to their surprise, the scientists found that degraded pastures in Inner Mongolia provided optimal nutrition for the Mongolian locust.
"Overgrazing leads to the increased loss of top soil and organic nitrogen – and eventually to plants with low nitrogen content," she said. "Since most nitrogen in plants is in the form of protein, this translates to a low-protein, high-carbohydrate diet."
In contrast to the notion that most herbivores are limited by protein, locusts performed best when eating low-protein plants collected from areas heavily grazed by livestock.
Her findings are not confined to China. Preliminary studies in Australia suggest that populations of Australian plague locusts and the Eastern plague grasshopper also prefer diets low in protein and high in carbohydrates.
"All three species are members of the same subfamily and prefer similar habitats," Dr Cease says. "So they may also prefer to eat plants in nutrient-depleted areas."
As a result of rising temperatures associated with climate change, plants that were previously poor hosts can promote locust outbreaks.
Wheat is one such grass, says another Sydney University biologist, Fiona Clissold. "Locusts grow faster and bigger on wheat at temperatures of 38 degrees or more," she said. "So, with warmer springs and summers, this increases the potential for locust outbreaks."
The same phenomenon has been observed in the US where higher temperatures have allowed a major pest species, the tobacco hornworm, to switch host plants and invade new habitats, escaping from natural enemies such as parasites and pathogens. "This is causing economic damage in areas that previously the pest could not colonise," Dr Clissold said.
Locust outbreaks, she said, result from a combination of factors associated with climate: food, temperature and the effects of population density.
"We can ascertain which nutrients maximise the growth, development, survival and reproductive output of locusts, but we cannot accurately predict under what conditions particular plants might be good hosts."
More than 70 per cent of agricultural land consists of grasslands, and this is likely to increase as biofuels replace petroleum-based fuels. As grasses are the main source of human nutrition, the pressure to increase production is immense, Dr Clissold says.
"The development of grass cultivars for livestock production and the biofuel industry seem to facilitate locust outbreaks," she said. "This is because reduced silica in grasses required for the biofuel industry increases the ability of locusts to gain nutrients. And what improves cow milk production also is great for locusts."
In the Mitchell grasslands, for example, native grasses have been replaced with buffel grass that retains water for longer. "This is likely to allow lipids to build up and increase the number of local locust populations migrating long distances and thus damaging crops," Dr Clissold said.
"It's very unlikely that we'll experience any significant activity in Victoria next season," said Victorian Plague Locust Commissioner Gordon Berg. "No major activity seems likely until at least before autumn 2014 – and even then that is a low probability as it would only occur if there was significant population build-up in the northern states."
Previous outbreaks have wrought havoc with crops, including cereals, vegetables and other horticultural commodities. When locusts succeed in laying waste to crops and pastures, the bill can run to hundreds of millions of dollars in lost earnings and damage to agricultural land.
Apart from spraying pesticides, what can be done to curb the insects? "There are a range of natural enemies of locusts, including parasites – for example, small wasps – and diseases such as fungal pathogens," Mr Berg said.
One fungal pathogen, which was isolated from locusts in Australia, has been developed into a bio-pesticide. It has reportedly fewer side-effects than some synthetic chemicals and can be used in environmentally sensitive areas, Mr Berg said.
At a glance, locusts could be mistaken for grasshoppers. They can be identified by the large dark spot on the tips of their hind wings and distinctive red shanks on their hind legs. Body colours vary from grey and brown to green. Male locusts can reach up to 30 millimetres in length, while females sometimes grow up to 42 millimetres.
Immature plague locusts, called hoppers or nymphs, have wings that are not fully developed, while the red of the hind-leg shanks is less developed than in adults. This makes them hard to distinguish from the developing stages of other locusts and grasshoppers.
Locusts in Australia come in four varieties. They include the spur-throated locust, the migratory locust, the yellow-winged locust and, most common, the Australian plague locust, or Chortoicetes terminifera.
A native insect, the Australian plague locust occurs naturally in north-west NSW and adjacent areas in Queensland and South Australia, a region known as the channel country. The locust is less common in Victoria and Western Australia.
Large-scale breeding in the Riverina often results in swarms crossing the border into Victoria in late spring, summer and autumn, infesting crops and pastures from Mildura to Corryong and south to the Great Dividing Range.
Solitary locusts tend to be relatively shy and timid insects. But in hordes, they become wanton crop destroyers.
The sudden switch from solitary to gregarious behaviour, a phenomenon known as phase change, is now understood. A team led by Professor Simpson, in conjunction with researchers at Britain's Oxford and Cambridge universities, has found the answer in the insects' neurochemical make-up.
The team uncovered a single neurochemical responsible for the locusts' swarming behaviour. This is serotonin, a neurotransmitter involved in the social behaviours of species across the animal kingdom, including crustaceans and rats. In humans, the compound can affect sociability.
"The fact that serotonin causes a transition from shy, antisocial animals into party animals means that, pharmacologically, gregarious locusts are on ecstasy or Prozac," says Professor Simpson.
Among other things, phase change was found to be caused by stimulating touch-sensitive hairs on the hind legs of crowded African desert locusts. In contrast, the antennae of the Australian plague locust were found to trigger phase-change during crowding.
"The two species come from different parts of the grasshopper family tree and have arrived at different mechanisms to achieve the same end – crowding induced gregarisation leading to swarming," Professor Simpson says.
His team's work on the neurological and neurochemical basis of swarming behaviour was conducted mainly by Dr Michael Anstey, who completed his doctorate at Oxford University supervised by Professor Simpson and Dr Stephen Rogers, a postdoctoral fellow at Cambridge University.
The group measured levels of 13 neurochemicals in swarming and non-swarming locusts.
"As locusts switched from being solitary to gregarious, the amount of serotonin in their central nervous systems increased," Professor Simpson said. "The next step was to determine if this relationship meant that serotonin was the cause of gregarious, and thus swarming, behaviour."
To check this, the researchers either added serotonin or prevented its production in locusts. The results, Professor Simpson said, demonstrated that serotonin was responsible for the change in behaviour. "Withholding or blocking the action of serotonin means that locusts will not become gregarious, while adding serotonin will cause the phase change from solitary to gregarious."
Might a product that inhibits serotonin's production help prevent locusts from forming swarms? "It's a good idea, but it would be difficult to create a locust control agent that interferes with serotonin," Professor Simpson said. Social behaviour in many animals depends on serotonin, he said, so if scientists used serotonin antagonists without a specific target, they would run the risk of affecting other processes in locusts, as well as affecting other animals.
"We'd need to be sure locusts have a unique serotonin receptor that causes phase change," Professor Simpson said. "That's something we have not yet identified."
Find out more about the Charles Perkins Centre at: sydney.edu.au/perkins/
Learn more about a potential locust crisis in Madagascar at: fao.org/emergencies/crisis/madagascar-locust/en/
For more on a recent grasshopper and locust conference in China, go to:
AusVELS Science; Biological sciences: ausvels.vcaa.vic.edu.au/Science/Curriculum/F-10
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