Thursday 19 December 2019

Somalia hit by plague of Locusts.

Somalia and neighbouring parts of Ethiopia are suffering the worst Desert Locust, Schistocerca gregaria, outbreak in 25 years according to the Food and Agriculture Organization. The Insects have already destroyed around 700 km² of crops, and are likely to do considerable further damage, as well as potentially spreading to neighbouring countries such as Kenya, Djibouti, Eritrea, South Sudan and Sudan. The large numbers of Locusts this year are thought to have been triggered by the exceptionally heavy rains across the region, which have brought moist conditions in which the Insects thrive. The outbreak is likely to prove difficult to control as the preferred method of dealing with such events, the spraying the Locusts with insecticide from aircraft, is effectively impossible in politically unstable Somalia.

A Desert Locust, Schistocerca gregaria. Food and Agriculture Organization. 

Desert Locusts typically have two or three generations in a year, but when conditions are favourable then five or more generations can occur. The females lay eggs in an ootheca (egg sack) in soft sand, where they can remain dormant until conditions are favourable or hatch within a few days. Hatchlings can reach maturity in about five weeks, eating as much as one-and-a-half times their body weight each day, when they are potentially able to breed again. 

The adult form of the Desert Locust has two forms, the more usual solitary form, which does not usually move far from the area where they emerge, and a gregarious form, which forms large swarms and can roam long distances. The change from solitary to gregarious Locusts can happen in individuals, which change from one to the other in response to a rise in population density, but the reverse, a change from gregarious to solitary Locusts, can only happen when a new generation reaches maturity.

 Solitary (top) and gregarious (bottom) Desert Locust nymphs. Compton Tucker/NASA/Goddard Space Flight Center/Wikimedia Commons.

The rains this year in East Africa are thought to have been made worse by the development of a meteorological phenomenon called a Negative Indian Ocean Dipole. Indian Ocean Dipole Phases are similar to the El Niño/La Niña climatic oscillation that affect the Pacific Ocean. Under normal circumstances equatorial waters off the east coast of Africa and west coast of Indonesia are roughly similar in temperature, however during a Negative Indian Ocean Dipole Phase the waters off the coast of Indonesia become significantly warmer. As the prevailing currents in the area flow west to east, this warm water is then pushed onto the shallower continental shelf of north Australia, where it warms the air over the sea more rapidly, leading to increased evaporation (which fuels rain) and a drop in air pressure over the east Indian Ocean and west Pacific. This in turn drives air currents over the Indian Ocean to flow more strongly west to east, leading to higher rates of  cooling off the coast of Africa (where more water is drawn up from the cool sea depths) and more warming off the coast of Indonesia, fuelling a feedback cycle that tends to remain through the winter season in any year when it forms. This leads to a particularly wet winter across much of Australia, as well as a potentially damaging heatwave in the north, while much of East Africa is at risk of drought (during a Positive Indian Ocean Dipole Phase the reverse happens, with drought in Australia and flooding in East Africa). 

 Areas of warming and cooling and air flow during a Negative Indian Ocean Dipole Phase. Australian Bureau of Meteorology.

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