A severe plague of locusts has infested about half of Madagascar, threatening crops and raising concerns about food shortages, a UN agency says.The UN's Food and Agricultural Organization (FAO) said billions of the plant-devouring insects could cause hunger for 60% of the population.About $22m (£14.5m) was urgently needed to fight the plague in a country where many people are poor, the FAO added.It was the worst plague to hit the island since the 1950s, the FAO said.FAO locust control expert Annie Monard told BBC Focus on Africa the plague posed a major threat to the Indian Ocean island.'Generation of locusts'"The last one was in the 1950s and it had a duration of 17 years so if nothing is done it can last for five to 10 years, depending on the conditions," she said."Currently, about half the country is infested by hoppers and flying swarms - each swarm made up of billions of plant-devouring insects," the FAO said in a statement."FAO estimates that about two-thirds of the island country will be affected by the locust plague by September 2013 if no action is taken."It said it needed donors to give more than $22m in emergency funding by June so that a full-scale spraying campaign could be launched to fight the plague.The plague threatened pasture for livestock and rice crops - the main staple in Madagascar, the FAO said."Nearly 60% of the island's more than 22m people could be threatened by a significant worsening of hunger in a country that already had extremely high rates of food insecurity and malnutrition," it added.An estimated 85% of people in Madagascar, which has a population of more than 22 million, live on less than a dollar a day.The Locust Control Centre in Madagascar had treated 30,000 hectares of farmland since last October, but a cyclone in February made the situation worse, the FAO said.The cyclone not only damaged crops but created "optimal conditions for one more generation of locusts to breed", it added.