A Nouakchott resident sent us these pictures of her rubber tree - before and after the locusts arrived

The UN Food and Agriculture Organization says the plague could surpass that of 1987, which caused $300m damage to food production.

Mauritania says a million hectares of its arable land (2.47 million acres) have been affected.

The locusts this week engulfed the Mauritanian capital, Nouakchott.

Agriculture Ministry official Mohamed Abdallahi Ould Babah said Mauritania's initial estimate that 800,000 hectares (two million acres) might need treating with pesticides had had to be revised upwards because the infestation was so severe.

"The locust invasion could be catastrophic for the country, coinciding with the beginning of the sowing season, the need to feed our livestock and our complete lack of resources," he said.

The swarms turned green trees to brown skeletons in a matter of hours and even ate the grass from the pitch of the main football stadium.

Residents lit fires and rattled tin boxes filled with stones to try to chase away the insects, reports Reuters news agency.

Pictures of the locust swarms

The UN Food and Agriculture Organisation has warned that as a result of the heavy rains, locusts are breeding at alarming levels in southern Mauritania and Senegal.

Further south, The Gambia - where more than 70% of the population is dependent on farming for its survival - has declared a national emergency.

In Mali, the northern Gao region was overrun by the locusts.

There are fears that they could spread as far as the western Sudanese region of Darfur, which has already been devastated by a humanitarian crisis.

It was in western Sudan that the last major plague started 16 years ago, affecting 28 countries and spreading as far as India.