A Monarch butterfly is seen at the oyamel firs (Abies religiosa) forest in Temascaltepec, Mexico.

Reuters/San Mateo Almomoloa, Mexico

After a staggering decline over the past two decades, the population of the iconic monarch butterfly is expected to recover following co-ordinated efforts across North American governments, Mexico's environment minister said on Thursday.

The monarchs, unique among butterflies for the length of their annual migration, are a major tourist draw to the temperate forests of central Mexico where millions hunker down for the winter.

The black-and-orange insects have been damaged by illegal logging and pesticide use that have destroyed the milkweed plants they depend on for food and to lay their eggs.

As a result, monarch populations plunged almost 90% to a record low of about 35mn two years ago, compared with a peak of roughly 1bn in the 1990s.

During the current season, which started earlier this month, authorities expect up to a four-fold increase of the delicate-winged insects in the pine and fir forests of central Mexico, where they arrive after a nearly 2,500-mile (4,000-km) journey that stretches as far north as Canada.

"We are calculating that three to four times more butterflies will arrive compared to last year," said Mexican Environment Secretary Rafael Pacchiano. The butterflies numbered 56.6mn last year.

He attributed the expected increase to a series of measures launched last year by a committee of officials from Mexico, US and Canada, which include setting aside more habit and better controls on logging and pesticide use.

The group also seeks to restore thousands of hectares of agricultural land in the US over the coming years to reduce the threat of extinction facing the insect and reach 225mn butterflies coming to Mexico each year.

The butterflies' ability to navigate their epic migration remains a mystery. While they are known to orient themselves by the position of the sun or the planet's magnetic field on cloudy days, it is unclear how new generations find their way to wintering sites they have never seen.