The migration of monarch butterflies from Canada to Mexico is threatened by the use of pesticides that have reduced food sources for their epic journey, Mexican experts warned Tuesday.

The orange-and-black butterflies consume milkweed as they fly across the United States on their way to their winter grounds in the forests of central and western Mexico.

But the availability of the plant has "fallen considerably" due to the heavy use of pesticides in Canadian and US soybean and corn fields, said Luis Fueyo Mac Donald, the commissioner of the National Protected Areas of Mexico.

The monarch butterfly population was the lowest in 20 years during the 2013-2014 winter season.

"We don't know what will happen this season. We don't know what kind of population will arrive and we are a bit alarmed," said Homero Aridjis, a member of the Grupo de Los Cien environmental group.

The warning came after a meeting of a high-level group of officials and experts for the protection of the butterfly. The group will meet next month with counterparts from Canada and the US.

The leaders of Canada, Mexico and the US launched a working group to ensure the conservation of the butterfly when they held a North American summit outside Mexico City in February.

In the past, experts have also blamed illegal logging in Mexico for the butterfly's shrinking population.

Experts say the butterfly population has fallen from one billion to 33 million in the past 18 years, though it is not considered to be at risk of extinction.