Monarch butterfly, female. K. D. Harrelson

A monarch butterfly on Butterfly Milkweed (Asclepias). Kimberly Sayers

Monarch butterflies (Danaus plexippus) overwinter in areas of the Mexican state of Michoacán. Millions of Monarchs gather in pine-oak forested areas of Michoacán known as “ The Monarch Butterfly Biosphere Reserve ”. The area is a UNESCO World Heritage site for the protection of the overwintering sites of the monarch butterfly, considered as a exceptional natural phenomenon. At this reserve, several colonies of butterflies mate before heading north to Canada in the early spring.To estimate the number of individuals in the population, researchers use aerial images of the forest to measure the area covered by the insects. This winter, the area in the Michoacán reserve covered by the colonies of butterflies was about 1.2 hectares, while last year at the same time they occupied an area of about 2.9 hectares. This is the third consecutive year showing a decline in the population of these remarkable migratory insects. During better times , the area covered by millions of butterflies belonging to about 12 colonies has been as large as 4.7 hectares.According to researchers, this decrease of the butterflies’ population is due to droughts, fires, the use of pesticides , herbicides, and a severe decline in the species of plants used by the adults to breed and for the larvae to feed on. The Monarch butterflies rely mostly on the availability of various types of milkweed they normally find at the breeding grounds and along the migration route. Wildfires, droughts, and the replacement of wild milkweed by commercial crops have decreased the growth of the common milkweed (Asclepias syriaca),and although there are several types of this kind of plant, there is not enough to allow the butterflies to complete the return journey between Mexico and Canada, reports Science Daily Craig Wilson , a researcher at Texas A&M University, says the severe drought in recent years in Texas and in most of the southern states is a major cause of the decrease in the Monarch butterfly populations. According to Wilson, the planting of milkweed in all states along the path of the monarch butterflies should become a national priority to make sure there is a future for this awe-inspiring insect.