Monarch Population Status

On February 26th, World Wildlife Fund Mexico in collaboration with SEMARNAT and CONANP and the Monarch Butterfly Biosphere Reserve (MBBR) announced the total forest area occupied by overwintering monarch colonies. Nine colonies were located this winter season with a total area of 4.01 hectares (up from 1.13 ha last winter; see our February 26th Monarch Population Status post).

Due to an unprecedented rain/sleet storm at the overwintering sites on 8-9 March, the status of the monarch population is uncertain at this time. The storm was accompanied by strong winds that knocked down many trees and buried large numbers of monarchs beneath several inches of sleet. Many froze in place in the trees. There are no firm estimates of the numbers of monarchs that either died or survived this storm. Observers with various backgrounds and experiences visiting different colonies have estimated the losses at 3-50%. Fortunately, many monarchs had already left the area.

Recolonization of the South Region (Texas, Oklahoma, and Kansas) this spring has been slow. The numbers of first sightings recorded by Journey North to date are similar to the numbers reported in 2013, the spring that produced the all-time low number of overwintering monarchs (0.67 hectares) in Mexico. However, first sightings are an imperfect indicator of what to expect for the rest of the season. The conditions in Texas for reproduction by these returning monarchs are key. Successful reproduction in the South Region depends on the temperatures in March-April, the number and distribution of milkweeds and nectar sources, the abundance of fire ants, and other factors. These conditions vary from year to year but have a significant impact on the number of first generation monarchs that move north from this area in May and early June.

The size of this first generation moving north will largely determine the degree to which the population will be able to recover from the losses incurred during the late-winter storm. Right now, based only on first sightings, the overwintering population is likely to be in the 1-2 hectare range. We will have a better sense of how the population is developing later this month. The long-range forecasts for May and early June favor movement northward by the first generation monarchs. The unknown at this point is the number that will be moving north. If the number is low, the overwintering numbers next winter could be close to 1 hectare again. We will post another population status update via our blog in the coming weeks.