Monarch butterflies are famous for their autumn migration over long distances from their extensive North American habitat to a small area in the mountains of central Mexico where they spend the winter in a dormant state called diapause. The following spring the great majority of butterflies undergoes a return flight to their North American habitat where they mate, lays eggs and die. Northern migration from Mexico may involve three generations before monarchs reach summering grounds in the northern United States and Canada. The eggs hatch producing caterpillars that rely entirely on milkweeds (Asclepius spp.) for food. As adults the butterflies sip nectar from flowers but are not dependent on milkweeds for this. In the fall, the new generation of butterflies makes the return journey. The butterflies undertake this amazing journey through the use of a Sun compass that relies on chemical circadian clocks situated in their antennae.
The evolutionary basis for this remarkable migration is uncertain but one hypothesis is that the migration and subsequent entering into a dormant state in the cool mountain wintering habitat enables the butterflies to escape attacks by a protozoan insect parasite (Ophryocystis elektroscirrha), which causes malformed wings and shortened life spans in monarchs. Cold temperatures also are needed to enable the butterflies to reverse the Sun compass so they can migrate north to their summer range
Recent studies indicate that if ambient temperatures in the winter habitat are not cold enough as could happen with global warming, the wintering butterflies may not make the directional switch in their compass that is essential for northward migration.
In addition, Oberhauser et al. suggest that global warming will make the Mexican forests too wet for Monarchs causing them to take a shorter route to the Gulf Coast. If so, the range shift can expose the butterflies to greater parasite infections. Studies have shown that non-migratory monarch populations in Florida have higher parasite loads than migratory populations. In one study of monarchs in Florida, over 70% were infected.
Monarchs from the western US react similarly. In a study of 15,000 western monarchs, 8% of those taking the traditional route to Mexico were infected with the parasite, compared to an infection level about four times higher in monarchs which ended their migration in California. Apparently the long, arduous trip to Mexico that monarchs have historically taken every summer and fall, may have the advantage of screening out weaker, infected butterflies during migration while the more adaptive individuals reach the wintering grounds.
However, in 2014, for the first time in history, monarchs did not return to large parts of their traditional winter habitat. In fact, they were found on only 1.65 acres of the 45 acres they historically occupied in Mexico, suggesting a serious collapse in population size. In 2015, the occupied wintering area increased slightly to 2.79 acres (Green Peace 2015). This modest increase in habitat used by wintering butterflies from 2014 to 2015 is, of course, encouraging but far too premature to conclude that the monarch has recovered.
One reason suggested for the population crash that is independent of climate is the recent heavy application of herbicides in the U.S. to genetically-engineered crops designed to tolerate herbicide doses at levels high enough to eliminate wild plants including native milkweeds that sustain the monarchs. The milkweed-dependent butterflies are forced to expend more energy searching further afield for healthy milkweed plants on which to deposit their eggs. Adverse weather (year-to-year changes) on wintering or breeding grounds, habitat loss, which represents a more or less permanent loss, and as mentioned, parasites, also likely were involved in reducing overall butterfly survival.
Butterfly counts in northern Michigan and Ontario show little decline suggesting that bulk of the population losses occurs further south including in the Gulf states.
As mentioned, a small proportion of the eastern North American monarch population is relatively sedentary, in that it remains in Florida (and to a more limited extent in other Gulf states) and goes no further to overwinter. Additionally, some migrating butterflies reach Florida but go no further. Besides higher temperatures that stimulate parasite increases, the butterflies also encounter the non-native Asclepius curassavica in central and southern Florida (and in southern Texas). This tropical species is quite attractive in flower and is widely sold by nurseries and also sometimes establishes wild populations. It also is attractive to local monarchs which lay eggs on the foliage as they would on native milkweeds.
In its Florida habitat, the tropical milkweed remains in leaf throughout the winter. Unfortunately this tends to tempt more monarchs to forgo migration and stay in Florida where winter temperatures can be higher than in traditional wintering grounds. There appears to be no convincing evidence that consuming tropical milkweed foliage adversely affects the butterflies beyond tempting them to modify their migratory habits and thereby incur higher parasite loads. A. curassavica is native to much of the Neotropics including Mexico where monarch caterpillars rely upon it. Warmer winter temperatures may permit the tropical milkweed to expand its distribution in the U.S. northward thereby tempting more adult monarchs to avoid the long-distance journey to Mexico and remain where parasite exposure is greater.
COMPASS (Green Peace) spring 2016.
Entomologytoday.org (Jan 16, 2015)
Oberhauser, K.S, R.K. R. Nail and S. Altizer. 2009. Monarchs In A Changing World. Biology and Conservation of an Iconic Butterfly. Comstock Pub. Assoc.