Bugs normally don’t garner much adoration from people, but monarch butterflies (Danaus plexippus) are an exception. Monarch butterflies are arguably the most beloved and instantly identifiable insect in the United Sates. Perhaps nowhere else does an insect receive higher regard than in Pacific Grove, CA, which has gone so far as to adopt the moniker Butterfly Town, USA. Stu Weiss, Chief Scientist at the Creekside Center for Earth Observation, has been studying monarch butterflies for over two decades. “They represent the insect world, which is the most diverse order of life on Planet Earth.” said Weiss “And they’re good ambassadors because they’re beautiful.”
Weiss uses a technique called hemispherical photography to analyze monarch butterfly overwintering habitat. While facing north, he aims his digital camera directly up towards the forest canopy. The camera is affixed with a 180° fish-eye lens that produces an image of the sky partially blocked by the trees. Weiss takes a series of these images at specific locations corresponding to a grid. With the help of some special software, he is able to map out how much sunlight and wind is able to penetrate the forest at specific locations. This information is important because overwintering monarchs prefer microhabitats that provide both plenty of sunlight, but are protected from the wind.
With the information gleaned from his hemispherical photography, Weiss is able to characterize the monarch habitat and provide guidance on the long-term management of the major monarch overwintering sites. The City of Pacific Grove placed Weiss in charge of developing the habitat restoration and management plan for the Monarch Grove Sanctuary, and he has overseen the planting of additional Eucalyptus trees to improve wind protection for the monarchs. While small in size, the Sanctuary is a vital destination for monarchs and a major tourist attraction boasting visitors from around the world.
In addition to their pleasing appearance, monarch butterflies are famous for their extended migrations. The type of lengthy annual north-south migration undergone by monarchs is more commonly associated with birds. Monarchs spent most of the year in close proximity to their host plant, milkweed. Each fall, as the days grow shorter and the milkweed recedes, monarchs journey to their overwintering grounds.
The Rocky Mountains split the monarchs into two populations. Those butterflies that spend their summers east of the Rocky Mountains descent upon forests located within the Trans-Mexican Volcanic Belt Pine-Oak Forests not far from Mexico City. The monarchs that spend their summers west of the Rockies wait out the winter months in forests along the California Coast. Overwintering monarch populations return to the same forests year after year, even though none of the individual butterflies have ever been to these locations before.
Depending on the climatic conditions, monarch butterflies have three or four generations per year. How the butterfly’s great-grandchildren are able to find the same specific forests year after year remains a mystery. New research indicates that monarch butterflies possess photosensitive proteins called Cryptochromes that allow them to set an internal circadian clock. When exposed to UV light, Cryptochromes are also sensitive to magnetism. This allows the monarchs to follow patterns in the Earth’s magnetic field which guide them in their far-reaching travels. Monarch butterflies may also return to specific groves because the overwintering conditions at those particular spots are similarly attractive year after year. As Weiss explained, “The presence of other monarchs is a really good indicator it’s a good place to be a monarch, kind of crowd sourcing. So if a butterfly’s flying around looking for a place to land, if there are other monarchs there, it’s a good indicator that hey, that’s a good place to land.”
In the spring, the monarchs go into a mating frenzy, and then they depart from there overwintering spots to find the emerging milkweed throughout their range. Female monarchs lay hundreds of eggs on milkweed plants. The eggs hatch and the caterpillars gorge on the host plant. As the caterpillars grow, they sequester compounds called cardiac glycosides which make them distasteful – even toxic – to predators. During the pupae life stage the caterpillars transform into butterflies within the solitude of their chrysalises. As flying adults they feed on sugary nectar from milkweeds and other plants with their long tube-like proboscises. This cycle of reproduction repeats throughout the spring and summer as the monarchs disperse, filling out their geographic range.
Because monarch butterflies are intimately tied to milkweed, they are especially vulnerable to habitat loss from development and agriculture. Monarchs also need to find dependable overwintering sites, and both the eastern and western overwintering populations experience threats to these locations.
Monarchs do have one advantage, which is that humans find them beautiful. Enticed by the monarch butterfly’s beauty, people have gone to great lengths to study them and provide for their needs. Erica Krygsman, Monarch Alert Monterey Co. Field Coordinator is one of those people. She has the colossal task of counting the overwintering monarchs. So how does someone count thousands of fidgeting butterflies as they hang from a tree?
“Well you don’t count them individually”, Krygsman said. “Basically you count the butterflies when they are in clusters in the trees, and you go by each group and you count…you count a small group and extrapolate to the rest of the cluster.” For the Monarch Grove Sanctuary location, Krygsman’s counts for the winter 2011-2012 hover around 10,000 butterflies. The detailed counts made by Monarch Alert help the scientific community trace the fluctuations in the western monarch butterfly population size and demographics.
According to Weiss, “monarch butterflies are listed as an endangered phenomenon, because of the mass of the migration, the length of the migration, and the concentration of butterflies in relatively few overwintering sites. They’re actually vulnerable. You could cut down 20 groves of trees in coastal California and pretty much kill off the overwintering monarchs in California, they’re that concentrated. In Mexico, they occupy maybe a few dozen hectares of high elevation fir forests that are under extreme pressure.”
You can visit the overwintering monarchs during the winter months at the Pacific Grove Monarch Grove Sanctuary, and take a tour of the Monarch Sanctuary offered by the Pacific Grove Museum of Natural History. Other good places to find overwintering monarchs are Natural Bridges State Beach and Pt. Lobos Nature Preserve.