At this time of the year, the Monterey Bay area—and especially Pacific Grove—hosts many Monarch butterflies during their annual winter hibernation.
The butterflies arrive in stunningly large numbers, but we in the Monterey Bay area see only Monarchs that live west of the Rocky Mountains—five percent of the total population.
The larger numbers flow between north Central United States and Canada in the summer and Mexico in the winter. The number of Monarchs has been declining alarmingly. Counting the individuals in a vast flight of Monarchs is quite impossible, so scientists measure them by the area they occupy, measured in hectares (2.47 acres), and then estimate the number of butterflies per hectare. In their prime years, some 450 million Monarchs roosted in Mexico. This year, lepidopterists have estimated that three million will arrive in Mexico.
The decline in the Monterey Bay area’s Monarch population has been less than that of the larger population, but still disturbingly great.
Several factors contribute to this decline, but the principal contributor appears to be loss of habitat. In particular, the loss of the milkweed plant that constitutes the source of both the principle food for Monarch caterpillars and the alkaloids that make the Monarch unpalatable to most predators. Large-scale farmers are using Monsanto’s Roundup® herbicide to wipe out a variety of weeds, including milkweed.
The decline of Monarch butterflies means we have fewer opportunities to see them in our gardens and other natural environments but the Monarchs are also an important source of food for some birds.
In the larger context, the impacts of commercial farming on the Monarch population are being repeated by impacts on a range of beneficial insects. The decline of honeybees, for example, has been attributed to a class of agricultural chemicals.
This is a large-scale problem that the public and private sectors, working together, should address over the long term. Meanwhile, home gardeners can support Monarch butterflies in two easy actions.
First, provide nectar sources for the adult butterflies by planting brightly colored flowers that are native to the Monterey Bay area.
Second, provide milkweed plants for Monarch caterpillars to feed on. The genus Asclepias contains about 140 milkweed species, including Asclepias californica (California Milkweed), which is native to most of this state.
To learn more, visit the Pacific Grove Monarch Butterfly Sanctuary (Off of Ridge Road near the Butterfly Grove Inn) for a docent-led tour, and attend the free Monarch Butterfly Talk at 1:00 p.m. at any Saturday, November-to-February.
The website for the Center for Food Safety illustrates the areas in which the agricultural chemical glysophate is being applied in the Monarch’s migration path.