Taking a stroll through my neighborhood with my sister, we passed a home with two magnificent butterfly bushes flanking a barn-style shed. The deep purple spikes attracted a frenzy of fluttering commotion - over a dozen swallowtails hovered over each bush. My sister remarked, "Wow, butterfly bushes really do work!" Although I know these plants to be marvelous for attracting their namesake, this season, more so than most, I've noticed a bevy of butterfly activity everywhere.
Paula Shrewsbury, writing for The University of Maryland Extension, offers her best guesses to explain this Mother Nature mystery. Because nature is a balancing act of incredible intricacies, many factors could explain the increase in the butterfly population this year. First, our weather plays a significant role. This season has seen substantial rainfall, and the wet soil is ideal for new growth. More vegetation means more nutritious food sources for caterpillars than in drier years. This boost in nutrition supports successful development and survival of caterpillars and their eggs resulting in a larger number of butterflies this year. On the other side of the food chain, there may be a reduced interest by some of the caterpillar's natural predators, such as birds; birds had other tasty options this year with the Brood II periodical cicadas!
But that is only one part of the mystery. While there has been an increase in the number of butterflies this year (particularly Eastern tiger swallowtails), I have not noticed any monarchs at all. Although weather and environmental factors worked for one group, they may work against another. The monarch is the only butterfly that, like birds, makes a north-south migration. Also like birds, their survival depends on conditions that allow them to eat, rest, and reproduce along the way. Taking cues from nature, around October monarchs embark on a journey south and westward, to Mexico or parts of Southern California - an impressive feat for a butterfly! Since we are east of the Rockies, most of our Monarchs travel to Mexico and hibernate in the oyamel fir trees - using the same trees year after year. Because this trip outlasts a butterfly's life cycle, the females lay eggs for the next generation along the way. The overwintering population usually returns to the US sometime in February or March.
Though it is normal for the monarch's population to fluctuate, the steep decline may be the result of the weather last year and this year. Last summer's drought and this spring's cool temperatures challenged the monarchs, slowing down their migration and impeding reproduction. Heavy rains and severe storms that hit our coastline did not help the monarch's voyage either. And while some butterflies may be experiencing a reprieve from predator threats, the use of pesticides in Midwest corn and soy fields is on the rise, choking out milkweed plants - a key source of food in an important habitat for monarchs.
Though these are only speculations, it goes to show how delicate and interdependent our ecosystem truly is. While we can relish the abundance of swallowtails in a season of favor, on the rare occasion we spot a monarch we can appreciate its achievement of survival, sometimes against the odds.
You can help support the butterfly population in your area by planting a few caterpillar and butterfly favorites in your garden, such as butterfly weed, coneflowers, joe pye weed, asters, parsley and dill.