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Beetles are fascinating, but there are so many that it is difficult to know where to begin – more than 350,000 species that scientists know of. (And many more that have not even been discovered.)
Some are common, such as lady beetles (or “ladybugs”) and fireflies. Others you may not have heard of, such as the salt creek tiger beetle (one of the rarest insects in the United States) or the titan beetle, which is 6 inches long.
There are things the species have in common, such as playing an important role in the ecosystem by helping decompose matter and control pests.
And the adults have hard wings – actually two sets of wings of which the back set are thin and used for flying and the front are hard, protective covers for the thin ones when beetles are at rest. Most often, beetles don’t bite. They don’t sting. They don’t spread diseases. Beetles do not breathe like we do, but have holes in the sides of their bodies that act like lungs.
One family of beetle is the leaf beetle, but again there are so many of them (35,000 at least) that it is still difficult to know where to begin. These beetles are small and round or oval. Often colorful or metallic, they eat, as you might have guessed, leaves, plant stems and flowers. Leaf beetles evolved millions of years ago as flowering plants rapidly diversified. You might have seen a spotted cucumber beetle or a lily leaf beetle or even a golden tortoise beetle.
But how about we drill down a little more? How about we discuss one species of leaf beetle – the half-inch swamp milkweed leaf beetle (Labidomera clivicollis)? I’ve written about red milkweed beetles, which are longhorns. These are a different family of beetle with different characteristics.
As you may have figured out, many leaf beetles are named after the plant on which they are found, and this one is no exception: It is often found on swamp milkweed, its host plant (though you might find them on other types of milkweed, too, despite it not being a top preference).
The female cements her orange eggs on the underside of milkweed leaves. In about a week, grubs hatch and feed voraciously day and night on the toxic (to most other insects) leaves. Milkweed sap contains heart toxins called cardiac glycosides. These molecules bind to and block vital cell proteins. Blocking these proteins causes vomiting and can be fatal. The swamp milkweed beetle, however, has a mutation that prevents cardiac glycosides from binding and causing harm.
Critters that feed on toxic plants tend to be brightly colored – a warning to predators that they are not safe to eat. Swamp milkweed beetles can consume sticky milkweed sap safely, but they prefer to eat as little as possible; it gums up their mouthparts. So they chomp through the milkweed veins, let the sap run out and eat the nontoxic leaf. Monarch butterflies store cardiac glycosides in their wings, but swamp milkweed beetles do not store this poison at all. These beetles rely on their coloring to scare away predators. (Other orange-and-black milkweed eaters are milkweed tussock moths and milkweed bugs.)
After the grubs grow through four molts, they drop to the ground and form a pupa in which they transform into an adult. The beetles overwinter as adults, sheltering in leaves – often the large wooly ones of mullein plants, a flowering plant in the snapdragon family.
Female swamp milkweed beetles sometimes eat the eggs of other swamp milkweed beetles. The grubs may eat their mother’s eggs, which is called siblicide. About 15 percent of swamp milkweed beetle eggs are infertile, though, which some scientists believe is a way females protect fertile eggs from being eaten.
Female swamp milkweed beetles mate with multiple males, which helps to ensure genetic diversity in the species. The mating process can last for 2.5 days, although the average is only 18 hours. By staying with the female for so long, the male ensures that his sperm outnumbers the sperm from other male beetles.
Solo males attack copulating males for an opportunity to deliver their sperm. When copulation is finished, the male guards the female (rather than eating) to keep other males at bay until she deposits eggs, a cluster of about 60. Because the female may have had multiple partners, any two eggs in a cluster may be full or half siblings. As soon as the female produces her eggs, the male goes off, searching for another female.
Reader photo
Sherry Aaron sent this photo of a Carolina wren in her yard.
Have you taken photos of our urban wild things? Send your images to Cambridge Day, and we may use them as part of a future feature. Include the photographer’s name and the general location where the photo was taken.
Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.
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