To commemorate Independence Day this Bug of the Week visits an eclectic collection of insects found in the meadow and united by the fact that their principal body colors match those found in Old Glory. The red milkweed longhorned beetle is one of several brightly colored insects such as caterpillars of the monarch butterfly (The Royals are in the House) and milkweed tussock moth (Bugs in Orange and Black) found on milkweed. The immature stages of these insects eat milkweed and obtain noxious chemicals called cardiac glycosides from its leaves. Although these chemicals are heart-poisons to birds and mammals, they do not harm the well adapted insects that consume them. The bright red, orange, and yellow colors of milkweed-eating insects warn predators not to partake of this nasty meal. With so many insects eating milkweed plants, it is a wonder that milkweed survives at all. The milkweed plant has some clever defenses of its own. In addition to nocuous compounds in its tissues, the sap from which milkweed gets its name is a sticky, white, liquid similar to latex paint. Insects attempting to eat this plant soon find their jaws gummed-up with a rapidly hardening gob of goop. However, the milkweed longhorned beetle has a good trick to disable the latex defense of the milkweed. Prior to eating the leaf, the beetle carefully snips the major veins of the leaf. This prevents the latex sap from flowing to the blade of the leaf where the beetle takes its meals. With the sticky defense disabled, the milkweed longhorned beetle is free to devour the leaf without fear of gummed-up jaws.
The “white” insect of our Fourth of July trio is the dogbane tiger moth. This is a relative of other tiger moths and their larvae such as the wooly bears visited on November 7, 2005 in “Dashing caterpillars predicting weather." Moths are a favorite meal for many predators including bats. Bats are visually challenged creatures but evolved a nifty way to locate prey with echolocation. By emitting a series of ultrasonic sounds and then “listening” to the returning signals, bats are able to detect and identify prey much the same way a sonar system is used by one submarine to detect and identify another. In this game of cat and mouse several species of moths including the dogbane tiger moth have evolved “ears” enabling them to hear the acoustic signals their hunters make. These moths respond to a bat’s signal by taking evasive flight or simply ceasing to fly. However, the dogbane tiger moth has gone one better. By vibrating a drum-like organ called a tymbal, the tiger moth produces a series of clicking sounds that can be heard by the hungry bat. These clicks can ward off an attack by the bat. How does it work? First, clicks appear to confuse or “jam” the echolocation signals used by the bat to locate its prey and foil its attempt to capture the moth. Second, the clicks may act as a warning that the moth tastes bad. Caterpillars of the dogbane tiger moth feed on the noxious weed dogbane (See Gold in the meadow, the dogbane beetle, Chrysocus auratus.). Nasty chemicals ingested and stored by the caterpillars provide protection to the dogbane tiger moth as an adult. Visual predators such as birds hunt by day and can see the dogbane tiger moth. They may learn to reject it as a food source. Bats that hunt at night are, well, as blind as bats, but their keen sense of hearing may help them learn that the click of a dogbane tiger moth translates into a bad or dangerous meal and one to be avoided.
The “blue” in our gang of three is the familiar bluet. This beautiful male damselfly can be found from Maine to California where he cruises vegetation near ponds and lakes looking for small insect prey that he catches on the wing. His mate is a rather drab looking damselfly dressed in tan or pale blue with a stripe running down the center of the back. Despite her plain looks, he seems to love her nonetheless and the couple is often seen flying in tandem. The female bluet deposits her eggs on submerged aquatic vegetation. To place her eggs in the right location, she may submerge herself completely beneath the water. The male who is joined with her in flight and at rest on vegetation draws the line at an underwater interlude. He releases his grasp when his lady submerges to lay eggs. This unusual behavior makes one wonder if those eggs really need to go so deep underwater or if the lady bluet just needs a break from all that male attention. The red, white, and blue crew at Bug of the Week wishes you all a happy Fourth of July!
Information for this week’s bugs came from the “Beginner’s Guide to Dragonflies” by Nikula, Sones, and Stokes and from “The Love of Insects” by Thomas Eisner. More information on milkweed longhorned beetles and dogbane tiger moths can be found at the following web sites.