Ever since my young cousins in New Jersey sent me photos of monarch butterfly eggs and tiny snow white monarch caterpillars with black heads (they don’t get colorful until later in development), I have been looking for the same on my milkweed plants. Unfortunately, the one and only monarch caterpillar I have found so far was dead, killed by a predatory stink bug. While great for the stink bug, this was, of course, tragic for the monarch caterpillar. We were unhappy as well since we have been encouraging milkweeds and planting all sorts of wildflowers in an attempt to create a safe haven for these beleaguered insects.
Initially, watching this menacing-looking bug drag the hapless carcass of the monarch caterpillar around, I was horrified and wanted to know who the culprit was. Identifying insects can be incredibly difficult. I tried some books and the internet, and I decided it was some sort of stink bug, perhaps an anchor stink bug. According to a “Featured Creatures” bulletin put out by the Florida Department of Agriculture and Consumer Services, the stinkbug “genus is recognized easily by the enlarged long and broadly oval scutellum (located behind the pronotum) … subapical spine on the front femora; and ventral pubescent patches on the males.” I didn’t know what any of that meant, so I resorted to iNaturalist and the awesome folks at bugguide.net for the final identification. My tiny predator did turn out to be an anchor stink bug (Stiretrus anchorago).
Now that I knew what an anchor bug looked like, I decided to at least learn what one of those seemingly obscure anatomical terms referred to. I call myself a naturalist and, upon finding out what a scutellum was, realized I’m a fairly ill-informed naturalist. All true bugs have a hard plate, called the scutellum, that is usually triangular, on their backs. The anchor bug’s scutellum is unusually large and U-shaped, something like a shield with a black anchor-like pattern on it. Now that I knew what to look for, the scutellum was, indeed, a very prominent and recognizable feature.
While refreshing my memory on insect anatomy, I was reminded of some of the differences between insects that are known as true bugs (some common examples are cicadas, water striders, stinkbugs and spittlebugs) and other insects, like beetles (not true bugs). It can be difficult to identify insects down to their species, but once you know what to look for, you should be able to distinguish a beetle from a true bug fairly easily. Beetles have chewing mouthparts whereas true bugs eat a liquid diet. True bugs have a beak, which they use to suck out the contents of whatever it is they are eating. The anchor bug uses its beak to harpoon its prey and then inject digestive enzymes that first immobilizes the prey and subsequently turn its insides into goo, which they then suck up with those same beaks. If you look carefully at the photo, you can see a tubular structure attaching the anchor bug to the caterpillar — that’s the beak, firmly implanted in the caterpillar. A number of pests (aphids, for example) are true bugs that use their beaks to feed on the fluids inside plants.
The easiest way to know whether you are seeing a true bug or a beetle is the wings on the back. Beetles have hard, leathery forewings that cover up and protect the hind wings. When at rest, the hard forewings meet in the middle of the back forming a line down the middle separating the two wings. These have to be lifted out of the way when the beetle flies. Only the first part of the forewing of a true bug is hardened (hence the scientific name, Hemiptera, which means half wing). At rest, the wings cross over each other so that from above they have a triangular shape. All true bugs also have a scutellum in between the wings, sometimes it is reduced in size and sometimes it is big and obvious, like in the anchor bug.
Anchor bugs live solitary existences, roaming the landscape in search of prey and are considered to be economically beneficial insects. As generalist predators, they are good biological controls of a variety of pest species (though they really aren’t common enough to make a big dent in a pest population). Look for them in your garden or a nearby field, they are strikingly beautiful bugs with their bold patterns of black and red, yellow or white. Now that I know who they are, next time I see one, I will welcome it gladly and not judge it for killing the occasional monarch butterfly, being, as they are, important members of our local backyard ecosystems.