Bug of the Week

Red Milkweed Beetle (Family Cerambycidae)


Salutations, BugFans,

Red milkweed beetles (Tetraopes tetrophthalmus) are one of the BugLady’s favorite beetles (sharp-eyed BugFans may have noticed that approximately every third insect is a BugLady favorite). She loves RMBs because they are in the very-spiffy Long-horned beetle family Cerambycidae (of the elegant antennae), because even when occupied with matters of the heart, RMBs are cagey and elusive, and because they look mighty fine on a milkweed leaf.

Milkweed Beetle

There are 12 to 15 variously-marked species of beetles in the genus Tetraopes, the Milkweed Longhorns, north of the Rio Grande, but only three of those are Eastern. They are sometimes called Milkweed Borers and Four-eyed beetles. MBs have divided up the available milkweed species, and most kinds of MB favor the particular species of milkweed with which they evolved. They are found in the grasslands, gardens and road edges where their host plants grow. The BugLady photographed the lovely butterfly-weed-loving Blackened Milkweed beetle (T. melanurus), with its heart-shaped spots, in New Jersey.

The RMB’s scientific name,Tetraopes tetrophthalmus, is redundant; both the genus and the species name mean “four eyes” and refer to the way the socket of an antenna divides each compound eye in two, resulting in a “lower eye” and an “upper eye.” http://bugguide.net/node/view/419000/bgimage.

Milkweed Beetle

Common milkweeds (Asclepias syriaca) are the center of an RMB’s life (though the BugLady found mention of RMBs on Swamp, Whorled, and Green milkweeds). In early summer, a female RMB lays her eggs at the base of a milkweed stem, sometimes inserting them into the stem (she may also oviposit in a nearby grass stalk).

Newly-hatched larvae/grubs locate milkweed roots, either by tunneling south beneath the “skin” of the stem or by burrowing through the soil. They dwell in the soil, feeding on milkweed root through early fall (some sources say that they burrow in and feed within the root). They overwinter in the root and may resume feeding briefly in spring before pupating. RMB larvae create earthen chambers to pupate in, from which they emerge a month later as adults.

Milkweed Beetle

Adult RMBs eat milkweed leaves, buds, and flowers. Like some other milkweed leaf feeders, they sever leaf veins “upstream” of their feeding site in order to minimize their exposure to the milkweed’s sticky latex; vein severing is shown to decrease latex consumption by up to 92%. When an RMB gets latex on its mouthparts, it cleans them immediately by rubbing its face against the leaf; if it delays, it risks having the latex harden, gluing its mouth shut.

Adult RMBs eat milkweed leaves, buds, and flowers and they can get away with being red and black in a green world because milkweeds are toxic, and so, therefore, are RMBs, and red and black are (chorus of BugFans) aposematic (warning) colors. Apparently, there are some “primitive” species of Tetraopes that are not “locked into” toxic host plants and that have less a conspicuous coloration.

Milkweed Beetle

When the milkweeds are thick with females, males (which are smaller than females) become picky, and they favor larger females (plus-sized females have “greater reproductive value” – they pop out more eggs daily and cumulatively). When males outnumber females, males become competitive. Bigger males not only tend to be victorious, they tend to exclude from their smaller brethren all nearby females, not just the maiden in question. Female RMBs don’t send out pheromone signals (perfumes) to attract males, and males don’t use vision to search for females; males simply fly to milkweed plants (the more flowers, the better) and start looking for a little action.

The talented folks at Rutgers photographed an RMB and analyzed its flight in extreme slow motion so they could build a flying model of it (and they wrote about it in an article entitled “CAD Modeling of Insects and Insect Flight Animation” http://soe.rutgers.edu/files/2012CADModelingofInsects.pdf). One possible application of this technology – the next “RMB” you peruse might be packing a minicam and perusing you right back.

One final endearing trait of RMBs – they vocalize. But, let’s let Richard D. Alexander tell it. In his 1957 paper called “Sound Production and Associated Behavior in Insects,” Alexander wrote that “The red milkweed beetle was first noticed by the writer to make a sound when it was picked up and held. The noise made in this situation is a rather noticeable, shrill squeaking, produced by rubbing together stridulatory [friction-producing] structures on the back of the pronutum and the front of the mesonotum [the upper surfaces of the first and second segments of the thorax]. Several individuals were taken to the laboratory and placed on the leaves and flowers of milkweed (Asclepias sp.) in a quart jar. By listening at the mouth of the jar, one could hear a soft, almost continuous purring noise [oh, Richard – thank you for that image!]  ... Apparently both sounds are made by both sexes."

Milkwood Beetle

Alexander goes on to say that squeaking is produced when MRBs are held in the fingers, are exploring a cage, are stuck in a milkweed blossom, are placed on or fall on their backs, or are fighting (one beetle is gripping another with its mandibles). RMBs combine squeaking and purring when held in a closed fist, and when two individuals meet and touch antennae or crawl over each other. They will purr when standing motionless; but when they are quietly going about the business of crawling and feeding on milkweeds, they may purr or squeak.

The BugLady saw very few RMBs during the searing summer of 2012 and is looking forward to bending an ear to a milkweed.


The BugLady