The False Milkweed Bug (Lygaeus turcicus) is a seed bug that, although quite similar in appearance to the Small Milkweed Bug (Lygaeus kalmii), is not strongly associated with milkweed.
As can be seen in the photograph above, the False Milkweed Bug is most often found feeding on yellow composites (Family Asteraceae). These bugs were all photographed at Shaw Nature Reserve on what seems to be this insect species’ favorite food plant, the False Sunflower (Heliopsis helianthoides).
There are several members of the Lygaeidae family that are aposematically colored and found in North American prairies. As mentioned, it seems that the False Milkweed Bug does not typically utilize milkweeds. The Small Milkweed Bug feeds on milkweeds as well as other plant taxa. The Large Milkweed Bug feeds exclusively on milkweed. There is obviously a great case of Mullerian mimicry (distasteful organisms appearing similar to one another to benefit from a an easily identified color or body type) going on here, but it gets pretty complicated.
What has happened to the False Milkweed Bug? Is this a case of a species that once fed primarily on milkweed and developed aposematic coloration but has since switched food preference? Or, is this a case of a palatable species mimicking (Batesian mimicry this time) the aposematic coloration of a truly noxious species? Thinking about this, it is easy to see the selective advantages that could result from either possibility.
First, a little background…
Some insects that feed on milkweed benefit by concentrating chemicals called cardiac glycosides that are toxic irritants to vertebrate predators. Cardiac glycosides are an irritant to vertebrate herbivores (livestock) and vertebrates that feed on insects that feed on milkweed and store these compounds in their tissues. However, they are not a significant problem for insects that feed on milkweeds – they simply pass through their guts (insects that store these specific toxins, for example the monarch, must have biochemical changes to avoid toxic effects). The milkweed’s primary defense against the seed bugs and other herbivorous insects is the milky sap that gets forcefully pumped from any mechanical damage that is inflicted on the plant. For this reason the milkweed is a pain for an insect to feed on.
For a seed bug, with its piercing-sucking mouth parts, feeding on the gummy sap of a milkweed is a significant hurdle. Assuming the False Milkweed Bug once fed from milkweed primarily and gave it up would be a significant advantage. Keeping the aposematic coloration, which would allow it to gain the benefit from its vile-tasting, similarly colored cousins, still feeding on milkweed, would be advantageous as well. With my brief observations, the False Milkweed Bug still behaves conspicuously – feeding and doing everything else it does out in the open, suggesting that the aposematic coloration is still working in this mimic-model system, whatever the source history ultimately may be.
-OZB
A Thousand Acres of Silphiums 