Milkweed bugs, belonging to the genera Oncopeltus and Lygaeus, are true bugs easily recognized by their bright orange or red and black coloration. This striking appearance makes them highly visible. They are specialized insects whose diet is highly focused on the milkweed plant, allowing them to thrive on a host that most other insects cannot tolerate.
Feeding Habits and Primary Food Source
The diet of the milkweed bug centers almost entirely on the seeds of the milkweed plant (Asclepias). Both adults and young, called nymphs, pierce the seed pods to access the developing seeds inside. This consumption is essential, as adults reproduce most prolifically when fed milkweed seeds, while other plant parts, such as buds and flowers, result in much lower reproductive success.
Milkweed bugs feed using specialized, straw-like mouthparts known as a rostrum or proboscis. They insert this tube into the seed and inject digestive enzymes to pre-digest the contents. The bug then sucks up the resulting liquefied meal. Nymphs, especially the youngest instars, often feed on the outside of closed seed pods because their mouthparts are too short to fully penetrate the thick walls of mature pods. They rely instead on exposed seeds or the thinner areas of the pod.
The Chemical Relationship: Diet and Defense
The commitment of milkweed bugs to their host plant goes beyond mere nutrition; the milkweed provides a powerful chemical defense. Milkweed plants contain toxic compounds called cardiac glycosides, or cardenolides, which deter generalist predators. The milkweed bug has evolved resistance to these toxins, allowing it to tolerate the compounds and sequester them within its body for protection.
The sequestered cardenolides make the insect distasteful or toxic to predators. The bright orange and black coloration of the milkweed bug acts as an advertisement of this toxicity, a phenomenon known as aposematism or warning coloration. Predators learn to associate the insect’s vibrant pattern with an unpleasant experience, leading them to avoid the milkweed bug.
Survival Without Milkweed
When milkweed seeds are unavailable, milkweed bugs display limited dietary flexibility. They may occasionally feed on seeds from other plants within the same family, such as dogbane, which also contain similar defensive chemicals. Non-preferred food sources include nectar, general plant sap, or seeds of unrelated plants, such as sunflower seeds used in laboratory settings.
Small milkweed bugs sometimes exhibit predatory behavior, feeding on smaller insects like monarch caterpillars, pupae, or honey bees when seeds are scarce. However, these alternative foods do not provide the necessary cardiac glycosides, which compromises the bug’s chemical defense system.