Butterfly weed, which has the scientific name Asclepias tuberosa, is a common and very visible plant in eastern North America. Vivid orange flowers appear in clusters during summer at the tips of stems, offering a banquet of abundant nectar to butterflies, hence the common name. In addition to attracting adult butterflies, the plant’s foliage also attracts a variety of bugs, including the larvae of monarch and queen butterflies. Another frequent insect visitor to butterfly weed is the milkweed bug.
There are two types of milkweed bug. The larger of the two is Oncopeltus fasciatus and the smaller is Lygaeus kalmii. Milkweed bugs belong to the order of true bugs—or Hemiptera. They undergo incomplete metamorphosis, which means that immature insects, known as nymphs, resemble miniature adults except for the absence of wings. The nymphs of both of these are bright orange and wingless.
As the nymphs grow they molt their hard exoskeletons and black wing pads begin to develop on the thorax. With the fifth and final molt, the fullly grown winged adults are produced.
Adults feed on nectar and the seeds of milkweed and other plants and are good fliers.
The large milkweed bug nymph is mostly orange with black legs, head and wing buds. The small milkweed bug nymph is more reddish-orange and has two black spots on the first segment of the thorax above the wing buds. Adult large milkweed bugs are orange except for black legs, a black triangle right behind the head, a broad black band across the middle of the bug, and black wing tips covering the rear of the abdomen. Adult small milkweed bugs are gray-black, with orange or red bands across the top of the thorax and bordering the inner wing edges to almost meet in the middle of the bug’s back, forming a loose X. In eastern varieties, the wing tips are deep black. In western varieties, there is a conspicuous white spot in the center of the black area.
Often the nymphs of milkweed bugs are highly congregated on the plants while they are feeding and are conspicuous to possible predators. However, an inexperienced bird, lizard or other predator soon learns to avoid them. The bugs concentrate toxic and unpleasant-tasting chemicals present in milkweed in their bodies. The bright coloration of the nymphs and the distinct patterning of the adults helps the predator remember what not to eat in the future. Such warning signals are known as aposematic coloration.
On butterfly weed, nymphs can be seen on stems, seed pods and the undersides of leaves. They congregate more at night. Once they start feeding, they are fixed in place by the sucking-piercing mouthparts embedded in the tissues of the plant. The flying adults can be found on all parts of milkweeds, and they also visit flowers and fruits of other plants.
The milkweed bug feeds on the sap of the butterfly weed plant using its piercing-sucking mouthparts. It also eats the seeds of the plant. Neither activity harms the butterfly weed plant. It is safe to allow the striking insects to remain on the plants.