The milkweeds (Asclepias spp.) are prominent plants in the Midwest landscape, inhabiting moist disturbed areas such as areas next to roadways. A large, prominent plant represents a large potential source of nutrition for insects and other herbivores. Prominent plants typically are defended against herbivores through many means. Milkweeds are known for their sticky “milky” sap that oozes when leaves or stems are broken. The milky sap can gum the mouthparts of insects and interfere with digestion.
In addition to the physical defense of the milky sap, the milkweeds produce toxic chemicals, cardenolides, that deter feeding. Cardenolides have medical uses as heart stimulants. However, animals that eat cardenolide-containing plants such as milkweed can suffer heart palpatations and regurgitate the offending plant material. Many insects will regurgitate food containing cardenolides.
Some insects that feed on milkweed, such as the Monarch Butterfly and the large Milkweed Bug, will sequester cardenolides from the milkweed. These warningly colored (aposematic) insects use their orange and black colors to advertise to predators that they are loaded with toxins.
Adults of the milkweed beetle, Tetraopes tetraophthalmus, are easily spotted on milkweed because of their brilliant red colors with black spots. These colors are a warning to potential predators. However, the milkweed beetles only contain small amounts of cardenolides. They may have defenses against predators that do not involve cardenolide sequestration.
The adult milkweed beetles avoid the milky sap by severing the leaf veins basal to their feeding site. This cuts off the supply of sap to large sections of leaf and allows the beetles to feed on leaf areas. Most insects that feed on milkweed (including Monarch caterpillars) practice some form of “trenching” around their feeding sites to disrupt the flow of sap.
The adult female Milkweed Beetles lay eggs in Summer in the stubs of milkweed stems from the previous year. Milkweeds have thick stems that die in the winter and break. The stub below the stem break remains through the next year. The stub is hollow and provides a protected area for egg laying. The female will chew a hole in the stub with her mandibles and deposit an egg mass inside.
The larvae hatch inside the stub and enlarge the hole. Once they escape, they drop to the ground and tunnel through the soil. The larvae feed on the roots and rhizomes. Larval feeding is relatively well tolerated by milkweed plants.
The Milkweed Beetle, gets its scientific name, Tetraopes tetraophthalmus, from the configuration of its antennae and compound eyes. The antennae of many longhorn beetles arise on the head near the compound eye. Often the area around the antenna forms a notch in the compound eye. In Tetraopes tetraophthalmus, each compound eye is split into two distinct regions bisected by the antennae. Tetra-opes and tetra-ophthalmus both mean “four eyes”.
Look for these brightly colored beetles on milkweed this Summer.