Four-Eyed Beetle

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.

Tetraopes tetraophthalmus. Note the "Four Eyes"

About jjneal

Jonathan Neal is an Associate Professor of Entomology at Purdue University and author of the textbook, Living With Insects (2010). This blog is a forum to communicate about the intersection of insects with people and policy. This is a personal blog. The opinions and materials posted here are those of the author and are in no way connected with those of my employer.
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5 Responses to Four-Eyed Beetle

  1. gisselle says:

    That picture of the beetle was just plain disgusting. If you wanted to put a picture of a beetle, why couldn’t you put a actual decent one? If you showed that picture to a little child,the child who scream for their life. But of course if they saw your face i bet they would have the felling of throwing up. Next time put a actual nice one that would not make kids scream cause when I looked at it I screamed my head off.

  2. Lessando says:

    Hi there. I am admirer of entomology. Would you know if anybody has done any study to prove that these beetle puncture the leaf veins to reduce/avoid the milky sap??? Or was it just your opinion????
    Thank you.
    Lessandro

  3. jjneal says:

    Yes. Dussourd published a study in 1999. The estimate was about 92% reduction in exposure. If you watch these beetles, you can see them cut the veins and sap exude before they feed on the leaf. They cut the veins and one of the observable results is reduction of latex flow to the leaf.

    Behavioral Sabotage of Plant Defense : Do Vein Cuts and Trenches Reduce Insect Exposure to Exudate ? David E Dussourd, Journal of Insect Behavior (1999), Volume: 12, Issue: 4, Pages: 501-515

  4. Pingback: Swamp Milkweed Beetle | Living With Insects Blog

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