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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Bugs in Orange and Black II: Large milkweed bug, Oncopeltus fasciatus


Tiny milkweed bug nymphs crowd a seed head in late summer.


This week’s episode of Bug of the Week continues our theme of demystifying insects dressed in the Halloween colors of orange and black.  In last week’s episode, we met the beautiful milkweed leaf beetle and learned of its clever game of tricking predators not to attack by donning the colors of orange and black. Today we meet the large milkweed bug, equally attractive garbed in these same striking seasonal colors.


Nymphs with black wing buds on their backs will soon develop into adults with fully formed wings.

Many insects that consume milkweed display vivid patterns of orange and black as both juveniles and adults. By feeding on milkweed they obtain nasty tasting chemicals and, in turn, they become distasteful to a wide range of predators. This phenomenon of developing a similar, easily recognizable color pattern by two or more nasty-tasting insects is called Müllerian mimicry, so named for the visionary German naturalist Fritz Müller. However, the milkweed leaf beetle does not store noxious chemicals from the milkweed. Its scam is to wear orange and black, thereby dissuading enlightened predators from an attack once they have learned that “orange and black” spells “nasty meal.”  This type of mimicry, in which warning colors of a distasteful species like the monarch butterfly are copied by a tasty mimic like the milkweed leaf beetle, is called Batesian mimicry. The great English naturalist Henry Bates first described this form of mimicry while studying butterflies in Brazilian rainforests.

Another charter member of the Müllerian mimicry gang is the large milkweed bug, for it too stores nocent cardiac glycosides after consuming milkweed plants that serve as its source of food. This year was spectacular for milkweed bugs and my butterfly weeds generated hundreds. In spring and early summer my milkweeds thrived and produced early clusters of seeds devoid of hungry milkweed bugs. However, by late summer and early autumn my milkweeds were colonized by teeming legions of these beautiful bugs. Where did the bugs come from and why did they suddenly appear in late summer?


In the waning days of summer, dozens of milkweed bugs groom and fatten up on milkweed seeds before starting their southward trek.

Most people don’t realize that large milkweed bugs, like monarch butterflies, undergo annual migrations throughout much of the range of milkweeds, from southern states and Mexico where they spend the winter, to northern states and southern Canada where they spend the summer. Large milkweed bugs cannot survive winter’s chill in northern climes. Their annual migration south is triggered by shortening day length, cooling temperatures, and declining quality of milkweed plants as food. Titers of a glandular product called juvenile hormone signal the milkweed bug’s ovaries to take a “time-out”, and trigger flight behavior that transports the milkweed bug to warm southern lands where milkweeds grow. Once the southward migration is complete, juvenile hormone levels rise, ovaries are switched on, and reproduction resumes.  In spring, the migratory pattern reverses and generations of large milkweed bugs leap-frog their way northward to colonize milkweeds as far north as Canada.  

As members of the seed bug clan, milkweed bugs insert a long slender beak into the ripening seeds within the developing pod. After injecting digestive enzymes into the seed, they suck liquefied food through the straw-like beak into their gut where nutrients will be used for growth, development, and reproduction. During her lifetime, the female milkweed bug may lay up to 2000 eggs. Small orange and black nymphs hatch from the eggs and eat seeds of milkweed. As nymphs grow and develop, small black wing buds become clearly visible on the body segments just behind the head. These wing buds enlarge as the insect feeds and molts, until the final transformation to the adult stage when wings are fully formed and ready for flight. With a light frost on the pumpkins this morning and a killing frost just around the corner, my nymphs better hurry and earn their wings to begin their trek south before winter’s chill brings an end to their milkweed revelry.


To access nutrients, the milkweed bug inserts needle-like sucking mouthparts through the husk of the seed head and probes for nutrient rich seeds.


The wonderful reference “The Pleasures of Entomology”, by Howard Ensign Evans, was used as a resource for this episode.