Friday, November 4, 2011

Large Milkweed Bugs (Oncopeltus fasciatus) at Fort Ancient

A few weeks ago I spent a little time hiking through the meadow by the Mound Trail at Fort Ancient. The grasses were brown and dry, and what was left of summer's bounty crackled and rattled with each breeze that worked itself through the tumble of spent flower heads and stalks. Autumn had drained the green from the landscape, and even the yellows had faded from the fields, but oranges and reds were still around to be found on the dry and cracked Common Milkweed plants...

An adult Large Milkweed Bug (Oncopeltus fasciatus) is surrounded by all five of its instars. The adult is at the top. It is the largest and has usable wings. Instars are nymphs, or immature versions of the bug. Instars differ in size, color and pattern. They also lack usable wings.

Large Milkweed bugs molt five times (nymphal instars) before they become adults. During these stages the nymphs look similar to the adults; however, if you look closely you can see each instar has its own color (from deep red to orange) and pattern. During the middle instars black wing pads start to form, but the wings are not usable until the fifth instar molts into an adult.

...also unique to the adult is the flame-red pattern on its face between its eyes...

...the adult Large Milkweed bug has a striking pattern of orange and black on its wings. Here you can see the veins that carry the hemolymph through the wings. On the final molt, the adult Milkweed bug pumps hemolymph through these veins to unfurl the wings. To see an adult who failed to open his wings, click here.

...a fifth instar almost looks like an adult, but lacks the defined face pattern, usable wings, and the eyes are much less "buggy!" Look to the right for a glimpse at an adult's eye.

One adult and several fifth instars mass together to form a color warning on an old Common Milkweed pod.

Since Large Milkweed bugs eat Common Milkweed sap, which contains toxic alkaloids, they do not taste good. A young bird only has to taste this bug once or twice to learn to avoid orange and black bugs! Large groupings of this color combination warn birds away. You may already know of another orange and black insect that has the same type of protection--the Monarch butterfly. Just like the Large Milkweed bug, Monarch butterfly caterpillars feed on Common Milkweed plants and concentrate the alkaloids in their tissue. When the caterpillars metamorphose into Monarch butterflies, they are toxic and also taste bad.

Another tidbit...I read on this site (click here) of an easy way to distinguish between a male and female milkweed bug--the female Milkweed bug has one black strip and two back dots on her abdomen, while a male has two thick black strips. I didn't know that before and never flipped one over to look. Next time I see one I'll take a peak.

...close-up of two later instars.

...close-up of an adult Milkweed bug--love the face tattoo.

This guy is not going to be hanging around much longer. These bugs migrate! Just like the Monarchs, they head south for the winter. Shorter days in autumn trigger diapause in the adults, which shuts down the reproductive system (source: "Migration: the biology of life on the move," by Hugh Dingle, page 139, click here to read more.). Shutting down the reproductive system saves energy and allows the Large Milkweed bugs and Monarch butterflies to migrate south for the winter. The same adult Milkweed bugs that overwintered in the south then migrate back north in the spring to lay the eggs of the next generation.

If you want to learn more about the Common Milkweed plant and all the insects that feed on it, check out this post by Marcia Bonta. I stumbled across it a few weeks ago and thought it would fit in here. For more photos of Large Milkweed Bugs, click here for an earlier post.


holdingmoments said...

Fascinating and informative post Kelly.
Excellent close up shots too.
Looks like a real Halloween bug with that colour combination. :-)

Wanda..... said...

I've seen a few adults in the field over the summer, but need to take a closer look next year. It's nice learning from you, as well as viewing wonderful photos and paintings, Kelly.

Marco Alpha said...

Hello Kelly,
Amazing so much of these creeping animals. They've got wonderful colors and you've caught them very well in your shots.
But when I see this I get the itching on my back,...bbbrrrr...!!

Greetings and a good weekend,

TexWisGirl said...

fantastic detailed shots of these unique bugs! how cool that they migrate! monarch cousins, all the way! and i like the word 'instar'. :)

Guy said...


You started with a beautifully poetic opening paragraph. Then you followed that up with stunningly beautiful photos of the Milkweed bugs showing a number of the different stages. This was followed with great information I at least had never heard, links and the title of what sounds like a fascinating book. A perfect post.


Montanagirl said...

Great post, Kelly! Your photos of this bug are fabulous. They'd make great Halloween bugs with that vivid orange and black coloring.

Betsy from Tennessee said...

Unbelievable, Kelly.... I've never seen those bugs before (or never noticed them)... Their colors certainly blend in with this time of year, don't they????

Thanks for posting..

Gillian Olson said...

Very colourful bugs and excellent shots.

KAT said...

My eyes are so happy ...your photos are amazing detailed and your info is great not too much and not too little...just perfect for the type of blog I love to read ! thanks

- KAT -

Carol Mattingly said...

Oooh. These are so creepy crawly which is exactly is perfect. Beautiful. Carol

Kelly said...

...thanks, everyone! I know what you is sort of creepy to see all these bugs clumped up together. They move around and climb over each other, and if you're not expecting it, it can be a huge surprise if you're eye-level with them! I went back to Fort Ancient yesterday to see if I could find a clump to see if I could find a male and female to check out the markings, but they were all gone. I guess they have moved on already...

Elva Paulson said...

Great bugs and words!

Kathiesbirds said...

Kelly, I do not believe I have ever seen these insects. What an education I got today. How pretty these are! I cannot believe the detail that you captured!