Small vs Large Milkweed Bug: Decoding the Differences

Milkweed plants are home to a variety of insects, two of which are the small and large milkweed bugs. These bugs, although similar in appearance, have some key differences in size and markings. In this article, we’ll explore the characteristics that set them apart and help you identify these fascinating little creatures.

Small milkweed bugs (Lygaeus kalmii) are orange and black with a distinct heart-shaped black patch on their back and two smaller black patches on either side. They grow to a length of about ½ inch and are known to feed on milkweed seeds, as well as nectar from the plants source. On the other hand, large milkweed bugs (Oncopeltus fasciatus) are slightly larger, with a pronounced black band across their middle and diamond-shaped black patches on their backs source.

As you explore milkweed plants, keep an eye out for these colorful insects. Understanding their differences will not only improve your knowledge of the natural world but also help you appreciate the fascinating interactions occurring within this unique ecosystem.

Identifying Milkweed Bugs

Physical Characteristics

Large milkweed bugs (Oncopeltus fasciatus) and small milkweed bugs (Lygaeus kalmii) are part of the true bugs (Hemiptera) family. Both species are characterized by their distinctive orange and black color patterns.

Large milkweed bugs:

• ½ to ¾ inch long
• Orange to reddish-orange with a black band across their back
• Two large diamond-shaped black patches on their wings
• Orange antennae

Small milkweed bugs:

• About ½ inch long
• Reddish-orange X on their back
• Reddish-orange band across the pronotum (shield-like plate between head and wings)
• Black antennae

The Milkweed Bug Life Cycle

Both large and small milkweed bugs undergo complete metamorphosis which includes four stages: egg, nymph, pupa, and adult.

Eggs:

• Light yellow, turning reddish before hatching
• Laid in small clusters on milkweed plants
• Hatch within a week

Nymphs:

• Go through five instars (growth stages)
• Resemble smaller, wingless versions of adults
• Develop wing pads and increased black markings with each molt

Pupa:

• Non-feeding stage before becoming adults

Adults:

• Develop wings for flying and mating
• Feed on milkweed plants, particularly seeds
• Travel south for overwintering (large milkweed bugs)

Habitats and Migration

Small and large milkweed bugs can both be found in North America, particularly in areas where milkweed plants are present. Milkweed is a vital component of their habitat, as both bugs feed on these plants, specifically the seeds. Here are a few notable characteristics of each bug in relation to their habitats and migration patterns:

• Small milkweed bugs are more commonly found in southern Canada, the United States, and parts of Central America¹.
• Large milkweed bugs are primarily found in North America and can be seen throughout the United States and southern parts of Canada².

It is interesting to note that, unlike monarch butterflies, small and large milkweed bugs are not major migratory species¹. Monarch butterflies, on the other hand, undertake an impressive two-way migration from southern Canada to central Mexico each year³.

In some regions, like Illinois, both small and large milkweed bugs can be found as permanent residents¹. Their presence is essential in supporting the ecosystem by helping to control milkweed populations. Additionally, they provide a food source for various predators.

To summarize, small and large milkweed bugs share similarities in habitat preferences and are mostly found in areas rich with milkweed plants. While they do not exhibit the same extensive migratory behavior as monarch butterflies, they still play a significant role in the ecosystem.

Milkweed Bugs and the Milkweed Plant

Seeds and Seed Pods

Milkweed plants produce seeds that are enclosed in seed pods. These pods open in the fall and release seeds with silky hairs allowing them to be carried away by the wind. You can often find two types of milkweed bugs, the large milkweed bugs (Oncopeltus fasciatus) and small milkweed bugs, on the seed pods. They both feed on milkweed plant parts, especially the seeds.

For example, large milkweed bugs are ¾” long and have a reddish-orange color with a black band across their back. Both adults and nymphs can be found clustering on the seed pods.

Milkweed Plant Selection

There are many different species of milkweed plants, such as common milkweed (Asclepias syriaca), swamp milkweed (Asclepias incarnata), and butterflyweed (Asclepias tuberosa). They all share characteristics like leaves, stems, flowers, and roots, but they may vary in size, shape, and color.

When selecting milkweed plants for your garden, you can consider their features and choose the ones that best suit your preferences. Here are some of their characteristics:

• Common milkweed (Asclepias syriaca):
  • Broad leaves
  • Pink flowers
  • Tolerant of various soil types
• Swamp milkweed (Asclepias incarnata):
  • Narrow leaves
  • Light pink to mauve flowers
  • Prefers wet soils
• Butterflyweed (Asclepias tuberosa):
  • Narrow leaves
  • Orange flowers
  • Prefers well-drained soils

In addition to the above species, you can also find other varieties like whorled milkweed (Asclepias verticillata), and even giant milkweed (Calotropis gigantea), which can grow up to 15′ tall and 15′ wide. By choosing the right milkweed plants for your garden, you can provide a habitat for milkweed bugs, monarch butterflies, and other pollinators.

The Role of Milkweed Bugs in the Ecosystem

Milkweed bugs, including both large and small varieties, play a significant part in the ecosystem. They mainly feed on milkweed plants and are often associated with monarch butterflies.

Large Milkweed Bug

The adult large milkweed bug (Oncopeltus fasciatus) is ¾” long, orange to reddish-orange, with a black band across its back. They are most commonly found clustering on seed pods of milkweed plants.

Small Milkweed Bug

The small milkweed bug (Lygaeus kalmii) grows up to ½ inch long, has a black color with a large red X-shape on its back, white margins on its wings, and a red band on its pronotum.

Insects that rely on milkweed:

• Monarch caterpillars
• Milkweed tussock moth
• Longhorn beetle
• Swamp milkweed leaf beetle

Milkweed bugs are part of a larger group of insects that rely on milkweed plants. Each of these insects has a unique relationship with milkweed. For example, monarch caterpillars feed exclusively on milkweed, as its toxic compounds called cardiac glycosides help protect them from predators. However, large and small milkweed bugs don’t consume the entire plant; they focus mainly on seeds.

Similarly, other bugs like the milkweed tussock moth caterpillar (Euchaetes egle) and the longhorn beetle (Tetraopes tetrophthalmus) also rely on milkweed plants for their survival.

Comparison of milkweed insects:

While milkweed bugs may seem like they only have a small role in the ecosystem, you must remember that they are closely connected to a larger group of insects that all rely on milkweed plants. This dependency creates a complex web of interactions essential for a diverse and thriving ecosystem. By understanding the importance of these bugs and their role in the environment, you can appreciate the value of preserving milkweed plants and supporting the overall ecosystem.

Reproduction and Predation

When it comes to reproduction, both small and large milkweed bugs have unique strategies to ensure the survival of their species. Let’s explore their reproductive behaviors and their interactions with predators.

Small milkweed bugs (Lygaeus kalmii) and large milkweed bugs (Oncopeltus fasciatus) are known to make their home on milkweed plants, where they feed on plant seeds and living tissue. During the breeding season, males and females engage in mating behaviors that involve the transfer of pair-forming pheromones.

In contrast, aphids like Aphis nerii or the oleander aphid have a different reproductive strategy. They are able to reproduce both sexually and asexually through a process called parthenogenesis. This allows them to produce offspring without the need for male fertilization. For most parts of the year, females reproduce asexually, giving birth to live young. However, when environmental conditions change, they switch to sexual reproduction, which leads to the production of male and female offspring.

Predators such as ladybird beetles and lacewing larvae feed on both milkweed bugs and aphids. These predators help maintain a balance in the ecosystem by limiting the population of these herbivores. Common predators to milkweed bugs include insects like assassin bugs and ants, as well as mammals like birds and rodents. Predation is a natural part of the ecological balance and ensures that milkweed bug and aphid populations do not grow out of control, causing potential harm to their host plants.

In conclusion, milkweed bugs and oleander aphids showcase distinct reproductive strategies that ensure their survival in their respective habitats. Although they share some predators, their individual reproductive behaviors differentiate them in how they respond to these challenges.

Interactions with Other Organisms

The Effect on Monarchs

Milkweed plays a crucial role as a host plant for the monarch butterfly. When caterpillars feed on milkweed, they ingest the plant’s toxic latex, which offers them protection against predators. However, milkweed beetles may damage the buds and hinder the growth of new leaves on the milkweed, leading to stunted and deformed plants. This could negatively affect monarch butterflies by reducing the available food and habitat for their caterpillars.

With regards to milkweed tussock moth caterpillars, they eat milkweed leaves and may consume entire plants, with the exception of major veins which contain the latex sap. Unfortunately, this can potentially limit the resources for monarch caterpillars, as the leaf consumption is sometimes especially noticeable.

Interactions with Other Insects

Milkweed plants are also attractive to various other insects, such as honeydew-producing aphids. Honeydew is a sugary liquid secreted by these aphids that can cover the milkweed leaves, providing a food source for sooty mold which can stunt plants’ growth. On a positive note, aphids’ presence can attract beneficial insects like lacewings that help control aphid populations. To preserve the beneficial insects, it is recommended to avoid using insecticides on milkweed plants.

Milkweed tussock moth caterpillars, with their dense hair (tufts), make a striking appearance compared to the hairless monarch caterpillars. After maturing, they spin a gray cocoon and pupate. Although these caterpillars are predominantly native to the eastern United States, they’ve also been reported in the Mediterranean region.

Potential Threats and Solutions

Both the small milkweed bug and the large milkweed bug feed on milkweed plants. However, their feeding habits can present challenges for milkweed and the insects that depend on them.

Toxic Compounds

Milkweed plants contain toxic compounds called cardenolides. These toxins are a defense mechanism against potential predators. Fortunately, milkweed bugs have evolved to tolerate these toxins. As they feed on milkweed, they not only consume these toxic compounds but also store them in their bodies. This strategy makes them unappetizing and potentially harmful to their predators.

Saliva

Milkweed bugs have specialized piercing-sucking mouthparts that allow them to feed on plant sap, seeds, and leaves. As they pierce the plant tissue and inject their saliva, they can cause damage to the plant, affecting its overall health and growth. Moreover, their saliva can facilitate the spread of potential diseases, leading to further threats to the milkweed plant.

Solutions

To protect milkweed plants and their dependent insect populations, consider the following:

• Plant native milkweed species in your area to strengthen their resistance to potential threats.
• Monitor milkweed plants for the presence of milkweed bugs and other pests and manage them accordingly.
• Maintain a diverse garden by including various plants to support different insects and provide natural predators to control pests.

By following these steps, you can create a thriving environment that supports milkweed plants and the insects that depend on them, like the monarch butterfly, without being overrun by milkweed bugs.

Reader Emails

Over the years, our website, whatsthatbug.com has received hundreds of letters and some interesting images asking us about these insects. Scroll down to have a look at some of them.

Letter 1 – Small Milkweed Bug Nymphs

Subject: Daniel – Small Milkweed Bug Nymphs?
Location: Hawthorne, California
November 18, 2012 5:26 pm
Well, we’re just two weeks later than when I discovered the Small Milkweed Bugs mating and here are some tiny guys feasting on a Mexican Milkweed seed pod. I’m guessing they are the nymphs?
Oh, and I have discovered Monarch eggs on the milkweed. Too small for my camera to photograph successfully, though!
Signature: Thanks, Anna Carreon

Hi Anna,
Thanks for sending us further documentation of the wealth of insect life on your garden milkweed plants.  Since we will be out of the office for Thanksgiving, we are postdating your submission to go live on Thanksgiving.  Have a great holiday.

Letter 2 – Small Milkweed Bug

On today’s hike we spotted a Small Milkweed Bug, Lygaeus kalmii, on the Anise when we were monitering the growth of the three caterpillars we saw yesterday. We brought the bug home and pulled out the digital camera with newly recharged batteries. Hogue write: “The Small Milkweed Bug is occasionally seen around the home garden where it has strayed from nearby milkweed. Although it may feed on other plants, especially those of the family Asteracea (sunflower, asters, and ragweeds), it normally feeds on the pods, stems and seeds of the milkweed. As this plant seems to be declining locally (Ed. note: in Los Angeles) in the face of human progress, the insect will not doubt become increasingly rare.”

Letter 3 – Small Milkweed Bug

orange and black beetle in backyard garden
Am enclosing pic of beetle we can’t identify. Have searched books, charts, and net without success. Thanks
Cosmo Zipeto

Hi Cosmo,
Finally that attachment worked. This is not a beetle. It is a True Bug, a Small Milkweed Bug, Lygaeus kalmii. They are immune to the toxins in milkweed, and are consquently toxic to predatory insects.

Letter 4 – Small Milkweed Bug

Think it’s a blister beetle
June 28, 2010
Good day. The pictured bug is from Mesa, Arizona. Scads of them have been around my peppers and tomatoes (which they have damaged or killed) since March. They seem to suck the juice out of fruit and stems. My peppers and tomatoes are organic. I don’t use chemicals in
the pepper patch. If this is a blister beetle, how can I get rid of them without chemicals? If it isn’t a blister beetle, what is it? Not much else I can tell you. They fly. They haven’t been around in the past 40 years. First year I’ve seen this many, if any of them.
Daniel
Mesa, Arizona

Hi Daniel,
This is a Small Milkweed Bug, Lygaeus kalmii.  It is a True Bug and not a Beetle.  Beetles have chewing mouthparts and they actually take bites out of things.  True Bugs have piercing and sucking mouthparts and as your email indicates, they suck juices.  Your letter is the first report that we have received that Small Milkweed Bugs are problematic in the garden.  According to BugGuide:  “Adults suck nectar from flowers of various herbaceous plants, and also feed on milkweed seeds(?). Also reported to be scavengers and predators, especially in spring when milkweed seeds are scarce.”   We do not normally give extermination advice.  We control problematic sucking insects in our own garden by spraying with a weak detergent like dish detergent in water.

Letter 5 – Small Milkweed Bug

Subject: Black and Red with White Spots
Location: 92595 [Wildomar, California]
April 19, 2014 4:11 pm
I just found this in my backyard and want to know if he will be bad for my vegetables, grapes and fruit trees. I’ve narrowed it down to either a Milkweed Bug or a Box Elder Bug… but don’t know.
Signature: Joseph Morabito

Dear Joseph,
You did a very good job of narrowing this identification to two similar looking species.  This is actually a Small Milkweed Bug, Lygaeus kalmii, and we do not believe it will cause any significant problems in your garden.  Citations on BugGuide include:  “Adults suck nectar from flowers of various herbaceous plants, and also feed on milkweed seeds(?). Also reported to be scavengers and predators, especially in spring when milkweed seeds are scarce. They have been reported feeding on honey bees, monarch caterpillars and pupae, and dogbane beetles, among others. The Life of a Californian Population of the Facultative Milkweed Bug Lygaeus kalmii.  Adults mainly feed on milkweed seeds, but they often consume nectar from various flowers. Harvard Entomology.”

Letter 6 – Small Milkweed Bug

Subject: BUG ID
Location: Portland, oregon
June 4, 2016 1:47 pm
Hi my name is Dez, I’m almost six. We are helping with research about pollinators and are trying to find out more about them so we can keep helping them. I’m also making a blog post about them.
We saw this bug here in Portland, Oregon and can’t identify it from anything we found online. Could you help?
Thanks very much,
Signature: Dez

Dear Dez,
We are pleased to hear about your concern with pollinators, and it is wonderful that you will be blogging about what you learn.  This is a Small Milkweed Bug, Lygaeus kalmii, a species that is generally found not far from its host plant, Milkweed.  According to BugGuide citing another article:  “Adults suck nectar from flowers of various herbaceous plants, and also feed on milkweed seeds(?). Also reported to be scavengers and predators, especially in spring when milkweed seeds are scarce. They have been reported feeding on honey bees, monarch caterpillars and pupae, and dogbane beetles, among others.”

Thank you so much for telling us what this bug is called and some of its features! I was so excited to get such a fast response!
Dez

Letter 7 – Small Milkweed Bug

Subject: What is this bug
Location: Ontario, canada
November 6, 2016 12:36 pm
It is black and red.
Signature: Karen

Dear Karen,
This is a Small Milkweed Bug.  It is a native species for you an no cause for concern.

Letter 8 – Small Milkweed Bug

Subject:  Beetle ID
Geographic location of the bug:  Denver, co
Date: 05/15/2018
Time: 06:01 PM EDT
Your letter to the bugman:  What is this bug?
How you want your letter signed:  Jon orsborn

Dear Jon,
This is not a beetle.  It is a Small Milkweed Bug, Lygaeus kalmii. 

Letter 9 – Small Milkweed Bug and Jumping Spider

Various bugs & spiders
Hello: You have a wonderful and informative website. I applaud your use of color pictures and your professional approach at identifing and informing the public about various creatures they encounter. From your site, I believe the first photo is that of a boxelder insect. I saw him on my garage door and wanted to take a picture. At that time, I had to leave to make it to an appointment. The garage door opened and closed and repeated the cycle when I returned. Still the boxelder remained. I got my camera and shot a couple of pictures. After dumping them to my computer, I could see that they were blurry and had no depth-of-field. I went back out to see if the insect was still there and he was. Then I set up the camera on a tripod and went back out. Much to my delight, the insect was still there. I shot the picture you see (entitled Boxelder.jpg). The next morning I went out and found the insect laying on the ground in front of my garage door. It was his last day on Earth and he survived long enough to become immortalized in pictures. The second photo is a house centipede, which your site helped me identify. Spider1.jpg is a small spider trying to hide in the keyhole of the deadbolt lock on my garage door. No doubt he was hoping I didn’t have a key. I have no idea what kind of spiders are in spider2.jpg and spider3a.jpg. Any identification would be appreciated. No insects or spiders were harmed in making the photographs. The photos are yours to use as you see fit. I am not making any silly copyright claims. They are in the public domain as far as I am concerned. Thank you, Jon

Hi Jon,
Thank you for the wonderful letter. Your bug is actually a Small Milkweed Bug, Lygaeus kalmii, one of the Seed Bugs. Your third spider is a Jumping Spider in the genus Phidippus, Family Salticidae. We are not sure about the other spiders.

Letter 10 – Small Milkweed Bug from Canada

Subject: Chris in Toronto/Boxelder?
Location: Toronto, Canada
November 29, 2012 10:31 pm
Hi.
Is this guy a boxelder?
I’ve been wondering for a while…the marking are a little different from other examples that I’ve seen.
Thanks.
Signature: Chris in Toronto

Hi Chris,
This is a Small Milkweed Bug, Lygaeus kalmii, from the Seed Bug family, not a Boxelder Bug, which is in the Scentless Plant Bug family.  While Boxelder Bugs are sometimes a harmless nuisance, the Small Milkweed Bugs get no complaints.

Letter 11 – Small Milkweed Bug Nymphs

Subject:  Elder of some sort??
Geographic location of the bug:  Vermont
Date: 07/02/2021
Time: 07:47 PM EDT
Your letter to the bugman:  Hi Daniel – thank you so much for your comments on my orchard Spider from June…Need your help again.  Saw these two on milkweed and I think I interrupted a lovefest..My guess is some type of Elder??
I am happy to send you as many images as you would like…Im a professional commercial photographer, but I love wildlife, astro and MACRO photography as my hobby. I take LOTS of bug pics!  Thanks again for your help!  Have a great weekend!
How you want your letter signed:  Caroline Minneci

Dear Caroline,
We are not certain what an elder is, other than a person of greater age.  These are definitely not elders.  They are immature Small Milkweed Bugs, Lygaeus kalmii, which are pictured on BugGuide.  They were also most certainly not mating, though they are a communal species.

Letter 12 – Small Milkweed Bugs

What’s this bug?
This picture was taken 1/8/08 at about 10am at Watertown, NY. The surface has a SE exposure and the outside temperature is about 62 deg. F. Thank you.
Bob Dell

Hi Bob,
These are actually Small Milkweed Bugs, Lygaeus kalmii. We mistakenly originally identified them as Boxelder Bugs, but actually magnifying the original thumbnail revealed our original error.

Letter 13 – Small Milkweed Bugs

PLease tell me what this bug is and where it’s coming from??
Tue, Dec 9, 2008 at 10:45 AM
Hello, we are purchasing a foreclosure home that has been vacant since May 2008, vacant except for an infestation of these little critters (see picture). They are only outside, seem to be hovering around the overgrown bushes and weeds and living in the gaps of the wood beams. By infestation I mean thousands. They are slow moving and huddle in groups.They are grey/black with red markings and about 3/4 inch long. They don’t appear to fly. If you could please tell me what they are, why they are there and if they are dangerous I would appreciate it.
Karen Wondergem
North Glendale, Arizona

Hi Karen,
These are Small Milkweed Bugs, Lygaeus kalmii, and they are perfectly harmless.  The insect feeds on the pods of Milkweed Plants, and it is possible that the previous owner had a butterfly garden.  It is also possible, though you did not indicate the actual circumstances, that this home is part of a new development that was originally natural open space.  According to Charles Hogue in Insects of the Los Angeles Basin:  “it normally feeds on the pods, stems, and seeds of the milkweed.  As this plant seems to be declining locally in the face of human progress, the insect will no doubt become increasingly rare.”

Letter 14 – Small Milkweed Bugs: adults and a nymph

Milkweed Bug
Hi Bug Man….
a search on your webpage suggests that these are Small Milkweed bugs. Is the one in the upper left hand corner also a small Milkweed bug (perhaps a WEE small milkweed bug)? Or is it another species altogether? The bugs were found, appropriately, on a swamp milkweed pod.
Thanks,
Jill

Hi Jill,
Great photo. These are all Small Milkweed Bugs, Lygaeus kalmii. The smallest is an immature nymph that will grow wings at the final molt.

Letter 15 – Small Milkweed Bugs Mating

Photo of mating Boxelder bugs
I LOVE your site!!! (You just helped us ID our first Robber Fly!) I thought you may be interested in this photo of a mating pair of Boxelder bugs. We took the photo on 09/05/05 at Table Rock Mountain in Western North Carolina.
Lori Martin
Maiden, NC

Hi Lori,
Sorry for the long delay but we had lots of Boxelder Bug photos when your letter arrived. We are catching up on old mail and we were pleasantly surprised to see you had misidentified your bugs. These are actually Small Milkweed Bugs, Lygaeus kalmii.

Letter 16 – Small Milkweed Bugs Mating

looking to identify cool insects
love your site! i live about 40 mi. due west of the Superstition Mountains in Gilbert AZ (SE of Phoenix by about 25 miles) I found these on an oleander. they stayed rear-to-rear for quite awhile, when one crawled, the other moved backwards; their coloring is wrong for the pic i found of Lovebugs, Plecia nearctica (but the Florida site said they are invaders from the West), but can you tell if they’re related, or what they are? thanks!
Elizabeth Boepple
p.s. Florida says they get swarms, but in 1o years here, i’ve only seen these two, in April 2007

Hi Elizabeth,
Florida Lovebugs are actually flies. Your mating Small Milkweed Bugs are True Bugs. Western specimens of the Small Milkweed Bug, Lygaeus kalmii, show white spots on the membranous wings as pictured in your photograph.

Letter 17 – Unnecessary Carnage: Small Milkweed Bug was difficult to kill!!!

What is this?
Location: Northeastern Colorado
December 8, 2011 12:20 am
Hi Bugman! I am having an issue with this bug around my house. I find them maybe every other day and they are almost impossible to kill. I step on them and they seem to just spring back to life. I finally killed it by drowning it in Windex (This was the only thing I had close to me). Please help! I am new to this area of the country and have never seen this bug before. I haven’t seen them except in the colder months after it has started to snow outside.
Signature: Tricia M.

Dear Tricia,
If you find these benign Small Milkweed Bugs “almost impossible to kill”, then perhaps you should just stop trying to kill them.  They will not harm you, your home nor your pets.  You may read more about Small Milkweed Bugs, Lygaeus kalmii, on BugGuide.

Letter 18 – What are the Large Milkweed Bugs Doing???

Subject: Large Milkweed Bugs Kissing?
Location: Hawthorne, CA
September 9, 2013 12:41 pm
Hi Daniel,
Here are two Milkweed Bugs that look as though they are kissing. Below them is a nymph. Are they kissing, protecting the nymph from my prying lens, or something altogether different? They sat like this for quite some time.
Signature: Thanks, Anna Carreon

Hi Anna,
We have no idea what these Large Milkweed Bugs are doing, but it is still a very lovely and interesting photo.  Perhaps one of our readers will know what is going on.