The large milkweed bug is a piercing insect that sucks the juices out of the leaves, stems, and seed pods of milkweed plants (Asclepias genus) and dogbane plants (Apocynum cannabinum). They are generally more of a nuisance in a garden setting rather than seriously harmful. They do not bite or sting. While they can disfigure the milkweed plants valued so much by those wishing to create a butterfly garden, they do not do any serious harm to the plants. The worst that you can say about them is that, if they show up in such massive numbers that they cover all of your milkweed plants, then they may interfere with your viewing of the monarch butterflies.
What Are Large Milkweed Bugs?
The large milkweed bug goes by the scientific name of Oncopeltus fasciatus. This flying, migratory bug is a member of the Lygaeidae family (the "seed bugs"). It is a true bug, meaning it belongs to the order, Hemiptera. Members of the Hemiptera order lack mouths with which to do their eating; instead, they have a proboscis. This proboscis is a long beak that they stick into the plant part that they are dining on to extract fluids from it.
Since, like the monarch butterfly, this bug is drawn to milkweed plants, a comparison is in order. Both of the monarch and milkweed bug gain protection from predators via the milky sap of the milkweed plant parts that they eat. This makes them poisonous to most predators, so they are generally left alone. Both are also black and orange, a coloration that warns predators ahead of time that they will not be tasty morsels.
The Lifecycle of Large Milkweed Bugs
The life cycle of the large milkweed bug is made up of three stages: egg, nymph, and adult. Entomologists refer to its life cycle as being gradual or "incomplete metamorphosis." This means that the nymph looks a lot like the adult, except that it is smaller. The entire lifespan of this insect is only about 12 weeks.
Mating occurs in the second half of summer, after which the female lays eggs on the milkweed (the seed pods are a favorite spot for egg-laying). When the nymphs hatch out (in about one week) they are bright orange. The nymphs go through sub-stages of development (known as instars) during which, among other things, their color changes. About four weeks after the egg stage, adults emerge.
A week into adulthood, the new generation mates. There can be several generations each summer in a warm climate; each new generation replaces an older one that has died out. But they can't survive cold temperatures, so, in cold climates, the last adult generation migrates to warmer climates as autumn progresses, overwintering in the southern Atlantic states of the U.S. and in the Gulf Coast states. If they run out of plant food (or if it gets too cold) they may overwinter in leaf litter and in garden refuse. They return to the North the following year, once the milkweed plants once again provide food for them.
Identifying Large Milkweed Bugs
Although the appearance of this bug can differ slightly from one region of its range to another, there are certain characteristics it retains regardless of where it is found that allow us to identify an adult large milkweed bug:
- It is orange, with three equidistant black patches distributed along the top of its body.
- The middle patch is a more or less even black bar.
- Body shape is an elongated oval.
- The large milkweed bug is about 1/2 inch to 3/4 inch long.
You may not think that an insect that measures, at most, 3/4 inch long deserves to have "large" in its common name. However, this is to distinguish it from the small milkweed bug (Lygaeus kalmii), which stays closer to 1/2 inch in length. The habits of the small milkweed bug are similar to those of the large milkweed bug. Even their coloration is similar. However, the former has five black patches rather than three, the two extra patches being on the outer side of each wing.
Where Do Large Milkweed Bugs Live?
The large milkweed bug can be found throughout most of North America and Central America. It frequents patches of ground that have been disturbed by humans in one way or another, including roadsides, home landscapes, and pastured land grazed by livestock. Such areas often contain the Asclepias plant species that large milkweed bugs feed on, as do the wild meadows that they also live in. These species include:
- Common milkweed (Asclepias syriaca)
- Showy milkweed (Asclepias speciosa)
- Butterfly weed (Asclepias tuberosa)
- Swamp milkweed (Asclepias incarnate)
- Sandhill milkweed (Asclepias humistrata)
What Problems Can They Cause?
At worst, if the infestation is big enough, they may temporarily displace some of the monarch butterflies that you would otherwise find on your milkweed plants. Large milkweed bugs can, in fact, appear in great numbers in your garden. In such cases, great swathes of your milkweed plants can be covered with them. Such infestations may be enough to crowd out the butterfly visitors for which you created your butterfly garden in the first place.
How to Prevent Milkweed Bugs
In warm climates, the best way to prevent a massive infestation of large milkweed bugs from occurring is through proper garden hygiene. When doing a cleanup in the garden in autumn, be sure to pick up any leaf litter and to remove the old, dead stalks of your plants. This will eliminate the opportunities for overwintering that large milkweed bugs may need. Since the bug migrates south out of the northern states for the winter and then flies back in the spring, Northerners can't prevent an infestation (unless they give up growing milkweed): They must either tolerate the bugs or remove them.
If you really must get rid of any large milkweed bugs that may be present, at least avoid using chemical pesticides, which will also harm monarch butterflies. In drastic cases, you may use insecticidal soaps safely. But really all that you would need to do is to pick them off by hand or blast them off with a spray from your garden hose when you are watering your plants.
Feir D, Beck SD. Feeding behavior of the large milkweed bug, oncopellus fasciatus1. Annals of the Entomological Society of America. 1963;56(2):224-229.
US Fish and Wildlife Service. “Inside Region 3.” Fws.gov. N.p., n.d. Web.
US EPA and OCSPP. “Protecting Monarch Butterflies from Pesticides.” (2019): n. pag. Web.