We all know that monarch caterpillars eat milkweed plants, but what about the other insects that share a food source with Monarchs?
Red Milkweed Beetle (Tetraopes tetrophthalmus)
Native range: Their range matches the distribution of Common Milkweed (Asclepias syriaca), basically the Northeast quadrant of North America
Harmful to Monarch caterpillars/eggs? No. They are herbivores and only eat milkweed. They are harmless to monarch larvae and eggs so you can leave them on your milkweed plant if you have enough to go around.
Identification: Red/orange in color with oval spots all over body and un-ringed antennae.
Life Cycle: Eggs laid on stems near ground or just below surface; larvae bore into stems, overwinter in roots, and pupate in spring; adults emerge in early summer
Small Milkweed Bug (Lygaeus kalmii)
Native range: Throughout US and Southern Canada
Harmful to Monarch caterpillars/eggs? Usually not. They are mostly herbaceous and will suck nectar from flowers and sometimes feed on milkweed seeds. However, they have been reported to be 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 (Root 1986).
If you feel like you MUST remove them from your plant, the best method to is to place them into a bucket of soapy water, NOT use pesticides that will harm other beneficial insects.
They adult is black with a broad orange/red band on forewing, forming an "X" shape. Their head is black with a dull red spot on top. In eastern specimens, forewings are all black, but in western specimens they have large white spots. (Bugguide.net)
Eggs are laid on milkweed in spring. One or more generations per year. Adults overwinter.
Large Milkweed Bug (Oncopeltus fasciatus)
Native range: Throughout North America and from Central America through Mexico and the Caribbean to southern areas in Canada.
Harmful to Monarch caterpillars/eggs? No, they do not feed on Monarch larvae or eggs so they are harmless and play a role in the ecosystem. However, they do feed on the seeds, leaves and stems of milkweeds (Asclepias). Damage to the plants can deplete resources for Monarch caterpillars. Planting more milkweed will ensure that they both have enough to eat.
Identification: Adults are overall black and orange- with black band in the middle and two large black spots in front and back. Nymphs are bright orange and develop black spots late.
Life Cycle: Eggs are laid in milkweed seed pods or in crevices between pods. About 30 eggs are laid a day, and about 2,000 over a female's lifespan, which lasts about a month during the summer. One or more generations per year. They can't survive cold winters, so they migrate south in the fall. They overwinter in the southern Atlantic and Gulf coast states where they feed and breed and gradually migrate north again in the spring and summer. (bugguide.net)
Swamp Milkweed Leaf Beetle (Labidomera clivicollis)
Native range: Widespread in North America east of the Rocky Mountains, south to northern Mexico (Riley et al. 2003)
Harmful to Monarch caterpillars/eggs? Technically, no, because they are herbaceous. They do feed on milkweeds, especially Swamp Milkweed (Asclepias incarnata), but also Common Milkweed (A. syriaca) so just plant more so there is enough to go around!
Life Cycle: They often overwinter as adults among leaves such as mullein (Verbascum). Adults mate on or around milkweed. Eggs are cemented to the underside of leaves. Larvae feed on leaves, and drop to ground to pupate (National Audubon Society Field Guide to Insects and Spiders)
Note: Unlike the Monarch, larvae and adults of the milkweed leaf beetle are not thought to sequester cardiac glycosides from their milkweed host.
Milkweed Tussock Moth Caterpillar (Euchaetes egle)
Distribution: From southern Canada and south through Texas and Florida in North America
Harmful to Monarch caterpillars/eggs? No, they are herbivores and only eat Milkweed. But they can defoliate an entire stand of Milkweed in a matter of days. Again, plant more milkweed!
If you must: Handle milkweed tussock moth caterpillars with gloved hands as the caterpillars have urticating hairs that can result in an uncomfortable rash.
Identification: Early instars appear slightly hairy and gray. Later instars sport tufts of black, white, and orange (sometimes yellow) setae (hairs). The head capsule is black. Larvae grow as long as 35 mm.
Life Cycle: There is one generation per year in the north and two or more in the south. Mature caterpillars occur from June onwards.
Monarch (Danaus plexippus)
And finally! Our beloved Monarch caterpillars. We want to plant enough milkweed for the survival of these creatures while still respecting the natural order and ecosystems in nature.
Please do not ever use pesticides on milkweed because you will not only harm the "pests" but also our beloved Monarchs which are in decline.
Large Milkweed Bug Profile
Missouri Botanical Garden
Milne, Lorus and Margery. National Audubon Society Field Guide to Insects and Spiders
Red Milkweed Beetle Profile
Small Milkweed Bug Profile