What Is That … on the Milkweed?


We were raised thinking, as the name states, that milkweed plants are unwanted weeds.  This summer we have noted increased numbers of milkweed plants growing around our cottage at White Lake.  This may be due in part to the North American campaign to foster milkweed plants in order to aid the recovery of migratory monarch butterfly populations since milkweed is the main host plant for their larvae (caterpillars).

There are two types of milkweed that we see near our cottage, the common milkweed which has broad leaves, pale pink flowers and grows in dry conditions such as road sides and the swamp milkweed with its narrow leaves, rose to purple flowers and grows in marshy areas and moist ditches.  Both species have a milky sap that contains a glycoside which helps to protect the plant from grazing as it is poisonous to many animals.  That said, there are several insects capable of feeding on milkweed.  In addition there are other small insects, arachnids and amphibians that use milkweed plants as relatively safe havens and a base from which to hunt insects.  The increased number of plants this year has allowed us to observe a broad range of beasties who call milkweed home.

We start our tour of milkweed lovers with the very visible and recently publicized monarch butterfly.  The yellow, white and black banded monarch caterpillar is distinctive, growing up to five centimeters in length.  There are two fleshy black filaments growing on the head and another longer pair at the tail.  These solitary caterpillars are most commonly found feeding on the underside of young leaves.  The egg-laying female butterflies ignore older leaves.  The larvae concentrate cardiac glycosides in their tissues from the milkweed leaves they ingest.  These heart toxins are also passed along to the pupae and adult butterflies, affording all life stages protection from predation by birds.  The following photograph was taken recently of a four centimeter long monarch caterpillar.

Next we turn to the milkweed tussock caterpillar.  These densely hairy caterpillars are covered with black, yellow/orange and white tufts and longer lashes.  Prominent black lashes and tufts extend from both ends and sides, white lashes are visible at both ends and yellow/orange tufts curve upward over the body of the caterpillar which can grow up to 2.5 centimeters in length.  Female butterflies lay numerous eggs on both younger and older milkweed leaves.  These gregarious caterpillars can defoliate plants, as can be seen with the following picture of four caterpillars munching through the leaves of this one plant.

Both the larvae and adults of the swamp milkweed beetle feed exclusively on milkweed.  The adult is a robust, humpbacked black beetle with orange/red markings on the margins and center of their wing covers.  They can grow to be eight to 11 millimeters in length.  We often see more than one adult munching away on a leaf as shown in the following picture.

Another insect we have observed feeding along the main leaf veins of milkweed are aphids.  They are very small insects (four to eight millimeters) that feed on plant sap and are often considered as pests, since typically large numbers of aphids feeding on plants can cause considerable damage.  Particularly interesting is the mutually beneficial or symbiotic relationship between aphids and ants.The aphids excrete large quantities of sugary waste, called honeydew.  Ants care for and protect aphids from predators and parasites in exchange for the sugar-rich honeydew which they harvest as part of their food.  The following photograph of a milkweed leaf shows the black ants tending aphids.

We regularly see an eastern harvestman, known to many as daddy-longlegs, perched on milkweed leaves.  The plants afford them some protection from being eaten by birds and make good platforms from which to hunt prey.

This year, we have spotted both spring peepers and grey treefrogs on milkweed plants.  They seem to glean the same benefits from milkweed plants as the eastern harvestman. The first picture is of a spring peeper treefrog.

The last picture is of a juvenile grey treefrog.

These are just some of the more visible and interesting milkweed residents.  We suggest the next time you see a milkweed plant that you take a few seconds to observe how many interesting beasties are on the plant.  Do not forget to look also at the underside of the leaves.


We relied on the following publications while researching for this article: Wetland Plants of Ontario by Steven G. Newmaster, Allan G. Harris and Linda J. Kershaw; The ROM Field Guide to Wildflowers of Ontario by Timothy Dickinson, Deborah Metsger, Jenny Bull and Richard Dickinson; Caterpillars of Eastern North America by David L Wagner; and, Beetles of Eastern North America by Arthur V. Evans.