Monday, August 12, 2019

Milkweed Munchers

Barb was dumping plant debris into the compost pile when she noticed this bug crawling around in it.  Lifting up some of the old scraps revealed even more of them.  They measured 3/8" (10 mm) and required magnification for final identification as small milkweed bugs, SMB - Lygaeus kalmii.

Box elder Bug
On first glance it resembles a smaller version of the box elder bugs that swarm later in the year. They tend to migrate into the house, occasionally using their plant-sucking proboscis to puncture the skin, causing slight irritation.

The distinctive coloration of SMB made the identification relatively easy.  It has a bright orange incomplete "X" on the back and the upper black marking is somewhat heart shaped.  The anterior edge of the pronotum (the plate covering the first portion of the thorax) has two small black spots.  L. kalmii also has a small red spot on the back of the head extending forward, visible only with magnification.

When I think of milkweed, I associate it with the poster child of lepidoptera, the monarch butterfly.    SMB feeds on milkweed seeds as well as the nectar from other forbs.  They can also predate on small insects when other food sources are depleted.

The adults lay their eggs on milkweed in the spring and the larvae require milkweed to grow.  They absorb and store the milkweeds toxin glycosides which are harmless to them but serve as a defense against predators.  Actually at least four other insects feed on milkweed without ill effect from the toxic chemical.

Large milkweed bugs - Wikimedia
The large milkweed bug, Oncopeltus fasciatus is nearly twice the size and has a different pattern of the same colors.  Both the adult and larval forms feed on milkweed juice and seeds when available.  O. fasciatus is commonly used in science due to ease of rearing and ease of dissection, a trait that is probably not of significant survival value to the bug but is a boon  to science and education. 

Milkweed beetle-Wikimedia
The milkweed beetle, Tetraopes_tetrophthalmus likewise stores the toxins and advertises its distastefulness.  All the above species share the black and orange aposematic coloration, as does the monarch, warning potential predators that they are toxic. After the eggs are laid on stems at or just below surface, larvae bore into stems, overwinter in roots, and pupate in spring.  The adults appear in the early summer to repeat the cycle.

Our final contestant lacks aposematic coloration.  The milkweed stem weevil,  Rhyssomatus lineaticollis, can be quite destructive of rare and endangered milkweed species.

Milkweed stem weevil - Ken at Bugguides
"Adults initially feed on the apical leaves and then, after feeding, female weevils walk to lower parts of the stem and chew several sequential holes in the stem, creating a continuous scar. Females lay a single egg per hole and larvae complete development inside the stem while feeding on pith tissue."
A mastication of caterpillars
We have previously written about the Milkweed Tussock Moth.  There are lists of special names for groups of animals such as a "murder of crows."   The "official" name for the group of caterpillars is "an army of caterpillars" but I like Kevin Furth's naming of a "mastication of caterpillars."  They are enough to make crow vomit as seen in this blog.

All insects that have milkweed in their diet at some part of their life cycle are poisonous to their predators because of the toxic cardiac glycosides contained in milkweed sap.  Brightly colored or not, don't eat them!

MN Members:
Got Milk-weed seeds?  MDC is collecting them for monarchs.  Store them in a plastic bag and we will get them to MDC at a later time.