Thursday, August 26, 2010

Milkweed puzzle

The common milkweed (Asclepias syriaca) in my field is a famously toxic plant. It produces cardenolide alkaloids in its foliage, cardiac toxins that affect sodium and potassium levels and interfere with heart contractions. It also produces a sticky latex sap and leaf hairs to foil herbivory.

It is common knowledge that the Monarch butterfly caterpillar (Danaus plexippus) which feeds on milkweed has evolved to tolerate the plant's toxins, sequestering them in its body, rendering it toxic to its own predators.

In the course of wandering my field with my camera for the last year, I have found that the milkweed plant is host to many more insects than Monarch larvae. Even if I can't find anything to photograph in the rest of the field, there's always something to be found on the milkweeds. And it turns out that many of these insects pull off the same trick--consuming the plant with impunity and using the toxin to ward off their own enemies. Some of them, in addition to tolerating the cardenolides, have evolved techniques to disarm the plants other defenses. Monarch caterpillars first clear an area of hair before feeding and also cut the latex canals in the leaf, reducing the pressure of the sticky sap. Anurag Agrawal's video discusses this topic in depth (

Another Lepidopteran, the Milkweed Tiger Moth caterpillar (Euchaetis egle) eats milkweed foliage.

The Large Milkweed Bug (Oncopultus fasciatus) lays its eggs on the milkweed and its nymphs, along with the adults, feed on the plant's juices and maturing seeds.

The Red Milkweed Beetle (Tetraopes tetraophthalmus) larvae bore into the stems and roots of the plant to feed.

The Swamp Milkweed Leaf Beetle (Lasidomera clivicollis) consumes the foliage and the flowers.

All these milkweed specialists advertise their own toxicity with bright, apsomatic coloration. This warns off predators that would otherwise eat them. It works so well that some non-toxic insects [again famously, the Viceroy butterfly (Basilarchia archippus)] wear the same colors to avoid predation.

The milkweed does depend on insects for pollination, and produces large, fragrant flower clusters. However with the exception of the occasional Monarch adult, the pollinators I've observed are different species than the milkweed specialists above.

It makes me wonder how this is working for the plant. It spends energy producing toxins and defenses that many insects have evolved to disarm. The plant toxins protect the insects from vertebrate predators, but the insect work-around must be fairly easily developed since it occurs in at least three orders. And while the milkweeds do survive to set seed, by the end of the summer they are pretty ragged. Are milkweeds just barely protecting themselves against these insect specialists, engaged in an arms race with them as Anurag Agrawal suggests? Do these specialists benefit the plant in some way? Or is the milkweed protecting itself against some other rapacious insect that is completely foiled by the plant's defenses? Vertebrate herbivores--geese, bison, or deer? Food for thought.

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