The Web of Life (84-10) Monarchs & Milkweed

 Monarchs & Milkweed

Various milkweeds employ cardiac glycosides to ward off herbivores. The larvae of monarch butterflies evolved the ability to eat milkweed with impunity, concentrating the glycoside compounds into their own body.

Adult butterflies use this weapon against birds that prey upon them. Blue jays quickly learn to take monarchs off the menu by noting their characteristic wing pattern.

The interaction between monarchs and milkweed is ultimately to the milkweed’s favor.

In an exclusive mutualism, monarchs lay their eggs on milkweed. The eggs hatch onto an instant meal, but one that might be an early demise.

If a young caterpillar hits a vein, viscid sap pours out, engulfing the newborn. The latex can drown the little crawler, or lock its jaws, making the gushing bite its last.

The tally by the time the caterpillars pupate is that 2/3rds having been killed by the milkweed. The 1/3rd that flutter away do so to an immediate victory meal.

The milkweed flowers with perfect timing: just as the butterflies emerge, so that the butterflies can enjoy some well-deserved nectar while pollinating the milkweed. In all, milkweeds manage the damage from their dependent pollinators.

The ferocity of milkweed’s latex brew is welcome in the floral neighborhood, as its toxic scent wards away pests, such as wireworms, which would otherwise infect nearby plants.

Viceroy butterflies ply on the poisons that monarchs amass by mimicking the monarch’s warning coloration, thus avoiding predation by masquerading as monarchs. This stratagem only works if adult viceroys emerge late in the season, after the birds have learned that monarchs are a digestive monstrosity.