From Seed to Seed:
Plant Science for K-8 Educators

Digging Deeper

Two Fascinating Relationships

Milkweed-Monarch. Not all plant-insect relationships involve pollination. Consider the relationship between the milkweed plant and monarch butterflies.

Monarch larvae feed on various species of milkweed and, in doing so, incorporate chemicals in the plant called cardiac glycosides into their bodies. The presence of this chemical makes the larvae and butterflies unpalatable to predators such as birds. The monarchs then seem to "advertise" their undesirability with their bright orange and black coloring. (What's more, another butterfly, the viceroy, has evolved to look almost identical to the monarch-so predators avoid the viceroy, even though it doesn't have the bitter chemicals and would actually make a tasty meal!)

Acacia-Ant. Perhaps the most fascinating case of a plant-insect relationship occurs in Mexico and Central America. There, the lives of the bull's-horn acacia and one species of ant are inextricably intertwined.

The bull's-horn acacia has large thorns, each with a swelling on its base that conceals a source of nectar. On the tips of each leaflet are tiny extensions containing oils and proteins. The ants live on and in the thorns, getting the nutrients they need from sugars in the nectar and the oils and proteins in the leaf tips.

In return, the worker ants patrol the entire surface of the plant, attacking and stinging any creature, large or small, that threatens the plant. And even more remarkable, the ants also girdle and kill any nearby branches that encroach upon or shade out the acacia, ensuring it room and light to grow! Mutualism par excellence! And neither organism appears to be able to thrive without the other.

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