The spines on the pads of the fragile prickly pear (Opuntia fragilis) and the prickles on the stems of the climbing rose (Rosa setigera) help convince herbivores to look elsewhere.

Chemical defenses of select Iowa plants

The milky sap of purple milkweed (Asclepias purpurascens) contains a toxic steroid – cardiac glycoside – that interferes with the sodium/potassium pump mechanism in cell membranes, thereby disrupting osmotic balance.

Locoweed (Exytropia lamberti) contains the alkaloid swainsonine, which causes enzyme dysfunction.

Terpenes in mints, such as narrowleaf mountain mint (Pycnanthemum tenuifolium), give them their minty aroma and inhibit the growth of attacking bacteria and fungi.

White camass (Zigadenus elegans) contains steroidal alkaloids, particularly zygacine, throughout the plant and is highly toxic. Only one species of bee is known to be tolerant of zygacine and capable of utilizing the pollen and nectar, thereby pollinating it.

photos c. by Dr. Thomas Rosburg

Plants make galls to isolate and limit the effect of herbivores. The dry gall on the goldenrod stem (above) was last season’s home for the larvae of the stem gall fly (Eurosta solidaginis). Maple spindle gall on this leaf (left) is caused by a species of mite.