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 At this time of year, the garden starts to look tired and ragged. This week I took the opportunity to reassure myself that chewed leaves aren’t a problem for garden plants. They have lots of ways to deal with chewing insects, and lots of leaves can be chewed without changing a plant’s beauty.

Canadian wild ginger

Humans aren’t that sharp about noticing chewed leaves. Research on aesthetic tolerance has found that 10 percent of the foliage in a garden can be damaged by herbivorous insects before the average gardener even notices.

Insects have been chewing the ginger leaves

     Plants notice, of course, and they’ve evolved a range of tools for turning away hungry animals. First there are mechanical barriers, such as the bark and thorns that some deploy. Without these obvious weapons, though, lots of other plants are ready with chemical defenses.

Closeup shows insect damage

 

Oxeye sunflower

     

 

 

 

 

 

 

     

 

 

We often hear about plants whose tissues are poisonous to leaf-eaters. Milkweeds (Asclepias spp.) can’t be eaten by any but monarch caterpillars because their sap contains toxic cardiac glycosides. Foxgloves (Digitalis spp.) poison browsing herbivores with digitalis in their tissues. 

    Other plants can generate chemical responses to browsing insects. Goldenrod, for example, sends out a chemical signal that keeps leaf beetle larvae from eating too much of each plant. A team at Cornell found that a chewed goldenrod plant sends out volatile organic compounds warning the larvae away from itself and its neighbors. The scientists found that the larvae preferred unchewed plants. 

Goldenrod can communicate with plants and insects

     The goldenrod’s chemical message also alerts nearby goldenrods to the danger so they’ll ready their own defenses. Each plant can lose up to 30 percent of its leaves and survive. In response to the chemical signal, the damage was spread out over stands of goldenrod. This seems like a win-win: the plants aren’t mortally wounded, and the insects get what they need.

    So as I walk around the garden noticing damaged leaves on just about every plant, I remind myself that there are enough leaves to go around. 

 

Leaf miner at work on a columbine

And I’m definitely one of those gardeners who doesn’t notice the chewed parts when I shift my attention to taking in the whole plant. Just as well.