Sharing

 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.

 

Striking a balance

There’s a tension built into welcoming native insects. I want to host native leaf-eaters, because they’re the base of the food web for the garden ecosystem. They turn the sun’s energy, embodied in plant tissues through photosynthesis, into food for carnivorous animals, including birds and beneficial insects such as ladybugs. But unchecked, they can also defoliate plants and cause an unsightly mess.

Ladybug eating an aphid

    Worldwide, herbivorous insects make up 37 percent of all animal species. If you’re like me, you don’t spot most of them at work. A leaf-eater has to be big and unmissable, like a Japanese beetle (Popillia japonica) or a tomato hornworm (Manduca quinquemaculata), for me to notice it. But although I don’t see them, they’re out there chewing.

Japanese beetle

    In a healthy garden ecosystem, their population is controlled by predators, including birds, reptiles, amphibians, small mammals, and especially other insects. That’s a reason to avoid pesticides. If I kill their prey, insect predators won’t stay in my yard.

     Problems happen when a nonnative insect with no local predators enters the system. In Japan, for example, Japanese beetles aren’t a problem. When they came to North America in 1916, they left behind the insect predators that kept their numbers under control. Nice for the beetles; less pleasant for rosarians. This story repeats with increasing frequency as global trade expands. In my area, we’re seeing native hemlocks (Tsuga canadensis) devastated by hemlock woolly adelgid (Adelges tsugae), another inadvertently imported nonnative insect.

Adelgid egg cases on a hemlock twig

     I’m trying to keep leaf-eater populations in balance by supporting a broad range of insects in the garden. I tolerate some leaf damage and hope it won’t become too obvious. Research into what’s termed “aesthetic tolerance” indicates that 10 percent of the leaves in a yard can be chewed before the average gardener even notices. 

     This spring as hosta leaves poke up and unfurl in shady spots, I’m noticing that several seem to emerge with notches or tears in the leaf margins. Did some insect get to them as they developed underground, or is this just a result of careless steps before their location showed? To me, the hosta leaves are still glorious. I don’t think I’ll be bothered by a few flaws.

Hosta leaves will still be dramatic with a few holes

     Theoretically, native plants should be even more attractive to native herbivorous insects than imports such as hostas. That was certainly my experience when I planted a native maple leaf viburnum (Viburnum acerifolium) in view of the window over the kitchen sink. Within a few weeks, I saw it completely defoliated. 

I’d chosen to add this shrub to the back of a bed to provide food and shelter for native insects. Well, they’d certainly made use of it. I decided to consider this a victory. Interestingly, that shrub was never stripped by hungry insects again. It seems to have settled into its niche, sustaining a few chewed leaves but thriving. So far, that seems to be typical.

Maple leaf viburnum-photo

Joshua Mayer

Insect superfoods

Visiting Coastal Maine Botanical Gardens this week brought some encouraging news about insect-friendly plantings. The designers had showcased many of the same flowers I’m growing to attract insect visitors, some inside the butterfly house near the entrance to the garden. Monarchs fluttered around this enclosed space, lighting on swamp milkweed and purple coneflowers. I spotted two species of caterpillars on stalks.

Monarch on purple coneflower in the butterfly house

   
    Planting for insects isn’t just for pollinators. It’s also intended to help leaf-eating insects, particularly caterpillars of moths and butterflies. 

Monarch caterpillar

Therefore Thomas Berger performed a useful service by cross-referencing lists of the best plants for these two purposes to derive a master list of insect “superfoods” posted by the Ecological Landscape Alliance. At the top of the section on herbaceous plants were goldenrods, asters, and sunflowers.

    Goldenrods support 11 species of specialist bees—bees that can only feed on a limited group of native flowers—and 115 kinds of lepidopteran caterpillars (caterpillars of butterflies and moths). Asters and sunflowers aren’t far behind. Goldenrods attract monarch butterflies and feed their caterpillars, making them second to milkweeds for supporting these fast-disappearing butterflies. 

    This is good news for my garden, because lots of goldenrods and asters are growing here already. I started some goldenrod from seed years ago. Patches of goldenrod here and there mark its spread around the yard. Unfortunately I didn’t keep a record of which species I’d planted. From photos, I suspect it was blue-stemmed goldenrod, Solidago caesia. Two years ago I bought some sweet goldenrod, Solidago odora, for a new perennial bed. Both native species have proven to be easy to grow and free-flowering.

    Goldenrods are members of the Compositae, also called Asteraceae, a useful family of plants because their central discs are made up many tiny flowers, each with pollen to offer. 

Visiting a daisy flower is an efficient way to collect pollen

The “petals” of the daisy-shaped blooms are actually strap-shaped flowers too. So an insect visiting one of these composite flowers gets lots of pollen with minimal travel. Goldenrod blooms are small, but if you look closely, you’ll see they share the family shape. 

Goldenrod's tiny daisy flowers

 

By the way, goldenrod doesn’t trigger hay fever. It has sticky pollen that doesn’t blow around like that of real culprits, such as ragweed.

    I’m growing some New England asters (Symphyotrichum novae-angliae) with pretty blue flowers. But aster volunteers are much more plentiful in the garden. In the past few years, a white-flowered type has popped up in lots of part-shaded spots with no effort on my part. I suspect it’s white wood aster, Eurybia divaricata.

Aster volunteers

    Last week I wrote about my bad luck growing sunflowers. I learned from Berger’s post that Helianthus annuus, the garden or field sunflower, was brought to New England by Native Americans as an agricultural crop and isn’t native here, although our local specialist bees can still use it. A native option is woodland sunflower, Helianthus divaricatus. Meanwhile, I’m growing oxeye sunflower, Heliopsis helianthoides, also called false sunflower, a different native genus but useful to pollinators too.

Oxeye sunflower