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

Pest control without pesticides

It’s nice to hear about solutions for insect problems other than spraying pesticides. I’ve been a fan of biocontrol—introduction of nonnative insect predators to reduce populations of problematic insects—since I was able to stop spraying my trees for winter moth (Operophtera brumata). 

Winter moth-photo Ben Sale

This lucky turn of events came about because UMass entomologist Joseph Elkinton built up populations of Cyzenis albicans, a tiny parasitic fly that kills moth larvae. As substantial fly populations were established, winter moth damage became a non-issue. The parasitic fly attacks only winter moth and doesn’t prey on native insects.

    Since the winter moth success, I’ve learned about another clever use of a specialist insect, Hypena opulenta, a moth native to Ukraine. This moth eats the leaves of black swallow-wort (Vincetoxicum nigrum) a pernicious vine from the same region that’s causing problems in New England and elsewhere. 

Black swallow-wort, a nonnative invasive vine

Among other negatives, swallow-wort tricks monarch butterflies into laying eggs on its leaves, where larvae starve because they can’t eat swallow-wort foliage. Struggling monarchs don’t need this added trouble.

A monarch nectaring on the real thing, native showy milkweeed

    Boston’s Arnold Arboretum started releasing the Ukrainian moths this spring and saw them defoliate swallow-wort significantly in a test plot. Biocontrol is particularly welcome in the case of swallow-wort, because it’s really hard to pull out. Stems break off at ground level and send up new stalks. Some towns have asked Boy and Girl Scouts to pick unripe pods of swallow-wort vines and dispose of them before they can spread their seeds to the wind. This still sounds like a good idea, because the moths aren’t expected to wipe out swallow-wort, just reduce and control its population and spread.

    Meanwhile, scientists at the University of Rhode Island are working to establish the best predators for red lily beetles (Lilioceris lilii) that chew lilies, especially the Asiatic species, into unsightly stumps covered with beetle excrement. They've released three kinds of parasitoid wasps and are achieving some success.

Red lily beetles at work on a lily stem-photo Charles J Sharp

    When I started gardening back in the 1970s, organic gardening guides promoted the idea of companion planting, claiming that interplanting with herbs or annuals would keep chewing insects from damaging vegetable crops. 


    Unfortunately, this conventional wisdom often turns out to be wishful thinking. Thomas Christopher, in a recent blog post, traces the advice back to Ehrenfreid Pfeiffer, an early 20th century soil scientist and advocate of biodynamic gardening. Pfeiffer conducted bogus experiments purporting to show that certain plants were compatible. The idea was picked up and spread by J.I. Rodale, publisher of Organic Gardening. The nasturtiums, alyssum and marigolds I still dot around my vegetable bed originate from this myth. Most gardeners never traced it back to its faulty source.

Marigolds are nice, but they're probably not protecting my vegetable plants

    Christopher directs readers to a reliable review of companion planting by garden designer Robert Kourik. The scientific papers he cites show a more nuanced picture. Many of the usually recommended flowering plants don’t help to defend food plants, but some do attract beneficial insects that can reduce populations of leaf-eaters. Another reminder of the importance of healthy skepticism about gardening advice.

Skip the fall clean-up

It used to be good gardening practice to clean up your flower beds in fall. We cut perennials to the ground with the theory that leaving their stems standing would provide shelter for insects. How gardeners’ perspectives have changed!

    Now much of what I do in my garden is aimed at providing that very shelter, as well as food, for native insects. That means I shouldn’t cut down my perennials in the fall. I’m unlearning the habit, although my pruner hand still tends to twitch when I see those “messy” stalks.

Ox eye sunflower seeds and stalks will provide winter food and shelter for insects

    There’s a conflict between some of my favorite garden authorities regarding this fall clean-up issue. At the popular blog Garden Rant, Elizabeth Licata writes that tree leaves shouldn’t lie in yards through the winter because they “smother plants and soil.” All I can say is that I haven’t found this to be a problem.

    Elizabeth doesn’t want to get involved in shredding leaves, which I can understand. It’s work, but for me it’s well worthwhile. I like converting leaves into shreds that make pretty, useful mulch. 

Shredded leaves make useful mulch

Because of what I’ve learned this year, though, I’m shredding less this fall and leaving more leaves whole.

    I just discovered that another trusted authority, Jessica Walliser, is contributing to a site and newsletter at Savvy Gardening. I’d already learned from Jessica’s book Attracting Beneficial Bugs to Your Garden that winter stalks, leaves, and berries provide needed shelter for over-wintering insects. Her post “Six reasons NOT to clean up the garden this fall” provides useful detail. 

    Did you know that some butterflies pass the winter here as adults, some in chrysalises, and some as caterpillars? 

Black swallowtail chrysalis. Photo by S. Detwiler

All of these forms could be depending on us to leave them some leaf litter or hollow stems for winter quarters. Jessica advocates letting whole leaves lie to avoid cutting up insects sheltering among them.

    Then there’s the matter of winter food for birds. I’ve spent a fortune on birdseed that’s mostly snapped up by nonnative English sparrows. Both Jessica and George Adams, author of Gardening for the Birds, point out that birds can find more varied food in our yards if we’ll garden with them in mind. I’ve been working on providing a bird menu of berries and seeds on native shrubs and perennials. 

Native winterberry provides food for birds

It’s interesting to hear that hibernating insects are also food for birds such as chickadees that forage for them through the winter. 

    There’s a seeming conflict between trying to help insects survive and offering them as food for birds. But in fact these are part of the same goal. The idea is to grow plants as food and shelter for a large and varied population of insects, so many that birds can eat their fill and still leave enough for other ecological roles, such as pollination and predation.

This green lacewing eats herbivorous insects

     Jessica’s book explains in fascinating detail how predator insects can help keep your garden’s population of leaf-eaters in balance. That’s yet another reason to leave the welcome mat out for insects in winter.