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

Post-pesticide paradise?

It’s been three years since I stopped authorizing pesticide spraying on our property. As this year's gardening season ends, I’m assessing the effects of that decision.

    After I pulled the plug, the garden wasn’t reduced to stubble by a plague of leaf-chewing insects. But neither did it remain unchanged. I'd sort of hoped that spraying all those years might have done so little that the garden wouldn’t look different without it.

Lush and green in 2013--but with pesticide spraying

    I started the spraying back in the 1990s when I noticed pinpoint white speckles on the leaves of Japanese pieris (Pieris japonica), an elegant early-blooming evergreen shrub. 

Marked leaves of Japanese pieris

A technician from Lueders Environmental diagnosed pieris lacebug, a tiny sap-sucking insect that originated in Japan and has spread around the gardening world. Decades of spraying kept the pieris leaves clean. When we stopped the spraying, the lacebugs came back. New foliage emerging the next spring was shiny and unmarked, but by the end of the growing season, all the pieris foliage was speckled again. 

    After we stopped spraying our many boxwoods (Buxus sempervirens) for psyllids, their leaves too weren’t as well formed and healthy-looking as they’d been with chemical protection. 

Psyllids have inscribed and curled these boxwood leaves

I’d thought the nonnative boxwood shrubs were problem-free. That was a false impression created by pesticide spraying.

    I’d agreed to use pesticides thinking I was protecting my trees and shrubs. Later we added dormant oil spray for hemlock woolly adelgid and Spinosad, a naturally derived insecticide, for winter moth. Those were nonnative insects with no local predators, I argued. I feared that they’d decimate my yard if they weren’t stopped.

A bleak scene: hemlocks dying from woolly adelgid infestation

    Around 2011 I started to get uncomfortable with this approach. Yes, I was targeting problem insects with each pesticide, and the chemicals had each been chosen for least toxicity for humans, mammals and birds. But my newfound concern for the native insects in the yard shifted the frame. I came to realize that no matter how carefully they were applied, these pesticides would always cause collateral damage, killing native insect bystanders that could be leaf-eaters, pollinators, or beneficial insect predators that I wanted to foster and encourage.
 

I didn't want to kill beneficial insects such as this ladybug, seen here eating an aphid

    After gradually cutting down on spraying, in 2016 I finally decided it was time to make a total break with garden pesticides. Regretfully, we had our hemlocks cut down so we wouldn’t need to spray for hemlock woolly adelgid, a particularly pernicious nonnative pest.

The fluffy white balls along these hemlock twigs are woolly adelgid egg cases

    Probably no one but me noticed when, without pesticide treatment, the leaf damage reappeared on the pieris and boxwood leaves. The leaves didn’t change color or drop off. The subtle damage just made me worry that worse was to come.

     Instead, this year the infestations seem less comprehensive. That could mean that without pesticides, the garden ecosystem is rebounding. In a healthy ecosystem, there’s a balance between leaf eaters and their predators. 

Dragonfly on St. John's wort: one of the predators I want to welcome

There’s also plenty of redundancy, so that if one population of insects has a hard year, there are others to fill their role in the community. I’d love to think that’s the ecosystem we’re building here.

Gypsy moth: more complicated than you'd think

While waiting last week for my computer to return from urgent care, I contemplated a talk about biological controls by Joseph Elkinton, a professor of environmental conservation at the University of Massachusetts, that I heard last November. 

Gypsy moth caterpillar

    What Elkinton said about the history of gypsy moth in North America turns out to be unexpectedly relevant. We had a gypsy moth surge in 2016, and this year there has been significant defoliation by gypsy moth caterpillars, especially in southeastern Massachusetts.

Voracious gypsy moth caterpillars shred leaves

    A complicated web of factors influences the gypsy moth population. It all goes back to acorns.

    Gypsy moth (Lymantria dispar), native to Europe, was introduced to Medford, Massachusetts by Étienne Léopold Trouvelot, an astronomer dabbling in silk worm breeding. He thought he could cross the gypsy moths with native silk moths to produce silk in the US. The insects escaped from his home in the 1860s, and over 50 years they slowly established themselves in the wild. They’ve spread as far as Minnesota and North Carolina.

Life stages of the gypsy moth

    Elkinton explained that until 1989, gypsy moths experienced population outbreaks about every 10 years. Throughout the twentieth century, predator insects were introduced in hopes of controlling the gypsy moth population, but none did enough.

Predatory wasps kill caterpillars, but not enough

     Meanwhile, research into the ecology of the gypsy moth uncovered a system of natural controls. Robert Campbell of the US Forest Service found that the white-footed mouse was the most significant predator of the caterpillars, with birds less of a factor.

White-footed mouse, Peromyscus leucopus, in captivity

     Mouse populations rose and fell with the supply of their main food, acorns, which fluctuated with weather conditions. The mice couldn’t expand their population enough to control gypsy moth outbreaks, when caterpillar numbers spiked exponentially.

    Outbreaks were brought to an end by Nuclear Polyhedrosis Virus (NPV), which eventually killed up to 99 percent of the horde of larvae.

    This cycle continued until another accidental foreign introduction in 1989: Entomophaga maimaiga, a Japanese fungus. Infected by the fungus, gypsy moth caterpillars suddenly died all over southern New England.

    So with mice, viruses and fungi controlling its population, why is gypsy moth surging again? The levels of fungus are highest in wet years. Last year’s severe New England drought allowed the gypsy moth population to escape from its fungal control. This summer, caterpillars chomped foliage until June, when they started to die off. Caterpillars examined by Elkinton’s team showed a combination of viral and fungal infections. Wet weather had turned the tide.

    This means, thank heavens, that we don’t have to make the difficult decision whether to spray for gypsy moth.

    The future outlook for New England trees is mixed. Gypsy moths won’t die out completely, and our trees face a number of stresses from drought and other nonnative insect attacks. A tree can survive one defoliation, but repeated losses deplete its energy stores. Elkinton worries about major losses of native oaks, which the caterpillars particularly enjoy. Then what will happen to the white-footed mice?

 

The best among bad choices?

This is the time of year when small gray-brown winter moths (Operophtera brumata) congregate on windows and storm doors at night, sometimes fluttering into the house. These nonnative insects have staged a major infestation in Massachusetts since 2001, and they have me in a dilemma.

Male winter moths hanging out on a window

    Starting around Thanksgiving, the moths mate, and the females lay their eggs on trunks and major branches of trees. They particularly like maples, oaks, crabapples, and blueberries. Those are some of the key species in my yard. The larvae, tiny caterpillars, hatch in spring, wiggle into buds, and eat young leaves. 

    Weakened by losing a large proportion of their leaves to the caterpillars’ depredations, the trees may die because they lack the energy to put out a new crop of foliage. I can see this happening to street trees in the neighborhood. Some have died; many are very stressed.

This birch lost many leaves to winter moth caterpillars

    I’ve been having my trees sprayed with a (somewhat) natural product called spinosad that kills winter moth caterpillars when they’re eating leaves in spring. I do it to save the trees, but I don’t feel good about it. Spinosad kills by its action on the caterpillar’s nervous system. It’s extracted from the bacterium Saccharopolyspora spinosa, which is fermented to derive the active ingredient. 

    Spinosad has low toxicity for mammals and birds and spares many insect predators and parasitoids that we count on to control leaf-eaters. The problem is that it kills not only winter moth larvae but other insects too. I was alarmed to learn that it can kill honeybees if it’s applied while they’re active, as when flowering trees are blooming in spring and bees are foraging. Once the spinosad dries, it becomes less dangerous to bees. 

I don't want to kill bees by spraying spinosad

    So which choice is correct, to spray or not to spray? I’m torn between wanting to save trees I love--losing the hundred-year-old oak near the house would be particularly sad—and recognizing that by spraying spinosad, I’m altering the balance of insects in my yard and killing off some natives participants in the local food web. I could let the winter moth caterpillars do their worst and see which plants survived, but so far I haven’t had the heart for such an austere approach. 

I'd hate to lose this red oak

    In the next few years, biological control may let me off the hook. Winter moth originated in Europe. Because it didn’t evolve here, it has no native predators. A team led by UMass scientist Joseph Elkinton has spent a decade releasing a predator of winter moth in New England.    

     The silver bullet is a European parasitic fly, Cyzenis albicans, which controlled winter moth outbreaks in Nova Scotia and the Pacific Northwest. The flies eat winter moth caterpillars when they pupate in the soil. They’re specialists, so they don’t affect other species. Their population is expanding. 

Cyzenis albicans

    I like the idea of getting rid of winter moths without chemicals. I wonder, though, about unforeseen risks of introducing nonnative insects to kill the nonnative insects we’ve already introduced accidentally.