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.

Hard choices

A garden club member recently asked me how I would deal with an infestation of gypsy moth caterpillars on a mugo pine (Pinus mugo). Last summer her tree was crawling with them. 

Nonnative gypsy moth caterpillars can cause massive defoliation

Would I use pesticides in this situation? Fortunately gypsy moth populations wax and wane on a predictable cycle. In wet years, like the one we just had, a fungus that kills the caterpillars surges. We can expect minimal gypsy moth damage this summer, so her tree should get a break. Her question raises a larger issue, though. When is it right to use pesticides?

    I was recently disillusioned to learn that the Arnold Arboretum is using a neonicotinoid pesticide to control massively destructive hemlock woolly adelgids in its vast collection of eastern hemlocks (Tsuga canadensis).

White cottony masses on this hemlock are adelgid egg cases

Following the lead of Great Smoky Mountains National Park managers, they’re taking a two-pronged approach to this nonnative pest. They’re spraying the trees with horticultural oil, which smothers the insects, and they’re drenching the soil around the trees with imidacloprid, which attacks their nervous system.

Gray trees in this view of the Smokies are hemlocks killed by adelgids-photo NPS

     In the Smokies, this treatment is reducing adelgid damage and even resulting in regrowth of some hemlock foliage (When it’s not applied to soil, the pesticide is injected into the tree’s bark). Hemlocks are foundational species in the park. Park managers decided that losing them all was unacceptable.

Drenching soil with pesticide-photo NPS

    I sympathize with their dilemma and the Arboretum’s. We resolve to limit our use of pesticides or ban them outright. Then a threat to a plant we can’t bear to lose makes us break our promise to ourselves. As an Arboretum staffer writes, “. . . we are an essential resource for a large urban population that for over 150 years has enjoyed . . . a majestic hemlock-dominated forest.”

Hemlock forest

    The problem, of course, is that all pesticides cause collateral damage. I stopped spraying hemlocks in my yard with horticultural oil for hemlock woolly adelgid two years ago when I realized that even spraying in early spring was killing some insect bystanders that I didn’t want to lose. 

Spraying for adelgids could kill bumblebees and other early foragers-photo Ivtorov

The USDA acknowledges that in the Smokies, imidacloprid affects insects in tree canopies and in the soil at their feet.

    It was human activity that brought the adelgid from Japan to North America. It has no natural predators here. Some human response seems to be needed before all our hemlocks succumb, but what should it be? Applying a pollinator-killing neonicotinoid seems like an unhappy choice. 

     One hopeful development in the Southeast: introduction of natural predators from Japan that eat only adelgids. 

Biocontrol beetle eating adelgids

Tiny beetles in the genus Laricobius have been released in the forest and succeeded in decreasing the adelgid presence without affecting native insects. The hope is that a new balance will develop between these predators and their adelgid prey. Foresters in some areas have been able to stop the chemical treatments. 

     The beetles will never kill every last adelgid, but they may allow hemlocks to survive. Was it worth using pesticides before the beetles became available? I guess so.

The native (hemlocks) are dead. Long live the natives!

Remember those hemlocks I removed to avoid spraying for hemlock woolly adelgid? 

All that's left

They left two big gaps in the garden’s tree canopy that weren’t there before. It’s disconcerting seeing fresh wood chip mulch where evergreen branches used to sweep the ground.

    Last week I finished planting new trees and shrubs in the newly opened spaces. 

For now, small plants and lots of open space

The hemlocks were native to eastern North America. I wanted to replace them with other plants from our region to offer food and habitat for native insects. 

    Between the vegetable bed and compost bins, two hemlocks used to stand along the side fence. I planted two conifers to replace the visual barrier the hemlocks provided. A northern white cedar, also called arborvitae (Thuja occidentalis ‘Smaragd’) will eventually grow 15 feet high and 6 feet wide.
 

Northern white cedar is native to our area

    A small white spruce (Picea glauca conica) went in nearby. This tree ranges across the northern reaches of the continent. You may know it as dwarf Alberta spruce, the little conical potted trees that are often sold as container plants or live Christmas trees. 

This white spruce stays small because it's grown in a pot

It starts small and grows slowly at first, but someday this spruce’s height may reach 10 feet.

    The dense foliage of both these trees provide year-round shelter for birds and mammals. Birds eat the seeds and forage for insects among the branches.

    In the same area, I added an American black elderberry (Sambucus canadensis). Doug Tallamy, the entomologist author of Bringing Nature Home, lists this small tree as highly desirable for providing native insects with nectar and leaves they’re adapted to eat. Birds will also enjoy its copious black fall fruits.

Elderberries

    In view of the vegetable bed, I planted a sweetshrub, also called Carolina allspice (Calycanthus floridus). Its distinctive dark red flowers are reputed to have an intoxicating sweet, fruity scent that attracts insects. 

Sweetshrub has curious dark red flowers

I also planted a mountain laurel (Kalmia latifolia 'Elf') with pink buds and white flowers. I love mountain laurel’s freckled blooms, and Tallamy lists this shrub too as a boon to native insects.

An older mountain laurel blooming in June

    In a back corner of the yard where three hemlocks used to grow, I put in two inkberries (Ilex glabra ‘Shamrock’). I’m hoping this bird- and insect-friendly native evergreeen will prove to be a tough, reliable substitute for nonnative boxwood (Buxus sempervirens).

Boxwood

Inkberry

    Nearby in a spot visible from the kitchen window, I place a spicebush (Lindera benzoin), another of Tallamy’s choices that’s praised for its handsome yellow fall foliage.

Spicebush in fall--photo by tgpotterfield

    In case you’re wondering, I did find some of these young plants at the shop at the New England Wild Flower Society. This means they were grown without pesticides, so they definitely weren’t treated with neonics. I bought three at a sale at a local nursery. I can’t be sure those are neonic free. They don’t attract pollinators, though, so I hope they won’t sabotage the yard’s insect population.

    I’m looking forward to finding out whether these natives will flourish. If they do, they’ll join an expanding list of plants that welcome a broad variety of insects and birds to the yard.

Native purple coneflower

 

Hemlock farewell

For years I’ve been protecting my hemlocks from a nonnative pest, the hemlock woolly adelgid, by having them sprayed with a pesticide. Now I’ve had a change of heart. Sadly, those hemlocks will have to go.

Our hemlocks need pesticide spraying to survive

     When I was researching The Sustainable-Enough Garden, I interviewed environmentalist Ellie Goldberg of Green Newton for advice on whether this treatment was consistent with an environmentally friendly approach. She didn’t favor the idea. She’d taken down her hemlocks rather than spray them. At the time I wasn’t ready to give up on mine.

     But this spring I’ve decided to follow Ellie’s example. Perhaps learning about neonicotinoid pesticide residue on nursery-grown plants has made me more sensitive to the effects of my actions on the insects around me. 

A praying mantis keeps garden insects in balance--photo by Scott Robinson

     Our eastern hemlocks (Tsuga canadensis) were planted by previous owners in two corners of the yard and have grown to about 30 feet tall. The adelgids are tiny imported insects that kill hemlocks by sucking their sap. That’s why you see so many hemlock skeletons and trees gradually losing their needles. 

Hemlocks succumbing to woolly adelgid damage

You can recognize infested trees by the lines of white egg cases along the twigs that look like tiny cotton balls.

Telltale white dots along the twigs are adelgid egg cases

     Leaving the hemlocks untreated wasn’t an option, because untreated trees die within a few years of infestation, and there’s no escaping the ubiquitous adelgids. I didn’t want to provide a staging area for further adelgid depredations in my neighborhood. I wanted to keep the hemlocks alive because I thought that as native trees, they must be hosting native animals in the yard.

A hemlock (not ours) shelters young Eastern screech owls--photo by Matt MacGillivray

So I signed up to have our hemlocks sprayed with horticultural oil. My justification for using the pesticide was that the adelgid is a nonnative introduction with no North American predators.

     The horticultural oil application involves spraying a mist of refined petroleum-derived oil combined with emulsifier into the hemlocks. The spray smothers the insects mechanically by coating them with oil rather than attacking their metabolism as many chemical pesticides do. When the oil dries on the trees in a few hours, it’s non-toxic. Horticultural oil was used on fruit trees as far back as ancient Greece, although back then they used plant oils rather than petroleum.

     If it’s applied in very early spring when the adelgids are active but most other insects aren’t, horticultural oil comes close to targeting only the adelgids. But in practice, the timing never turns out right. It’s not the company’s fault; weather and scheduling get in the way. If they spray now, the oil will also suffocate insects I’m trying to foster in my yard. 

Bees on swamp milkweed are welcome guests

     The only alternative is to remove the hemlocks. I asked arborist Kevin Newman’s team to cut them down in the next few weeks. I’m sad to do it, but it seems better than continuing to spray at the wrong time. To replace the food and shelter that the hemlocks have provided for wildlife, I'll need to plant other native trees or shrubs in their place.

Life after pesticides

Two years ago I resolved to make a break with pesticides. Well, not a total break, because my hemlocks are still sprayed with horticultural oil for nonnative woolly adelgid, and I still spray for winter moth with spinosad, a natural product derived from bacteria. That was supposed to be all.

    Since I made that decision, I’ve reminded Lueders, the company that does the spraying, that I don’t want horticultural oil sprayed on anything but the hemlocks, and I want them sprayed only in early spring, when insects other than the adelgids are dormant and not in danger of being suffocated by the oil.

White adelgid egg sacs along hemlock twigs. These insects will kill a tree within 5 years

    I convinced myself that I wasn’t going to notice any difference when they stopped spraying other shrubs and trees with the horticultural oil. My reading told me that native insects I’d been spraying for were not going to kill the shrubs and trees. I imagined that the damage they’d do if untrammeled would hardly be noticeable. 

    It’s not turning out to be quite that simple. I’ve already noticed discolored and dying leaves on boxwood. I pruned out some of the worst-looking branches. A few weeks later, more leaves show dots and discoloration. 

Boxwood foliage is yellower than normal, and leaves are stippled with bites

I’d imagined that box (Buxus sempervirens) grew healthily in my garden. More accurately, it looked healthy as long as it got annual spraying.

    With a sinking heart, I noticed last week that scale insects are back on the undersides of the leaves of a variegated kiwi vine (Actinidia kolomikta). I knew they were there last year and hoped I’d eliminated them by swabbing them off the leaves with rubbing alcohol.

White cottony covering protects scale insect on kiwi leaves

    Lueders inspected in March and warned that scale was also affecting two magnolias. I’d noticed black staining on the branches, probably from mold that grew on the insects’ honeydew secretions. 

    So now what? Lueders recommends spraying more horticultural oil. They’d resume treating the magnolias and boxwood in early spring as they used to do, and they’d add a second oil application to the boxwood shrubs in fall.

    The problem is that I’m trying to enable a full population of insects to live in my yard, finding their own balance between leaf-eaters such as the mites and scale and their insect predators. If I keep interfering by applying insecticides, beneficial insect predators won’t be attracted to the garden. 

Dragonflies help maintain balance by eating other insects

Spraying in fall when lots of insects are active would also kill bystanders that are needed as part of the full food web.

    For a start, I think I’ll remove the kiwi vine and replace it with something else that can grow on a north-facing wall. I could try replacing boxwood shrubs with native inkberry (Ilex glabra), which has similar small, shiny evergreen leaves. I’ll hold off on making a decision about the magnolias. 

     I’m thinking of this period as analogous to the transition from conventional to organic farming. It could take a few seasons for a natural balance to be established.

    Here’s hoping that won’t mean losing those lovely magnolias whose flowers raise our spirits in early spring.

Star magnolia (Magnolia stellata) in late April