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.

Good news for trees

Spotting a little green caterpillar on a weed I pulled from the perennial bed reminded me that this is the time when winter moth larvae (Operophtera brumata) make their way up trunks of trees such as the old oak that stands over this area of the garden.

Winter moth caterpillar

    That caterpillar was in for a surprise, though. Thanks to a team led by University of Massachusetts entomologist Joseph Elkinton, a parasitoid tachinid fly, Cyzenis albicans, is laying its eggs on leaves consumed by winter moth caterpillars. The fly’s offspring will kill most of the caterpillars that eat those leaves. What used to be an epidemic of nonnative pests defoliating trees in eastern Massachusetts has turned into no big deal.

    We used to notice clouds of male winter moths when they appeared in late fall, often lighting on windows and doors and sometimes making their way into the house. 

Winter moth caterpillars in November 2016

The wingless females can’t fly, but they can climb tree trunks to lay their eggs in bark crevices. The larvae emerge in March and devour leaf and flower buds, or they produce strands of silk that allow them to “balloon,” floating to other trees to spread the destruction. 

     Winter moth surged in Massachusetts in 2003 and found no native predators. At the height of the moths’ population boom, they affected most of our street trees, favoring the plentiful maples and oaks. It was alarming to walk down the street and see every tree leaf riddled with holes.

Birch leaf chewed by winter moth caterpillars, 2015

    To protect ornamental trees, the usual approach was to apply pesticides, and I did that for several years. I was telling myself that the treatment was relatively benign, because it used spinosad, an agent derived from naturally occurring soil bacteria.

Although it's a natural product, spinosad kills bees as well as winter moths

     Elkinton’s team took a different tack. In Europe, where it originated, winter moth has numerous insect predators. The scientists tested these to see which would be safe for release in New England, seeking an insect that targeted only winter moth and wouldn’t harm native insects. For the past 14 years, they’ve been releasing the parasitoid fly Cyzenis albicans to kill winter moth caterpillars. 

Cyzenis albicans-photo James K. Lindsey

    This year Elkinton has declared victory. After years of painstaking monitoring, his team has determined that winter moth populations are down to manageable levels. The parasitoid fly has become established, maintaining sufficient populations to keep winter moth under control.

    Predators need prey. If the flies eradicated all the winter moths, they’d have no food for their larvae. They would die out too. With both maintaining a stable presence, they can continue their interdependent dance.

    This is one of those scientific and ethical dilemmas where human beings have created a problem—in this case by unintentionally importing nonnative winter moths—and there’s no perfect response. If we did nothing, we’d likely lose many trees. 
  

     If we kept spraying pesticides, even targeted ones, we’d inevitably cause collateral damage to other insect species. That's why I stopped applying spinosad. So far, introducing nonnative Cyzenis albicans seems like the happiest solution for a thorny environmental problem.

Winter moths won't be troubling our oak anymore

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.

High life for hummingbirds

Last year I gave up on my hummingbird feeder. I was concerned that I might be offering moldy or infected sugar water, because I couldn’t get around to refilling the feeder every day in hot weather. And truth to tell, I wasn’t seeing any hummingbirds using the feeder.

    Because they beat their wings constantly, hummingbirds have to keep eating whenever they’re awake, and at night they lower their metabolic rate to use less energy. They’re welcome in the garden community because their quest for nectar makes them important pollinators. They also eat lots of insects for protein, consuming up to half their body weight daily.

A hummingbird on the job, pollinating while it sips nectar

    This year I focused on growing flowers that would attract hummingbirds. Conventional advice is to offer a succession of red, orange and pink flowers with long tubular shapes from spring through fall. Recent research clarifies, though, that the birds seek out the location of known nectar sources rather than searching by flower color.

    Until this year, I’d rarely seen a hummingbird in the yard. This year I hoped to lure some to visit the back deck so I’d have a chance of seeing them. 

    Sure enough, we’ve seen hummingbirds near the house this summer. Yesterday one visited the cobalt-blue flowers of a hummingbird sage (Salvia guaranitica) growing in a big pot on the deck. 

That blur in the center of the photo is the hummingbird--best I could do

He hovered to dip his beak in the thin tubular flowers before darting away to perch on a nearby tree branch—the first time I’ve seen a hummingbird sitting still.

    I’ve also seen a hummingbird feeding from a tall meadow rue (Thalictrum rochebrunianum) that has sprays of tiny shallow lavender flowers, not trumpet-shaped at all. I guess the birds’ choice of flower shape must be a preference rather than an invariable rule.

Surprisingly, these flowers attracted a hummingbird

    So what accounts for more hummingbirds visiting the yard and coming near the house this year? I can’t be sure, but I have some guesses. First, I now have three honeysuckle vines (Lonicera sempervirens) blooming along the back and side of the house near the deck. 

Honeysuckle has everything hummingbirds like

Their flowering has lasted longer this summer than usual, perhaps because it’s been such a wet year. There’s that hummingbird sage, which started blooming in August and is covered with flowers. I’ve also learned that the old-fashioned magenta-flowered rhododendron (Rhododendron catawbiense) facing the deck is popular with the birds. Virginia bluebells (Mertensia virginica), whose trumpet-shaped flowers also offer nectar for hummingbirds, had a good year in a shady spot around the corner from the deck.

Virginia bluebells' blooms start pink and change to blue

    The yard also provides nesting material for hummingbirds: spider webs and dandelion fluff. I can’t take any credit for those; they’re just here. 

    Another important factor could have cleared the way for hummingbirds. Because they eat so many insects, they’re highly affected by pesticides. This is my first year with no pesticide spraying at all. I wasn’t thinking of hummingbirds when I made the decision to stop the last pesticide application, for winter moth. If it’s made a better life for hummingbirds, I’m glad.

Hummingbird sage draws other visitors too

Fighting caterpillars from within

Winter moths are appearing on the storm doors again,

so I was eager to attend a recent talk by Professor Joe Elkinton. Elkinton is an entomologist at UMass who not only studies infestations of nonnative insects, he actually does something about them. His speech about gypsy moths and winter moths alerted me to a gardening quandary I’ll need to resolve.

Birch leaves chewed by winter moth caterpillars

    Gypsy moths (Lymantria dispar) came to Boston in 1868, imported by Étienne Léopold Trouvelot, an astronomer in Medford, Massachusetts who was trying to mate them with silk worms. In an all-too-common scenario, they escaped from his custody and traveled across Massachusetts defoliating trees over vast areas. The last major outbreak was in 1981.

Life stages of the gypsy moth

     In an interesting ecological twist, gypsy moth populations were decimated soon afterward by a newly introduced fungus. This year’s drought changed the balance by limiting the fungus, so gypsy moths surged. 

    Elkinton said that biological control—introduction of organisms to kill unwanted insects such as gypsy moth—was tried as far back as 1905. In this approach, insects that control the pests in their homeland are brought over to do the same here. 

     Early on, one introduced parasitoid killed native insects, such as the beautiful luna moth (Actias luna).

The luna moth is a North American native

A parasitoid, unlike a parasite, ultimately sterilizes or kills its host. 

    That brought me to what I wanted to know from Elkinton’s talk. How can we be sure we won’t cause more problems by releasing new nonnative insects to control the ones we don’t like? His answer was, “Host range testing.” This means carefully evaluating the insect you’re planning to release, checking whether it will use native insects as hosts. Entomologists in this field only release parasitoids or predators that specialize in one nonnative insect, unlike goofballs like Trouvelot. 

    Winter moth (Operophtera brumata), also from Europe, got to the US in 1950 and has been a major problem in eastern Massachusetts since around 2001. The larvae, little caterpillars, tunnel into buds and eat young leaves of most of our deciduous trees. Elkinton has been releasing a tiny tachinid fly, Cyzenis albicans, to parasitize winter moth larvae. 

Winter moth caterpillar

     This is a long process. It takes three to five years after introduction to establish a population of the fly in a new location. To determine whether their efforts are succeeding, his team and volunteers harvest 120,000 caterpillars in May and check whether each contains flies.

    Elkinton revealed that my neighborhood has an established population of C. albicans. He predicted there will be enough of them next summer to make a significant dent in the winter moth population.

    Thereby hangs a sustainability dilemma. If I continue having my trees sprayed to kill winter moth caterpillars, I’ll kill the little flies inside the larvae as well. I’d like to let the flies do their job, but if they don’t, the caterpillars will defoliate the trees. I think I'll take a chance and skip the spraying.

Last foliage of the year. This ornamental plum could be defoliated by winter moth next summer if I guess wrong.

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.