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.

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.

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

Less is more

Happy New Year! Amy Andrychowicz advises in the Savvy Gardening Newsletter that gardeners write “garden reflections” at the end of each year, documenting what went well and badly in this year’s garden and what can be improved next year. Here are some of mine:

    What went well? A lot that wasn’t in my control. We got plenty of rain! That kept the garden lush and flourishing. 

Rain made everything grow

    Also good was that the destructive nonnative winter moth dwindled my area. Tachinid flies (Cyzenis albicans) released by UMass professor Joe Elkinton are killing winter moth larvae. I counted three or four moths at the front porch lights this fall, nothing like the clouds of them we’ve seen in previous years. With so few parent moths, I predict few caterpillars will show up next spring to defoliate and weaken our trees.

There should be less winter moth damage next year

  
    Thanks to a much-abbreviated work schedule, I was in the garden most days all season. That gave me a chance to start addressing a list of dozens of tasks and projects I’d put off over the past decade. 

    This year I got to tear out a big patch of lawn and replace it with new perennials, mostly native species. It doesn’t look like much yet, with little plants surrounded by wide stretches of mulch. In three years when it’s filled in, I’m expecting a lush habitat for birds and native insects, as well as a pretty sight from the back of the house and the deck.

With luck, these little plants will grow wide and tall

    Restricting my container choices to neonicotinoid-free plants also turned out well, better than I’d expected. Maybe a smaller palette of choices prevented over-complicated compositions.

Simpler container combinations did better

    What didn’t go so well? Despite the plentiful rain, my vegetable garden didn’t produce a lot. Two problems stick out. First, I planted intensively with not much space between plants and rows because I’ve got so little sunny ground. That meant overcrowded plants didn’t get all the sun they needed. I didn’t leave enough space for paths, so while trying to spring lightly in and out of the beds, I stepped too close and packed down soil where roots were trying to grow.

Tight quarters in the vegetable garden

    This was the year I noticed my trees were crowded too, bent out of shape by competing to reach the light. That’s because of mistakes I made twenty years ago planting them too close together, like those vegetables. Having done that, I could have pruned more aggressively than I did. Back when the trees were young, I was afraid I’d kill them if I cut back their young branches to direct their growth. Now I know better.

Cramped trees

    The takeaway? Less is more. Make the hard choices and plant what I really have room for. Plan for the full-grown size, whether of 12-inch-tall bean plants or trees that will reach 60 feet. Failing that, don’t be too sentimental to thin out the extras when it’s time. Will I live by this insight and refrain from overcrowding the new perennial bed? Time will tell.

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.