Btk spray kills native butterflies, moths, imperils nestlings

I don’t know where the myth arose that there are no native butterflies or moths out and about in our area in late May and early June, and that this is a time an aerial spraying of the pesticide Btk to knock back the European Gypsy Moth (EGM) can occur without adverse effect on other caterpillars.

Because, it’s said, they’re not around.

But they are.

Late May and early June is precisely when I and a group of naturalist friends share our photographs of the arrival of our first Monarch, the charismatic butterfly we’re all waiting to welcome as it ends its arduous migration from its Mexican wintering grounds.

We might or might not share our sightings of the other species that are flitting around, but sometimes they impose themselves on our consciousness. I remember one year when there was an explosion of Red Admirals, so many I worried about the numbers smashing against the car as I drove down the highway. Googling back, I found the news stories from May, 2012.

So, what is around us in spring? For an answer, I went to two sources. The first is the Toronto Entomologists’ Association’s online records, which can be narrowed down to Simcoe County. I didn't use the earliest date, rather the one after the earliest 10 per cent of the records have been discarded - to eliminate outliers.The second, to be more specific to North Simcoe, are the first sightings recorded by Victoria Harbour naturalist Jim Charlebois up to the end of May. TEA is the first date in parenthesis, Charlebois the second:

Red Admiral (April 27, May 11), American Lady (May 2), Northern Azure (May 5, June 5); Silvery Blue (May 29, May 22), Eastern Tailed Blue (May 24, May 22), Common Ringlet (June 7, May 24), Monarch (June 14, May 24), Viceroy (June 14, June 6), Northern Crescent (June 17, May 16), Clouded Sulphur (July 1, May 24), Pearl Crescent (June 17, May 23).

Alan McNaughton of TEA told me if you see a butterfly or moth in the spring, it will be mating, and there will be eggs or caterpillars on its host plants. They don’t waste time, he says. The exceptions are the ones that overwinter as adults, that might come out on a warm day, too early for mating - Mourning Cloak, Eastern Comma, Grey Comma and Question Mark are examples. 

EGM, as its name implies, is a non-native insect, that feeds on a very wide range of trees, with oak a particular target. Outbreaks occur every 7-10 years and last 1-3 years before the population collapses naturally due to a fungus, a virus and other predators/parasites. It’s endemic - it’s not going to disappear from our environment and spraying will not reduce its numbers in the following year.

Tiny Township has been hard-hit in the last two years. And despite pressure from residents, Tiny Township Council declined to engage in an aerial spraying program. The Severn Sound Environmental Association pointed out the negative impact on native caterpillars and the food web. The County of Simcoe said municipal aerial spraying is not cost-effective.

Disappointed by government inaction, the Federation of Tiny Township Shoreline Associations has facilitated an aerial spraying of private properties this year. Residents and cottagers are being invited to sign up with an air services firm, and the township has agreed to provide a waiver forgiving any drift of the spray onto public lands.

Bacillus thuringiensis is a bacterium that resides in the soil. There are various strains, some of which are used in genetic engineering of crops. Bacillus thuringiensis kurstaki is the one that targets Lepidoptera (moth and butterfly) caterpillars and no other insect order. Described as "safe," it does not affect humans or any creatures other than the caterpillars of moths and butterflies. It kills the caterpillars of all Lepidoptera species; it does not affect adult moths and butterflies and it does not kill on contact; it has to be ingested. 

Btk gets sprayed from the air in two applications, a few days apart. The caterpillar ingests the Btk that's on its host foliage. The Btk proteins punch holes into the wall of the caterpillar's gut, the contents of which spill into its blood (hemolymph). Within a few days, the insect dies from infection and starvation. The pesticide is then supposed to degrade within 3-5 days with exposure to ultraviolet light, and no longer present a threat. 

Proponents of the EGM-busting treatment say native butterflies and moths will not be affected because the spraying is targeted at the trees where the EGM is feeding. There’s a video on the federal Invasive Species Centre website in which an expert from Natural Resources Canada explains that Btk does not affect Monarchs. “We’re not spraying Monarch butterfly habitat, and usually the spraying for Gypsy Moth is done before Monarchs have arrived from the south and started to lay eggs and have the caterpillars out there.”

But it’s clear the Monarch can arrive in our part of the province by the end of May. And it’s clear that our open woodlands are also Monarch habitat: I see adults and caterpillars all the time on my summer walks through the county forest. These are not large expanses of closed canopies as may be the case in other parts of Ontario where there have been major outbreaks. The spray cannot be aimed with the relative precision that applies in farming, where there’s a specific area with a specific crop that is consumed exclusively or primarily by a specific target butterfly or moth.

How to ensure that the Btk does not drift down to the Milkweed in the clearing that is hosting the Monarch, or to the edge of the trail where vetches and clovers are consumed by the Eastern Tailed Blue, or to the nettles that support the Red Admiral?

How to avoid any other Lepidoptera that, like the European Gypsy Moth, favour the oak and may be present at the same time? The genus Quercus has been found to be the number one plant for supporting caterpillars by Doug Tallamy, professor of entomology at the University of Delaware. His research identifies 557 species of butterflies and moths with caterpillars that feed on oak in the mid-Atlantic region.

What of other trees, part of our forest mix - the Willow and Poplar that also support Mourning Cloaks and Viceroys, the Cherry species that support Northern Azures?

And what of the birds? A theme in my approach to gardening is that we need to landscape for insects in order to support the food web. Flowers are a welcome and essential part of the package but insects are fundamental.

And one reason is that birds rely on one particular order of insect to feed their nestlings - Lepidoptera caterpillars that are soft and digestible and rich in protein and other nutrients. Even the birds that normally visit our feeders for seed turn in spring to caterpillars for their young, with moths a more important part of the diet than butterflies, Tallamy told me when I contacted him last week.

(Cherish your moths when you next see them. They get exhausted by lights left on at night. If you need one, make it motion-activated or use a yellow bulb).

The oft-cited Tallamy statistic is that one chickadee pair needs to forage 6,000-9,000 caterpillars to raise a brood. Multiply that by however many chickadees are in your yard, and however many other birds and you will realize how important it is to make sure you have caterpillars on the menu.

Here, from the Ontario Breeding Bird Atlas, are the dates some familiar bird species start nests in Southern Ontario: Mourning Dove (March 15), Northern Cardinal (April 15), House Finch (April 15), Black-capped Chickadee (April 16), White-breasted Nuthatch (April 20), Downy Woodpecker (April 30), Song Sparrow (April 30), Eastern Phoebe (April 30), House Wren (May 10), Red breasted Nuthatch (May 15), Chipping Sparrow (May 15), Blue Jay (May 15), Purple Martin, Swallows (May 15), Ruby-crowned Kinglet (May 20), American Robin (May 24), Ruby throated Hummingbird (May 24), Evening Grosbeak (May 24), Warblers (14 May - May 31), Indigo Bunting (May 31).

“The end of May is peak breeding bird activity,” Tallamy told me in an email. “You will not find a time when more warblers etc. are foraging for their nestlings. This is not debatable. It is also the time when most geometrid caterpillars, the Lepidoptera that fuel breeding bird food webs, are out as caterpillars. That's why the birds breed then.”

I was not aware of the Btk controversy in these parts until I received a letter from Tiny cottager Nancy Moysiuk, detailing her experience in 2019 when her neighbourhood in Toronto was sprayed. She gathered all the Monarch eggs and caterpillars she could find in her yard, about 50 in all, on the day before the first spraying (May 27) and and brought them in along with Milkweed foliage for feed. When the foliage wilted beyond being useful, she gathered fresh Milkweed.

Unfortunately, this was a few days after the second spraying (June 7) and despite her washing the foliage, the caterpillars died, the first on June 11, the last on June 23. It seems she may have gathered the second lot of foliage within the 5-day official window for Btk degradation. The unfortunate end of all the caterpillars she tried to save appears consistent with Btk ingestion.

You can read her letter and see her photos of what happened below. Despite 2019 being a banner year for many of us who enjoyed plentiful Monarch sightings, the population in Nancy’s Toronto yard did not rebound. Those caterpillars she “saved,” it should be noted, were the only ones to be observed. Outside, it seems likely that Monarchs were laying eggs during and after the spraying and if the resulting caterpillars ended up as smudges on the underside of a leaf, they went unnoticed.

I’m not about to downplay the distress Tiny residents have gone through for the past two years. Many are my friends and I’ve heard the tale of their trees being defoliated, some dying, caterpillars everywhere, frass raining down from trees, complete disruption of outdoors enjoyment. Is it a comfort that the population is expected to crash after next year if 2021 shapes up like the last two? That most trees will rebound, even after two or three years defoliation, if not subjected to drought or other stress? Probably not.

I contacted several experts in writing this blog. How does one balance the weakening or killing of trees by EGM against the impact of Btk on non-target Lepidopterans and the food web that depends on them? Good questions, they replied to my inquiries, questions that go beyond the research that has been done. One noted that trees die naturally and many fauna and floral species benefit from dead and dying trees, another that we may have a changed ecosystem but not necessarily a less valuable ecosystem.

Is spraying a useful intervention to bring balance and sustainability to our environment? The problem is that the pesticide is not specific and its effects may be wide-ranging. We just don’t know. We do know that birds, butterflies and moths are in decline. My view: that the precautionary principle should apply.

A final suggestion from Tallamy, that the weather be monitored before a decision to spray is made. “Cool, wet springs favour the outbreak of a fungus that controls gypsy moth really well… We are an El Nina year which is predicted to bring a cool, wet spring to the north east.”

Nancy Moysiuk Letter
Ken Thompson
- 16 February 2021 at 03:34pm

Thank you for this essential information.
Beth
- 17 February 2021 at 07:52am

Thanks for the link to the Toronto Entomologists’ Association’s online records. Very interesting!
Yana
- 20 February 2021 at 10:33am

Last Spring I noticed a lot of caterpillars on my property just die one day. Every caterpillar I found was dead either on the tree leaves or on the ground. Later in the Summer I did not see many butterflies on my property how I normally do. We have Monarchs are other butterfly species feed on our wild flowers yet last summer that was not the case. I was wondering what has happened up until I read this article. I had no idea such spraying was taking place. Would you happen to know if there is a way to start a petition or revoke consent of such spraying by private property owners? I do not think it is Legal for such spraying to take place without property's owner consent as it directly affects the property and its environment.
Janet Davis
- 2 May 2021 at 11:42am

Kate... I wrote a blog last summer about my experience with gypsy moth control in Muskoka. I used a homemade horticultural oil to spray the egg masses. My blog doesn't allow links in comments, and yours might not either so just Google thepaintboxgarden and gypsy moth and Janet Davis. Btk also kills the larvae of little moths and butterflies that aren't on our radar because they're not the pretty butterflies we like to think about. Killing native lepidoptera larvae has huge ramifications for bird life, as you've said.
Kate
- 2 May 2021 at 01:26pm

Hi Janet - what an informative blog. Your methods are inventive and make sense. I look forward to hearing how this year works out for you and your oaks.
The link is: https://www.thepaintboxgarden.com/tag/forest/
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