Scientists with B.C.’s Ministry of Forests have set late April — or early May — for a targeted aerial spraying effort, hoping to keep an invasive insect species from establishing a foothold in the province.
Last seen in the 1990s, spongy moths were detected again on Salt Spring Island in 2021; monitoring efforts the following year revealed a growing population of the insects which — while caterpillars — are voracious defoliators, according to Canadian Food Inspection Agency biologist Dave Holden.
Holden and members of the Spongy Moth Technical Advisory Committee hosted an online information session Wednesday, Feb. 7, outlining the problems the moths bring and answering questions about the province’s eradication program — and how it will unfold on Salt Spring.
The spongy moth is the common name for Lymantria dispar, renamed from “gypsy moth” by the Entomological Societies of both Canada and the United States in 2022. The name references the insect’s “spongy” egg masses, often found in the branches of the same deciduous trees the hatched insects will devour — and which have been known to cause rashes when handled.
Recently, the egg masses have been found more often in human-made shelters — including under vehicles and even in bird houses, which has helped them spread from Ontario into B.C.
“It’s a perfect hitchhiker,” said Holden. “Often the egg masses are laid in concealed areas, sometimes in the wheel wells of cars.”
The spongy egg masses are roughly the size of a quarter and are difficult to spot, Holden said, and it’s generally understood that moths now found in B.C. came from established populations in Ontario in late 2020 — catching a ride behind vehicle bumpers and under RVs.
“With Covid restrictions, we saw a huge number of introductions into B.C., because [snowbirds] weren’t moving south, because the borders were shut down,” said Holden. “They were travelling to the west. That’s what we’re trying to ‘mop up.’”
Infestation impacts seen in Ontario range from the destruction of individual trees to defoliation “across the landscape,” according to Holden, who said they generally attack deciduous trees — oak being their “favourite,” he said — followed by alders, birches, arbutus, apple and pear trees. One of the biggest concerns for scientists are the vulnerable Garry oak ecosystems both on Salt Spring and Vancouver Island; but when they build up in an area, according to Holden, the conifers are eaten next. And while deciduous trees are often resilient enough to come back after a year or two, he added, a defoliated conifer usually doesn’t survive.
“I’ve seen when they’re super hungry, they’ll actually eat the bark off the tree,” said Holden. “But typically they’ll eat down to the mid-rib of a leaf.”
This past summer’s efforts to slow the infestation through a high-density trapping program on Salt Spring were successful, according to acting provincial forest entomologist Timothy Ebata, and as a result of data gathered the area to be sprayed is relatively small — a 48-hectare patch roughly centred on Elizabeth Drive, which will be treated from the air over three early mornings, likely in late April or early May.
The exact date, he said, would be determined by population growth modelling based on measured air temperatures — picking the precise moment when the insects will be at their most vulnerable.
“We were able to get the boundary down to this basically bare minimum,” said Ebata. “We have really good intelligence in terms of where we think the epicentre of this infestation is.”
The spraying will be conducted by relatively low-flying aircraft — usually fixed-wing, according to Ebata, but sometimes a helicopter — using a GPS-controlled system. The pilots will fly in a line, he said, and the equipment turns on and off as boundaries are crossed, recording the whole event as a digital file to be reviewed later.
Ebata said despite the 300-metre “drift area” buffer zone allowed by their permit, his experience was that even the overspray tended to stay within 100 metres of the treatment boundary. Spraying will not occur at all if winds exceed 8 km/hr.
“It’s very precise,” said Ebata. “On the ground, we monitor the deposition of the product using ‘spray cards’ that are placed before the treatment, and then examined after.”
In addition to public outreach efforts like postcards and newspaper ads, Ebata said there would be signs placed in the neighbourhood well in advance of the spraying days.
Unlike ground-based spraying, such as took place near Lee’s Hill in 2007, aerial applications are advantageous partly because they require a lower concentration of the bacteria-based insecticide preferred by the province’s Spongy Moth Technical Advisory Committee — Foray 48B, an Organic Materials Review Institute listed product certified in 2018 for use in organic production.
The program this spring will use about four litres of product per hectare, said Ebata, “about a coffee cup’s amount over a typical city lot.”
“It has no known toxic effects on any organisms other than lepidopterans,” said Ebata — the order of winged insects that includes butterflies and moths. “That’s because the mode of action of the bacterium is very unique to alkaline environments — alkaline guts, especially.”
That does mean other caterpillars within spray areas can be affected, according to invertebrate conservation biologist Jennifer Heron, but most species present are non-native or not at risk — such as the European Skipper or cabbage white butterflies, she said — and pollinators like bees, wasps and beetles aren’t affected by the bacteria at all.
Ebata did say there had been some human health effects reported from people directly hit by sprays — described as similar to seasonal allergies, he said, such as runny noses and itchy eyes, which can be avoided by not being outside in the targeted area for the hour or two it takes for the spray to dissipate.
“We’re working very hard to get the word out,” said Ebata. “If you think you’re sensitive to aerosolized products and you want to avoid the treatment, you should stay indoors with the doors, windows and vents closed to completely avoid contact.”
Ebata reiterated there were no toxic or detrimental effects of ingesting the product, and that groundwater would be unaffected; once it lands on a hard surface, it dries within 10 minutes, he said, and will fully break down within seven days — sooner, he added, if it rains.
For more information, visit gov.bc.ca/spongymoth-news or call the “mothline” at 1-866-917-5999.