NORTHWEST COAST NATURE

The Problem of Introduced Species : The Story of Spraying the Spongy Moth

LUNA LOISEAU-TREMBLAY

AS THE GRAY DAYS OF JANUARY start to lighten up, and the shades of green and browns deepen with the dampness of the season, a closer look shows that there are signs of spring all around us. While the winter months feel long and dark, before we know it there will be wildflowers popping up and pollinators flying around doing their business.

As most of us are well aware, habitat loss is the number one concern impacting biodiversity around the globe, the ravages of which are seen and felt in almost every ecosystem on the planet. The mention of invasive species is common—with many folks discussing scotch broom or himalayan blackberry—but it is important to keep in mind that the onslaught of introduced species is the second biggest issue regarding loss of native biodiversity on a global level. In this area, species such as knotweed, broom and invasive blackberry are commonly associated with this issue but there are many introduced species of concern that should also be considered.

One of these species is the Spongy moth (formally the gypsy moth) Lymantria dispar dispar, an introduced member of the moth subfamily Lymantriinae, the tussock moths. The Spongy moth is native to Europe, North Africa and Asia but is now found in North and South America. This particular insect is responsible for a large amount of damage to a variety of hard and softwood tree species in Canada, including species of Oak, Maple, Alder, Birch, Hawthorne and fruit trees.

The Spongy moth inflicts the most damage during the caterpillar stage, the part of its life cycle in which it is known to completely defoliate host trees. The defoliation of its canopy can impact a tree’s growth and reproduction, making it more susceptible to other parasites and diseases. Economically, this impacts the aesthet- ic of the forest, which stretches into ecotourism. In urban and residential areas, poor tree health results in tree removal or replacement—with negative consequences on the property value of the affected areas. Ecologically, the defoliation of a canopy heavily impacts tree species composition and other wildlife species that use these trees, since poor tree health can affect forest dynamics through the lessening of available habitat and decreased food production. Spongy moth caterpillars can outcompete other species of Lepidoptera (moths and butterflies) or do enough damage to an ecosystem that it is difficult to support other insect species.

The BC government has been monitoring the Spongy Moth for years and while different control measures have been used to treat affected areas, the main method on Vancouver Island and surrounding areas is the aerial spraying of Btk, or Bacillus thuringiensis var. kurstaki, a species of bacterium used as a biological pesticide against pest caterpillars. In order for Btk to work on the target species, it must be administered so that the moth caterpillar will ingest the pesticide through the eating of foliage. Btk is lethal to caterpillars in the order Lepidoptera, which includes all butterflies and moths. However, according to the BC government, Btk is not toxic to any non-target species and their website states that “Btk has no known toxic effects on humans, other mammals, plants, birds, fish, or honeybees or other beneficial insects”.

While this may or may not be true, several points should be made. Stating that non-target species are not impacted is misleading, as Btk is lethal to all species of moths and butterflies. Their life cycle follows the same life cycle as that of the Spongy moth, so any caterpillars of any other species in the area that is subjected to aerial spray are also doomed to being reduced in numbers. In addition, amphibians are extremely sensitive to any toxins in the ecosystem, and it is unclear what the impact of Btk is on this group of organisms. While information referencing Btk as a biological control agent states that it is nontoxic to beneficial insects such as ladybugs and bees, it is clear that more studies need to be done to ensure that is actually the case.

The BC government does emphasize that while it isn’t toxic to humans, anything that has been exposed to spray should be washed thoroughly, including garden produce. Windows should be closed and Btk should not be inhaled within an hour or more of being sprayed in an area. Btk is sprayed between April and June, but is most effective during the spring, which is when non-target pollinators (including butterflies) be come abundant. While the pesticide does not appar ently impact butterflies and moths in the adult (flying stage), pupae, or chrysalis stage, spring is when most species are in the caterpillar stage of their life cycle, thus being the most susceptible to the effects of Btk, while foraging for food.

Understandably, there is not enough space in this ar ticle to get into an in-depth discussion regarding this issue, however several perspectives should be consid ered with regards to the use of Btk as a biological pes ticide. A field assessment in 2003 was done to see how much impact spongy moth eradication control mea sures had on non-target lepidoptera species and it was discussed that non-target butterflies and moth abun dance was significantly reduced, along with non-target and target (spongy moth) species, over several years after the spray.

Other studies have pointed out that while there are clearly concerns around non-target species of butter flies and moths, the decision not to spray may result in worse impacts. While this could be true, much of this also depends on other factors that contribute to how bad Spongy moth outbreaks are in different areas. As the BC government website points out, this species often attaches its egg masses to outdoor household ob jects, wood or anything transported in a vehicle, mak ing it very easy for it to spread.

According to the Ministry of Forests of BC, the 2022 season for monitoring Spongy moths trapped a record amount in the province. Because of this, treatment for Spongy Moth will be done from April to June 2023 and will cover Victoria, Courtenay, Campbell River, and Port Alberni. According to the Ministry website, the biological insecticide used will be Foray48B, which contains the active ingredient Bacillus thuringiensis var. Kurstaki (Btk). I would strongly encourage anyone who is interested to learn more to visit the BC government website—easily found through www2.gov.bc.ca—and search 'invasive forest pests and pathogens'. Residents within spray zones are invited to submit comments before February 21.

More information can be found here: https://news.gov.bc.ca/releases/2023FOR0001-000018