The insect invader that grows in your berry blossoms

Acres of raspberries. Fields of strawberries. Roadsides filled with blackberries. All could be at risk from one bug the size of a pencil eraser: the strawberry blossom weevil.

“Could” is the operative word. So Michelle Franklin is working to find out exactly what this weevil is up to in fields across the Lower Mainland.

As the small fruit entomologist at the federal research station in Agassiz, Franklin is responsible for investigating insects that affect fruits like berries. After starting work in July of 2020, one of the first things she began researching was the strawberry blossom weevil, or Anthonomus rubi.

The strawberry blossom weevil was first discovered in North America in 2019 in Abbotsford. Weevils as a whole are common in the valley: the long-snouted insects are cousins of beetles and found on plants throughout the region. Most are nocturnal, and all have larvae that eat plant roots.

But when a backyard berry grower brought in some dead, closed flower buds to Tracy Hueppelsheuser, an entomologist with BC’s plant health unit, was surprised. Inside the dead buds were the larvae of a weevil, something that hadn’t been seen in the valley before. Hueppelsheuser sent the dead buds containing the larvae across the country, to the Canadian National Collection of Insects, Arachnids, and Nematodes in Ottawa. There, taxonomists discovered the weevil was a species new to the valley, introduced somehow from Europe. (Franklin is currently working with researchers in Europe to figure out where the weevil may have come from, and how it may have come into Canada.)

Franklin’s research on Anthonomus rubi started last year, with field work identifying where the weevil was living and how it was affecting plants. So far, it has been found across the Lower Mainland, from Delta to Hope. To find them, Franklin uses sticky yellow cards to trap the weevils (they love the colour yellow). She also uses the “tapping method” that drops the weevils onto a white sheet for identification. And finally, she looks for dead flower buds.

That’s where you can find weevil larvae. The little berry munchers focus their life cycle on plants in the Rosaceae family: raspberries, strawberries, blackberries, and roses. (Franklin’s research found the weevil was well-established in wild blackberries, roses, and thimbleberries, with fewer weevils found in cultivated berry fields.) After spending the winter in the dead leaves that cushion the ground, the insects emerge in the spring and begin to feed on the growing plants. In May, they mate. The females find green, unopened flower buds and, using their long snouts, chew a hole through the petals. Then, they back up and lay a single egg inside. They close the hole again, and chew through the stem at the base of the bud. It dies, and the eggs hatch into larvae, consuming the bud from the inside out.

Franklin and her team are also trying to learn the potential consequences of the weevil’s future spread. Sampling blackberry bushes at the Agassiz Research Station, Franklin and her team found that anywhere between 33% and 54% of closed buds had weevil larvae or eggs inside. As the season went on, the extent of the weevil’s presence became even more apparent.

“It becomes quite visually apparent that you’ve got weevils in that field, because those brown buds stand out against the rest of the plant,” Franklin said. “Later on in the season you’ll see the fruit develop that wasn’t affected by the weevil, and you’ll see the brown buds that stay on the plant.”

Although some weevils have been found to lay their eggs in open flower buds, allowing fruits to grow, those berries were misshapen. And the occurrence was also extremely rare, Franklin said.

In their native range in Europe, the weevils can significantly damage strawberry production, sometimes reducing yields up to 60% depending on the type of plant and the spring weather. In raspberry production, that yield loss ranges from 10% to 80%. (There, they use pheromone traps to lure the pests away from cultivated berry fields.) The impact on the Fraser Valley is still unclear. But finding out is extremely important, as 75% of raspberries and 5% of strawberries are grown in the valley.

“We want to start to understand the biology of this pest so that we can start to make some recommendations for what growers can do in terms of management,” Franklin said. “But before that, we really need to help growers understand what the impact of this pest is in their field.”

Right now, there are no approved pesticides for growers to use against the strawberry blossom weevil before the plants start to bloom and the insects lay their eggs. An emergency pesticide was approved for use last summer, but would only reduce weevil numbers for the coming season. The province is suggesting growers remove blackberry bushes around their property to reduce the amount of egg-laying habitat for the little insects.

There is some good news on the pest management front, however: BC already has a natural enemy for the strawberry blossom weevil: a parasitoid wasp that has been found together with weevil larvae in blackberry buds. It’s not clear yet how much of an impact the wasp will have on the invasive species, but Franklin said it’s an exciting development.

“We’re still in the process of evaluating how important this natural enemy is,” Franklin said. “But it’s quite exciting to discover this natural enemy that’s associated with our weevil.”

That’s the theme of our understanding of the Fraser Valley’s strawberry blossom weevil right now: we just need more information. That will be coming soon. Franklin and her team are partnering with more than 30 institutions across Canada, including other federal research stations, universities, and provincial bodies. Together, they will work to capture weevils across the country and find out how widespread it is.

She’s looking for help from the community as well. The project has set up a page on the citizen science website iNaturalist looking for images of strawberry blossom weevils found in people’s home gardens. Come summer, people can check their own plants for the bugs by laying a white sheet or container beneath a plant, then striking its top. They can then identify and report any weevils that drop to the sheet or container on the ground.

“It’s quite neat to have such a large group effort to work on this species,” she said. “I’m sure this is going to be an insect that will be part of my research program for a long time to come.”