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Alberni growers face cutworm

Alberni Valley farmer John Oosterom surveyed what was left of his hay field after armyworms devoured his second cut in 2017. Bob Collins photo

A mild winter has provided favourable conditions for a surge in winter cutworm (Noctua pronuba) populations on central Vancouver Island.

Alberni Clayoquot Regional District sustainability planner Amy Needham says populations are approaching outbreak levels in areas such as the Alberni Valley, though the exact cause is unknown.

The appearance of the worm, the larval form of the large yellow underwing moth, has caused concern because of its similarity to true armyworm (Mythimna unipuncta), which infested farms on Vancouver Island and the Fraser Valley in 2017.

Winds carried true armyworm into the province that year, spawning the outbreak, whereas winter cutworm is an established species, having spread west after its initial introduction in Nova Scotia in 1979.

The species overwinters as larvae and emerges as adult moths in the spring. Cold-hardy, it has been known to emerge despite snow cover to forage on grasses and a variety of storage crops, brassicas and berries.

Together with the Alberni Farmers Institute, the regional district is planning a program of Light traps to monitor emerging populations beginning in March. It is hoped monitoring will indicate the potential for a naturally occurring pathogen to crash the population, reducing the threat to Vancouver Island growers.

Most outbreaks do not lead to economic damage. Just one economically significant outbreak of the pest has occurred, when extension agents reported thousands of caterpillars around homes and in hay fields in Michigan in 2007. The population eventually crashed, with a pathogen suspected of clipping its wings.

True armyworm has not been reported in BC since the 2017 outbreak, though the Western Yellowstriped Armyworm made an appearance the following year in the southern Interior.

Warmer winters have raised pest pressures for growers, creating more favourable conditions for invasive species to gain a foothold and establish themselves. With some species, warmer weather increases the potential for more generations each season, complicating management.