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Bug of the Month – September 20, 2018

Redbacked cutworm adult (photocredit: John Gavloski, Manitoba Agriculture, Food and Rural Development)

Most of the cutworm species that are pests in Canada overwinter locally, typically as eggs or larvae (see table).  A small number of species (armyworm, black cutworm) re-establish each year in Canada when prevailing winds blow adults northward from the USA. 

Pale western cutworm and redbacked cutworm are two of the species that overwinter as eggs. Uncultivated fields with broadleaf perennial and winter annual weeds may be particularly attractive to female redbacked cutworms, which will lay their eggs beginning in early August in the top 1 cm of loose soil.  The embryo within the egg may develop prior to the onset of winter temperatures, but the larvae won’t emerge from the egg until the following spring.

The eggs of army cutworm and glassy cutworm are laid in loose soil later in summer and hatch the same year.  The young larvae overwinter and emerge the following spring to begin feeding, which puts crops are at greatest risk to damage by overwintered larvae in early spring. Crops seeded later in spring are at a lower risk of damage; crops seeded in autumn (e.g., winter wheat, winter triticale) and alfalfa are at a higher risk of damage in the following spring. 

Approximate seasonal occurrence of lifecycle stages for different species of cutworms. Crop damage is caused by larvae. Chart originally compiled by J. Otani and J. Gavloski, expanded and updated by K. Floate.

Detecting eggs and young larvae in fields in late autumn can be difficult.  And high numbers of cutworms in autumn may not predict high numbers of cutworms the following spring.  Depending upon winter temperatures, snow depth and moisture levels in spring, many of the cutworms entering winter as eggs or larvae may not survive.  For these reasons, scouting fields in beginning in early spring remains the best strategy to reduce crop losses due to cutworms. 

More information on the identification and control of pest cutworm species is available in the book “Cutworm pests of crops on the Canadian Prairies: identification and management guide”, which can be downloaded for free in either English or French.