Adult beetles normally walk to the new food source. Invading beetles are usually concentrated in a moving front only a few yards wide. Occasionally, crop damage may occur in patches throughout the crop when volunteer canola, rapeseed, mustard or weeds belonging to the mustard family were abundant the previous year and the canola, rapeseed or mustard is direct seeded with conservation tillage equipment. The larvae and adults feed on the cotyledons, true leaves, petioles, and stems of seedlings. Depending on the size of the seedlings and on beetle abundance, damage will vary from the loss of small portions of the cotyledons and true leaves to complete defoliation and death of plants.
Small seedlings are more susceptible to damage than larger seedlings.
When red turnip beetle populations are low, only plants at the edges of the fields are at risk. But when adult beetles are numerous, they invade the crop, gradually moving inward from the margins, destroying the crop as they go. In the larval stage, the red turnip beetle is not a major threat to canola, mustard, or rapeseed since it normally completes development before the host crop germinates. In August, the summer generation adults re-invade canola, rapeseed or mustard fields and feed on the flowers, pods and stems of plants until harvest. However, adult densities at this time of year normally are not high enough to cause significant damage.