By Kaeley Kindrachuk, TechAg, Crops Extension Specialist, Outlook and Erin Campbell, PAg, Crops Extension Specialist, North Battleford
July 2020
An increased number of adult diamondback moths (DBM) have been caught in field traps in some areas of the province this spring. This means that there is an increased risk of earlier larval feeding damage to crops like canola and mustard. Each year, strong southerly winds bring adult DBM from the United States and Mexico. Females lay eggs on crops and larvae can start feeding very soon after hatching. Typically, we see two or three generations of DBM in canola or mustard crops. In years with earlier arrival such as back in 2001, there is potential for up to four generations of DBM living on crops per year and earlier feeding on the crop which will increase losses. The DBM takes about 30 days (3.5 to four weeks) to develop from an egg to adult, but this may also depend on the growing conditions, particularly the temperature.
Photo 1: Diamondback moth larvae feeding
on cabbaging canola. Note the damage is a bit
bigger than the flea beetle shot-hole damage.
Typically, we see damage to crops in flowering and podding stages; with earlier arrival, damage can occur to seedling and cabbaging stages of the crop. When scouting for early feeding damage, look for leaf mining caused by new larvae for about a week after they hatch. The larvae exit leaves to feed externally after this time. The damage caused by young larvae can look similar to flea beetle shot-hole damage (Photo 1). If larvae are numerous, they may eat the entire leaf, leaving only the veins.
As the larvae mature, they will feed on the underside of leaves giving the characteristic “window-paning” effect of leaf tissue looking translucent (Photo 2). DBM larvae can feed on leaves, buds, flowers, seed pods, the green outer layer of the stems and occasionally, the developing seeds within the older seed pods of canola and mustard. The amount of damage varies greatly, depending on plant growth stage, larval densities and their size.
1 
Photo 2: Window-paning effect on canola
leaves due to diamondback moth larvae
feeding on the underside of the leaves.Economic thresholds for DBM depend on the stage of the crop. Threshold numbers are based on crop stands averaging 100 plants/m2. In areas where plant stands are thinner, the action threshold should be adjusted. A nominal threshold of 25 to 33 per cent defoliation with larvae still present, can be applied for canola in the seedling and rosette stage. In immature and flowering canola, the threshold is 100-150 larvae/m2 (10-15 larvae/ft2) or in plants with flowers/pods 200-300 larvae/m2 (20-30 larvae/ft2).
For more information on DBM, updated moth numbers or any other insect pest, contact your nearest crops extension specialist or contact the Agriculture Knowledge Centre at 1-866-457-2377.