Diamondback moth larvae have been found earlier than usual in some areas, says Scott Hartley, pest management specialist with Saskatchewan Agriculture.
“The larvae were found by Agriculture Canada researchers who were monitoring for another canola pest. They also found a fair amount of parasitism in some of the samples, which is good news,” said Hartley.
“The bad news is that the early presence of diamondback in the province may result in more than one generation this summer if conditions are favorable.
“A full life cycle for the diamondback moth varies from 21 to 51 days. This is dependent on climate, largely temperature and humidity, and availability of food. Because of this relatively short life cycle, at least two generations can be expected in a hot growing season.”
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The moth blows north each year from the United States.
Voracious appetite
Diamondback larvae are less than 1.3 millimetres in length. They feed on leaves, buds, flowers, seed pods, the green outer layer of the stems and sometimes the developing seeds within the older seed pods of canola, mustard, flixweed, rape and cabbage.
When leaves begin to die in July, second generation larvae feed on floral parts and the surface or tips of seed pods. The damage reduces seed-set, pod-fill and predisposes pods to shattering. The second generation usually causes the most yield loss because the larvae feed when flowering and pod formation is at a peak.
The larvae are easily identified because when they are disturbed they violently wriggle backwards and may drop from the plant, suspended by a silken thread. After several seconds, they will climb up and continue feeding.
Look for damage on plants growing on ridges and knolls. Check for larvae on the crop at least twice a week during the growing season and apply insecticides if larval numbers warrant.
The action threshold for canola or mustard in the early flowering stage has not yet been established, but Hartley said insecticide applications at this stage are likely required at larval densities of 10 to 15 larvae per 1/10 sq. metre, or one or two larvae per plant.
At the advanced pod stage, the threshold for canola is 20 to 30 larvae per 1/10 sq. metre, or about two to three larvae per plant.
Cool, windy weather reduces adult activity and females often die before they lay all their eggs. Heavy rainfall can drown small larvae and reduce numbers by more than half.
