By: Brooke Fiala, Crop Lab and Field Technician, Regina and James Tansey, PhD, Provincial Specialist Insect/Pest Management
The Diamondback moth (DBM) cannot survive Canadian winters in large numbers, so populations here are associated with new immigration to Saskatchewan. Adult moth populations from the southern United States and Northern Mexico migrate north in the spring. DBM can travel with high altitude winds over 1000 kilometres per day while remaining in continuous flight for multiple days. It is still unknown how DBM can survive the high altitudes and low temperatures they encounter in their travels. DBM is distributed in many cropping regions of the world.
Weather is a factor that determines the timing of DBM’s arrival and the eventual size of populations. For a serious infestation to occur, immigration must be early in the season with high numbers and the weather must be dry.
The life cycle of DBM includes four stages: egg, larva, pupa and adult. Their development is dependent on weather conditions and food availability. It usually takes 32 days to become an adult, though it can vary from 21 to 51 days to complete a generation, depending on temperature and host plant quality. Multiple generations can develop throughout the growing season, allowing all life stages of the DBM to be present in a field at once. The larval stage is damaging to crops.
The preferred hosts of DBM are cultivated brassica crops, including canola, mustard, broccoli and cabbage. However, if arrival is early, before producers seed in the spring and cultivated crops are ready, DBM can use brassicaceous weeds. The seedlings and rosettes of many of these weeds overwinter, allowing them to be ready in the spring for the first arrival of DBM. In addition, these plants, like their crop plant relatives, contain glucosinolates which stimulate female DBM to lay eggs and larval feeding.
Cold and windy weather reduces adult activity, while heavy rainfall can drown larvae. DBM populations are also regulated by the availability of host plants and parasitoids. In Canada, three major parasitoid species target different life stages of DBM; Diadegma insulare and Microplitis plutellae target larvae and D. subtilicornis targets the prepupal and pupal stages.
Although biological factors typically control DBM populations, insecticide should be used when these factors are insufficient and populations reach economic importance. However, insecticide use isn’t without risk. Frequent use of insecticide has led DBM to become resistant to all major classes of insecticides in several parts of the world. For example, the diamondback moth was the first insect to become resistant to DDT in 1953. Although widespread insecticide resistance has not yet been reported in Canada, rotation of different modes of action should be considered to reduce the probability of selecting for resistance.
To scout for DBM in canola crops, pull plants from a 0.1 square metre (approximately one square foot) area of the crop. Shake these plants onto a container (some scouts like to use a plastic bucket or cat box), or clean surface and count larvae that fall from the plants. Consider control if you see 25-33 percent defoliation on seedlings and larvae are present; 100-150 larvae per square metre (20-30 larvae per square foot) are found at late flowering to pod stages. Always check at least five sights within a field and use the average of your counts.
See the Saskatchewan Guide to Crop Protection for specific control product recommendations. See too, current results of DBM monitoring in Saskatchewan.