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Diamondback Moth Update

By James Tansey, PhD, Provincial Specialist Insect/Pest Management and Brooke Fiala, Crop Lab and Field Technician, Regina

Diamondback moths (DBM) cannot survive Canadian winters in large numbers, so early season populations are associated with migration to Saskatchewan from the southern United States and Northern Mexico. Adults migrate north in the spring. Preferred hosts of DBM are cultivated brassica crops including canola, mustard, broccoli, and cabbage.

Weather is a factor that determines the timing of arrival and eventual size of populations. Early season arrival in substantial numbers can lead to significant populations. Population increase also benefits from relatively dry conditions.

Diamondback moth larvae
Diamondback moth larvae

Although arrival was detected by the ministry’s network of sentry sites in early May, trap catches remained relatively low until late June. The period from late June to early July saw a significant increase in catches that continued through late July. This increase was likely associated with reproduction in and migration from fields in Saskatchewan.

Each female can produce up to 200 eggs, so under good growing conditions, populations can increase rapidly. It usually takes 32 days for DBM to develop from egg to 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.

Cold and windy weather reduces adult activity, while heavy rainfall can drown larvae. DBM populations are also regulated by 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. These natural enemies of DBM can significantly reduce populations. Although biological factors typically control DBM populations, insecticide should be used when these factors are insufficient and populations reach economic importance.

Diamondback moth pupa
Diamondback moth pupa

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 20-30 larvae per square foot (200-300 per square metre) at late flowering to pod stages. Always check at least five sites within a field and use the average of your counts.

See the Saskatchewan Guide to Crop Protection for specific control product recommendations. Given the time of year, pay careful attention to the pre-harvest interval of the product you choose. See too, current results of DBM monitoring in Saskatchewan.

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