Thursday 27 July 2017

Weekly Update (July 27, 2017; Wk 13) - Diamondback moth

Diamondback moth (Plutellidae: Plutella xylostella) - Throughout April and May, both forward and backward wind trajectory data was compiled weekly to identify potential DBM arrival events from southerly source areas including Mexico and southwest USA or the Pacific Northwest.  This week, two biofix dates were selected as "starting points" used to apply the DBM model based on Harcourt (1954).  By selecting and presenting mapped model outputs for both a biofix date of May 1 (Fig. 1) AND May 21 (Fig. 3), the predicted number of generations of DBM can be estimated across the Canadian prairies as of July 24, 2017. The following maps indicate that potentially two generations have been completed across most of the prairies for both biofix dates (Fig. 1 and 3). 

Using Biofix of May 1 - Based on the biofix date of May 1, 2017, the model predicts two generations of DBM (e.g., areas highlighted in yellow) whereas southern Alberta populations are potentially in the third generation (Fig. 1).  The second map (Fig. 2), showing predicted results for Long Term Normal (LTN) data, indicates that populations in southern Alberta and the Peace River region are ahead of normal development (based on May 1 introductions)
Figure 1.  Predicted number of generations of Diamondback moth based on a biofix date of May 1, 2017.

Figure 2.  Predicted number of generations of Diamondback moth based on a biofix date of
May 1, 2017, but using Long Term Normal (LTN) data.

Using Biofix of May 21In the following scenario using biofix date of May 21, 2017 (Fig. 3), the number of generations of DBM are marginally behind the early May introduction presented above in Figure 1.
Figure 3.  Predicted number of generations of Diamondback moth based on a biofix date of May 21, 2017.

REMINDER - Once diamondback moth is present in the area, it is important to monitor individual canola fields for larvae.  Remove the plants in an area measuring 0.1 m² (about 12" square), beat them on to a clean surface and count the number of larvae (Fig. 4) dislodged from the plant. Repeat this procedure at least in five locations in the field to get an accurate count.  The economic threshold for diamondback moth in canola at the advanced pod stage is 20 to 30 larvae/ 0.1  (approximately 2-3 larvae per plant).  Economic thresholds for canola or mustard in the early flowering stage are not available. However, insecticide applications are likely required at larval densities of 10 to 15 larvae/ 0.1  (approximately 1-2 larvae per plant).

Figure 4. Diamondback larva measuring ~8mm long.
Note brown head capsule and forked appearance of prolegs on posterior.

Figure 5. Diamondback moth pupa within silken cocoon.

Biological and monitoring information for DBM is posted by Manitoba Agriculture, Food and Rural DevelopmentSaskatchewan AgricultureAlberta Agriculture and Forestry, and the Prairie Pest Monitoring Network.  

More information about Diamondback moths can be found by accessing the pages from the new "Field Crop and Forage Pests and their Natural Enemies in Western Canada: Identification and Field Guide".  View ONLY the Diamondback moth page but remember the guide is available as a free downloadable document as both an English-enhanced or French-enhanced version.

Figure 6. Diamondback moth.

Across the prairies, provincial staff coordinate diamondback pheromone trapping during the growing season:

● Low numbers of moths have been reported across Saskatchewan for the 2017 pheromone monitoring.  
● Manitoba Agriculture and Rural Initiatives posted low DBM counts which can be reviewed here.  
● Alberta Agriculture and Forestry has a live 2016 map reporting Diamondback moth pheromone trap interceptions.  A copy of the map (retrieved July 20, 2017) is below for reference.