Alfalfa weevil

Alfalfa Weevil (Hypera postica) – Model output indicates that hatch is almost complete with less than 5% of the population predicted to still be in the egg stage (prairie average; Table 1). Across the prairies, populations are predicted to be 12, 45, 29 and 10% in first, second, third and fourth instars, respectively.  

Table 1.  Predictive model output estimates for Hypera postica development (% of total population for each location) at selected sites across the Canadian prairie as of June 14, 2020.

Larval populations in the Peace River region are expected to be mostly in the second instar (Fig. 1).  Last week, the presence of third instar larvae was limited to areas southeast of Saskatchewan and extended into Manitoba (Fig. 2).  This week third instar larvae should be occurring across southern and central regions of the prairies (Fig. 3).  Fourth instar larvae should be abundant across southern Manitoba and southeastern Saskatchewan.

Figure 1. Predicted percent of populations of alfalfa weevil (Hypera postica) in the second instar stage across

the Canadian prairies as of June 14, 2020. 

Figure 2. Predicted percent of populations of alfalfa weevil (Hypera postica) in the third instar stage across

the Canadian prairies as of June 14, 2020. 

Figure 3. Predicted percent of populations of alfalfa weevil (Hypera postica) in the fourth instar stage across

the Canadian prairies as of June 14, 2020. 

Monitoring

The larval stage of this weevil feeds on alfalfa leaves in a manner that characterizes the pest as a “skeletonizer” (Fig. 3, lower left).  The green larva featuring a dorsal, white line down the length of its body has a dark brown head capsule and will grow to 9 mm long (Fig. 3, upper right).  

Figure 3.  Developmental stages of the alfalfa weevil (Hypera postica); overwintered adult (upper row L-R), eggs, larvae, larva feeding or "skeletonizing" alfalfa leaf (lower row L-R), pupa within lacey cocoon, pupa, and newly emerged adult.

Alfalfa growers are encouraged to check the Alfalfa Weevil Fact Sheet prepared by Dr. Julie Soroka (AAFC-Saskatoon).  Additional information can be accessed by reviewing the Alfalfa Weevil Page extracted from the "Field crop and forage pests and their natural enemies in western Canada - Identification and management field guide" (Philip et al. 2018). The guide is available as an English-enhanced or French-enhanced version.