Thanks to Dr. Hector Carcamo (AAFC-Lethbridge) for the following update (24May2016):
A mild winter and a very warm and early spring have contributed to what looks like the worst outbreak of pea leaf weevil in southern Alberta. Weevil densities appear to be high enough to be threatening stand establishment of peas this year.
Normally, adult feeding damage has little consequence whereas the main concern is related to larval feeding damage to nitrogen fixing nodules. That's NOT the case this year - several growers are spraying for PLW in peas because small seedlings are suffering very high defoliation resulting in stand reductions – something seen previously only in seedling alfalfa stands. Additionally, this year some stands treated with a registered neonicotinoid insecticide have required foliar spraying; the insecticide seed treatment may not provide sufficient protection when weevil densities are extremely high. Growers hoping to prevent yield losses may consider supplementing with nitrogen during the seedling stage but be cautious - the economic returns should be carefully considered, given input prices. Finally, be mindful that PLW adults emerge from overwintering over several weeks so sprayed fields may be repeatedly invaded - continued monitoring is a necessity!
Weevils have natural enemies such as ground dwelling beetles that feed on their eggs. The best way to protect natural enemies is to avoid foliar applications of insecticide unless damage reaches the economic threshold of 30% of seedlings with damage on the clam leaf (i.e., assess using 10 transects of 10 seedlings, half near the edge and half inside the field).
Thanks to Scott Hartley (Sask Ag) who noted (25May2016) that PLW surveying is underway in the southwestern Saskatchewan (i.e., as far north as Kindersley / Outlook and east to mid-way between Swift Current and Moose Jaw). Preliminary reports include high levels of feeding in several areas in the southwest. Crops were generally seeded earlier this year than in the past few years. Seed treatments are considered the best control for the pea leaf weevil but, if 30% of plants are showing damage, a foliar insecticide could be required but this strategy is not as consistently effective as a seed treatment. Foliar insecticide may be necessary if plants are very young and therefore more vulnerable to heavy feeding pressure. Remember plants can outgrow PLW damage once they are past the 6th node stage.
Also refer to previous PLW posts for additional information!
Weekly Update from May 18, 2016
Insect of the Week from 2015
Pea leaf weevil in Central Alberta in 2015
The PPMN's Pea leaf weevil monitoring protocol