By Carter Peru, AAg, BSc, Integrated Pest Management Agrologist
Pea leaf weevil is a pest of concern in field pea and faba beans in Saskatchewan. The survey for pea leaf weevil in Saskatchewan records damage on pea plants caused by the adult weevils, which leave a characteristic "notching" on the leaves of pea plants after feeding (Photo 1). Only field pea crops are assessed during the pea leaf weevil survey to ensure accuracy in the development of the pea leaf weevil notching damage severity map.
Assessing notching damage during the pea leaf weevil survey is an effective way of monitoring pea leaf weevil populations. However, it is the damage caused by the larval stage that causes economic damage. Pea leaf weevil larvae feed on the nitrogen-fixing nodules, which can inhibit nitrogen fixation, thereby reducing the nitrogen supply to the plant.
The best confirmation of the presence of this pest is the weevils themselves. However, they are hard to spot in fields with lower infestation levels, especially in new areas, due to their tendency to hide in the soil near the plant base, their similar colour to the soil, and their habit of "playing dead" when disturbed. During the 2018 growing season, there was low notching damage throughout the province, with the exception of the south-west corner, which had some
higher feeding damage. (See the 2018 pea leaf weevil map.) Another monitoring resource can be found on the Prairie Pest Monitoring Network Blog. The Prairie Pest Monitoring Network Blog provides weekly updates using models to project predicted status of pea leaf weevil populations.
The 2019 pea leaf weevil survey has begun and will continue into June. Data from this survey will be used to generate the 2019 pea leaf weevil map, which will be posted to our maps page on our website in the fall.
What can you do?
The pea leaf weevil map should be used as a general guideline and not used to make decisions to apply an insecticide. Individual fields should be scouted before the decision to apply an insecticide is made. Detailed information on the pea leaf weevil and how to survey in your own pea field is available as part of an updated protocol available on the Prairie Pest Monitoring Network blog. There is no specific protocol for faba beans, though pea leaf weevils cause the same characteristic notching on the faba bean leaves.
Insecticide seed treatments are the most effective control for the weevil.
It is important to scout your peas and faba beans this year for evidence of the pea leaf weevil, and use that information to make your seed treatment decision for next year.
Although foliar insecticides are registered for the pea leaf weevil on peas, research has indicated the results are inconsistent with respect to yield benefits. If considering a foliar spray, the economic threshold is one notch on the clam leaves per three plants prior to the sixth node stage of crop growth. Damage to the clam leaf is an indicator that the feeding is recent; older damage might indicate that the feeding has already stopped and therefore an insecticide will not be effective. If your peas are past the six node stage, there is no recommended action to be taken. However, it is still important to note the notching damage for your field records.
Research has also shown that higher nitrogen levels in the soil alleviate damage to the plant from larval feeding on the root nodules. Thus, if soil nitrogen levels are sufficient for the crop, there may be no yield decrease associated with larval feeding.