By: Danielle Stephens, PAg, MSc, Integrated Pest Management Agrologist
If you aren’t a pea grower in southwestern Saskatchewan you might not have heard of the pea leaf weevil. This insect pest of pea and faba bean has been in the province for many years, though, primarily in southwestern and south central Saskatchewan. The survey for pea leaf weevil in Saskatchewan records the damage done to the pea plants by the adult weevils that feed and leave a characteristic “notching” on the leaves of pea plants (Photo 1). One or two notches in a field could be caused by other weevils that could feed on peas in this manner, but this isn’t very likely to be picked up in our survey.
The best confirmation of presence of this pest is the weevils themselves. However, they are hard to spot in fields with lower infestations, 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 2016 season there was confirmation of the weevil in Saskatoon and large amounts of notching were reported in other areas outside of our survey results. In response, the 2017 pea leaf weevil survey was expanded to determine how far east the pea leaf weevil can be found.
That said, the survey is not finished and we don’t have a complete distribution picture yet. If you grow peas or faba bean, you should check your fields for the typical notching symptoms, especially in the south and central areas of the province.
Detailed information on pea leaf weevil and how to survey for the pea leaf weevil in your own pea field, is available as part of an updated protocol available from the Prairie Pest Monitoring Network. There is no specific protocol for faba beans, though the same characteristic notching will be evident on the faba bean leaves.
What can you do?
Although the notching is the visible characteristic that indicates the presence of the pea leaf weevil, it does not usually cause an economic impact. Adult feeding on the leaves is outgrown past the sixth node stage. It is the larvae of the pea leaf weevil that cause the yield loss by feeding on the root nodules of pea and faba bean host crops.
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.