By Dale Risula P.Ag., Provincial Specialist for Special Crops
We are approaching that time of year when pea leaf weevil becomes a noticeable pest in field peas planted throughout most of Saskatchewan. Pea leaf weevil was first observed in Alberta; however, it soon moved into Saskatchewan, appearing in fields in the southwest part of the province. Since then it has become much more prevalent and now occupies much of the province, right up to the Manitoba border.
Pea leaf weevil is a small insect measuring between 3.5 and five mm in length as an adult. The adults have stripes from their head to the bottom of their abdomen. They are difficult to find in field conditions since they blend in well with soil colour.
It is much easier to find and evaluate damages from adult weevils, as they typically chew notches along the edges of pea leaves. The shape of the notch is very distinctive to field pea weevil when compared to other insects such as cutworms. Depending on the severity of notching, it can cause significant crop damage, especially if the clam leaf of the plant is damaged.
The major damage comes from larvae that feed on the root nodules of pea plants where nitrogen fixation is taking place. This is also suspected to have an impact on the plant's ability to produce higher protein.
Pea weevils are able to overwinter in Saskatchewan in shelterbelts and protected areas. They overwinter as adults and invade fields from the edges moving inward. Producers who have grown field pea for some time have selected seed treatment as the preferred method of controlling the insect, which seems to provide the best measure of control.
Scout for pea leaf weevil when pea plants are between the two and six node stage, as the plant is most susceptible to pea leaf damage at this point. Once past the six node stage, the plant becomes less vulnerable to damages caused by pea leaf weevil larvae. Adults can emerge again at this later stage and they have the ability to take flight and move into deeper areas in the field.
Scouting is fairly simple and consists of assessing five areas of the field, keeping assessed areas 25 metres apart and within two metres of the field edge. Count the number of notches on 10 to 20 plants at each of the five locations. To get better results, repeat these steps 100 meters into the field.
The Ministry of Agriculture and its collaborators conduct the survey for pea leaf weevil each year and collect data showing the intensity of the insect across the province. Once the data is collected it is put together and released as a pest forecast map on the ministry's website. We are in need of growers who are willing to grant permission for surveyors to conduct the survey on their land. Please contact your nearest Crops Extension Specialist if you wish to provide authorization or use the online signup form. The survey typically takes place near the end of May or beginning of June.
For more information, visit our pea leaf weevil webpage, check the Prairie Pest Monitoring Network Blog or contact your nearest Crops Extension Specialist.