Higher Pea Leaf Weevil Risk

EDMONTON - Mar 31/17 - SNS -- Wet weather during August has increased the risk of pea leaf weevil infestation in field pea and fababeans crops in southern Alberta, says Neil Whatley, crop specialist, Alberta Agriculture and Forestry.

The pea leaf weevil was reported in southern Alberta in 1997 and remained for several years south of highway 1. Since 2013, this insect's geographic range has greatly expanded into central Alberta, extending as far north as Sturgeon County, north of Edmonton, with lower levels of feeding reported in east central Alberta.

"Given that 2016 survey levels were high in the aforementioned areas, there's a high risk of infestation in the same areas if winter and spring conditions are favorable. A potential predictor of population increase is precipitation in August. As many areas with high weevil populations in 2016 experienced August precipitation, pea and fababean producers in these areas are advised to plan control strategies for the 2017 crop year."

Research shows that treating pea seed with a systemic insecticide product is the most effective control measure to prevent pea leaf weevil damage.

"Properly inoculated annual legume crops, like field pea and fababean, produce most of the nitrogen they require for growth through the growing season via nitrogen fixation carried out by nodules on plant roots," says Whatley.

"Hence, field pea and fababean are generally grown on nitrogen deficient soil without much, if any, additional synthetic nitrogen fertilizer. When the pea leaf weevil insect pest feeds on the nodules of pea and fababean seedlings, this natural nitrogen source is greatly compromised, inhibiting optimal pea and fababean growth throughout the remainder of the growing season as well as decreasing crop yield."

Spring weather conditions can alter the timing and severity of pea leaf weevil damage, says Whatley.

"Weevils arrive early to pea and fababean fields if warm temperatures above 20 degrees C persist for more than a few days in late April or early May, potentially corresponding with higher yield losses. Alternatively, if cool weather occurs during the same period, yield is generally not as compromised especially when the crop advances past the 6th node stage before the weevils arrive. In either case, field scouting is required to make control decisions on a field by field basis. It's also advised not to seed into cold soil."

Yield losses may occur when there are more than 30% of seedlings (3 out of 10 plants along a seed row; assess groups of 10 plants in multiple rows) with feeding damage on the clam leaf before the 6th node stage in peas. The clam leaf is the most recently emerged leaf.

"Most research hasn't shown that control of weevils using foliar insecticide prevents yield loss. The ineffectiveness of foliar spraying probably arises because weevils have already laid enough eggs to significantly damage root nodules when sprays are applied or because healthy weevils immigrate after spraying. According to research on the Prairies, nodule protection is more effective when pea seed is treated with a systemic insecticide product prior to seeding. Fababean may be similarly protected, but this requires investigation."

If feeding damage is only apparent on the older, lower leaves and not on the newer clam leaf, says Whatley, the weevil has probably already laid eggs and spraying would be of no value.

"As such, producers should scout for damage on the clam leaf and not on lower leaves. Since pea leaf weevils migrate into field pea and fababean fields, foliar damage is initially observed along field edges. Foliar insecticides applied early in an infestation to field edges may be a sound economic decision; however, additional on-farm research will provide more clarity. Limited spraying would also reduce the risk of affecting beneficial species, such as ground beetles, that may help manage pea leaf weevil populations via predation."