Crop Production News
By Maryna Van Staveren, Summer Student, Ministry of Agriculture, Moose Jaw
The 2017 Saskatchewan Pea Leaf Weevil Survey has indicated that this invasive pest has spread across southern Saskatchewan and will most likely be an ongoing issue during the 2018 growing year. Although the adult weevil causes some damage to the clam leaves of the crop, most of the economically significant damage is done by larvae that feed on the nitrogen-fixing root nodules. The greyish-brown adults overwinter in perennial legume stands, ditches and shelterbelts. They emerge in the spring and move to pea and faba bean fields to lay their eggs. Pea leaf weevils produce one generation per year in Saskatchewan. The timing of their occurrence in the field and the severity of their damage depends on spring temperatures.
Adult pea leaf weevils from the previous year will fly to pea or faba bean fields once the temperature remains above 17 C for more than a few days in late April or early May. Once the adults arrive to the host field, they disperse by walking along the field. Throughout the summer, females can lay up to 1,600 eggs at the base of pea plants or in the soil. As larvae hatch, they undergo five instars and burrow deeper into the soil, where they begin to feed on the Rhizobium nodules of the legume roots. The resulting yield loss is caused by inhibition of nitrogen fixation by the plant.
The larvae have off-white bodies, curl into a c-shape when disturbed and have a brown head. Pea leaf weevils undergo pupation in the soil and emerge as adults in late July through September. The newly emerged adult pea leaf weevils feed on any pulse crops available in order to prepare for their overwintering. These overwintering adults will emerge the following spring and continue the cycle.
The adult’s slender body is a greyish-brown colour with three light-coloured stripes running length-wise on its back. Due to the adult’s colour, they are easily camouflaged with the soil, so scouting for these pests is a challenge. To better monitor pea leaf weevil populations, scouting for adults should be done in late May to monitor the adults that have overwintered. Weevils are most active during the warmest hours of the day, such as late-morning or midafternoon. Inspection should be done between the second and fifth node stage of the crop. To monitor your fields, select five locations on the edge of a field. Select 10 plants in those five locations and examine them for any c-shaped notches. The economic threshold is reached when three out of 10 plants have notches in the clam leaf. However, after the sixth node stage, the crop will no longer be susceptible to damage.
Ideally, control measures should be taken before adult females start laying their eggs. Insecticide seed treatment is the most effective option to control these pests. Seed treatments such as Cruiser Maxx Vibrance Pulses, Cruise 5 FC and Stress Shield 600 protect against foliar feeding and also prevent Rhizobium nodule damage. Foliar insecticide applications have been shown to have inconsistent effects on weevil populations and damage. Any eggs that have already been laid will survive. If foliar insecticide application is required due to severe feeding on the leaves, refer to the Government of Saskatchewan 2018 Guide to Crop Protection for registered insecticides for specific crops.
Integrating several management tools along with insecticidal seed treatments will lead to the best outcome for control of the pea leaf weevils. Seeding early during cooler temperatures when the weevil’s activity is limited will give the crop a head start. More advanced pea plants can have enough nodules to prevent severe yield losses from the damage done by the larvae. During fall, planting field peas along the field edges will trap the adult pests. Natural enemies such as ground beetles, rove beetles and other insects feed on pea leaf weevil eggs, creating another reason to avoid foliar insecticide applications.
More information on the pea leaf weevil can be found on the Prairie Pest Monitoring Network blog.