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Pea Leaf Weevil Emerging (05/28/20)

Feeding injury by the adult pea leaf weevil (PLW) was recently found in field peas in southwest and northwest North Dakota

Feeding injury by the adult pea leaf weevil (PLW) was recently found in field peas in southwest and northwest North Dakota (Sources: R. Buetow DREC and C. Keene WREC). Adults are weevils about 3/16 inch long, slender and greyish brown with a broad snout (Figure 1). Pea leaf weevil is an economic insect pest in field peas and faba beans. Clover and alfalfa are secondary hosts, but larvae do not fully develop on these crops.

Since PLW was discovered on field peas during 2016 in Golden Valley County, North Dakota, PLW has increased its distribution in the state. It now occurs in 13 counties in western North Dakota (see map on next page). Thanks to funding support from the Northern Pulse Growers Association for the survey work.

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Scouting should occur when field pea or faba beans have just emerged in the spring and temperatures have warmed up above 63˚F. Instead of looking for adult weevils, it is easier to look for their feeding injury, half-moon leaf notches on the lowest leaves of the plant along field edges. Notches are small, half-moon-like semicircles in a symmetric pattern found around the leaf margins (Figure 2). Leaf feeding by the adult does not typically result in yield loss as the crop usually compensates and recovers. However, larvae feed on the nitrogen-fixing bacteria within root nodules and cause significant damage to nodules (Figure 3). This damage also reduces soil and plant available nitrogen for the current and future crops, which results in poor plant growth and lower crop yields.

For Integrated Pest Management (IPM) of PLW, cultural control and chemical control strategies are often used.

Cultural control for PLW include:

  • Late-seeding of field peas (by 10 days) reduces foliar leaf feeding injury by weevils because the pea emergence is delayed after the peak emergence and dispersal of weevils in spring.
  • Use of no-till systems: Research comparing no-till and conventional till plots found that PLW was more attracted to conventional tillage. As a result, conventional tillage had higher leaf feeding injury by weevils and larval density on root nodules. Larvae also developed faster and the new generation of adults emerged earlier in conventional tillage than no-till. Another benefit of no-till is that this tillage system supports higher populations of the beneficial predators, such as ground beetles (Coleoptera: Carabidae).

Chemical Control:

Insecticidal seed treatments reduce defoliation from adults, egg laying and larval feeding on the  root nodules. There are two active ingredients), registered in field peas: thiamethoxam (Cruiser 5F, Cruiser Maxx) and imidacloprid (Attendant 600, Dyna-Shield imidacloprid 5, Gaucho 600, Senator 600FS, other generics). There are no accurate risk forecasting models, so the decision to treat seed before planting is based on past field history of PLW populations and damage levels.

The Economic Threshold for a foliar insecticide application is recommended when 30% of the plants have half-moon shaped feeding notches in leaves from the seedling through the 6th node growth stage. However, optional foliar insecticidal control timing is difficult, because insecticides should be applied before the female PLW has the opportunity to lay eggs. Research studies have shown that insecticide seed treatments are more effective than foliar sprays, because of the long window of weevil emergence in the spring and multiple movements into fields.

For more information, read the NDSU Extension publication E1879 Integrated Pest Management of Pea Leaf Weevil in North Dakota.

(Note: Mention of any products are giving as examples only and do not imply an endorsement nor discrimination against any products not mentioned by the author or university.)

 

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 TJ Prochaska

Extension Crop Protection Specialist

NDSU North Central Research Extension Center

 

Janet J. Knodel

Extension Entomologist

 

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