Thursday, 25 June 2020

Predicted pea leaf weevil development

Pea Leaf Weevil (Sitona lineatus–  As of June 21, 2020model runs for pea leaf weevil (PLW) indicate that oviposition has peaked at Swift Current and Lacombe.  The following two graphs illustrate that larval numbers continue to increase at Swift Current and Lacombe as the eggs continue to hatch.
Figure 1. Predicted pea leaf weevil (Sitona lineatus) phenology at Swift Current SK.
Values are based on model simulations (April 1-June 21, 2020).

Figure 2. Predicted pea leaf weevil (Sitona lineatus) phenology at Lacombe AB.
Values are based on model simulations (April 1-June 21, 2020).

Pea leaf weevils emerge in the spring primarily by flying (at temperatures above 17 ºC) or they may walk short distances. Pea leaf weevil movement into peas and faba beans is achieved primarily through flight.  Adults are slender, greyish-brown measuring approximately 5 mm in length (Fig. 3, Left).  

The pea leaf weevil resembles the sweet clover weevil (Sitona cylindricollis) but the former is distinguished by three light-coloured stripes extending length-wise down thorax and sometimes the abdomen.  All species of Sitona, including the pea leaf weevil, have a short snout.  

Figure 3.  Comparison images and descriptions of four Sitona species adults including pea leaf weevil (Left).

Adults will feed upon the leaf margins and growing points of legume seedlings (alfalfa, clover, dry beans, faba beans, peas) and produce a characteristic, scalloped (notched) edge.  Females lay 1000 to 1500 eggs in the soil either near or on developing pea or faba bean plants from May to June.

Larvae develop under the soil and are “C” shaped and milky-white with a dark-brown head capsule ranging in length from 3.5-5.5 mm (Figure 4).  Larvae develop through five instar stages.  After hatching, larvae seek and enter the roots of a pea plant.  Larvae will enter and consume the contents of the nodules of the legume host plant. It is the nodules that are responsible for nitrogen-fixation which affect yield plus the plant’s ability to input nitrogen into the soil. Consumption of or damage to the nodules (Figure 5) results in partial or complete inhibition of nitrogen fixation by the plant and results in poor plant growth and low seed yields.

Figure 4. Weevil larva in soil (Photo: L. Dosdall).

Figure 5. Pea nodules damaged by larval feeding (Photo: L. Dosdall).

Biological and monitoring information related to pea leaf weevil in field crops is posted by the province of Alberta and in the PPMN monitoring protocol.

Also refer to the pea leaf weevil page within the "Field Crop and Forage Pests and their Natural Enemies in Western Canada: Identification and management field guide" - both English-enhanced or French-enhanced versions are available.