MSU Ag Alert submitted by Kevin Wanner
Pea leaf weevils (PLW, Sitona lineata) are now feeding on emerging field pea crops in Montana. PLW is an introduced pest that is established in the Pacific Northwest and western regions of Montana. Its range is expanding eastward following the expansion of pulse crop acreage. When it first becomes established in a new area, populations can be high for the first few years, after which it becomes a periodic pest. PLW moves into emerging pea fields from surrounding overwintering grounds such as alfalfa fields, roadsides or sheltered areas.
Adult feeding damage to seedlings is easily visible as scalloped leaf edges (figure below) and often appears first on field edges. At high populations defoliation of small seedlings can cause significant damage, but by the six-node stage the plants become more tolerant and some defoliation can be tolerated. After mating the females lay eggs in the soil and the larvae burrow into the nitrogen-fixing root nodules. The degree of yield loss from larval feeding has not been well quantified, however, producers from Alberta report that yield loss of 30{f2533179b7c7e7cbdbc11018732de14c82f3d44c9f1e829e9a046cc47141a2e6} is not uncommon.
PLW scouting should start at the 2- to 3-node stage. Foliar insecticide treatments may be warranted when 30{f2533179b7c7e7cbdbc11018732de14c82f3d44c9f1e829e9a046cc47141a2e6} or more of plants have feeding injury (one or more feeding notches appear on the clam leaf pairs) at the 2- to 3-node stage. A row of 10- 20 seedlings should be examined at several locations (including the interior of the field) to establish an average number of plants with feeding damage. If notching is occasional and the insects are hard to find on a warm sunny day treatment may not be required. After the 6-node stage applications are not recommended. Timing the foliar spray and persistence of the insecticide is critical for good control, to prevent incoming females from laying eggs in the field (larvae are not exposed to foliar insecticides as they burrow into the nitrogen fixing root nodules). Systemic seed treatments such as Cruiser 5FS are a good control option for both larvae and adults, however, the seed is treated and planted before the pest can be surveyed to determine economic levels. In this case decisions to treat seed are based on the previous year’s PLW population level in the area.
Further treatment information including insecticides can be found on the online High Plains IPM Guide, http://wiki.bugwood.org/HPIPM:PC_Pea_leaf_weevil
An educational video posted on YouTube provides a good overview of PLW management: http://www.youtube.com/watch?v=5yZXnBXgMkg
Source: MSU Ag Alerts
