Saturday, 28 May 2016

Pea Leaf Weevil Monitoring

The pea leaf weevil (Sitona lineatus) is an insect pest native to Europe but established in Alberta in 2005 and southwest Saskatchewan in 2007. Host plants include peas and faba beans and pea leaf weevils can cause considerable damage and yield loss in these crops. Losses result from the larval stage feeding on plant roots and the reduction of nitrogen fixation by preferential feeding on root nodules. Losses can also result from adults feeding upon leaves which is characterized by scalloped notches along the edge of pea leaves.

Pea leaf weevil overwinter as adults which become active early in spring.  The adults move from overwintering habitats to legume host plants to feed, generally from late May through to early June.  During this time, pea leaf weevil populations are best estimated by plant damage assessments. More details on pea leaf weevils and monitoring protocols can be found here.

Figure 1. Pitfall trap containing RV antifreeze solution and covered with wire mesh.

Pheromone-baited pitfall traps are used to detect and monitor pea leaf weevil. Pitfall traps intercept arthropods at ground level.  Each trap consists of a plastic container (500 mL) that is submerged in the soil so the opening sits at ground level (Figure 1).  Each trap is baited with an aggregation pheromone lure then ~175 mL of a solution of RV antifreeze and water (ratio of 1:1) is poured into the base of the pitfall trap.  The solution both kills and preserves the arthropods that are attracted and fall into the trap (see Figure 1). The pheromone, developed by the University of Alberta, is suspended from the wire mesh and attracts the pea leaf weevils (see Figure 2).  The pitfall trap is then covered with wire mesh to exclude field trash and small vertebrates.  All arthropods  are collected weekly from traps then preserved in 95% ethanol for later identification. Back in the lab, the arthropods are processed by selecting out weevils, ground and rove beetles which are pinned or pointed to enable identification.

Figure 2. Pheromone suspended from wire mesh using green twist tie.

The monitoring of pea leaf weevils is important since its distribution has recently changed to include a more northerly range.  Pea leaf weevil was initially discovered in Alberta near Lethbridge in 2005.  Since then, its distribution increased to include southern Alberta to just north of Edmonton and east into southwestern Saskatchewan. In 2015, pea leaf weevil damage on faba bean was observed in central Alberta and helped confirm an increase in range compared to its previous limit. The 2015  risk map for pea leaf weevil is available here. More information on the impact of pea leaf weevils feeding on faba beans can be found here.

- Hannah