Dr. Meghan Vankosky and Dr. Hector Carcamo, Agriculture and Agri-Food Canada – Fall/Winter (December) Pulse Beat 2021
The pea leaf weevil, Sitona lineatus, is an invasive pest in Canada. Following its initial detection in southern Alberta in the late 1990s, its range has expanded north and eastward. Pea leaf weevils now occur across all agricultural areas of Alberta (including the Peace River region) and Saskatchewan.
It was first detected in Manitoba in 2019 in the Swan River Valley and has since expanded. This fall, adult pea leaf weevils were found in pea and faba bean fields throughout western Manitoba (Figure 1).
The primary feeding and reproductive hosts of pea leaf weevil are field peas and faba beans. However, adult weevils will feed on a variety of secondary hosts, including clovers, alfalfa, chickpeas and soybeans. Damage to the secondary hosts is not typically economical except at unusually high densities on establishing alfalfa fields. Adult pea leaf weevils emerge from overwintering sites along field margins close to their secondary hosts (e.g., forage alfalfa or clover fields) in the spring. Initially, they feed on secondary host plants before dispersing into fields where field pea and faba bean seedlings are emerging. After arriving in pea and faba bean fields, adults feed, making ‘u’-shaped notches along the leaf margins (Figure 2). Within a few days of feeding, adults mate and females begin to lay eggs; weevils are long-lived and individual females may lay over 1,000 eggs from June to August. When the eggs hatch, first instar larvae chew their way into the root nodules of pea and faba bean plants, where they feed on the bacteria that fix nitrogen inside the nodules. Larval feeding damage is generally considered to be more serious than adult damage in terms of yield loss. After completing their development, larvae pupate in the soil and a new generation of adult weevils emerges, usually beginning in late July or early August. New generation adult weevils feed on any remaining green pea and faba bean plants and then disperse to find secondary hosts where they feed until temperatures drop and overwintering begins.
A variety of management options for pea leaf weevil have been studied since its detection in the late 1990s and research projects continue to address pea leaf weevil management. Currently, the best management practice for pea leaf weevil is the use of neonicotinoid seed treatments. The insecticide works systemically to protect seedlings from excessive adult feeding until the seedlings are established. Weevils that consume the insecticide in the foliage are paralyzed for a short time, feed less and lay fewer eggs during the critical vulnerable crop stage compared to weevils that feed on untreated plants. Only about 30% of the adult population dies after consuming foliage from insecticide-treated seed.
In addition to insecticide seed treatments, some foliar insecticides include pea leaf weevil on their labels. However, studies conducted in Alberta and Saskatchewan have found that foliar insecticides do not reduce weevil populations, do not reduce the amount of damage caused by adult or larval weevils and do not protect against yield loss in peas or faba beans. In Alberta and Saskatchewan, it has been recommended that farmers consider applying insecticide seed treatments if they plan to plant peas or faba beans in regions with high population densities in the previous growing season. This is inferred from high levels of foliage damage (more than 30% of seedlings with damage on the clam leaf for peas and 15% for faba beans) observed in spring. Based on surveys conducted in the spring, information about the distribution and estimated density of adult pea leaf weevils is available provincially and from the Prairie Pest Monitoring Network (prairiepest.ca), usually in January. A protocol for scouting fields for adult weevils in the spring is available from the Prairie Pest Monitoring Network.
There are also some options for cultural control of pea leaf weevil. Because the earliest planted pea fields tend to be most damaged, delaying planting may reduce the risk of damage (so long as crops are not planted too late as to impact yield). Fields with very high levels of soil nitrogen, such as manured fields, may have a lower risk of yield loss. However, supplementing nitrogen post-seeding to faba beans is not recommended because the amount of fertilizer needed would be too high and cost-prohibitive. Management options that use trap crops and pheromone traps to attract and kill adult weevils in the spring and fall are currently under investigation in Alberta. The impact of predation on pea leaf weevil populations is also being investigated, with a focus on generalist ground beetle species that are often encountered in field pea and faba bean fields.
For advice about managing pea leaf weevil in Manitoba, farmers are encouraged to consult with John Gavloski from Manitoba Agriculture and Resource Development.