The DAMAGE done by adult pea weevils (Sitona lineatus) is characteristic; adults chew half-moon circles on leaf margins and also chew on growing points. When the notches line up, it looks like a scalloped edge on the leaf. Early infestation by adult weevils can cause up to 50% defoliation to seedling field pea, and part of the problem with this pest is that it can cause damage, either above or below-ground, for the majority of the growing season. Larval feeding within roots inhibits nitrogen-fixation and can lead to poor plant growth.
BIOLOGY of this pest is unique in the PNW. Adults are the overwintering stage, and given mild winters, they do not enter diapause like they do in other regions. As soon as your peas pop up, the pest is present and welcomes the snack. Larvae feed underground, and can be a serious problem from April through June. A new study from WSU indicates there may be biologically relevant associations between pea weevil, pea aphid, and pea enation mosaic virus (PEMV).
HOST RANGE Unfortunately, other cultivated Fabaceae are at risk as well. Clovers, alfalfa, fava beans, chickpeas, and other legumes are all suitable hosts, and damage to these crops is not uncommon.
SCOUTING Leaf damage will be evident, as noted above, but be aware that larvae may be in the ground already. Dig the top 1-3″ of soil and look for creamy-white grubs with a brown head capsule. Sweep netting can be used to sample adults if crops are tall enough.
ID Adult pea weevils are elongate, light brown weevils with three light stripes on the dorsal surface, and their snouts are shorter than other weevils. Grubs are legless, milky white with a brown head, and have tiny hairs protruding from each bulging body segment.
MANAGEMENT In high-production areas, there is a reliance on chemical, foliar or soil applied controls to target different life stages. However, there is increasing interest in alternative management strategies such as nitrogen applications 1,3 that may reduce larval attraction and suppressive soils to enhance entomopathogenic nematodes and fungi.
1 Vankosky, M.A., et al., Integrated management of Sitona lineatus with nitrogen fertilizer, Rhizobium, and thiamethoxam insecticide. Agronomy journal, 2011. 103(3): p. 565-572.
2 Williams, L., D. Schotzko, and L. O’Keeffe, Pea leaf weevil herbivory on pea seedlings: effects on growth response and yield. Entomologia experimentalis et applicata, 1995. 76(3): p. 255-269.
3 Héctor A Cárcamo, Meghan A Vankosky, Asha Wijerathna, Owen O Olfert, Scott B Meers, Maya L Evenden; Progress Toward Integrated Pest Management of Pea Leaf Weevil: A Review, Annals of the Entomological Society of America, Volume 111, Issue 4, 24 July 2018, Pages 144–153, https://doi.org/10.1093/aesa/say007
4 Basu, S., Clark, R.E., Bera, S., Casteel, C.L. and Crowder, D.W. (2021), Responses of pea plants to multiple antagonists are mediated by order of attack and phytohormone crosstalk. Mol Ecol. . https://doi.org/10.1111/mec.16103
5CABI datasheet with links to 50+ references. Invasive Species Compendium https://beav.es/3tx