Economic thresholds and economic injury level for pea aphid in tannin and low tannin faba bean

Pea aphids, Acyrthosiphon pisum, are one of the major insect pests of faba bean (Vicia faba). Infestations of pea aphids on faba bean can occur at any plant growth stage and result in drastic yield losses (up to 100%). There was no established guideline to determine when to manage pea aphids on faba bean in Saskatchewan. We conducted a field study to develop an economic threshold for pea aphids on faba bean at six sites in the Saskatoon area in 2019 and 2020. No choice experiments were performed to determine pea aphid growth rate on tannin and low tannin faba bean. Under field conditions, the population of pea aphids on faba bean doubled in approximately 5.25 ± 0.19 days. The average economic threshold (ET) was 34–50 aphids per main stem of a plant. This ET provides a 7-day lead time before aphid populations are expected to exceed the economic injury level (EIL) of 680–984 cumulative aphid days or 96 to 142 aphids per faba bean main stem. All ETs were estimated based on the cost of registered insecticides and the average of three benchmark prices (high, medium and low) of faba bean from January 2018 to July 2021. Laboratory no-choice bioassays demonstrated no significant effect of faba bean tannins on pea aphid growth. Therefore, economic thresholds established in this study are applicable to both tannin and low tannin varieties of faba bean.