Behavior and damage potential of the newly emerged pest Myochrous armatus (Chrysomelidae: Eumolpinae) on soybean plants in Brazil

Myochrous armatus Baly, 1865 (Coleoptera: Chrysomelidae) has expanded its occurrence significantly into soybean-growing areas of Brazilian Cerrado and became an important early-season soil pest. Experiments were performed under field conditions from 2020 to 2023 to assess several aspects: (1) population behaviour over crop season and offseason; (2) day/night behaviour during the early stages of soybean plants and (3) injury to plants and potential damage to soybean yield to establish the Economic Injury Level (EIL). Adults of the M. armatus population presented a season-long abundance during the specific period from late October to January, with major captures in November and December (end of Spring), either inside or on the edge of soybean fields. Weeds and volunteer plants of corn and cotton hosts adults and eggs. These findings suggest an univoltine life cycle. Behavioural assessments revealed that M. armatus does not exhibit a specific day, night or crepuscular behaviour, performing deeds at any time. The majority of insects (40% to 70%) were found in the soil throughout the day and night. Adults feed on soybean plants by scraping or cutting the stem, cotyledon, petiole and apical sprouting. Their preference is initially for the stem and cotyledon (up to 14 days after emergence), and later for the petiole (after 21 days of emergence). After 7 days of coexistence at V1-V3 soybean stages, we observed a potential yield reduction of 35% for each insect per plant. Dynamic EIL estimations are between 0.4 and 1.9 adults per row metre, depending on the grower productivity expectation, control costs and soybean market value. Our results are fundamental for establishing Integrated Pest Management for M. armatus in soybean and other crop systems.