PEST ALERT: Heavy Leafminer Activity in Magic Valley, ID

Published online: Jun 10, 2010 PNWPestAlert
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Heavy leafminer activity has been observed in several sugar beet fields across the Magic Valley. Maggots feed between the upper and lower leaf surfaces, leaving irregular transparent windows, blotches, and winding tunnels. Eggs are tiny, white, and cigar shaped and are laid on the underside of leaves and cotyledons; they may be laid singly or in groups of two or more. Eggs may be readily observed without magnification. Under very warm conditions, eggs may hatch within four days, but will take longer to hatch under the cooler conditions that we have experienced this spring. There are no formal economic thresholds for insecticide treatment decisions for leafminers, but treatment should be considered if fields are under heavy pressure. To scout a field, sample ten plants in each of several representative areas across a field. Record the average number of eggs and mines per plant observed in each area. Foliar insecticides should be considered within a few days after the first appearance of mines or before most eggs have hatched. Alternatively, systemic granular insecticide should still be effective if applied (and activated) shortly after most larvae have tunneled into leaves. Insecticide seed treatment should also provide control against early-season leafminer attack, which is expected to be more damaging than later-season attack. For current information on registered insecticides, consult the Pacific Northwest Insect Management Handbook ( This information was provided by Dr. Erik Wenninger, UI Extension Entomology Specialist. To view the full alert, and to download any attached files, please go to