Leafminers

The sugar beet leaf miner is becoming more of a concern to area farmers. I asked Dr. Bob Stoltz, University of Idaho Entomologist, to describe the lifecycle of the leaf miner, description of crop damage, and control options.

Dr. Stoltz: The beet leaf miner overwinters as a pupa in the soil. Adults emerge in spring and lay white, sausage shaped eggs on the undersides of the leaves – usually 3-5 together. The adults are grey or grey- green and about ¼ inch long. The eggs hatch and the larvae burrow into and feed between the two layers of the leaf. The feeding area is small and narrow at first but widens to a blotch as the larva matures. Larvae feed for 2-4 weeks. Mature larvae drop into the soil to pupate and spend about 3-4 weeks before emerging as new adults.

There are about three generations per year in southern Idaho. The first and third generations (May and August) tend to harm beets the most. Small seedlings are most susceptible to damage but with warm weather can outgrow the damage caused by the leafminers.

There are no economic thresholds established for this insect. Controls should be applied based on how much leaf area loss the plant can withstand at the particular stage of growth. If seedlings lose 25% or more of their leaf surface, sprays could be considered. Later in the year, the same number may be used but if the damage is on the older, outer leaves, the plants can probably withstand that damage without yield loss. In many cases the larvae are parasitized and sprays may not be needed.

Several materials are available for leafminer control. Those listed in the PNW Insect Management Handbook are Temik, phorate, Lorsban, diazinon, pyrethrins and horticultural oils.