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This is an urgent alert for producers planting fall crops, especially brassicas. The yellowmargined leaf beetle (YMLB, Microtheca ochroloma), is a small 5-mm long insect with dark brown elytra (hard forewings) that has a yellow margin all the way. Producers usually experience large numbers of these migratory insects during cool season, right when the crucifers are in the ground and growing. Eggs are laid loosely in clusters under leaves or on soil surface. YMLB eggs may look like lady beetle (beneficial insect) eggs but the latter are upright and in a tight cluster. In our experience, these insects will buildup in soil within two years if uncontrolled.
YMLB larvae are grayish with a dark head and plenty of short body hair or setae. In comparison, the lady beetle larvae have several stiff spines on their body and they may be multi-colored. YMLB pupae look like debris in the soil and is a clever defense strategy to escape predation. Spined soldier bugs and lacewings love to feed on the YMLB larvae when they are abundant on leaves; however, lack of foliage may trigger the insects to move deep down the roots following soil cracks.
YMLB larvae and adult prefer to feed on turnips and Napa cabbage; the pest can cause complete devastation as shown below. Besides complete leaf skeletonization in turnips, the adult YMLB make notches on the leaf margins of cabbages and kale. In other words, YMLB feeding injury is different from caterpillar feeding in that the latter feed toward the center of leaves near the midrib and less toward the margin.
Another characteristic worth mentioning is the fact that several YMLB larvae can be seen feeding in clusters on leaves; larvae and adults may drop off leaves if you disturb them too much – so make sure to look under the plants for these dark insects. Direct scouting of plants is perhaps the best way to look for these pests and one adult YMLB per plant is the action threshold. We have experienced larvae to feed on leaves and tunnel the stems during warm days during the winter; under a high pest pressure, untreated turnips were 100 percent damaged within two weeks. Don't try to control YMLB with Bt sprays; YMLB must be controlled with broad-spectrum organic insecticides used in rotation or combination with Bt products. For knowing more about the lifecyle of the pest, scouting methods, and management strategies, please watch this 12 minute webinar recording and call a commercial horticulture regional extension agent for updates. There is also an IPM training module for producers and gardeners regarding YMLB.
So don't delay, scout your crucifer crops today!
Ext. Entomologist and SARE Coordinator
Research Fellow, AU Entomology and Plant Pathology
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