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Numerous past articles in the IPM Communicator have described the biology and behavior of the yellowmargined leaf beetle (YMLB, Microtheca ochroloma). Basically, it is an insect that was first detected in Mobile, AL, in 1947 and has made a comeback as a major pest of brassica crops. YMLB larvae and adults prefer to feed on turnips and napa cabbage which result in complete defoliation of plants and yield loss. YMLB larvae are dull brown with several grayish areas and a dark head along hairs all over the body. YMLB larvae only have three pair of thoracic legs and feed in large aggregations during the early stages. Larvae and adults are very good at dropping of the plant to the ground or to the central whorl of leaves to hide when approached. The picture below shows the effect of YMLB larvae on leaf and root growth; turnips with high YMLB counts usually have poor growth and weak root systems.
Since YMLB prefers to feed on turnips, we have been testing it as a trap crop to protect cabbages per the research report by Dr. Balusu and Dr. Fadamiro (published in the Journal of Economic Entomology, 2015). In the past two years, we have completed large test plots studies with the turnip-cabbage system and evaluated some major organic insecticides that may help kill YMLB larvae on the trap crop. Goal is to prevent the larvae from moving to the main crop (cabbage) where they can be very difficult to control. A damage rating system of 0 to 6 was used to rank the feeding levels on turnip leaves (1 = less than 10 percent defoliation to 6 = over 90 percent defoliation); results are as follows.
Results from Fall 2015 study indicated two timely applications of Monterey Garden Insect Spray with Spinosad (0.5 oz/qt) reduced YMLB feeding on turnip by over 50 percent compared to the untreated check. PyGanic (natural pyrethrin, 18 oz) and Grandevo (microbial insecticide containing Chromobacterium, 3 lb) also slowed down YMLB feeding. Venerate, a new microbial insecticide, was not effective in reducing leaf feeding. None of the organic approved insecticides completely eliminated the YMLB larvae; adult beetles are even harder to kill with these products. Since this test was done at a new location (Clanton, AL) with no prior history of YMLB, we were surprised as to how quickly the pest moved in and found the turnip trap crops necessitating the treatments (see picture below).
Results from the 2016 also study provided consistent results to prevent YMLB outbreak. This time we included a chemical standard (Mustang Max) and an organic standard (Entrust) in the test for comparison. There was very high pest feeding pressure as the larvae colonized turnips in the early growth stages. Two weekly applications of Pyganic (18 oz), Monterey Spinosad (0.5 oz), and Entrust (3 oz) reduced leaf feeding significantly compared to the untreated check. The picture below quickly tells you the story. Overall, we think that there is great potential in the popular organic insecticides for YMLB control and research will continue on successful deployment of 'integrated' strategies such as trap crops and insecticides. For further details, please check the YMLB page on Alabama Vegetable IPM website and contact a Commercial Horticulture Regional Extension Agent for correct pest diagnosis.
Ayanava Majumdar, Ext. Entomologist
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