Corn blotch leafminer (CBL) is an occasional corn pest that feeds within corn leaves. CBL feeding does not typically result in economic yield loss. Since CBL feed within the leaf, insecticide applications are not an effective in controlling this pest
What to Watch For
Corn blotch leafminer (CBL), which is actually the larva of a small black fly, is a frequent but minor pest of corn. During the growing season females may insert eggs into corn leaves, which upon egg hatch the larvae begin tunneling into the corn leaves. Larvae are pale-green to yellowish-white and about 1/4-inch long when fully grown. Mature larvae chew out of the leaf and drop down to the soil to pupate. Roughly four to six weeks are required to complete the entire CBL life cycle. There can be several generations per year, but typically the first generation does the most leaf damage.1
Larvae feed on internal leaf tissue, leaving transparent tunnels or mines (Figure 1 and 2 ). The tunnel from one larva usually widens as the larva grows, resulting in a broad blotch in the last third to half of the mine.2 Numerous mines on a single leaf can merge giving the leaf an overall bleached appearance.3 In heavy infestations when the entire leaf is mined, the foliage dries up and shrivels, giving the plant a frosted appearance.4 As corn matures, leaves thicken and larvae from later CBL generations generally tunnel in only the lower or upper half of the leaf, causing less damage.
Impact on Your Crop
There are no sampling methods or economic thresholds for CBL.3 While there is not much research on the effects of CBL, entomologists believe that CBL feeding does not typically cause economic damage.
Potential losses from CBL can be estimated using hail damage data.3 Using this data, a 4% yield potential loss in 7-leaf corn would require 60% defoliation (Table 1, page 2). To incur a 5% loss in yield potential in 12 or 17-leaf corn, mining damage would have to be 40 and 25%, respectively.5 This is the best information currently available to estimate potential yield loss, but it is not based on CBL feeding damage. Hail damage is based on whole plant damage versus CBL feeding that occurs on leaves generally in the lower portion of the plant canopy.
Tips to Manage
Foliar insecticide applications are not effective for CBL control because the egg and larvae are inside the leaves, protected from insecticides. Adult flies emerge over several weeks and would require multiple insecticide applications. In addition, broad-spectrum insecticides may kill predators and parasites which are important for suppression of CBL and other corn pests such as spider mites and aphids. CBL populations are normally suppressed by numerous species of parasitic wasps.1,2,4
1 Wright, B. 2010. Corn blotch leafminers damaging corn in central Nebraska. CropWatch. University of Nebraska - Lincoln. https://cropwatch.unl.edu
2 Steffey, K.L., Rice, M.E., All, J., Andow, D.A., Gray, M.E., and Van Duyn, J.W. 1999. Handbook of corn insects. Entomological Society of America.
3 Obermeyer, J. and Bledsoe, L. 2003. Corn blotch leafminer or why are corn leaves turning white? Pest & Crop. Issue 16. Purdue University. https://extension.entm.purdue.edu
4 DiFonzo, C. 2012. Corn blotch leafminer. CDD #038. Michigan State University. http://www.msuent.com
5 2013. Corn loss adjustment standards handbook, 2014 and succeeding crop years. Page 84. FCIC-25080 (11-2013). USDA Federal Crop Insurance Corporation. https://www.rma.usda.gov 180828120703
Web sources verified 9/12/18