Corn Blotch Leafminer Identification and Management

Key Points

• Corn blotch leafminer (Agromyza parvicornis) is an occasional insect pest of corn that feeds within corn leaves.
• Feeding on corn leaves by the corn blotch leafminer (CBL) has not resulted in corn yield loss in recent years, as reported by the Crop Protection Network from 2020 through 2025.¹
• Insecticide applications are not effective in controlling CBL larvae that are feeding, as they are protected within the leaf structure.

Corn Blotch Leafminer Identification and Life Cycle

Corn blotch leafminer (CBL) is considered a sporadic, minor pest of corn. The adult female is a small, gray to brown fly, about ¼ inch long, that inserts eggs into corn leaves during the growing season. After the eggs hatch, the larvae tunnel into leaves. Larvae are pale green to yellowish-white and about ¼ inch long when fully grown (Figure 1). Mature larvae leave the tunnel and drop down to the soil to pupate. About four to six weeks are required to complete the CBL life cycle. Several generations of this pest can occur per year, but typically the first generation does the most leaf damage. Corn blotch leafminers are more likely to attack seedlings in cool springs.^2,3,4,5

Corn Blotch Leafminer Damage Symptoms and Economic Thresholds

Larvae feed on internal leaf tissue, leaving transparent tunnels which are also known as mines (Figures 2 and 3). The tunnel from one larva usually widens as the larva grows, resulting in a broad blotch in the last one-third to one-half of the mine.³ Numerous mines on a single leaf can merge, giving the leaf an overall bleached appearance.⁴ In heavy infestations in which the entire leaf is mined, the foliage dries up and shrivels, giving the plant a frosted appearance.⁵ The leaves of mature corn are usually thicker than the leaves of seedling corn, so larvae from later CBL generations tend to feed only on a portion of a mature leaf rather than the entire leaf, and are less likely to cause damage.²

Sampling methods and economic thresholds have not been established for CBL. Research on the effect of CBL leaf-feeding on the yield potential of corn is very limited. Because feeding by CBL usually occurs on the lower leaves—which are not contributing as much to growth to the corn plant as the leaves in the upper canopy—entomologists believe CBL damage is usually non-economic.⁴

Potential yield losses from CBL can be estimated by using the same chart used for estimating yield loss due to defoliation from hail damage. Using this data, a 4% yield potential loss in 7-leaf corn would require 60% defoliation (Table 1). To incur a 5% loss of yield potential in 12- or 17-leaf corn, mining damage would have to cover 40 or 25% of the leaf area, respectively.⁶ This is the best information currently available to estimate potential yield loss, but it is not based on CBL feeding damage. Unlike CBL feeding, hail damage estimates are based on whole-plant damage.⁴

Table 1. Estimated potential corn yield loss due to percent leaf defoliation at various growth stages.

Corn Blotch Leafminer Management

Foliar insecticides applications are not effective for CBL control because the eggs and larvae are inside the leaves, protected from insecticides. Adult flies emerge over several weeks and would require multiple insecticide applications to manage. In addition, broad-spectrum insecticides may kill helpful predators and parasites that suppress CBL and other corn pests such as spider mites and aphids. Corn blotch leafminer populations are normally suppressed by numerous species of parasitic wasps.^2,3,4

Sources

¹Reisig, D., Graham, S., Bateman, N., et al. 2026. Corn invertebrate loss estimates from the United States and Ontario, Canada – 2025. Crop Protection Network. CPN-2019-25. https://cropprotectionnetwork.org/publications/corn-invertebrate-loss-estimates-from-the-united-states-and-ontario-canada-2025
²Wright, B. 2010. Corn blotch leafminers damaging corn in central Nebraska. University of Nebraska–Lincoln, CropWatch. https://cropwatch.unl.edu/unl-cropwatch-july-9-2010-corn-blotch-leafminers-damaging-corn-central-nebraska/
³Steffey, K.L., Rice, M.E., All, J., Andow, D.A., Gray, M.E., and Van Duyn, J.W. (Eds) 1999. Handbook of corn insects. Entomological Society of America.
⁴Krupke, C. and Obermeyer, J. 2020. Why are my corn leaves turning white? Purdue University Extension, Pest&Crop Newsletter. 2020.17. https://extension.entm.purdue.edu/newsletters/pestandcrop/article/why-are-corn-leaves-turning-white/
⁵DiFonzo, C. 2012. Corn blotch leafminer. Michigan State University Field Crops Entomology Program. CDD #308. https://www.canr.msu.edu/fieldcropsent/uploads/files/38CornLeafminer.pdf
⁶Federal Crop Insurance Corporation, United States Department of Agriculture. 2023. Corn loss adjustment standards handbook, 2024 and succeeding crop years. Page 86. FCIC-25080 (11-2023). https://www.rma.usda.gov/sites/default/files/handbooks/2024-25080-Corn-Loss-Adjustment-Handbook.pdf
Web sources verified 07/07/2026. 1215_64527