An occurrence of Corn Blotch Leafminer (CBL) was reported June 26 in Christiana Township (Dane County) by David Fischer, UW-Extension Dane County Crops and Soils Agent. The feeding was found on V12-V15 corn on the older, lower leaves, while the youngest, uppermost three leaves of each plant were found uninjured. Fischer also reported a large area of CBL feeding in the Black Earth area on June 28.

 

 Corn with CBL feeding in field

 

CBL is a tiny fly, about ¼″ long that lays its eggs on corn leaves. Similar to the Alfalfa Blotch Leafminer, the adults will make pinhole punctures in the leaves during feeding and oviposition. When the eggs hatch, the larvae tunnel (or “mine”) into the corn leaves, eating out the mesophyll, or middle layer of cells in the leaves. This feeding creates a translucent “windowpane” in which the larvae can sometimes be seen.

 

 Adult CBL feeding and oviposition scars on corn leaf visible as pockmarks. 

 

 Translucent window in corn leaf caused by CBL larval feeding in mesophyll layer. 

 

 

 

When mature, the larvae drop off the leaves and pupate in the soil. There are several generations in a year, each generation lasting from three to six weeks. CBL feeding is rarely known to cause economic levels of damage, and since damaging populations are rare, studies have not been conducted to evaluate insecticide efficacy against this insect feeding on corn. Even if leafminers are causing significant leaf feeding, it does not mean insecticides should be applied. Control efforts would need to be targeted to adult flies before they lay eggs, because the fly maggot is protected inside the leaf from insecticide contact. Targeting flies of overlapping generations and expense of repeated application, coupled with absence of economic injury level data do not justify the expense of insecticide treatments. An outbreak of this pest occurred on corn in Nebraska in 1995. A hypothesis for that outbreak was that beneficial wasp numbers were reduced that year and CBL may have escaped biological control. At any rate, this article provides a diagnostic guide for leaf damage symptoms you may be seeing in the field at this time.

 

Wright, Robert J. “Corn Blotch Leafminer.” NebGuide. University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources. July 2006. http://www.ianrpubs.unl.edu/epublic/live/g1635/build/g1635.pdf

 

Ratcliffe, Susan; Gray, Michael; and Steffey, Kevin. “Corn Blotch Leaf Miner.” University of Illinois Extension. 2004. http://ipm.uiuc.edu/fieldcrops/insects/corn_blotch_leaf_miner/index.html