Weekly Scouting Report - August 4, 2014
I scouted corn fields in the Streator area this week. Disease pressure has increased slightly in the past two weeks. I could see differences by hybrid at this point. Gray Leaf Spot and Common Rust are the two main diseases that I’ve found, with Northern Leaf Blight spotted occasionally. Corn leaf aphids were the most prevalent pest I found. I did see a few rootworm beetles and Japanese beetles. Below are some pictures I took of corn leaf aphids and an explanation of what this pest can do to corn.
The corn leaf aphid is a blue-green or gray, soft-bodied, spherical insect about the size of a pinhead [1/16 inch (1.6 mm) in length]. It has approximately 9 generations per year. Female corn leaf aphids do not lay eggs, as do most other insects, but give birth to living young. These young, called nymphs, resemble the adults except that they are smaller and are sexually immature. Adults and nymphs can often be found clustered within the whorls or upper parts of corn plants over isolated or wide areas of a field.
Most corn leaf aphids are wingless. However, as populations increase, some develop delicate, filmy wings. These wings enable them to fly to uninfested plants to start new colonies.
Like other insects, aphids shed their skin as they grow. These numerous white to gray discarded skins give the appearance of a white mold or ash on leaf surfaces. Aphids also secrete a sticky, sugary substance, known as honeydew (excrement).
Corn leaf aphids do not generally appear until mid-June or early July. Since they are not cold hardy, they migrate each year into the mid and upper Midwest from southern areas of the USA. It is feasible that some overwinter in the lower Midwest.
Heavily infested corn leaves may wilt, curl, and show yellow patches of discoloration. When tassels and silks are covered with honeydew, the pollination process may be disrupted. Also, excessive aphid feeding within the whorl prior to tassel emergence appears to be directly related to incomplete kernel development and/or barren ears.