No. 16/July 10, 1998
Corn Leaf Aphid Populations: Reports of Infestations Are Down
In most seasons, after the 4th of July we begin to receive many reports of corn leaf aphid infestations. So far, that has not been the case this year. The excessive precipitation across much of Illinois has likely suppressed aphid densities in many fields. However, donít be surprised to see "pockets" of corn leaf aphids during your scouting ventures for corn rootworm beetles. Provided is a summary of corn leaf aphid biology, management strategies, and some information about thresholds for commercial and seed production cornfields.
Corn leaf aphids do not survive the winters throughout most of the north central states. The aphids usually migrate or are carried into Illinois on storm fronts and prevailing winds during June and July. When these aphids "drop" into cornfields, they move into the protective whorl area to begin the business of establishing colonies. The reproductive powers of corn leaf aphids are impressive. About nine generations of this insect occur each year in the Midwest. Producers generally begin to notice the aphids in clusters that appear on leaves and in the whorls of corn plants. As the number of aphids increases on a given plant, winged females become more apparent and eventually fly from infested plants to less crowded plants to begin new colonies. Corn leaf aphids secrete a sticky substance referred to as "honeydew." As densities increase, leaf surfaces and tassels often become black and sooty as mold begins to grow on the honeydew.
When monitoring fields for this pest, look for bluish green, soft-bodied insects the size of a pinhead (Figure 3). Their legs and short cornicles ("tailpipes") near the rear of the abodomen are completely black. When scouting fields for corn leaf aphids, examine at least 5 sets of 20 plants per field.
Figure 3. Corn leaf aphid, winged and wingless forms.
The presence of predators such as lacewings and lady beetles also should be noted (Figure 4). Unfortunately, specific information is not available regarding how many predators and diseased aphids represent that level at which an insecticide is not required. Parasitized aphids will be smaller, brown, and less mobile. Diseased aphids will be shriveled and possibly moldy. Because corn leaf aphids must shed their skins when they molt, white to grayish cast skins are often found. Donít confuse cast skins with diseased or parasitized aphids.
Figure 4. Common predators of corn leaf aphids (clockwise from upper left): lady beetle adult, lady beetle larva, lacewing adult, lacewing larva.
If 50 percent of the plants during the late-whorl to early tassel stages have light to moderate infestations (50 to 400 aphids per plant) and plants are under drought stress, a treatment may be warranted. Please note that plants must generally be under moisture stress for a rescue treatment to make much sense. Obviously, most fields in Illinois donít fit this category at this point. If the pollination process is well over halfway complete and the plants are under no moisture stress, corn leaf aphids will pose less of an economic threat. Corn that is under serious moisture stress after pollination may still suffer yield losses if plants are heavily infested (upper leaves and tassel completely coated). For many replanted fields that are still several weeks from pollination, dry conditions could return in late July and early August, potentially setting the stage for late-season corn leaf aphid problems. Bottom line--don't write this insect off, just yet.
For those in the seed industry, consider the following advice. According to the Seed Corn Pest Management Manual for the Midwest (published by Purdue University), fields should be monitored when they reach the whorl stage of development. It is during the whorl stage that yield losses are most likely to occur if aphid densities are high. In addition, information from this manual indicates that "corn leaf aphid control is most effective 2 to 3 weeks prior to tasseling, after which it is rarely advisable."
Insecticides suggested for corn leaf aphid control include: dimethoate (see product label), Lorsban 4E at 1 to 2 pints per acre, malathion 57% EC at 1-1/2 pints per acre, and *Penncap-M at 2 to 3 pints per acre. Penncap-M is a restricted-use insecticide and can be applied only by certified applicators. If corn leaf aphids are the only target insect and residual activity is not a main concern, malathion 57% EC should provide satisfactory levels of control.
Mike Gray (firstname.lastname@example.org), and Kevin Steffey (email@example.com), Extension Entomology, (217)333-6652