Aphids in Corn and Sorghum
Jenny Rees, extension educator in Clay County, said growers are reporting greenbugs (Figure 1) in sorghum and corn leaf aphids (Figure 2) on corn in south central Nebraska.
Greenbugs should be monitored closely for the next couple of weeks in case economically damaging populations develop (Figure 3). Predator populations, particularly lady beetles and greenbug parasites, may be found in many fields. The greenbug parasite is highly effective in controlling greenbugs if it gets started early.
The adult parasite (Figure 4) is a small wasp that lays eggs inside greenbugs. The immature stage (larva) of the parasite develops internally and ultimately kills the greenbug. Just before completing development, the larva causes the greenbug exoskeleton to swell and change to a tan color. This is the parasite pupal stage, called a mummy. The wasp will emerge from the mummy in one to two days.
Because parasites and predators can be highly effective in controlling greenbugs, delay use of insecticides until economic thresholds are reached.
Most insecticides registered for greenbug control usually provide excellent control. Insecticide-resistant greenbugs have occasionally been present in Nebraska, but there have not been any recent reports of insecticide failure in Nebraska.
For more information on greenbug management refer to the UNL Extension NebGuide Management of Greenbugs in Sorghum (G838).
Plants 6 inches tall to boot stage: Treat if greenbug colonies are beginning to cause red or yellow leaf spotting on most plants; before any entire leaves are killed, and if parasite numbers are low (less than 20% of greenbugs are mummies).
Boot to heading: Treat if greenbug colonies are present on most plants and have killed one lower leaf and if parasite numbers are low (less than 20% of greenbugs are mummies).
Heading to hard dough: Treat if greenbug colonies are present on most plants and have killed two normal-sized leaves and if parasite numbers are low (less than 20% of greenbugs are mummies).
Corn Leaf Aphids
Corn leaf aphids are beginning to appear in corn fields. Their survival is favored by dry weather, which inhibits the activity of fungal pathogens which often help control these insects in a year with more frequent rainfall.
Corn leaf aphids cause the greatest amount of injury while they are feeding within the whorl. Sometimes , if they are very abundant, they can interfere with pollination. They are less likely to cause damage after pollination unless they are very abundant.
There are no research-based treatment thresholds for corn leaf aphids in corn after pollination. Treatment would be most likely to increase profit in later planted field, or fields under drought stress with high populations of aphids associated with leaves or tassels killed by previous aphid feeding.
See the Insect Management section of the Guide for Weed Management in Nebraska with Insecticide and Fungicide Information (EC 130) for insecticides to be used for corn leaf aphid control in corn.