Earlier this summer, Iowa State University specialists found aphids in V4 corn in central Iowa. This early establishment is unusual because most aphids that feed on corn have to migrate from the southern United States every summer. ISU Extension entomologist Erin Hodgson and her graduate students also found soybean aphids in vegetative soybeans at most of the ISU Research Farms in June. Although these aphids can overwinter in Iowa, larger colonies have not recently formed until after soybeans bloom. Pea aphids have also been reported in alfalfa in northeast Iowa this summer.
All of these aphid detections prompted Hodgson to provide the following summary of scouting guidelines and insecticide treatment thresholds for aphids in field crops.
* Soybean aphid: This is the only species in Iowa that will colonize on soybeans. Scout weekly from plant emergence until seed set. Aphids prefer to feed on the undersides of leaves and will establish on the newest leaves (see Photo 1). If a large colony develops, they will feed on stems. Initial infestations are patchy and located near field edges, but winged aphids can quickly disperse within and between fields. Commercial fields that have reached uniform infestation should be closely monitored in August.
Photo 1: Turn over soybean leaves and count the number of soybean aphids present, to estimate soybean aphid density. Photo ISU Extension
The economic threshold for soybean aphid is well established for the north central region of the United States. Consider applying a foliar insecticide when the average density exceeds 250 per plant. Populations should be increasing and most of the plants have to be infested (greater than 80% of the plants) to justify an application. This threshold is appropriate until plants reach mid-seed set (R5.5—See Photo 2). Spraying at full seed set (R6) or later has not produced a consistent yield benefit.~~~PAGE_BREAK_HERE~~~
Photo 2: At mid-seed set (R5.5 growth stage of soybean plant) soybeans have seeds that are expanding in the pod. Photo by ISU Extension
Symptoms of infestations in soybeans include shortened plant height, curled leaves with yellow edges, excessive honeydew on leaves and the presence of ants. Excessive honeydew may also promote mold growth, reducing photosynthesis. The distinguishing soybean aphid characteristic is cornicles or black "tailpipes" projecting from the rear of the abdomen.
* Aphids in corn: There are several species of aphids that can feed on corn. They prefer to feed on small grains, but will use corn as a host. Aphid infestations in corn have been sporadic in Iowa the last five years, but should be monitored after silking. A widespread outbreak occurred in northwest Iowa in 2011. Populations are typically aggregated at field edges, but winged aphids can move to field interiors. Aphids will colonize the stalk, leaves and ear (See Photo 3).
Photo 3: Pay special attention to aphids on corn if they are at or above the ear leaf. Photo by ISU Extension field agronomist Brian Lang
Currently, there are no treatment thresholds for aphids in corn if the corn is past the tasseling stage. But regular sampling will help you make educated decisions about a foliar application at this time. Consider a foliar application when most of the plants are infested (greater than 80 percent), and aphids are have exceeded 500 to 1,000 per plant. An insecticide spray may be warranted if aphid honeydew and sooty mold are evident above the ear leaf of the corn plants and the plants have not yet reached hard dent (which is the R5 growth stage of corn plants).~~~PAGE_BREAK_HERE~~~
* Aphids in alfalfa: Like aphids in corn, there are several species of aphids that will feed in alfalfa. Although considered secondary pests, aphid infestations in alfalfa fields can build up to high densities. You should scout fields weekly to monitor for aphid arrival and spread within a field.
Photo 4: Pea aphids can be pink or green and they can infest alfalfa fields Iowa. Photo by ISU Extension field agronomist Brian Lang
Treatment thresholds for aphids in alfalfa depend on the aphid species that is present in the field and the size of the alfalfa plants. Use Table 1 as a management guideline. Depending on the cutting cycle for hay harvest, the timing of alfalfa harvest would be an effective control strategy instead of applying insecticide.
Table 1: Treatment application guidelines for insecticides for aphids (per stem) in alfalfa.
* General aphid management tips for soybeans, corn and alfalfa in Iowa. Ideally, droplets of the insecticide spray should make contact with the aphids for the greatest knockdown of the aphid infestation. Increasing the volume applied and the pressure of the spray will improve the efficacy of foliar insecticides. For applications that are applied with a ground driven sprayer, use 20 gallons of water per acre and 40 pounds of pressure per square inch. Be aware that some foliar insecticides have a 60-day pre-harvest interval. Check the label of the particular product you are considering applying and the calendar when making insecticide product selections.