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| The presence of lady beetles indicate that soybean aphids may be in your field. |
Go to the following pages to study:
Description, identification, and the life cycle of the soybean aphid
Symptoms and signs of soybean aphid activity
Agronomic impact of soybean aphids
Management of soybean aphids
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Scouting Soybean Aphids
Yield-damaging aphid populations can occur in any field and can not be detected from the road. There are a few factors that increase the likelihood of aphid problems. For example, smaller fields with wooded borders containing buckthorn are most likely to be the first to develop high populations.
Scouting recommendations
Start checking soybean fields in mid-June and continue to estimate aphid numbers weekly until aphid populations decline. A good rule of thumb would be to scout fields until the middle of August.
Estimate aphid populations on 20-30 plants following a W-pattern throughout the field. Early in the season, aphids tend to be located on new growth. Examining the upper trifoliate is a quick way to determine the percentage of plants with aphids. When counting aphids, count all aphids (adults and nymphs), but do not count cast skins, or confuse potato leafhoppers for aphids.
As vegetative growth ceases, aphids tend to move down the plant toward the pods, stems and leaves lower in the canopy, presumably tracking the movement of nitrogen into pods.
In the early reproductive growth stages, look at both sides of the leaf, stems, and pods throughout the canopy for aphids.
The presence of lady beetles, honeydew, and ants also indicate high aphid populations. Unfortunately, at this time, predators are relatively rare in many fields. Ants tend to be more abundant on field edges. Within the canopy, large numbers of aphids on the stems and pods indicate populations approaching, if not exceeding, 250 aphids per plant.
Action thresholds
The threshold for soybean aphid is 250 aphids/plant. This threshold is assumed to be an aphid population that is below the level where unacceptable yield loss occurs, but high enough that a high probability of reaching yield-damaging levels exists.
This threshold is based on field averages and not hotspots or field borders. Hot spots often collapse. However, once a high percentage of the plants in the field have aphid colonies, rapid population increases on an aphid per plant basis can occur.
Observations from Wisconsin and Iowa show that soybean aphids tend to leave soybean in the second to third week of August. Many reasons exist for this sudden decline in aphid populations, and many still believe that photoperiod may be a major reason. However, there are other hypotheses. For example, the translocation change in the plant during pod set and seed filling may play a key role. Soybean aphids are really good at tracking the nitrogen flow in the plants and move towards the richest and best food quality. Another reason can be that high aphid numbers can lead to a generation of alatoid nymphs, which develop into winged adults and leave the field in a few days. The latter indicates that they have overrun the carrying capacity of the plant which is not good from a yield standpoint.
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