Scouting for Soybean Aphids
Scouting for soybeans can be a
particular challenge for
three reasons.
- Soybean aphids are fairly small (<1/16”) and present a visual
challenge, especially if a person is not accustomed to their appearance
or if a person
has bifocals.
- Economic infestations may occur from mid vegetative stages
through podfill. This long scouting window and the re-distribution of
aphids among fields
as the summer progresses makes repeated visits a necessity.
- Aphids distribution
on the plant changes during the course of the summer. While it would
be nice to sample specific leaves, there is no stable sampling unit
except the entire plant.
What to look for on soybean
Aphids and their signs on soybean
- Aphids – Search the underside of new leaves on main
stem and branches. Aphids on the stems indicate a heavy infestation that
probably exceeds threshold.
- Cast skins – Soybean aphids go through
4 molts before reaching the adult stage. The shed skins litter lower
leaves and the stem like
dandruff
in heavy infestations.
- Honeydew – Aphids defecate excessive plant
sap that drips on lower leaves. These drips glisten in the sun. Walking
through heavy infestations
can be a sticky experience from the honeydew. Note: This honeydew will
dry to give clothes a sticky, starch-like stiffening.
- Sooty mold – A
grey mold may eventually colonize the honeydew turning the surface of
the leaf dark grey. In severe infestations, the field
takes on an unmistakable grey cast.
Symptoms on soybean
Click for larger image
Click for larger image
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- Stunting – When heavy infestations occur during vegetative
growth, plants may become stunted. In severe cases, height and the
number of nodes on the stem may be significantly reduced.
- Lack of canopy closure – As with stunting, leaf development
and expansion can be retarded or drastically altered by heavy infestations.
- Distorted leaves – Leaf expansion may be affected, especially
during drought, to produce cupped leaves, curled edges, or shortened
veins that may be mistaken for herbicide injury.
- Potassium deficiency-like chlorosis – Upper leaves may exhibit
a yellowing along leaf edges and in between veins that’s reminiscent
of potassium deficiency symptoms on lower leaves. This situation occurs
where soils are already low in potassium and it’s thought that
aphid feeding robs new growth of potassium. Not a very reliable symptom
with other causes.
|
Indicators that soybean aphids may be present on a plant
Click for larger image
Ants
tend aphids for their honeydew. They move aphids between plants and protect
them from ladybugs. |
Click for larger image
Adult ladybugs, which feed on aphids, can be a conspicuous sign of early infestations. |
Colonization of Soybean Fields: 2001 vs 2002
Soybean Aphid Shift Within the Plant
New Ulm - 2002
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