Foliage Feeding Caterpillars
Green Cloverworm, Cabbage Looper, Velvetbean Caterpillar, Thistle Caterpillar, and Alfalfa Webworm
Control of these different caterpillars is normally not recommended
until greater than 30% of the foliage is destroyed prior to bloom, or
when 20% of the foliage is destroyed after bloom, pod set or fill has
been reached. This usually requires an average infestation of 4 to 8
larvae per row foot.
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Grasshoppers
In the northern plains, grasshopper egg hatch normally begins
in late April to early May. Most grasshoppers emerge from eggs
deposited in uncultivated ground. Soybean growers should expect to find
grasshoppers feeding first along bean field margins adjacent to
non-crop sites where the nymphs are hatching. Later infestations may
develop when grasshopper adults migrate from harvested small grain
fields. Grasshoppers will feed upon leaves and pods, chewing holes in
them. A result of these migrations is soybean fields becoming sites for
significant egg laying.
Many of the grasshopper infestations in soybeans will be the
heaviest on the field margins. Treating these areas may lessen the
total numbers of grasshoppers successfully entering a field.
Grasshopper control is advised whenever 50 or more small
nymphs per square yard can be found in adjacent, non-crop areas, or
when 30 or more nymphs per square yard can be found within the field.
When 20 or more adults per square yard are found in field margins or 8
to 14 adults per square yard are occurring in the crop, treatment would
be justified.
Border Treatment In most years, treating either the crop margin
or the border area surrounding the crop is adequate for control. A
border treatment of 150 feet beyond the crop edge should be adequate in
most situations, depending on the size of the grasshopper source area,
but season long control may require up to a 1/4 mile border treatment
when the population source is large. Under extreme pressure, control
may be difficult and multiple border treatments may be required. Using
insecticides with the longest residual activity would be most
effective. The residual activity of the treatments will vary with the
chemical and environmental conditions. It is important to monitor the
border areas and crop margins after treatment to make sure grasshoppers
do not re-enter the field. See more information on our row crop grasshopper page.
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Bean Leaf Beetle
This beetle emerges from overwintering, moving into bean fields as the
seedlings emerge. The white larvae develop in the soil, feeding on the
roots and nodules. New adults emerging in August feed on foliage and
pods. Feeding injury to leaves appears as small round holes between the
leaf veins. Injury to pods appears as lesions similar in size and shape
to leaf feeding holes. The injury to pods results in secondary
infections by fungi and bacteria, causing rotting and discoloration.
Treatment would be recommended when 3 to 7 beetles per sweep are found.
Soybean Aphid
Aphids suck fluid from plants. When infestations are large,
infested leaves are wilted or curled. The aphids excrete honeydew, a
sweet substance that accumulates on surfaces of lower leaves and
promotes the growth of sooty mold. This aphid colonizes tender leaves
and branches from seedling to blooming. Later, as the growing point
slows, the aphids slow their reproductive rate, move down to the middle
and lower part of the plant, and feed on the undersides of leaves.
Towards the end of the season the colonies begin to rapidly increase in
number, again. These increases are followed by a migration to the
overwintering, alternate host, buckthorn.
The critical growth stage for making most soybean aphid treatment
decisions appears to be the late vegetative to early reproductive
stages (Vn to R3). Assessing aphid populations at this time is
critical. Conclusions from 2001- 2003 management programs found that
the best results from an aphid treatment occurred from mid-July to
early-August. Treatment to manage soybean aphid would be recommended at
growth stages R1 to R4 when aphids are abundant on most plants
(guideline: aphids number 25 or more per sampled leaflet OR 250 total
aphids per plant).
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Seedcorn Maggot
Seed corn maggots
attack soybean seed, preventing sprouting or weakening the seedlings.
The adult flies emerge in spring when soil temperatures reach 50o. When
cool, wet conditions occur during planting, the slow emergence of the
seedling extends the period of time it is vulnerable to feeding by the
maggot.
Spider Mites
Feeding damage by
mites in soybeans first appears as small yellow spots ("stipples"). As
feeding activity increases, leaves become yellow, bronzed, brown, and
eventually shed from the plant. Mites usually become a problem when
hot, dry weather occurs. These environmental conditions stress the
plant, whether mites are present or not. If conditions continue,
treating for mites is no guarantee plants will recover.
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