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Insect Management
Soybean Insect Management
Root and Stem Feeders
Stem, root and seedling feeders, which are often the hardest to sample and are not detected until after they have caused damage.
 Threecornered Alfalfa Hopper - The
threecornered alfalfa hopper adult has a distinct triangular shape, from
which it derives its name, and is bright green in color. Adults are
about one-fourth inch in length. Nymphs have 12 pairs of spines along
the top of the body. First instar nymphs are extremely small and
translucent. As the nymphs grow, they become green like the adult
although the later instar nymphs can also be brown in color.
Threecornered alfalfa hoppers overwinter as adults in a reproductive
diapause state. That is, they are active on warm days but do not
reproduce. They are often found in the winter beneath pine trees where
they reside under plant debris in field although the damage occurred
several weeks of unfavorable weather and move up into pine trees and
feed on warm days. In the early spring they leave pine and move into
alternate hosts such as clovers, vetch, dock, wild geraniums and other
hosts for the first generation. As the first generation of nymphs
becomes adults, they move into soybeans in May and June. Usually two to
three generations occur in soybeans.
Damage to soybeans caused by threecornered alfalfa hoppers is caused
by their unique feeding behavior. Nymphs and adults are phloem feeders
and will often feed in a circular pattern around a stem or petiole
resulting in girdling. This girdling disrupts the vascular flow. Plants
girdled on the mainstem near the soil surface may die or may survive the
damage only to break over later in the season due to high winds and/or
rain. When the latter occurs, growers are often prone to treat the field
although the damage occurred several weeks before. Plants are
susceptible to the hopper feeding that causes this type of damage until
they are about 10 inches tall. Scouting for this damage should be done
from emergence until plants are 10 inches tall. During this period,
treatment should be considered if 50 percent of the plants are girdled
or if less than four to six ungirdled plants per row foot remain in
conventional rows (30- to 38-inch rows) and hoppers are present. Later
as the plant develops, the hoppers will move up into the canopy and
cause girdling on vegetative branches and petioles. However, this damage
does not appear to cause economic damage in Arkansas. Studies in
Louisiana have shown economic yield losses when these pests girdle stems
attaching blooms and pods. The grape colaspis is considered to be a minor pest
of soybeans. Larvae and adults are common in soybean fields throughout the
state but rarely at economic levels. In recent years, however, growers in
Lee, Monroe, St. Francis and Woodruff counties have had a serious problem
with this pest.
The adult is a small light brown-colored beetle about one-fourth inch
long. The adult is a foliage feeder but has not been known to cause
economic injury. In contrast, the soil-dwelling larval stage can cause
problems. The larvae or grubs are white to tan in color with a brown head
capsule and cervical shield. Grubs can be up to about one-third inch in
length. These grubs feed on the roots and underground stem portions of the
plant. This feeding can cause stunting or even loss of stand when
populations are high. The damage remains unnoticed in many cases until
after larvae have finished development. Also, the damage is often mistaken
for soybean cyst nematode damage. In severe cases, reports of growers
losing three stands of soybean plants in one field in a season have been
observed. At the present time there are no effective insecticides for
control.
 Cutworms - Several species of cutworms are found in Arkansas.
Occasionally, cutworms can destroy a stand of soybeans. Damage is
generally spotty in the field. Infestations most often occur during early
wet seasons in conjunction with heavy vegetative cover. With the increase
in reduced- and no-till throughout the state, this pest has the potential
to become more of a problem. In reduced- and no-till situations, it is
important to rid the field of grass and weeds three to five weeks prior to
planting to help reduce the chances of a problem.
Cutworm feeding is easily detected by walking and looking for seedling
plants which have been damaged or cut off just above or below the soil
surface. Usually a small hole will be seen around the damaged plant. This
is where the larva resides during daylight hours. Often the larva will cut
the plant and drag the top part of the plant back to its lair. If you
suspect cutworms, dig up the soil around the plants to find if cutworms
are still present. Spot treatments should be made when 30 percent or more
of the plants are damaged or if plant stand counts indicate less than four
to six plants per row foot (30- to 38-inch rows) and larvae are present.
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