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Insects and Pests of Field Crops: Soybean Insects and Pests
| Corn |
Soybeans | Forages
| Cereals | Dry
Edible Beans | Pub 811: Agronomy Guide> Insects and Pests of Field Crops > Soybean Insects and Pests Excerpt from Agronomy Guide for Field Crops
Table of Contents
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Symptom
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Soybean Aphid |
Japanese Beetle Adults |
Two-Spotted Spider Mite | Stink Bug | Tarnshied Plant Bug |
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Gaps in the stand
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| Roots pruned, plants wilting, stunted or absent |
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| Tunnelling into cotyledon, embryo or hypocotyl |
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Holes in leaves
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| Round holes in leaf |
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| Leaves are skeletonized |
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Leaves distorted or discoloured
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| Small white stipples on upper surface of leaves, leaves turning yellow |
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| Leaves puckering or plant stunting (leaves may be sticky) |
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Pod injury
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| Pods have holes, feeding scars or are clipped off |
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| Pod is pierced or kinked and seed is dimpled or blemished |
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Description: The soybean aphid is a small (pinhead-size), pale yellow aphid with black cornicles ("tailpipes") and a pale yellow tail Plate 82 . Adults may be winged or wingless. Nymphs are smaller than the adults and are wingless. Eggs on buckthorn are small, football-shaped and yellow when first laid but turn a dark brown similar to the colour of the buckthorn branch. Eggs are usually laid along the seams of the buckthorn bud.
Plate 82. Soybean aphids are small, pale green to yellow and may be winged or wingless. They can be found on the underside of leaves and stems.
Life History: The soybean aphid, a pest originally from
Asia, was first discovered in North America in 2000 and in Ontario in
2001. This insect has two hosts that it requires to complete its life
cycle. The soybean aphid survives as eggs on the twigs of buckthorn species.
In the spring, nymphs hatch from these eggs, and the aphids undergo two
generations as wingless females on the buckthorn. The third generation
develops into winged adults that migrate to soybean plants. The aphids
then continue to produce wingless generations until the soybean plants
become crowded with aphids and the plants experience a reduction in quality.
Once crowded, winged forms are produced to disperse to less-crowded soybean
plants. There can be as many as 18 generations of aphids per year on soybeans.
Like most aphids, the soybean aphids are all female, born pregnant and
give birth to live nymphs. Males are only born in the fall so that the
females and males can mate to produce the egg on buckthorn.
Damage: Aphids have piercing-sucking mouthparts that suck juices and nutrients from the plant. Lower populations of aphids can live and feed on soybeans without causing yield loss. Once populations reach threshold levels, especially in dry years when the plants are stressed, aphids can cause the plants to abort flowers, become stunted, reducing pod and seed production and quality. Yield loss by soybean aphid is greatest when soybeans are in the early R stages (R1-R2), when flowers can abort and impact pod establishment. Peak infestations during the pod fill stage (R3) and beyond can result in smaller seed size and a reduction in seed quality. Aphids also excrete a sticky substance called honeydew, which can act as a substrate for grey sooty mould development. This insect may also be a vector of soybean mosaic virus.
Scouting Technique: Early-season aphid infestations
tend to concentrate on the newly emerging leaves and upper trifoliates
of the plant. Later in the season, once into the reproductive stages of
soybeans, the aphids tend to migrate down to the middle or lower canopy,
possibly due to heat and predator abundance experienced at the top of
the canopy. Because of this movement within the canopy through the season,
taking full plant counts is still the best method to estimate the number
of aphids per plant and relate that to the threshold.
Scout each field every 7-10 days from early June until early September
or until the crop is well into the R6 stage of soybeans. Scout fields
more frequently (every 3-4 days) as aphid populations approach the threshold.
Look at 20-30 random plants across the field. Avoid field edges. Estimate
the number of aphids per plant in that field. A minimum of two field visits
is required to confirm that aphid populations are increasing.
Action Threshold: The threshold for soybean aphids is
250 aphids per plant and actively increasing on 80% of the plants from
the R1 up to and including the R5 stage of soybeans. This threshold gives
an approximate 7-10-day lead time before the aphids would reach the economic
injury level, where cost of control is equal to yield loss. When soybean
aphid populations are not actively increasing above 250 aphids per plant,
natural enemies are keeping up with the aphid population. More aphids
per plant are needed once soybeans are in the R6 stage. Beyond the R6
stage, economic return from any insecticide application is not likely.
Soybean aphid colonies typically start on the underside of the leaves.
Once populations begin to increase on the plants, aphids can then be found
on the stems and pods of the plant. This is usually a good indication
that aphids have reached threshold.
Management Strategy:
Plate 83. Ladybird beetle larvae are a natural enemy of the soybean aphid.
Plate 84. Black mummies (right) are the result of the aphids being parasitized by parasitic wasps (inset). These are just one of the many natural enemies of soybean aphid.
Description: The adult mite is barely visible to the naked eye, roughly 0.5-1.0 mm (1/25 in.) in length, rounded, eight-legged and yellowish-brown with two dark spots on the sides of the abdomen Plate 85. The larvae look like the nymphs and adults but have six legs instead of eight. Nymphs look similar to the adults, with eight legs. Over-wintering females are orange/red.
Plate 85. Two-spotted spider mites are barely visible to the naked eye. Adults are eight-legged, yellowish-brown, with two dark spots.
Life History: Spider mites generally over-winter as
adult females in sheltered areas, such as plant debris and field margins.
Harvested wheat fields underseeded to red clover are another important
over-wintering site. Red clover provides food for mites until freeze-up,
allowing the mites to survive in the field. In late-April, as the weather
turns warm, mites become active in search of food and egg-laying sites.
Spider mites disperse by crawling, so infestations tend to spread slowly
from field edges. Non-mated female mites will mass at the top of the plants
and spin webs that serve as a "balloon," allowing strong winds
to pick them up and carry them off to another site. Spider mite females
can reproduce without mating. A single unmated female can be the start
of a new colony. Under hot, dry, windy conditions, infestations can spread
very quickly. There can be up to seven generations per year, with generation
development overlapping. Frequent rain and cool weather reduce mite populations
in soybeans.
Damage: Mites feed on individual plant cell contents
on the underside of leaves through stylet-like mouthparts. Each feeding
site causes a stipple. Severe stippling causes yellowing, curling and
bronzing of the leaves Plate 86. Eventually, the
leaf will dry up and fall off. Upon close examination, fine webbing on
lower surfaces of the foliage can be seen. Damage is more severe in hot,
dry weather and usually occurs in mid-July (after winter wheat harvest).
Spider mites usually start at the edges of the field, but windy days can
carry them in from other sites, with pockets starting up deeper into the
field. From the road, these pockets may have been confused for drought
stress. High-risk factors include neighbouring winter wheat stubble fields,
hay fields and ditch banks and fencerows that harbour over-wintering mites.
No-till fields of soybeans following winter wheat underseeded to red clover
are also at risk.
Plate 86. Spider mite infestations tend to start from the field's edge, with infested leaves turning brown and dropping off the plant.
Scouting Technique: Scout fields weekly, starting the
first week of July. Infestations tend to occur shortly after wheat harvest
and when municipalities mow road sides. Infestations usually move in from
the edge of fields as hot spots. Look for tiny white stipples on the upper
surface of leaves in the mid-canopy. Pull these leaves from the plant
and shake them onto a white piece of paper to see the actual mites moving
around. You will need a 10X hand lens to actually see the mites.
Action Threshold: Four or more mites per leaflet or
one severely damaged leaf per plant prior to pod fill indicates that control
is necessary.
Management Strategies:
Soybeans are able to compensate for large amounts of foliage loss due to insect feeding, and often little effect on yield is observed. Soybean plants not only continue to put out new leaves at the top to compensate for the feeding but leaves positioned below the feeding injury sites actually grow larger, increasing their surface area, since they are getting more sunlight through the canopy. However, the most critical stage for soybeans is bloom (R1) to pod-fill (R4), when seed development is highly dependent on photosynthesis. Should large amounts of defoliation occur throughout the plant during these stages, yield can be affected, particularly in dry years.
To estimate damage thresholds for leaf-feeding insects on soybeans, determine the percentage of defoliation occurring in each soybean field. In 10 areas of the field, pick trifoliate leaves from five plants in the middle of the canopy. Discard the least and most damaged leaflets from each trifoliate collected.
Compare the leaflets that are left with Figure
13-2, Defoliation Chart for Soybean Leaf-Feeding Insects, and determine
the average percentage of defoliation. Defoliation is often overestimated.
Most of the defoliating insects feed on the tops of the plants and field
edges first so that upon first inspection of the field, it appears that
there is a lot of defoliation. Make sure to inspect trifoliates from the
middle of the canopy to get a good assessment of defoliation.
After determining the level of defoliation in each field, use Table 13-3, Standard Damage Thresholds for Soybean Insect Defoliation, to determine whether control is necessary, depending on the stage of the crop.
Figure 13-2. Defoliation Chart for Soybean Leaf-Feeding
Insects
Source: University of Illinois and Purdue University, 1982.
Text Explanation for Figure 13-2. Defoliation Chart for Soybean Leaf-Feeding Insects
Description: The bean leaf beetle (BLB) adult is around 5 mm (1/5 in.) in length, with or without four black spots (parallelogram shaped) found on the wing covers Plate 87. Adult beetles can vary in colour but are most often yellow-green, tan or red.
Plate 87. Bean leaf beetle adults vary in colour, but always have a small black triangle visible behind the head. They may or may not have four spots.
A small, black triangle is visible at the base of the wing covers (the prothorax - behind the head). The margins of the wing covers have a black border.
The bean leaf beetle is often confused with the spotted cucumber beetle or lady beetles. A small black triangle is visible at the base of the wing covers (behind the head) of the bean leaf beetle.
Life History: There are two generations of BLB per year, not including the over-wintering population that enters the soybean crop from their over-wintering sites in early spring. The BLB overwinters in the adult stage in woodlots, leaf litter and soil debris. In late-April, the over-wintering adults become active and begin feeding on nearby alfalfa fields until the first cutting of alfalfa or soybeans emerge. Mated females then lay lemon-shaped, orange-coloured eggs in small clusters in the soil at the base of the soybean plants. Egg-laying occurs until mid-June. There is then a distinct period between the end of June to mid-July when there is little to no adult activity in the field, since most of the population is now in the egg and larval phase. Newly hatched larvae feed on roots and other underground plant parts for about 30 days before pupating. The first generation adults begin to emerge from the soil in mid-July and feed on the soybean foliage and pods. This generation lives for approximately 1 month, laying eggs that will become the second generation adults. A second generation of adults emerges mid- to late August and feeds on the pods until the plants senesce. The adults then migrate to alfalfa fields if available or move to their over-wintering sites.
Damage to Soybeans: Defoliation injury by bean leaf
beetle adults is generally not serious in Ontario. The exception is damage
caused by over-wintering adults to young soybean plants (V1-V2). Adult
feeding appears as small round holes between the major leaflet veins.
Cotyledons and seedling plants can be clipped off by heavier populations.
Late-season pod feeding is another concern. BLB feed on the surface of
the pod, leaving only a thin film of tissue to protect the seeds within
the pod. These pod lesions increase the pod's susceptibility to secondary
pod diseases such as alternaria. Pods may also be clipped off the plant,
but this is not the primary cause of yield loss. The most important concern
is that BLB is a vector of bean pod mottle virus. The virus causes the
plant and seed to become wrinkled and mottled, reducing the quality of
the seed.
Damage to Dry Edible Beans: BLB prefer soybeans, though
they can cause injury in dry edible beans, particularily in dry years
when populations are high. BLB rarely enter the edible bean crop until
mid-season. Defoliation typically does not reach threshold in Ontario.
Adult feeding appears as small round holes between the major leaflet veins.
Cotyledons and seedling plants can be clipped off by heavier populations.
Late-season pod feeding is the main concern in dry edible beans. BLB feed
on the surface of the pod, leaving only a thin film of tissue to protect
the seeds within the pod. These pod lesions increase the susceptibility
to secondary pod diseases such as alternaria. Pods may also be clipped
off the plant. However, this is not the primary cause of yield loss.
Soybean Seedling Stage: Select at least five sampling
sites from across the entire field at random. At each sampling site, slowly
walk down 4.5-6 m (15-20 ft) of row and carefully count all beetles. Do
not disturb the plants, but check closely enough that you can see the
underside of the leaves. Calculate the average number of beetles per metre
(foot) of row.
Beyond Soybean Seedling Stage: In 10 areas of the field,
pick trifoliate leaves that are fully expanded from the centre of the
plant canopy from five plants. Discard the least and most damaged leaflets
from each trifoliate collected. Determine the percent defoliation that
has occurred, using Figure 13-2, Defoliation Chart
for Soybean Leaf-Feeding Insects.
Soybean R5-R6 Stage: Thresholds have not yet been validated
for Ontario. Thresholds are based on the percent of pods with feeding
damage. Assess pods on 20 plants in five areas of the field. Avoid the
field edge. Determine the number of pods with feeding injury or clipping
and make note of the presence of adults.
Prior to the Dry Edible Bean Pod-Fill Stages: Determine
the level of defoliation taking place. In 10 areas of the field, pick
trifoliate leaves that are fully expanded from five plants in the middle
of the plant canopy. Discard the least- and most-damaged leaflets from
each trifoliate collected. Determine the percent defoliation that has
occurred, using Figure 13-2, Defoliation Chart for
Soybean Leaf-Feeding Insects, as a guide.
Dry Edible Bean Pod-Fill Stages: Thresholds are based
on the percent of pods with feeding damage. Assess pods on 20 plants in
five areas of the field. Avoid the field edge. Determine the number of
pods with feeding injury or clipping and make note of the presence of
adults.
Soybean Seedling Stage (VC-V2): Thresholds for bean
leaf beetle are 16 adult beetles per foot of row in early seedling stages.
If plants are being clipped off, take action.
Soybean V3-R4 Stage: If the defoliation exceeds the thresholds stated in Table 13-3, Standard Damage Thresholds for Soybean Insect Defoliation a rescue treatment may be warranted.
Soybean R5-R6 Stage of IP, Food Grade and Seed Fields:If
10% of the pods on the plants have feeding injury and
the beetles are still active in the field, a spray is warranted. However,
remember to consider days-to-harvest intervals.
Japanese beetles can also defoliate soybeans. See Japanese Beetles. For other foliar feeding insects in soybeans, including redheaded flea beetle, corn rootworm adults, grasshoppers, thistle caterpillars and others, follow the same scouting techniques discussed under Defoliating Insects and Table 13-3, Standard Damage Thresholds for Soybean Insect Defoliation.
Action Thresholds for Dry Edible Beans: Substantial yield loss does not take place until up to 35% defoliation occurs before bloom and 15% after bloom. Thresholds during pod fill have not been validated in Ontario yet. However, with higher value and stringent quality standards in dry edible beans, if 5%-8% of the pods inspected have feeding scars, control may be necessary. Ensure that adults are still presently active in the field before a spray is applied.
Management Strategies for Soybeans: Plant fields with
a history of early-season bean leaf beetle activity and fields planted
earliest in the area with insecticide-treated seed. Before applying a
foliar insecticide, determine the level of soybean aphid and or spider
mite pressure in the field. Certain insecticides can have more impact
on the natural enemies than on intended pests and can cause aphid or spider
mite populations to flare up.
Management Strategies for Dry Edible Beans: Treat the
seeds planted in fields with a history of early-season bean leaf beetle
activity or fields planted earliest in the area with insecticide seed
treatments. Use foliar insecticides when defoliation thresholds have been
reached.
Description: There are two types of stink bugs that
can injure beans: Southern green stink bugs and brown stink bugs. The
Southern green stink bug adults are large (about 1.8 cm (3/4 in.) long),
light-green, shield-shaped bugs with fully developed wings. Brown stink
bugs are smaller than the green stink bug, approximately 1 cm (1/3 in.)
in length, and are a mottled brown-grey in colour. Adult stink bugs are
shaped like a shield Plate 88 . The nymphs (juveniles)
can look very different from their adult stage, having very short, stubby
wing pads, and are often a different colour than the adults. In particular,
green stink bug nymphs have a flashy display of black, green, orange and
yellow. Eggs are laid in tight clusters, are yellowish white and barrel-shaped.
Plate 88. Green stink bug adult. Stink bugs pierce the pods and suck on the seed, causing the seed to shrivel and dimple.
Life History: Southern green stink bugs do not over-winter in Ontario but are blown up from the southern U.S. by mid-summer. Brown stink bugs can over-winter in Ontario and may be present in other crops in early summer. Stink bugs are first found in soybean fields during August and early September.
The brown stink bug adult should not be confused with the spined soldier bug, which is a beneficial and feeds on caterpillars and other insect pests. To tell these two apart, look at their feeding beak or needle-like mouthpart. The beak of the brown stink bug is slender to pierce through delicate plant tissue. The beak of the spined soldier bug is thicker so it can harpoon into its insect prey. The soldier bug adult also has more pointed ("spined") shoulders than the brown stink bug, though this may be hard to notice unless you have them side by side to compare.
Damage: Both adults and nymphs have piercing and sucking
mouthparts for removing plant fluids. Stink bugs feed directly on pods
and seeds. However, their injury is difficult to assess because their
mouthparts leave no obvious feeding scars on the outside of the pod. Instead,
they inject digestive enzymes into seeds, causing the seed to dimple or
shrivel. The feeding wound provides an avenue for diseases to gain entry
into the pod. Seed quality is reduced. Indirect effects can include delayed
maturity (green bean syndrome) of injured plants, though stink bugs are
not the only cause for green bean syndrome.
Scouting Technique: Use the drop-cloth technique in
row plantings, and the sweep-net technique for narrow row and drilled
beans.
The drop-cloth method involves using a 90 cm (36 in.) long piece of white
cloth positioned on the ground between two rows of soybeans. Vigorously
shake the plants over the cloth in each of the two rows. Count the number
of adults and nymphs and divide the number by 6 to obtain the average
number of stink bugs in a 30-cm (1-ft.) row. Repeat this in at least four
more areas of the field. Be careful not to disturb the plants prior to
shaking them on the cloth.
Using a 38-cm (15-in.) diameter sweep net, take 20 sweep samples (in
a 180°-arc sweep) in five areas of the field. Determine the average
number of adults and nymphs per sweep by dividing the total count by 100.
Action Threshold: Control may be warranted in IP food
grade and seed soybeans if an average of one stink bug per 30 cm (1 ft)
of row or 0.2 bugs per sweep is found during the late R5-R6 stages.
Management Strategies:
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