Insects and Pests of Field Crops: Soybean Insects and Pests

 

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Pub 811: Agronomy Guide> Insects and Pests of Field Crops > Soybean Insects and Pests


Order OMAFRA Publication 811: Agronomy Guide for Field Crops

 

Table of Contents


Introduction

This section describes insects and pests that affect only soybeans. The insects and pests listed below affect soybeans as well as other crops

Japanese beetle
June beetle
Seedcorn maggot
Slugs
Tarnished plant bug

Table 13-2, Soybean Insect Symptoms in the Field, shows insects and pests that could be causing the symptoms in the field.

Table 13-2. Soybean Insect Symptoms in the Field
  Insects and Pests
Symptom
Grubs (June or
Japanese Beetles)
Seedcorn
Maggot
Slugs
Bean Leaf Beetle
Gaps in the stand  
Roots pruned, plants wilting, stunted or absent
x
 
 
 
Tunnelling into cotyledon, embryo or hypocotyl
 
x
x
 
Holes in leaves 
Round holes in leaf
 
 
 
x
Leaves are skeletonized
 
 
x
 
Leaves distorted or discoloured  
Small white stipples on upper surface of leaves, leaves turning yellow
 
 
 
 
Leaves puckering or plant stunting (leaves may be sticky)
 
 
 
 
Pod injury 
Pods have holes, feeding scars or are clipped off
 
 
 
x
Pod is pierced or kinked and seed is dimpled or blemished
 
 
 
 

 

Table 13-2. Soybean Insect Symptoms in the Field
  Insects and Pests
Symptom
Soybean
Aphid
Japanese
Beetle Adults
Two-Spotted Spider Mite Stink Bug
Tarnshied Plant Bug
Gaps in the stand 
Roots pruned, plants wilting, stunted or absent
 
 
 
 
 
Tunnelling into cotyledon, embryo or hypocotyl
 
 
 
 
 
Holes in leaves 
Round holes in leaf
 
x
 
 
 
Leaves are skeletonized
 
x
 
 
 
Leaves distorted or discoloured 
Small white stipples on upper surface of leaves, leaves turning yellow
 
 
x
 
 
Leaves puckering or plant stunting (leaves may be sticky)
x
 
 
 
 
Pod injury 
Pods have holes, feeding scars or are clipped off
 
 
 
   
Pod is pierced or kinked and seed is dimpled or blemished      
x
x


Soybean Aphid (Aphis glycines)

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.

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:

  • There are several natural enemies, including the lady beetles (ladybugs) Plate 83 and Plate 84, minute pirate bug, syrphid fly larvae and parasitic wasps that are helpful in controlling this pest. A pathogen can also infect the aphids but requires warm, moist conditions to become established.
  • When soybean aphid populations are not actively increasing above 250 aphids per plant, natural enemies are keeping up with the aphid population. Do not use an insecticide in this case, as it will kill the natural enemies and enable the aphid population to increase above threshold levels.
  • Before applying an insecticide to control aphids, scout for spider mites to ensure that populations are not present. If they are, select the appropriate insecticide that will kill the mites and the aphids, so that the mite population is also controlled and will not flare up shortly after application.

Plate 83. Ladybird beetle larvae are a natural enemy of the soybean aphid.

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.

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.

 

Two-Spotted Spider Mite (Tetranychus uricae)

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.

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.

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:

  • If mite numbers exceed the action threshold, an insecticide may be necessary.
  • Use border sprays to keep early infestations under control. This will help prevent the spread of mites to other parts of the field and may reduce the need for further treatment.
  • If rain is in the forecast, delay spraying. Prolonged wetness will usually reduce the number of mites to insignificant levels.
  • Use of drought-tolerant varieties will minimize the effect of spider mites. Natural enemies help keep mites at low levels when conditions are unfavourable for the mites. Natural enemies of mites include ladybird beetles, thrips and predaceous mites. Cool temperatures and high humidity can promote the development of a pathogen that can provide natural control.

Defoliating Insects

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
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

 

Bean Leaf Beetle (Certoma trifurcata)

dry edible beans, soybeans

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.

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.

Table 13-3. Standard Damage Thresholds for Soybean Insect Defoliation
Soybean Development
% Defoliation
Pre-bloom (i.e., vegetative stages)
30%
Bloom (R1) to pod-fill (R4)
15%
Pod-fill to maturity (R5-R6)
(unless pod feeding observed)
25%

 

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.

Scouting Techniques:

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.

Damage Thresholds:

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.

Pod-Piercing Insects

Green Stink Bug(Nezara viridula)

Brown Stink Bug (Euschistus servus)

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.

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:

  • Apply foliar insecticide if thresholds are reached.
  • Be aware of the preharvest intervals for products.
  • Some natural enemies parasitize or feed on stink bug eggs.

 


For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca
Author: OMAFRA Staff
Creation Date: 30 April 2009
Last Reviewed: 30 April 2009