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Soybeans – Insects

Insects of concern in soybeans include below-ground feeders, sap feeders, as well as defoliators. Crops should be scouted throughout the growing season to monitor for damage, and control measures only taken if economic thresholds are reached, to help protect beneficial insects present in the field.

 

Resources

Cutworms

Cutworm, Photo Credit: Canola Council of Canada

DAMAGE
  • Cutworms are the larvae of several different types of moths and they overwinter in the soil as pupae.
  • The first generation of cutworm larvae is the most damaging. Most sever young plants from roots near the soil line as they feed at the base of the leaves; others feed on the roots and underground stems of cut plants. One larva can kill many plants until it pupates in the soil.
  • Below-ground feeding cutworms (pale western and red-backed cut worms) cut plants off at-or-near the soil surface.
  • Foliar feeding cutworms (dingy and army cutworms) feed above-ground, consuming the plant foliage.
  • Soybeans do not have growing points below-ground and therefore cannot regrow after being cut off by cutworms.

LIFE CYCLE
  • Cutworms are an early-season insect pest.
  • Several species of cutworms can be present in pulse crops and their lifecycles can differ. Cutworms may take up to 65 to 87 days to complete all larval stages. However, damage may not be inflicted during this entire period.
  • Moths move up from the southern U.S. each spring; females lay eggs on weed and grass hosts in fields or margins. 
  • The mature larvae are up to 46 mm long, smooth and ranges from purplish to brown in colour. Patterned with grey lines and spots
  • Adults are 20 mm long body with long, narrow, usually dark forewings which are pale near the tips with three black dashes on each forewing. Wing span varies from 38-50 mm.
  • There are one or two larval generations per year before populations die off in the fall.

CONTROL
  • Crops should be scouted regularly (weekly) in early spring to June.
  • Monitor germinating crop for expanding thinned or bare areas.
  • Look for areas with missing plants and wilted, notched, gouged or shot-holed plant tissue. Individual cutworms tend to feed within the seed row, leaving a gap when the affected plants are clipped off or die.
  • Scouting for cutworms is often more successful in the evening when they come out to feed. Cutworms are nocturnal and spend the day hiding under debris or in the soil.
  • Dig around in suspect areas with a trowel or a shovel. Sift soil or scoop soil into a container and shake loose any potential larvae.
  • Cutworms may be found anywhere from just below the soil surface to four inches deep. They move deeper into the soil during the heat of the day, so it may be necessary to dig deeper or revisit the field under cooler conditions.
  • Pheromone traps are available to detect emergence of adults. Monitor for expanding thinned or bare areas, especially sandy ridges and knolls. At a minimum ten sites, mark off 0.1m2 area (1 ft2) and examine the top 2-3 cm of soil along the edges of the affected areas for larvae. Depending on the crop, other species of cutworms may be present.
  • Apply necessary foliar treatments in the evening just before larvae emerge from the soil to feed. Only infested areas need to be treated.
  • Natural predators: ground beetles will attack exposed larvae on soil surface; similar to other cutworms and armyworms, eggs and larvae are attacked by insect parasitoids and predators.

ECONOMIC THRESHOLD
  • The economic threshold for cutworms of two to three larvae/m2 in top 7 cm of soil.

REFERENCE

Grasshopper

Grasshopper, Photo Credit: Shelley Barkley

DAMAGE
  • Grasshoppers feed on the foliage of almost any crop, however soybeans are not a preferred food source.
  • Growers may notice grasshoppers moving into fields are actually feeding on weeds, rather than soybeans, however, there may be some feeding on soybeans, and field edges may see higher infestations as grasshoppers move in from field borders.
  • Up to 40% defoliation in vegetative stages of soybeans could result in a yield reduction of 3 – 7%, similarly at pod set and seed fill stages, 20% defoliation could cause similar losses.
  • Growers should watch to ensure grasshoppers are not feeding on pods, as action may need to be taken sooner if pods are being affected.
  • Feeding on early developing pods can result in yield loss and cause delay in maturity as the plant tries to compensate for the lost biomass.

LIFE CYCLE
  • Overwinter as eggs in pods (8-150 eggs/pod) laid in soil and hatch the following spring when the temperature reaches 4.5°C.
  • One generation per year.

CONTROL
  • To monitor, start from a corner of the field, sample at least 20 sites along a line to the field centre, then to one side. Count the number of nymphs that jump in a 1 ft2 area as you approach each site (e.g. every 100 steps). Divide the total number of grasshoppers counted by two for number/m2.
  • Check field margins for grasshoppers moving in from roadsides and headland. Numbers will be higher in field margins and a thick crop will deter the insects from moving further into the field as they prefer more open and bare areas. Also check around wet areas in drought seasons.
  • If control is needed, the optimal timing is when nymphs are at the third instar, which is usually about mid-June. At this stage the grasshoppers become mobile, consume more, and most of the hatch should be complete.
  • Target younger instars in order to use the lowest recommended rates and to reduce the area requiring treatment. 
  • Once grasshoppers reach adult stage, insecticides are much less effective in protecting crops
  • Natural predators include birds, small rodents, coyotes, parasitic and predatory insects, as well as the pathogenic fungus Entomophthora grylli Fresenius, and the microsporidian parasite Nosema locustae Canning.

ECONOMIC THRESHOLD
  • There are no economic thresholds established for grasshopper in soybeans, but there is information on effects of defoliation on soybean yield. Generally, soybeans can tolerate quite a bit of defoliation, and over 25% leaf defoliation would need to be reached to consider control of grasshoppers.

REFERENCE

Green Cloverworm

Green Gloverworm, Photo Credit: Clemson University Bugwood.org

DAMAGE
  • Green cloverworms are only an occasional pest of soybeans.
  • Adults feed on nectar from flowers, while larvae feed on leaves, occasionally defoliating plants. They also attack seed pods.
  • Young worms scrape leaf tissue, creating a transparent skin, or window on the leaf surface. Older cloverworms eat holes in the leaves.
  • Damage can be confused with grasshopper defoliation, or damage from other worms.

LIFE CYCLE
  • Adults migrate up from the southern U.S. and females lay eggs singly or in clusters on available host plants on arrival.
  • Larvae pupate in the soil.
  • Mature larvae are 14 mm long, pale green caterpillars with a white stripe down each side and three paler white lines down the back. They walking in a looping motion, and thrash violently when disturbed.
  • Adults are 14 mm long moths that form a triangle when at rest; forewings charcoal gray with patches of brown and silver; wing span of 25 – 35 mm.  Head appears to have a snout.

CONTROL
  • Shaking plants over a sheet to dislodge worms can help identify what is present. While they may be confused with an alfalfa looper because they loop when they walk, they are different in that they will wiggle vigorously if disturbed.
  • Economic infestations are uncommon due to the number of natural predators including several species of parasites, predatory insects, and final pathogens that attack eggs and larvae.
  • Harvesting will kill or remove many eggs, larvae, and pupae present at the time.

ECONOMIC THRESHOLD
  • Use an economic injury level of 22.5 green cloverworms per metre of soybean row under normal to above-normal precipitation conditions.  During drought, when canopy is seriously impaired, use an economic injury level of 10 green cloverworms per metre of soybean row.

REFERENCE

Seedcorn Maggot

Seedcorn Maggot, University of Kentucky

Seedcorn Maggot Adult, Photo Credit: Pest and Disease Library, Bugwood.org

DAMAGE
  • The adult seedcorn maggot looks similar to a common housefly and will emerge from eggs in the soil in late May. They are 6 mm long, gray flies, wings overlap when at rest. Larvae is white, tough skinned, and 8 mm long.
  • They will burrow into both the seed and the stem of developing dry bean plants, resulting in thinned stands and wilted plants.
  • Damage is worse when germination and emergence is delayed, so ensuring planting as early as possible into a warm, shallow seedbed encourages fast emergence and will help limit damage.

LIFE CYCLE
  • Overwinters as reddish brown pupae in the soil of harvested host crops.
  • The adult then lays eggs in moist cracks in the soil that hatch after two to four days when temperatures are 10°C.
  • Females lay their average of 250–300 eggs in batches of 25–30 in freshly-tilled soils high in moisture and organic matter. Will also lay eggs on hosts infested with other root maggots.
  • Usually one generation but two overlapping generations are possible during warmer or extended summers.

CONTROL
  • Scout for the presence of all adult root maggots starting in mid to late May using yellow sticky cards or sweep nets.
  • Sprays are not effective and seed treatments are the only control registered to protect against the seedcorn maggot.
  • Seed treatments are registered for beans, peas, soybeans, and corn.  
  • Several species of predators and parasitoids attack all root maggot life stages and provide the greatest protection against root maggot depredation. Ground beetles and rove beetles feed on eggs, larvae and puparia; nematodes attack the larvae. Some fungal and bacterial pathogens also provide some control.
  • Rotate host crop with non-host crop. Increasing the seeding rate can decrease root maggot damage. Spring and/or fall tillage may reduce fly emergence by exposing puparia to predation. Depending on the crop, severely damaged crops could be ploughed under and immediately reseeded.

ECONOMIC THRESHOLD
  • None established.

REFERENCE

Soybean Aphids

Soybean aphids, Photo Credit Daren Mueller, Iowa State University, Bugwood.org

Soybean Aphid Infestation, Photo Credit Ames Herbert, Tidewater AREC, Bugwood.org

DAMAGE
  • A variety of different aphids can be present in many crops.
  • The soybean aphid is very small, 1.5 millimetres (mm) long, pale yellowed with distinct black cornicles.
  • They are most often found on the underside of the newest leaves. If found on the stem or petioles, it is usually an indication that the number of aphids on the plant is very high, as this tissue is a less desirable food source and normally aphids only migrate there if crowded.
  • Soybean aphids suck out plant juices from the plant. This can cause a reduction in vigour, growth rates, leaf puckering, and reduced pod and seed counts. All of these factors may ultimately result in yield loss.
  • The damage is increased if the plant has other stressors, such as drought. Additionally, aphid honey dew, produced as a waste product during feeding, promotes the development of grey, sooty mould on leaf surfaces, reducing photosynthetic capacity of plants.
  • This aphid is a vector for soybean mosaic virus.

LIFE CYCLE
  • Soybean aphids are not known to overwinter in Canadian prairies.
  • Blown in on warm, southerly winds from the United States (U.S.) in June, or early July.
  • Depending on weather conditions there may be from seven to 15 generations per season, and if conditions favour multiple generations, then populations may increase to levels where control is warranted. Populations can increase very rapidly because of their ability to clone themselves, so when identified, populations should be monitored closely.

CONTROL
  • Several species of predators such as green lacewing, snakefly and parasitoids as well as pathogenic fungi are capable of controlling population levels of less than 200–250 aphids/plant.
  • Insecticides are available to protect crops once the economic threshold is reached. Foliar treatments should be made within 7–10 days of reaching economic threshold.

ECONOMIC THRESHOLD
  • Economic thresholds depend on the growth stage of the crop, and the presence of other stressors.
  • Average of 250 aphids per plant, when the population is increasing, and the plants are in the R1 (beginning to bloom) to R5 (beginning seed) growth stage.
  • Soybeans can tolerate quite a bit of feeding, and some research shows that 670 aphids per plant are required to cause enough damage to cover insecticide costs. However, the trigger is set at 250 as aphid populations can increase very quickly and it allows for some application time to control them prior to economic damage being sustained.

RESOURCE

REFERENCE

Two Spotted Spider Mites

Two Spotted Spider Mites, Photo Credit: Frank Peairs, Colorado State University, Bugwood.org

DAMAGE
  • Two spotted spider mites are more closely related to spiders than to insects, and will overwinter in soil, debris, and grass.
  • They are extremely small and difficult to see on the plants, and damage is usually noticed before the mite.
  • Two spotted spider mites 0.5 mm long, eight legs, greenish, yellowish to orange oval body with two dark spots on their abdomen and eight legs. Visible to the unaided eye only as small specs. 
  • Larval mites have only three pairs of legs; nymphal stages have four pairs of legs and resemble adults, but are smaller and black spots are paler.
  • Motile stages create webbing on the undersides of leaves where they puncture cells to feed on cell contents, causing stippling, yellowing or browning of leaves. Leaves may dry and drop which can further reduce crop yields.
  • With the cells turning yellowish and eventually brown, this leaves a mottled, sandblast effect on the leaf when observed up close, which may look just like a general yellowing from farther away.
  • Infestations start at the field edge and move inwards. Extended hot, dry conditions favour rapid population build up and acerbate feeding injury.
  • Since mites often overwinter in grasses and clovers, there may be an edge type effect from the mites moving in from edges of the field.

LIFE CYCLE
  • Overwinter in protected sites as eggs, immatures or adults depending on food hosts and habitat.
  • Immatures and adults move to emerging plant hosts in the spring and, depending on temperature and moisture conditions, produce many overlapping generations before seeking overwintering sites in the fall.
  • They disperse by spinning a silk thread that’s caught by the wind.

CONTROL
  • In dry, warm springs, examine host crops weekly along the margins adjacent to grassy areas, residential areas, production greenhouses and other areas mites may have overwintered. Check for feeding injury and characteristic webbing on undersides of leaves. Also check regrowth when hot dry conditions persist.
  • Shaking leaves showing symptoms over a white piece of paper and looking for small dark specs coming off helps to identify mites. Also look carefully at the undersides of leaves for eggs, they may be noted by minute webbing covering them.
  • The use of a 10 times magnifying lens aids in identification and verification of a mite infestation.
  • Natural predators include several species of predatory mites and insects feed on all stages of the mite.
  • Minimize plant stress through improved irrigation, fertilization, and cultural practices such as timely harvest is beneficial.
  • Consider a spray if mites are numerous and plants are beginning to develop a bronzed appearance. A second spray may be required 7–10 days later to kill any mites that hatched from previously laid eggs. Border or spot sprays may be all that is needed. To avoid mite flare-ups, avoid using products against other pests that kill mite natural enemies.

ECONOMIC THRESHOLD
  • There is limited information for economic thresholds, but recommendations based on visual damage suggest an insecticide application if 25% of the leaves show discolouration prior to seed set, or 10% of the leaves if pods have set.
  • Since spider mites often move in from field edges, growers could consider the potential for spot treatment if populations appear to be severe.

REFERENCE

Thistle Caterpillar

Thistle Caterpillar, Photo Credit: Shelley Barkley

DAMAGE
  • Thistle caterpillars are sporadic pests in soybeans.
  • Caterpillars grow to be two to three cm long, and are brown and black with yellow stripes down each side. They are covered in branched spiny hairs, giving them a bristly appearance.
  • They are often found where they feed on leaves, inside a webbed area formed by cupping leaf margins together.
  • Adults will feed on nectar of flowers, while the larvae feed together in leaf-nests created near the terminals of host plants.

LIFE CYCLE
  • Adults are occasionally blown up from southern U.S in vast numbers that settle on weed hosts. Their hosts include Canada thistle, sunflowers, canola, mustard, borage, soybeans, burdocks, knapweeds, wormwood, and many other hosts.
  • Caterpillars form a goldish/brown chrysalis that hangs from the plants, and the painted lady butterfly will emerge.
  • The pupa/chrysalis is suspended by silk threads from plants.
  • Several generations of thistle caterpillars can occur in one season.
  • At least one larval generation per year; two if summer temperatures favourable.
  • Populations die off in the fall.

CONTROL
  • If the butterflies are very abundant in a canola, mustard, or sunflower crop, inspect the crop weekly until caterpillars are noticed feeding on the plants. In sunflowers, examine 10 randomly selected plants from 10 areas of the field and record the number of damaged plants. Calculate the percentage of infested plants.

ECONOMIC THRESHOLD
  • There are no established economic thresholds based on the number of caterpillars, rather a focus on the amount of defoliation that is occurring. At vegetative stages prior to flowering, defoliation should be greater than 50% to warrant control; bloom 40%, bloom-pod fill—20%, and pod fill harvest—35%.

REFERENCE

Wireworms

Wireworm, Photo Credit: Shelley Barkley

Prairie Grain Click Beetle, Photo Credit: Shelley Barkley

DAMAGE
  • Wireworms are the immature stage or larvae of click beetles.
  • Wireworms are small-segmented, tan coloured worms that live in the soil and will feed on a variety of plant hosts, including field peas, dry beans, lentils, chickpeas, and soybeans. Although wireworms prefer grassy plants (cereal crops), wireworm damage has been noted in other crops including canola, chickpeas, and potatoes.
  • Wireworms tend to be more abundant in moist soils and in lower, damper areas of a field.
  • Wireworms feed on germinating seeds and on the underground parts of stems of young seedlings, causing reduced crop emergence and thinned stands. Damage is often blamed on poor quality or dry soil, and occurs only in the spring.
  • Larvae will migrate near the germinating soil in early spring, and move back down deeper in the soil as soil temperatures climb, later in the summer.
  • Damage can be seen in the field by wilting plants, resulting from wireworms chewing on the stems, and may appear similar to cutworm damage. The distinction is that wireworm feeding often looks more like the stem is shredded, and it is usually still attached to the roots, whereas cutworm feeding cuts the stem right off. Wireworms tend to shred the plant tissue below the soil surface.
  • Wireworms can eventually cause the death of the plant.
  • Damage may not be noticed early enough in the season to reseed.

LIFE CYCLE
  • Only one generation per year, however the larval stage can last four to 11 years depending on species.
  • Both new adults and larvae overwinter. Larvae pupate in earthen cells, and the new adults remain in the cell until the following spring.
  • The mature larvae are elongated, cylindrical, wiry body, 10 – 20 mm long; last abdominal segment is flattened with a keyhole-shaped notch.
  • Adult wireworms are click beetles that do not do economic damage to crops. They will characteristically make an audible clicking sound when they try to right themselves when placed on their backs (hence common name – click beetles).

CONTROL
  • Controlling wireworm requires the use of an insecticidal seed treatment.
  • Predators of wireworms include ground beetles, rove beetles, and larvae of stiletto flies.

ECONOMIC THRESHOLD
  • None established.

REFERENCE

Other Insects

Lygus Bug
  • Because of their wide plant range, lygus bug potentially can be found in all pulse crops.
  • Lygus Bug

Adult Lygus Bug, Photo Credit: Shelley Barkley

Fifth Instar Nymph, Photo Credit: Michael Dolinski

 

Special thanks to Saskatchewan Pulse Growers and Manitoba Pulse and Soybean Growers.