Insect pest management in soybeans

Soybean variety trial

Soybean variety trial
Photo: H Brier

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Major pests

Minor pests

Soybeans can be attacked by pests at any stage from seedlings to close to harvest, but are most attractive from flowering onwards. It is important to note that soybeans are very tolerant of insect damage at many stages of crop development, and that noticeable damage (particularly leaf damage) does not necessarily translate to yield loss.

Soybeans can tolerate up to 33% leaf loss (providing terminal and auxiliary buds are not attacked) without yield loss but their ability to compensate for pest damage decreases as pods develop. Soybeans set a large number of reserve pods and can compensate for insect damage during early podding by diverting energy to fill these reserve pods. If developing seeds are damaged the plant diverts more energy to undamaged seeds, making these bigger and heavier.

Seeds damaged by pod-sucking bugs during early pod-fill are often lost at harvest, or are graded out post harvest, as they are lighter than undamaged seeds. Seeds damaged from mid pod-fill onwards are similar in weight to undamaged seeds, not lost at harvest or able to be graded out without resorting to colour sorters.

Crops remain susceptible to late bug damage until the pods harden just prior to harvest. As a result, late bug damage is a major factor affecting seed quality. As a rule of thumb, only 2% seed damage is tolerable for soybeans targeting the culinary market.

Major pests of soybeans

Helicoverpa

Helicoverpa armigera, Helicoverpa punctigera

Helicoverpa can severely damage all crop stages and all plant parts of soybeans. Of the summer legumes, soybeans are the most attractive to helicoverpa during the vegetative stage and can even be damaged during the seedling stage. In sub-coastal and inland southern Queensland, summer legumes are at greatest risk from H. armigera from mid-December onwards. However, spring H. armigera outbreaks are more likely in coastal regions.

Identification

Helicoverpa larvae can be confused with loopers, armyworms or cluster caterpillars. Refer to the A-Z pest list for identification of pests.

Damage

  • Helicoverpa spp. defoliation is characterised by rounded chew marks and holes (loopers make more angular holes).
  • Helicoverpa will also attack auxiliary buds and terminals in vegetative crops. High populations in seedling or drought-stressed crops can cause considerable damage if vegetative terminals and stems are eaten. This type of damage results in pods being set closer to the ground. Such pods are more difficult to harvest.
  • In drought-stressed crops, the last soft green tissue is usually the vegetative terminals, which are thus more likely to be totally consumed than in normally growing crops.
  • Once crops reach flowering, larvae focus on buds, flowers and pods. Young larvae are more likely to feed on vegetative terminals, young leaves and flowers before attacking pods.
  • Small pods may be totally consumed by helicoverpa, but larvae target the seeds in large pods.
  • Crops are better able to compensate for early rather than late pod damage, however in dry land crops, where water is limited, significant early damage may delay or stagger podding with subsequent yield and quality losses.
  • Damage to well-developed pods results in the weather staining of uneaten seeds due to water entering the pods.

Monitoring

  • Beat sheet sampling is the preferred sampling method for medium to large helicoverpa larvae. Small larvae should be scouted for by opening vegetative terminals and flowers.
  • Inspect crops weekly during the vegetative stage - damage to vegetative terminals is often the first visual clue that helicoverpa larvae are present.
  • Soybeans should be scouted for eggs and moths to pinpoint the start of infestations and increase the chance of successful control.
  • Inspect twice weekly from early budding until late podding.
  • Sample six widely spaced locations per field. Take five one-metre samples at each site with a standard beat sheet. Convert larval counts/m to larvae/m2 by dividing counts by the row spacing in metres.

Beat sheet sampling may only detect 50% of small larvae in vegetative and podding soybeans, and 70% during flowering, as they feed in sheltered sites such as leaf terminals. Many of these small larvae will be lost to natural mortality factors before they reach a damaging size and in most crops, and this mortality will cancel out any sampling inefficiencies.

Thresholds

In vegetative crops, thresholds for many leaf feeding pests are expressed as % tolerable defoliation or % tolerable terminal loss. Before flowering, soybeans can tolerate up to 33% leaf loss without loss of yield. However recent data shows that helicoverpa populations inflicting less than 33% damage can cause serious yield loss, because the larvae not only feed on leaves, but also attack terminals and auxiliary buds. The data indicates an economic threshold of approximately 7.5 helicoverpa larvae per square metre (7.5/m2) in vegetative soybeans.

Helicoverpa thresholds for podding soybeans currently range from 1-2 larvae/m2 (depending on crop value and pesticide cost).

Table 1: Economic threshold chart for Helicoverpa spp. in podding soybeans, based on a measured yield loss of 40 kg/ha for every larva per square metre (Rogers unpublished data). Cross-reference the cost of control versus the crop value to determine the economic threshold (ET) (e.g. if the cost of control = $40/ha and the crop value =$700/t, the ET = 1.4 larvae/m2.) Spray only if the helicoverpa population exceeds the threshold.
Cost of control # = Value of damage ($/ha) Thresholds # (larvae/m2) for conventional pesticides at soybean crop values listed below ($/ha)
$450 $500 $550 $600 $650 $700 $750 $800
$20 1.1 1.0 0.91 0.83 0.77 0.71 0.67 0.63
$25 1.4 1.3 1.1 1.04 0.96 0.89 0.83 0.78
$30 1.7 1.5 1.4 1.3 1.2 1.1 1.0 0.94
$35 1.9 1.8 1.6 1.5 1.3 1.3 1.2 1.1
$40 2.2 2.0 1.8 1.7 1.5 1.4 1.3 1.3
$45 2.5 2.3 2.0 1.9 1.7 1.6 1.5 1.4
$50 2.8 2.5 2.3 2.1 1.9 1.8 1.7 1.6
$55 3.1 2.8 2.5 2.3 2.1 2.0 1.8 1.7
$60 3.3 3.0 2.7 2.5 2.3 2.1 2.0 1.9
$65 3.6 3.3 3.0 2.7 2.5 2.3 2.2 2.0

Chemical control

  • Prior to flowering, biopesticides, particularly Helicoverpa nucleopolyhedrovirus (NPV), are recommended in preference to chemical insecticides. This helps conserve beneficial insects to buffer crops against helicoverpa attack during the susceptible reproductive stages, and avoids flaring of other pests such as silverleaf whitefly and mites.
  • For best results, all ingestion type products require thorough plant coverage. For biopesticides, addition of Amino Feed® or an equivalent product is recommended.
  • For chemical control and current registrations refer to How to find the right insecticide .

Cultural control

  • Where possible, avoid successive plantings of summer legumes.
  • Good agronomy and soil moisture are crucial as large, vigorously-growing plants suffer less defoliation for a given helicoverpa population and have less risk of terminal damage.
  • In water-stressed crops, terminals are more attractive to larvae than wilted leaves. Vigorously growing plants with adequate available moisture are better able to replace damaged leaves and compensate for flower and pod damage.

Natural enemies

The number of natural enemies or beneficials varies with crop age, from crop to crop, region to region, and from season to season. The combined action of a number of beneficial species is often required to have a significant impact on potentially damaging helicoverpa populations. It is therefore desirable to conserve as many beneficials as possible.

Natural enemies of soybean pests include predators of eggs, larvae and pupae, parasites of eggs and larvae and caterpillar diseases.

Predatory bugs and beetles that attack helicoverpa eggs and larvae include:

Parasites include:

With the exception of the egg parasites and Microplitis, most parasites do not kill helicoverpa until they reach the pupal stage. Predatory earwigs and wireworm larvae are significant predators of helicoverpa pupae.

Naturally occurring caterpillar diseases frequently have a marked impact on helicoverpa in summer legumes. Outbreaks of NPV (Nucleopolyhedrovirus) are frequently observed in crops with high helicoverpa populations.

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Pod-sucking bugs

Pod-sucking bugs can move in at budding but significant damage is confined to pods. While pod-sucking bugs start breeding as soon as they move into flowering crops, nymphs must feed on pods to complete their development. Pod-sucking bugs cause shrivelled and distorted seed, and can severely reduce yield and seed quality. Pod-sucking bugs can even damage seeds in pods that are nearing harvest maturity. Late bug damage reduces seed quality but not yield. As only 2% seed damage is tolerable in culinary soybeans, bug thresholds are based on seed quality, not yield.

A number of pod-sucking bugs can attack soybeans and include:

  • Green vegetable bug
  • Redbanded shield bug
  • Large brown bean bug
  • Small brown bean bug.

The green vegetable bug (GVB) and the brown bean bugs are equally damaging to crops, while the damage potentials of the redbanded and brown shield bugs are 0.75 and 0.2 of that of a GVB respectively. Nymphs of all species are less damaging than adults. While first instar nymphs cause no damage, subsequent instars are progressively more damaging with the fifth and final instar being nearly as damaging as adults. To determine the damage potential of mixed bug species populations, convert all species (adults and nymphs) to GVB adult equivalents (GVBAEQ) .

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Green vegetable bug (GVB) (Nezara viridula)

Pest status: This species is the most damaging pod-sucking bug in soybeans due to its abundance, widespread distribution, rate of damage and rate of reproduction. Very high populations are frequently encountered in coastal Queensland.

Risk period

  • Adult bugs typically invade summer legumes at flowering, but GVB is primarily a pod feeder with a preference for pods with well-developed seeds.
  • Nymphs are unable to complete their development prior to pod-fill.
  • Soybeans remain at risk until pods are too hard to damage (i.e. very close to harvest).
  • Damaging populations are typically highest in late summer crops during late pod-fill (when nymphs have reached or are near adulthood).

Damage

  • Pods containing well-developed seeds are most at risk.
  • While GVB also damages buds and flowers, soybeans can compensate for this early damage.
  • Damage to young pods cause deformed and shrivelled seeds and reduce yield.
  • Seeds damaged in older pods are blemished and difficult to grade out, reducing harvested seed quality, particularly that destined for human consumption (edibles).

GVB can even damage seeds in ´close-to-harvest´ pods (i.e. pods that have hardened prior to harvest). Bug damaged seeds have increased protein content but a shorter storage life (due to increased rancidity). Bug damage also reduces seed oil content. Bug damaged seeds are frequently discoloured, either directly as a result of tissue breakdown, or because of diseases such as Cercospora (purple seed stain), which may gain entry where pods are pierced by bugs.

Sampling and monitoring

  • Crops should be inspected for GVB twice weekly from flowering until close to harvest.
  • Sample for GVB in early to mid-morning.
  • Beat sheet sampling is the most efficient monitoring method.
  • The standard sample unit consists of five one-metre non consecutive lengths of row within a 20 m radius.
  • Convert all bug counts per row metre to bugs/m2 by dividing counts per row metre by the row spacing in metres.
  • At least six sites should be sampled throughout a crop to accurately determine adult GVB populations.
  • GVB nymphs are more difficult to sample accurately as their distribution is extremely clumped, particularly during the early nymphal stages (1-3).
  • Ideally, at least 10 sites (with five non-consecutive row metres sampled per site) should be sampled to adequately assess nymphal populations.

Thresholds

Pod-sucking bug thresholds in edible or culinary soybeans (destined for human consumption) are determined by seed quality, the maximum bug damage permitted being only 2%. GVB thresholds typically range from 0.3-0.8/m2 depending on the crop size (seeds per m2) and when bugs first infest a crop. Because thresholds are determined by % damage, the larger a crop (the more seeds per unit area), the more bugs required to inflict critical (threshold) damage, and the higher the threshold. See table 2.

Table 2: Pod-sucking bug thresholds
Days to harvest maturity Crop size (seeds/m)
500 1000 1500 2000 2500 3000 3500 4000 4500 5000
21 0.15 0.29 0.44 0.58 0.73 0.88 1.02 1.17 1.31 1.46
28 0.11 0.22 0.33 0.44 0.55 0.66 0.77 0.88 0.98 1.09
35 0.09 0.18 0.26 0.35 0.44 0.53 0.61 0.70 0.79 0.88

For crushing and stockfeed soybeans with lesser quality requirements, the threshold is doubled

Chemical control

  • Bugs should be controlled during early pod-fill before nymphs reach a damaging size.
  • Pesticides are best applied in the early to mid-morning to contact bugs basking at the top of the canopy.
  • For chemical control and current registrations refer to How to find the right insecticide .

Cultural control

  • Avoid sequential plantings of summer legumes as bug populations will move progressively from earlier to later plantings, eventually building to very high levels.
  • Avoid cultivar and planting time combinations that are more likely to lengthen the duration of flowering and podding.

Natural enemies

  • GVB eggs are frequently parasitised by a tiny introduced wasp Trissolcus basalis . Parasitised eggs are easily recognised as they turn black.
  • GVB nymphs are attacked by ants, spiders and predatory bugs.
  • Final (fifth) instar and adult GVB are parasitised by the recently introduced tachinid fly (Trichopoda giacomellii) .

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Redbanded shield bug

The Redbanded shield bug (RBSB) (Piezodorus oceanicus) was previously classified as Piezodorus hybneri and more recently as P. grossi.

Pest status

Major, widespread, regular. RBSB is 75% as damaging as GVB in summer pulses but is usually not as abundant. However, it is more difficult to control with current pesticides. Adults are similar in shape to GVB but are smaller and paler, with pink, white or yellow bands.

Damage

Damage is similar to that caused by GVB, with early damage reducing yields, and later damage reducing the quality of harvested seeds.

Thresholds

Convert to GVB equivalents to determine damage potential.

Monitoring

As for GVB. Beat sheeting is the preferred sampling method. Look for the distinctive twin-row egg rafts which indicate the presence of RBSB.

Chemical control

  • No insecticides are specifically registered against RBSB in Australia.
  • Recent trials suggest pesticides currently registered against GVB are ineffective against RBSB.
  • Control can be improved, albeit to only 50-60%, with the addition of a 0.5% salt (NaCl) adjuvant.

Natural enemies

  • Spiders, ants, and predatory bugs are major predators of RBSB, particularly of eggs and young nymphs with mortality of these stages sometimes exceeding 90%.
  • Eggs may be parasitised by the tiny wasp, Trissolcus basalis.
  • Adults are infrequently parasitised by the recently introduced tachinid fly.

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Brown bean bugs

Large brown bean bug (Riptortus serripes)

Small brown bean bug (Melanacanthus scutellaris)

Pest status

As damaging as GVB. More frequent on the coast.

Host range and risk period

As for GVB.

Damage

Both large and small brown bean bugs are as damaging as GVB. Damage is similar to that caused by GVB, with early damage reducing yield, while later damage reduces the quality of harvested seed.

Monitoring

  • Sample crops early in the morning.
  • The beat sheet method is not totally satisfactory as both brown bean bugs are very flighty, particularly during the hotter parts of the day.
  • Crop scouts should familiarise themselves with the appearance of flying brown bean bug adults and include these in sampling counts.

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Silverleaf whitefly

Silverleaf whitefly (SLW) (Bemisia tabaci biotype B) poses a threat to soybeans in tropical and subtropical coastal regions. However, the recently released SLW parasite Eretmocerus hayati, together with native parasites and predators, can reasonably be expected to stabilise SLW populations, provided they are not disrupted by the overuse of non-selective pesticides.

Pest status and host range

Major risk in susceptible crops. Of the summer pulses, soybeans and navy beans are preferred SLW hosts. Significant populations of SLW adults are frequently seen in mungbeans but nymphal development on this crop is very poor.

Risk period

Summer pulses maturing during late summer and autumn are at greater risk of attack because invading SLW have had more time to increase from low over-wintering populations. As a rule, the earlier crops are infested, the greater the risk. Crops remain attractive to SLW until mid pod-fill. As the crop matures, leaves become unattractive to SLW and adults leave the crop to find more attractive hosts.

Damage

  • SLW can reduce plant vigour and yield by the sheer weight of numbers removing large amounts of plant photosynthate from the leaves.
  • Severe infestations in young plants can stunt plant growth and greatly reduce a crop´s yield potential.
  • Later infestations can reduce the number of pods set, seed size, and seed size uniformity, thus reducing yield and quality. As a rule, the impact of SLW is worst in drought stressed crops.
  • In heavily infested soybeans, both pods and seeds are often unusually pale. While seed colour is unlikely to be of concern in grain soybeans (harvested seeds being naturally pale), pod and seed discolouration are a major marketing problem where pods are picked green (e.g. vegetable soybeans and green beans).
  • SLW can also secrete large amounts of sticky honeydew. Adult females produce more honeydew than other stages and nymphs produce more honeydew when feeding on stressed plants. Honeydew is not a major problem, but the sooty mould which develops on honeydew shields leaves from sunlight and reduces photosynthesis.
  • The impact of sooty mould is greatest during early to mid pod-fill when SLW activity is greatest at the top of the canopy, i.e. on the leaves with the greatest photosynthetic activity. Rain and overhead irrigation wash honeydew off leaves, lessening the risk of sooty mould.

Monitoring

  • SLW eggs, nymphs and resting adults are mainly found on the underside of leaves.
  • Flying SLW adults are readily observed when crops with high populations are disturbed.
  • The presence of honeydew and sooty mould may also indicate SLW attack, but can be due to aphid feeding.

SLW eggs are laid on younger leaves, so by the time eggs develop to large nymphs in crops with high growth rates, leaves with the greatest visible SLW nymphal activity are further down the plant. This may be as many as 5-7 nodes below the plant top. As vegetative growth slows, however, plant nodes with greatest nymphal activity move progressively upwards to the canopy top.

Thresholds and chemical control

There are no validated thresholds for SLW and no pesticides are specifically registered for SLW control in summer pulses in Australia. Use the softest options possible for other pests early in the life of the crop, to encourage SLW parasites and predators.

Cultural control

  • Where possible, avoid successive plantings of summer pulses to prevent movement from early to late crops.
  • Avoid planting summer pulses in close proximity to earlier maturing SLW hosts such as cotton and cucurbits.
  • Where damaging SLW populations are evident in other crops early in the season (early summer), or in regions with a history of consistently damaging widespread SLW activity, consider planting a pulse type less attractive to SLW (e.g. mungbeans or adzukis) (Vigna sp.), rather than soybeans.
  • Control SLW weed hosts such as rattlepod and milk thistle.
  • Irrigate crops to reduce moisture stress which makes crops more susceptible to SLW damage. Overhead irrigation also washes off sooty mould and drowns adult SLW.
  • Narrow leafed and smooth leafed (less hairy) cultivars may be less attractive to SLW. However, the latter attribute may leave crops more vulnerable to aphid attack.

Natural enemies

SLW nymphs are parasitised by native species of Encarsia and Eretmocerus (both very small wasps). In 2005 CSIRO released the exotic parasite Eretmocerus hayati in the Bundaberg and Childers region. It has successfully established and spread up to 20 km from the original release sites, with high levels of parasitism reported. The parasite has now also been released in other areas of Queensland and in conjunction with native SLW parasites, will hopefully help stabilise SLW populations.

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Minor pests

Brown shield bug

Dictyotus caenosus

Pest status and damage

  • Minor pest in Australia.
  • The BSB damages only 20% as many seeds as the GVB (i.e. BSB = 0.2 GVB).

Monitoring and control

  • Beat sheeting is the preferred sampling method.
  • Sample crops early to mid-morning when bugs are likely to be at the top of the crop.
  • Look for the distinctive egg rafts (small twin rows or small irregular rafts containing 10-16 eggs), which indicate the presence of BSB.
  • No insecticides are specifically registered against BSB in Australia. BSB are likely to be controlled by pesticides targeting VB.
  • When possible avoid sequential plantings of summer legumes.

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Caterpillars

Cluster caterpillar Spodoptera litura, often referred to as 'spods'

Pest status and damage

As damaging as helicoverpa but less frequent.

  • Can cause significant damage to coastal soybeans in Queensland during flowering and podding.
  • Small larvae window leaves, but older larvae chew holes in leaves.
  • Older larvae may also attack flowers and pods.

Monitoring and control

  • As for helicoverpa.
  • Look also for egg masses and clusters of young larvae.
  • In pre-flowering crops, control is warranted if defoliation exceeds (or is likely to exceed) 33%. (See helicoverpa).
  • Tolerable defoliation drops to 15-20% once flowering and podding commences.
  • Cluster caterpillars are not controlled by NPV and are difficult to control with Bt (Bacillus thuringiensis) unless very small.

Natural enemies

As for helicoverpa and loopers.

Bean podborer (Maruca vitrata)

Pest status and damage

Not usually a pest in soybeans, but tunnelling has been reported in soybean stems in coastal regions such as Bundaberg.

Monitoring and control

  • Look for tunnelling and associated larval frass in soybean stems
  • No thresholds are set as this pest is not regarded as a problem in soybeans.
  • Report any unusual heavy podborer infestations in soybeans to The Department of Employment, Economic Development and Innovation's Entomology (Field Crops) team.

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Etiella (lucerne seed web moth) Etiella behrii

Risk period and damage

  • Spasmodic but important pest of specialist soybeans in drier regions (e.g. natto soybeans on the Darling Downs) due to near zero damage tolerance.
  • Crops may be infested from flowering onwards, but are at greatest risk during late podding.
  • Because etiella larvae consume far less than larger caterpillar species such as Helicoverpa, seeds are usually only partially eaten out, often with characteristic pin-hole damage.
  • This damage is difficult to grade out and its unattractive appearance reduces seed quality.

Monitoring and control

Techniques are being developed to monitor moth activity with light traps or lures, as the moth is this pests´ most vulnerable stage. No pesticides are currently registered.

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Loopers

Green loopers - Soybean looper (Thysanoplusia orichalcea), Tobacco looper (Chrysodeixis argentifera), Vegetable looper (Chrysodeixis eriosoma).

Brown loopers - Bean looper or Mocis (Mocis alterna), Sugarcane looper (Mocis frugalaris), Mocis trifasciata and Pantydia spp.

The following applies equally to green and brown loopers:

Risk period and damage

  • Crops can be attacked at any stage but are greatest risk during flowering and podding.
  • Summer legumes such as soybeans are least tolerant of defoliation at these stages.
  • Loopers do not attack the flowers and small pods of soybeans.
  • Looper leaf damage is different to helicoverpa damage, with the feeding holes being more angular rather than rounded.

Monitoring and control

  • Use a beat sheet .
  • Inspect crops weekly during the vegetative stage and twice weekly from very early budding onwards until crops are no longer susceptible to attack.
  • In pre-flowering crops, looper control is warranted if defoliation exceeds (or is likely to exceed) 33%. Tolerable defoliation drops to 15-20% once flowering and podding commences.
  • Loopers are not controlled by products containing Helicoverpa NPV.
  • Small loopers (under 12 mm) can be controlled with Bt.
  • For chemical control options refer to How to find the right insecticide .

Natural enemies

  • Loopers are frequently parasitised by braconids (Apantales sp.) with scores of parasite larva developing per looper host.
  • Predatory bugs , tachinid flies and ichneumonid wasps also attack loopers.
  • The use of Bt will help preserve beneficial insects.
  • Outbreaks of looper NPV are frequently observed in crops with high looper populations. However, larvae are usually not killed by virus until they are medium-large (instars 4-5). Looper NPV is not the same as helicoverpa NPV.

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Soybean moth

Soybean moth (Aproaerema simplexella) is common in soybeans but is usually only present in low numbers with only the occasional leaf slightly webbed and folded to provide a shelter for larvae. However, they can occur in very high numbers and on rare occasions can destroy crops by denuding all the leaves.

Damage and control

  • Larvae initially feed inside leaves (i.e. mine leaves) for about four days, and then emerge to feed externally, folding and webbing leaves together.
  • The most obvious symptom of damage is the webbing and folding together of leaves. The larvae normally only cause cosmetic damage.
  • Infestations are favoured by hot, dry weather, with crops under severe moisture stress most at risk.
  • Scout crops regularly for the early warning signs of rare plague events - numerous small, pale patches (leaf-mining) on the leaves and large numbers of soybean moths around lights at night.
  • Indicative threshold is based on defoliation (i.e. 33% pre-flowering and 15-20% during early pod-fill).
  • Control will rarely be required and no specific registrations exist for soybean moth.

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Legume leafspinner

The Legume webspinner is also known as the bean leafroller (Omiodes diemenalis).

Risk period and damage

  • Widespread in coastal regions but rarely at damaging levels.
  • Crops are usually at greatest risk during early podding.
  • Larvae are leaf feeders, webbing leaves together.
  • Silken webs and frass are indicative of webspinner attack, but other leaf webbers cause similar symptoms.

Monitoring and control

  • Larvae will be sometimes detected when beat sheet sampling.
  • Inspect webbed leaves and look for the characteristic frass.
  • The threshold is based on tolerable defoliation, i.e. 33% pre flowering and 15-20% during early pod-fill.
  • Control is rarely required.

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Red-shouldered leaf beetle

The Red-shouldered leaf beetle is also known as Monolepta (Monolepta australis).

Risk period and damage

  • Common in sugar cane areas. Can arrive suddenly in large numbers, inflicting rapid defoliation and flower loss.
  • Soybeans are at greatest risk during flowering.
  • Infestations are most likely after heavy rainfall events.
  • Monolepta attack leaves and flowers, high populations (e.g. more than 50/m2) will shred leaves and denude crops of flowers.

Monitoring and control

  • Monolepta are readily assessed visually or with a beat sheet but can be difficult to count as they are extremely flighty. Estimate the number of groups of 5 or 10 beetles on the sheet to get a ´ball park´ population estimate.
  • Check crops after heavy rainfall that may trigger the mass emergence of adults.
  • Thresholds are not yet established but populations greater than 20/m2 can cause significant damage in flowering crops.
  • Defoliation thresholds are the same as for leaf feeding caterpillars.
  • Plant legume crops away from larval hosts of Monolepta such as sugar cane.
  • Spot treatment of borders may be sufficient.
  • Consult How to find the right insecticide for control options.

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Lucerne Crownborer

Lucerne crownborer (Zygrita diva)

Risk factors and damage

  • Soybean crops in the tropics, or growing in abnormally ´hot´ summers, or in close proximity to lucerne are at greatest risk.
  • Proximity to lucerne increases the risk of early infestation.
  • Larval feeding has little impact on yield but prior to pupating, plants are internally ringbarked or girdled above the pupal chamber causing plant death above the girdle and plants in thin stands may lodge before harvest.
  • In southern Queensland, this usually occurs after seeds are fully developed with no yield loss. In tropical regions, larval development is more rapid and there can be considerable crop losses.
  • Crownborers are very damaging to ´edamame´ soybeans where green immature pods are harvested by mechanical pod pluckers. The stems of infested plants are weakened and snap off, contaminating the harvested product.

Monitoring and control

  • Break open stems to look for larvae and eaten out and brown discoloured pith.
  • There are no effective chemical controls as larvae in the stems are protected from insecticide.
  • Avoid planting susceptible crops close to lucerne.
  • If in an at-risk region, consider later plantings to shorten crop development.
  • In the tropics, consider winter plantings.
  • Avoid thin plant stands to reduce the lodging of damaged plants.
  • Currently there are no pesticides registered for lucerne crownborer in soybeans. Trying to control the only vulnerable stage, i.e. the adults in early vegetative crops, would greatly increase the risk of silverleaf whitefly attack.

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Soybean aphids

Soybean aphid (Aphis glycine)

Damage

  • Not a major threat to soybeans but populations should be monitored. In the unusually cool summer of 2007-08 severe aphid outbreaks occurred in the Bundaberg region.
  • More prevalent on the coast than inland.
  • Cast off (white) aphid skins are evidence of past infestations.
  • Heavily infested plants may be covered in sooty mould growing on honeydew secreted by the aphids.
  • Heavy infestations can reduce yield significantly and delay harvest maturity.
  • Infested plants can have distorted leaves.
  • Crops become less attractive to aphids after early podding.
  • The adult, winged-form of the aphid is able to travel long distances on prevailing wind currents.

Monitoring and control

  • Look for aphid colonies on the upper stems, leaflets and terminal leaves.
  • In heavily infested crops, cast off aphid skins, sooty mould, and large ladybird populations are indicative of soybean aphids. The latter two can also indicate significant whitefly activity.
  • Chemical control is rarely required due to the significant impact of natural enemies, especially ladybird beetles and hoverfly larvae.
  • Soybean aphids can be controlled with systemic pesticides but no products are specifically registered for this pest in soybeans.
  • In the United States, the soybean aphid threshold is set at 250 aphids per plant from budding to podding. As a rule of thumb, once soybean aphids are present on the main stem, populations are in excess of 400 aphids per plant.

Two-spotted or red spider mite (Tetranychus sp.)

Damage

  • Can cause severe damage, particularly during hot, dry weather.
  • Mite outbreaks are often the result of using ´hard´ pesticides to treat other pests, where the killing of their natural enemies flares mite numbers.
  • Heavy infestations at pod-fill lead to leaf drop and early senescence.
  • Seed size and yield may be reduced by as much as 30% in severe cases.
  • Mites first occur on the lower leaves and gradually move to the top of the plant as the population builds up.
  • They make fine webbing on the underside of the leaves, and feed by a rasping and sucking action.
  • Infested leaves take on a speckled appearance.
  • In severe cases the leaves turn a yellow-brown before they wither and drop from the plant.
  • Consult How to find the right insecticide for control options.

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Mirids

Green Mirid (Creontiades dilutus) and Brown Mirid (Creontiades pacificus)

Risk period and damage

  • Budding, flowering and early-podding crops are at greatest risk while no damage has been observed in more advanced pods (Queensland Primary Industries and Fisheries trials - now part of the Department of Employment, Economic Development and Innovation).
  • Low populations (less than 1 per m2) of green mirids are often present in vegetative crops but there is no evidence they cause ´tipping´ of vegetative terminals or yield loss.
  • Mirids attack buds, flowers and small pods.
  • Soybeans are less susceptible to mirids compared to other pulses due to the synchrony of flowering and because they produce up to four times as many flowers as are necessary to set enough pods to produce a high yield (4 t/ha or more).
  • Trials have shown no yield loss in crops with up to 5 mirids/m2.

Monitoring

  • Mirids are very mobile pests and in-crop populations can increase very rapidly.
  • Crops should be inspected twice weekly from budding onwards until post flowering.
  • In row crops, the preferred method is beat sheeting, as this method is the most effective for helicoverpa and pod-sucking bugs.
  • Sample five one-metre lengths of row (not consecutive) within a 20 m radius, from at least six sites throughout a crop.
  • Avoid sampling during very windy weather as mirids are easily blown off the sheet.
  • Thresholds for soybeans are 3-4 mirids/m2.

Control

  • Shortening a crop´s flowering period reduces the risk of mirid damage.
  • Flowering periods can be shortened by planting on a full moisture profile and by watering crops just before budding.
  • Consider planting crops in at least 50 cm rows (as opposed to broadcast planting) to facilitate easier pest sampling.
  • Spraying for mirids is unwarranted in most crops unless populations are in excess of 5/m2. Unnecessary spraying for mirids in soybeans increases the risk of flaring silverleaf whitefly.
  • Trials have shown that the addition of salt (0.5% NaCl) as an adjuvant can improve chemical control of mirids at lower chemical rates. Reducing pesticide rates (typically by 50-60%) reduces their impact on beneficials and reduces the risk of flaring helicoverpa.
  • Consult How to find the right insecticide for control options.

Natural enemies

Spiders , ants, predatory bugs and predatory wasps have been observed attacking mirids in the field. Naturally occurring fungi (e.g. Beauvaria ) may also infect and kill mirids, but are rarely observed in the field.

Further information

Last updated 16 March 2011