Emerging insect threats in northern grain crops

Author: Hugh Brier and Melina Miles | Date: 31 Jul 2015

Take home messages

  • Monitor crops frequently so as not to be caught out by new or existing pests.
  • Look for and report any unusual pests/damage symptoms – photographs are good.
  • Just because a pest is present in large numbers in one year doesn’t mean it will necessarily be so next year – it maybe the turn of another spasmodic pest, e.g. soybean moth, to make its presence felt.
  • However be aware of cultural practices that favour pests and rotate cops each year to minimize the build-up of pests and plant diseases.
  • For Lucerne crown borer - Monitor and report any early beetle activity, Split stems to determine the first signs of larval feeding. Be aware of the damage symptoms of other stem borers, notably etiella and soybean stem fly.
  • Soybean stemfly - Check plants regularly for any stem damage and distinctive stemfly exit-holes.  Look out for and report any suspected stemfly activity.  Be aware of other-stem tunnelling pests.  Use crop rotations to avoid a build-up of soil borne diseases, the damage symptoms of which are often wrongly attributed to stemfly.  Please contact project DAQ00196 entomologist Hugh Brier before spraying (0428 188 069).
  • Etiella - Once larvae are inside pods or stems, they cannot be reached with insecticides.  The thresholds are indicative therefore of what would be recommended if etiella behaved as helicoverpa, i.e. if larvae were reachable by insecticides after they are first detected. However the indicative thresholds are useful as they show that what would be concerning levels of helicoverpa attack (e.g. 5-6 larvae/m2) are of no economic consequence for etiella. In practice, more emphasis needs to be put on early detection of the infestations.  In vegetative crops, the early warning signs are damaged and dying auxiliary buds, as well as increasing moth activity.  In lentils, there is a sweep net threshold of 1-2 moths per 20 sweeps.  However lentils are a much shorter and softer crop than mungbeans and soybeans.  In heavily infested mungbeans on the Downs, etiella moths were extremely difficult to catch in sweep nets, making it difficult to estimate moth density. The alternative would be to use traps, be they pheromone, bait or light traps. Further development is required to refine the design/use of these to make them more user-friendly. Agronomists and growers should monitor their mungbean and soybean crops this coming summer and report the first signs any suspicious activity.

Introduction

Recent seasons have seen a plethora of seemingly new pests and unusual damage in pulse and grain crops. The most notable examples are etiella up to 60/m2 in vegetative and podding soybeans and mungbeans, severe scarab damage in sorghum and winter cereals, bean podborer west of the Divide, the appearance of soybean stemfly in regions adjoining the Downs, well south of its ‘normal’ range, and plague numbers of a mystery planthopper in in mungbeans, sorghum and millet last summer. Key questions directed to GRDC’s Northern Gains IPM project DAQ00196 are:

  1. What damage are these pests doing?
  2. Is their damage economic?
  3. Will they continue to escalate? and
  4. Can they be economically controlled?

This paper describes each of these pest problems, and discusses possible management options, including the economics of doing so.

However before delving into specifics it is worthwhile reflecting on the bigger picture.  These reflections can be put into context with the following questions; (a) “What have we done to deserve these pests”? and (b) “Will these pests decimate our industries”? 

The answers to the first are “Maybe nothing, except growing crops that host these pests”.  However in some cases we may have inadvertently adopted cultural practices that favour the pests, such as planting the same crop in successive years in the same paddock.  It may also be (in some cases definitely so) that the seasons favoured the pests development, be it due to non-crop hosts, or a decline in the predatory and parasites insects (beneficials) that attack them. 

The answer to the second question (Will these pests decimate our industries?) is “Hopefully not”!  It should be remembered that pest populations can fluctuate widely, and that some major pests such as green vegetable bug and silverleaf whitefly, were conspicuous by their low activity last summer. Just because a pest is abundant one season, or for a number of season’s, doesn’t mean it will be a major issue every year.  It maybe that some of the ‘crazy’ seasons (climatically) we have experienced lately have led to a disconnect between pests and the beneficial insects that normally regulate their numbers. This phenomena has often been observed with exotic incursions (e.g. with soybean aphid and soybean loopers), where massive populations observed during the early phase of the incursion were eventually stabilized (in most seasons) by native beneficials. On the other hand, it may be that warmer seasons have favoured pest development and in some cases, worsened their damage, as is the case with lucerne borer.

Etiella

Etiella background

In the GRDC Northern Region, etiella (Etiella behrii) has (until recently) been regarded as a significant peanut pest, but only a spasmodic pest of other summer pulses.  Etiella normally attack pods, but in 2013 and 2014, larvae caused significant damage to the auxiliary buds (precursors of the floral buds) and stems of vegetative soybeans in southern Qld and northern NSW.  Similar but not as widespread stem damage was reported in mungbeans.  In some South Burnett soybean crops in 2014, etiella activity continued well into late podfill, populations peaking at > 40 larvae/m2. Significant etiella activity carried through into the spring/early summer of 2014/15, historically high pod damage being widely reported in spring-planted mungbeans from Central Qld to Central NSW. Fortunately, etiella activity was much lower in most 2015 summer-planted crops. 

Etiella R, D&E

Then project DAQ00153 and now DAQ00196 responded to the etiella outbreaks by:

  1. Documenting etiella activity and densities in commercial crops including describing (and photographing – see below) the damage in detail and differentiating the damage from that inflicted by other stemborers and podborers.
  2. Quantifying the damage with the aim of developing economic thresholds
  3. Evaluating a number of management options, including insecticides already registered or under permit in the crops in question, or registered in other crops against etiella.  Note that because they are not registered for etiella control in summer pulses, they cannot be identified in this paper.
  4. Extending these findings where appropriate to industry, e.g. through GRDC Updates or the BeatSheet Blog.

Figure 1. (clockwise from top left) Damaged axilliary buds and stem in vegetative soybeans, Etiella larva in soybean pod showing only 1 seed nearly completely damaged, Etiella larvae in mungbean pod showing multiple seeds partially damaged per larva, Infested mungbean pod (magnified) showing microscopic Etiella entry hole, and damaged pod with clearly visible and much larger larval exit hole.

Figure 1. (clockwise from top left) Damaged axilliary buds and stem in vegetative soybeans, Etiella larva in soybean pod showing only 1 seed nearly completely damaged, Etiella larvae in mungbean pod showing multiple seeds partially damaged per larva, Infested mungbean pod (magnified) showing microscopic Etiella entry hole, and damaged pod with clearly visible and much larger larval exit hole.

Etiella findings/ vegetative soybeans

  • Assessments of  soybeans severely damaged at the mid vegetative stage showed larvae initially feed inside the plants axilliary buds, before tunnelling in the plant’s pith. About 75% of plants were infested in a Branchview crop, larval density in infested plants averaging 4 per plant, or 60 per square metre.  The resultant damage killed a large proportion of the main stems, but the first signs of attack were the pale damaged and dying axilliary buds.
  • Data from this site showed a significant (P <0.001) 70% reduction in main stem length but only a 21 % reduction in total stem length (main stem + side branches).  This resulted in a commensurate but significant (P = 0.001) 20% reduction in total pod numbers, equating to (in this crop) 9 pods fewer per damaged plant. 
  • This suggests a theoretical threshold of approx. 2 infested plants per m2or, for a plant density of 25 plants/m2, a threshold of 10% infested plants. This assumes an etiella per plant density as recorded in this trial site, namely 4 larvae/infested plant. In practice at lower % plant damaged levels, larval density per plant may well be less than 4/plant, resulting in less damage per plant, and therefore a higher tolerable % plants damaged threshold.
  • Note that for grass blue butterfly larva e which also damage auxiliary buds, stems and vegetative terminals, the threshold is 25% of plants damaged.
  • The term ‘theoretical threshold’ is used, because larvae would have to be controlled before move from the auxiliary buds into the stems, inside which they are safe from insecticides.

Figure 2.  Impact of Etiella (Etiella behrii) damage to the final size (as measured by stem length and pod numbers) of vegetative soybeans at harvest.  Note that total stem length and total pods were positively correlated (P<0.001).

Figure 2.  Impact of Etiella (Etiella behrii) damage to the final size (as measured by stem length and pod numbers) of vegetative soybeans at harvest.  Note that total stem length and total pods were positively correlated (P<0.001).

Etiella findings/ podding soybeans

  • Etiella damage was also studied in a late podfill crop of South Burnett soybeans. The majority of larvae fed on only one (1) seed, consuming the seed totally, and confining their activity to one seed cavity. This means there is very little impact on seed quality as the seed remnants are lost at harvest. In soybeans, one seed totally eaten equates to 2kg/ha per larva/m2. In contrast, the yield loss per helicoverpa /m2 is 40kg/ha 
  • Because the yield loss per etiella is so low the theoretical threshold is commensurately high.  For example for a crop value of $600/t, and control costs of $36/ha, the respective thresholds for etiella and helicoverpa in podfilling soybeans are 30 and 1.5 larvae/m2 respectively.
  • The term ‘theoretical threshold’ is used, because larvae would have to be controlled before they burrow into pods. Recent DAF trials show that once inside pods, larvae inside pods at the time of spraying are unaffected by insecticides.
  • There is anecdotal evidence that when insecticides were applied under now lapsed permits, that the best results were achieved when they were applied at the first sign of significant moth activity.  This agrees with the guidelines for etiella control in lentils in Southern Australia.
  • Assessment of damaged pods from infested spring mungbeans shows that as in soybeans, each larva only damages one pod, but that on average, each larva partially damages nearly 4 seeds.  Because seeds are only partially damaged they are therefore far more likely to end up in harvested seed, and therefore have the potential to downgrade seed quality.
  • Harvested etiella seed damage was as high as 9% in some spring crops but was overshadowed by seed staining as high as 25% (due to weathering of the 1st pod flush).  Nonetheless, growers still received $1000/t for manufacturing beans.
  • Further samples are being assessed to determine the impact on seed quality for a crop where etiella damage is the quality-limiting factor.
  • Regarding potential thresholds, an average ‘good mungbean crop’ with 10 pods/plant, and 9 seeds/pod, and 25 plants/m2, would require 12 etiella larvae/m2 to give 2% damage (@ a yield of 1.6t/ha). Critical etiella numbers to give 2 % seed damage (assuming all damaged seeds were retained at harvest) would be higher or lower in higher and lower yielding crops respectively.  In practice, larval thresholds would be lower than that required to inflict the critical 2% damage level, as the potential quality downgrade would be much greater than the cost of control. 
  • Once larvae are inside pods or stems, they cannot be reached with insecticides.  The above thresholds are indicative therefore of what would be recommended if etiella behaved as helicoverpa, i.e. if larvae were reachable by insecticides after they are first detected. However the indicative thresholds are useful as they show that what would be concerning levels of helicoverpa attack (e.g. 5-6 larvae/m2) are of no economic consequence for etiella.
  • In practice, more emphasis needs to be put on early detection of the infestations.  In vegetative crops, the early warning signs are damaged and dying auxiliary buds, as well as increasing moth activity.  In lentils, there is a sweep net threshold of 1-2 moths per 20 sweeps.  However lentils are a much shorter and softer crop than mungbeans and soybeans.  In heavily infested mungbeans on the Downs, etiella moths were extremely difficult to catch in sweep nets, making it difficult to estimate moth density.
  • The alternative would be to use traps, be they pheromone, bait or light traps. Further development is required to refine the design/use of these to make them more user-friendly.
  • Agronomists and growers should monitor their mungbean and soybean crops this coming summer and report the first signs any suspicious activity.  As a final observation, the Peanut Company of Australia reported negligible etiella in their 2015 peanut intake, hopefully a good omen for next summer.

Etiella findings/ podding mungbeans

  • Assessment of damaged pods from infested spring mungbeans shows that as in soybeans, each larva only damages one pod, but that on average, each larva partially damages nearly 4 seeds.  Because seeds are only partially damaged they are therefore far more likely to end up in harvested seed, and therefore have the potential to downgrade seed quality.
  • Harvested etiella seed damage was as high as 9% in some spring crops but was overshadowed by seed staining as high as 25% (due to weathering of the 1st pod flush).  Nonetheless, growers still received $1000/t for manufacturing beans.
  • Further samples are being assessed to determine the impact on seed quality for a crop where etiella damage is the quality-limiting factor.
  • Regarding potential thresholds, an average ‘good mungbean crop’ with 10 pods/plant, and 9 seeds/pod, and 25 plants/m2, would require 12 etiella larvae/m2 to give 2% damage (@ a yield of 1.6t/ha). Critical etiella numbers to give 2 % seed damage (assuming all damaged seeds were retained at harvest) would be higher or lower in higher and lower yielding crops respectively.  In practice, larval thresholds would be lower than that required to inflict the critical 2% damage level, as the potential quality downgrade would be much greater than the cost of control. 

Conclusions and where to from here for etiella?

  • Once larvae are inside pods or stems, they cannot be reached with insecticides.  The above thresholds are indicative therefore of what would be recommended if etiella behaved as helicoverpa, i.e. if larvae were reachable by insecticides after they are first detected. However the indicative thresholds are useful as they show that what would be concerning levels of helicoverpa attack (e.g. 5-6 larvae/m2) are of no economic consequence for etiella.
  • In practice, more emphasis needs to be put on early detection of the infestations.  In vegetative crops, the early warning signs are damaged and dying auxiliary buds, as well as increasing moth activity.  In lentils, there is a sweep net threshold of 1-2 moths per 20 sweeps.  However lentils are a much shorter and softer crop than mungbeans and soybeans.  In heavily infested mungbeans on the Downs, etiella moths were extremely difficult to catch in sweep nets, making it difficult to estimate moth density.
  • The alternative would be to use traps, be they pheromone, bait or light traps. Further development is required to refine the design/use of these to make them more user-friendly.
  • Agronomists and growers should monitor their mungbean and soybean crops this coming summer and report the first signs any suspicious activity.  As a final observation, the Peanut Company of Australia reported negligible etiella in their 2015 peanut intake, hopefully a good omen for next summer. 

Bean podborer (Maruca vitrata)

Bean podborer is an example of a pest that fluctuates widely with the seasons.  Huge numbers (>100/m2) were observed in the wet summers of 2012 and 2013, but numbers crashed in the very dry summer of 2014 and were lowish again last summer.  Then GRDC project DAQ00153 took advantage of these outbreaks and generated efficacy data that secured the registration of AltacorÒ (chlorantraniliprole) against this pest.  Altacor is very effective against podborers, killing larvae hidden inside the buds.  However larval death is not immediate.  While feeding stops very quickly, larvae remain moribund for 3-4 days (shrunken and darkened), before dying.  This moribund state is also observed in helicoverpa and other caterpillars.

The key to successful control is to monitor the crops closely from early budding and target larvae before they move from the flowers to the pods. Cultural controls include getting rid of legume weed hosts such as sesbania.  Other favoured hosts include pigeonpea and adzukis.

Figure 3. Adult and larva of bean pod border (Maruca vitrata)

Figure 3. Adult and larva of bean pod border (Maruca vitrata)

Figure 3. Adult and larva of bean pod border (Maruca vitrata)

Lucerne crown borer (Zygrita diva) (LCB)

Lucerne crown borer has been on the increase in recent years, with reports of up to 90% of plants infested and early plant deaths in some crops. Areas infested ranged from coastal Qld to Central NSW, and included the Downs. Damage was worse in early planted (Oct/Nov) crops. In some crops, larvae girdled plants prior to pupation as early as February when crops were only at the pod set stage. The resultant plant deaths severely reduced yield. This premature pupation was triggered by prolonged high temperatures and low rainfall, both of which lead to crop stress which is a major trigger for early pupation in crown borer.

Unfortunately because of its biology and feeding behaviour, there are no silver bullets for managing this pest, nor are there for very similar overseas beetle pests, e.g. the soybean stem borer (Dectes tetanus) in the USA. 

Figure 4. Adult, larva and effect of Lucerne crown borer (Zygrita diva)

Figure 4. Adult, larva and effect of Lucerne crown borer (Zygrita diva)

Figure 4. Adult, larva and effect of Lucerne crown borer (Zygrita diva)

Figure 4. Adult, larva and effect of Lucerne crown borer (Zygrita diva)

Biology and damage

LCB overwinter as pupae in soybean stubble and beetles emerge in the spring/early summer.  They are not strong fliers, and soybeans in paddocks planted to soybeans last year, or adjacent to such paddocks, are at greater risk.  Proximity to other hosts such as lucerne or pigeon pea is also a risk factor. Adult LCBs lay eggs in the outer stems of young plants.  Hatching larvae tunnel immediately into and feed on the stem pith, i.e. not the vascular tissue.  Larval feeding in the pith by itself does not affect yield.  However, the drying down of crops as they approach harvest maturity triggers LCB pupation, and early maturing cultivars are at increased risk.  Heat stress earlier in the life of the crop can also trigger pupation, especially in early planted crops. 

Larvae internally ringbark (girdle) the vascular tissue in the lower stem to plug the pith tunnel, before pupating in the tap root. If pods are not yet set or filled, yield losses can be severe.  However, yield losses are far lower where pupation occurs when pods are nearly or fully filled, provided damaged plants do not lodge before harvest. Late pupation is the norm in cooler summers.  Note that other pests can tunnel in soybean stems, notably etiella and soybean stem fly.

Chemical control

Chemical control of LCB larvae is impossible, as they feed in the pith and cannot be reached, even by systemic insecticides or those with ‘upwardly xylem mobility’. No insecticides are registered against LCB but dead LCG adults (beetles) have been reported in crops sprayed that are targeting Monolepta (a leaf-feeding beetle pest).  However as crops are often invaded by LCB beetles over an extended period of time, multiple sprays would be required to give satisfactory protection. 

Cultural control

Cultural control offers the greatest hope of managing this pest effectively and sustainably, without creating secondary pest problems.  Specific strategies include.

  • Crop rotation: Don’t plant soybeans in the same ground as last year, and where possible, locate crops as far as possible from last year’s plantings. Similarly avoid plantings into or close to lucerne.  Crop rotation is also a key strategy to reduce the build-up of inoculum of soil bore diseases such as charcoal rot and phomopsis.
  • Weed control: Eliminate weed hosts such as phasey bean, sesbania and budda pea.
  • Time of planting: Avoid early plantings which greatly increase the risk of early damage.
  • Post-harvest cultivation:  Aim to split stems and burry stubble to a depth of at least 10 cm.  Splitting tap roots destroys LCB’s overwintering refuge, and burying stubble to reduce the emergence of beetles in the following spring.  This is a key strategy as it attacks the pest at its most vulnerable stage.  While any cultivation runs counter to zero till philosophy, strategic cultivations would also reduce the build-up of inoculum of key soil bore diseases such as charcoal rot and phomopsis. 
  • Minimize lodging: Have thicker plant stands in which girdled plants are more likely to be supported by adjoining plants.
  • Minimise water stress: In irrigated crops, don’t allow crops to become stressed, as stress can trigger early plant girdling.  In dryland crops, potential crop stress is a major reason to avoid early plantings, as larger plants need more water.
  • Harvester set up.  Set harvester up to pick up any side branches that have drooped or plants that have lodged due to LCB damage.
  • Monitor and report any early beetle activity
  • Split stems to determine the first signs of larval feeding
  • Be aware of the damage symptoms of other stem borers, notably Etiella and soybean stem fly.

Key crown borer messages

  • Monitor and report any early beetle activity
  • Split stems to determine the first signs of larval feeding
  • Be aware of the damage symptoms of other stem borers, notably etiella and soybean stem fly. 

Soybean stemfly (Melanagromyza sojae)

A major outbreak of this pest occurred in soybeans in the Casino region of NSW in 2013. To the author’s knowledge, the only previous reported Australian outbreak was in the Proserpine region in 2009.  Since 2013, stemfly populations have declined in the Casino region, most likely due to parasitism, significant levels of which were observed in late summer of 2013.  Stemfly have also been detected in other soybean growing regions, including the South Burnett in SE Qld, but not in damaging numbers.  It is likely that the pest will be always present in coming seasons, but hopefully only periodically in really damaging numbers.  To date, there have had no reports from the Downs but it is possible the pest is present in this region in numbers not high enough to date to draw attention.

Soybean stemfly adults are small (2 mm) and black with reddish eyes and are very similar to bean fly (Ophiomyia phaseoli) which is a major navy bean pest. Eggs are laid in the leaves and larvae tunnel down the petioles to reach the stem.  Unlike beanfly, stemfly larvae tunnel in the stem pith and make a distinctive exit hole before pupating. Note that stemfly damage look very similar to that caused by crown borer and etiella. Note also that the feeding in the pith has little if any effect on plant health.

Many infested crops near Casino exhibited a ‘sudden death’ syndrome during early podfill (leaf yellowing and plant death).  However, the real culprit in many instances was most likely soil borne disease such as charcoal rot and phomopsis.  These diseases are triggered by plant stress and inoculum build-up due to successive soybean crops in many paddocks.

Figure 5. Adult, larvae, parasite and exit hole of Soybean stemfly (Melanagromyza sojae)

Figure 5. Adult, larvae, parasite and exit hole of Soybean stemfly (Melanagromyza sojae)

Figure 5. Adult, larvae, parasite and exit hole of Soybean stemfly (Melanagromyza sojae)

Figure 5. Adult, larvae, parasite and exit hole of Soybean stemfly (Melanagromyza sojae)

Figure 5. Adult, larvae, parasite and exit hole of Soybean stemfly (Melanagromyza sojae)

Control

There are no well-defined stemfly thresholds. In navy beans the beanfly threshold is one tunnel per plant in seedling plants.  But in soybeans, stemfly normally attack older plants. Only spray if stemfly are present in ‘reasonable’ numbers (numerous larvae per plant) and there are increasing unhealthy plant symptoms that are NOT disease related.  Note that diseases such as charcoal rot and phomopsis are manifested by poor root development, distinctive stem discolouration and leaf discolouration and death, and eventual plant death. 

If you do spray, target the larvae before they reach the stems.  Once inside the stems, larvae cannot be controlled as they are feeding on non-vascular tissue. Note that Casino crops that were sprayed with the beanfly rate of dimethoate (800 mL/ha) in 2013, experienced an explosion of white fly numbers, from already very high levels. 

If you do spray, please leave an unsprayed strip to evaluate the efficacy of the spray on the pest and plant health, and its impact on secondary pests such as whitefly.

Take home messages

Check plants regularly for any stem damage and distinctive stemfly exit-holes.  Look out for and report any suspected stemfly activity.  Be aware of other stem tunnelling pests.  Use crop rotations to avoid a build-up of soil borne diseases, the damage symptoms of which are often wrongly attributed to stemfly.  Please contact project DAQ00196 entomologist Hugh Brier before spraying (0428 188 069).

Large mystery planthopper Oteana lubra (Cixiidae)

Very high numbers of a large planthopper Oteana lubra (no common name and formerly Oliarus lubra) were reported last summer in mungbeans on the Darling Downs and in North West NSW.  The bulky hoppers are 9-10 mm long and pale brown/grey with a fluffy white rear end (see figure 6).  They have been observed in previous years in low numbers (usually <1/m2) in mungbeans and soybeans. However this year, populations in excess of 100 per 20 sweep net sweeps were observed in some crops.  This most likely equates to an absolute population in excess of 20/m2. All hoppers sampled were adults, i.e. there were no nymphs.  This is because the nymphs of this planthopper  group are root feeders, often of grasses.

Figure 6. Oteana lubra (Cixiidae)

Figure 6. Oteana lubra (Cixiidae)

Damage

Damaged pods suggest they are not as damaging as first feared. Close examination of mungbean pods from heavily-infested crops revealed numerous feeding stings on the external pod wall, but extremely few stings on the seeds, or on the inside pod wall (see figure 7). Plotting the data showed no relationship between the number of external stings, the number of stung seeds (the very few present may have been stung by podsucking bugs), and the total number of filled seeds per pod.

To further assess damage, caged mungbeans (2 rows x 1.5m) at the R4 stage were infested with O. lubra at a density of 20 adults/m2. After 2 weeks, no live hoppers were present in the cage, suggesting mungbeans are not a suitable for O. lubra, or that the adults were short lived.  Seed samples from this site are still being assessed for damage.

Figure 7. Damage by Oteana lubra showing stings on pod surface, but no stings inside pod or on seeds

Figure 7. Damage by Oteana lubra showing stings on pod surface, but no stings inside pod or on seeds

Figure 8. Number of external stings per pod made by planthopper Oteana lubra, ranked from lowest to highest, versus the number of stung seeds, and the total number of filled seeds per pod.  Note the lack of correlation suggesting no impact on yield or quality.

Figure 8. Number of external stings per pod made by planthopper Oteana lubra, ranked from lowest to highest, versus the number of stung seeds, and the total number of filled seeds per pod.  Note the lack of correlation suggesting no impact on yield or quality.

Other hosts of adult large planthopper Oteana lubra include (DAF and other records):

  • Chickpeas at Breeza, NSW
  • Faba beans
  • Ex cotton or lucerne (?) at Warren, NSW
  • Sorghum & Millet – Downs 2015
  • Sugarcane at Bundaberg, Qld
  • Rice
  • Potato at Gatton, Qld
  • Sandalwood near Eulo and Pittsworth, Qld
  • Eucalyptus citriodora near Moree, NSW

Control

Dimethoate at 800mL/ha is registered) for jassid/leafhopper control in mungbeans and a range of other grain legumes.  In soybeans, the registered rate of dimethoate for Jassids/leafhopper is 340 mL/ha.

If any crops are sprayed next summer, please leave an unsprayed strip so that the impact of spraying can be assessed. In view of the assessments of damaged pods from heavily infested crops, spraying may be unnecessary.  This is even more reason to leave unsprayed strips.  Please also report any outbreaks to Hugh Brier or Melina Miles.

Take home observations for all above pests

  • Monitor crops frequently so as not to be caught out by new or existing pests.
  • Look for and report any unusual pests/damage symptoms – photographs are good.
  • Just because a pest is present in large numbers in one year doesn’t mean it will necessarily be so next year – it maybe the turn of another spasmodic pest, e.g. soybean moth, to make its presence felt.
  • However be aware of cultural practices that favour pests and rotate cops each year to minimize the build-up of pests and plant diseases.

Acknowledgements for summer pulse research in DAQ00196

GRDC and DAF for providing financial support for Northern Grains IPM project DAQ00196. We are also grateful for the assistance and feedback provided by numerous agronomists, growers and Pulse Industry personnel (including Pulse Australia) throughout the GRDC Northern Grains region.  Finally, we acknowledge the invaluable technical support of Joe Wessels, Wendy Sippel, Adam Quade and Kaela McDuffy, and of Scott Campbell and the Kingaroy Research Station farmhand staff.

Scarabs on the Downs, what do we know about them and can they be managed?

Overview of the problem to June 2015 (Melina Miles)

In February 2015, DAF Entomologists were contacted by several agronomists about the severity of damage to sorghum being caused by scarab larvae in the Jondaryan and Pampas areas. The agronomists reported that the problem had worsened over the past 5 years with persistently affected paddocks suffering up to 80% loss of sorghum. Losses in winter cereals have been recorded previously (Jondaryan, Jandowae and Clifton 2010). Anecdotally, cotton does not appear to be affected, but one record of severe damage to cotton in CQ was recorded in the mid-2000s. Sorghum and summer pulses and, in some seasons chickpea, are severely affected. The frequently observed pattern of crop loss in sorghum is that the crop establishes well, but within 4-6 weeks it is evident that plants loose vigour and show symptoms of wilting, yellowing and retarded growth are observed. 

Figure 9. Scarab larva (left), in situ in the roots of sorghum (centre) and wheat (right) 

Figure 9. Scarab larva (left), in situ in the roots of sorghum (centre) and wheat (right)

Figure 10. Aerial view of scarab damage to sorghum (2015).

Figure 10. Aerial view of scarab damage to sorghum (2015).

View video of affected fields at https://youtu.be/q6MptMKvT9U

Previous reports of scarab damage in the region have been sporadic. DAF entomologists have visited a number of farms over the past 3-5 years to inspect scarab damage. In all these instances there was a strong relationship between scarab damage and a recent history of pasture and/or grassy weed infestations in the affected fields. There is also anecdotal evidence that sorghum on sorghum rotations are more likely to have severe scarab problems in the affected regions. Historically, scarab damage in crops has been sporadic and associated with wetter than average seasons. Sustained scarab pressure in the same cropped fields over many years is unusual.

Between February and May 2015, DAF entomology in Toowoomba received reports of severe scarab damage in mungbean, maize, sorghum, and millet. During May, there was concern by agronomists, who had had significant crop loss in sorghum, in regard to the prospects of chickpea crop to be sown back into the affected fields. Reports were concentrated on the area between Macalister and Jondaryan, with a small number from further south into northern NSW.

Species status and biology of scarabs. (Coleoptera: Scarabaeidae)

At this point there is not a definitive identification of the species common in the affected fields.

Peter Allsopp (pers comm) has suggested that the species may be the Black Soil Scarab (Othnonius batesii), based on the incidence of this species the 1960-1970s on the Downs. Adult beetles are needed to confirm the identity of the scarab.

If the species is O. batesii, then this is what is known about it from the work 40+ years ago:

  • It is a native species, more commonly associated with damage to pasture.
  • It has a 2 year lifecycle, which may be extended to 3 years if food quality or conditions are poor (fallow, no grasses, dry conditions)
  • Beetles emerge from October to March. Females lay eggs into the soil. Eggs are present from November – March.
  • There are 3 larval instars. 1st instar larvae feed on organic matter and are present from December to October. 2nd and 3rd instar larvae feed on plant roots and are present all year.
  • Larvae feed on the roots, impacting plant vigour and root expansion. This limits access to soil moisture and nutrition and consequently plant survival and yield.
  • Larvae pupate after 18 months and develop into beetles. The beetles remain in the soil until rainfall in spring-summer (13 mm+) which softens the soil making it possible for them to emerge.
  • Beetles may disperse, or remain in/close to the fields from which them emerge; they mate and lay eggs.
  • Cultivation (with a mouldboard plough) was found to reduce larval densities by 50-90%.

Management of scarabs in other crops, and options for the grains industry

The most comprehensive management plans for scarab pests in Australia have been devised by the sugar industry for the cane grub species. The sugar system is dominated by a need to minimise canegrub damage to cane crops for up to three to five years. Consequently it is comprised of a number of options designed to detect and assess risk, to suppress (trap cropping) and control (insecticide and fungal pathogen) canegrub populations. Management in cane integrates a number of options.

Current scarab research

Scarabs are not one of the priority pests identified for inclusion in the activities of the current GRDC investment in Northern Region entomology (DAQ00196). However, in response to the outbreak in 2015, the DAF entomology team has commenced research on this pest.

Current activities:

  1. Species identification and clarification of lifecycle
    1. Collections of larvae from affected fields have been made and are being reared through to adult beetles in the laboratory so that the species/s can be identified.
    2. Survey larval populations through the year. Determine timing of larval development and risk to summer and winter cropping.
    3. Mapping distribution of damage in affected fields using drone technology
      1. The use of the drone to inspect fields aerially at field scale has been invaluable in identifying affected areas of crop and the spatial distribution and infestation severity. A new drone with ground-link GPS capability has been purchased to further facilitate the mapping of affected areas so that long term monitoring of patches with different crop and management histories is possible.
      2. On a practical level, mapping patches of scarab infestation will assist in targeting strategic tillage or in-furrow insecticide application and may overcome the negatives associated with cultivating the entire field where only a portion is currently infested (potential application)
      3. A small plot trial (in conjunction with Pacific Seeds) evaluating the efficacy of seed dressings and in-furrow treatments to protect establishing sorghum (completed May 2015).
        1. This trial has shown that there are both seed dressing and in-furrow insecticide treatments that result in significantly improved crop establishment and development. These products are currently not registered for scarab control, so data generated from this research will be used to support label extensions. These results will now be built on in 4.
        2. Small plot chickpea and wheat trial to evaluate the impact of at-sowing treatments (seed dressing, sowing depth) on crop establishment. This trial will be sown into the same heavily infested field in the Jondaryan area in May 2015.
          The aims of this experiment are to:
          1. Determine the potential value of seed dressings in establishing chickpea – potentially generating data for permit/s and registration of products
          2. Evaluate the benefit or risk of deep sowing chickpea in scarab-infested fields
          3. Determine the susceptibility of wheat to scarab damage and characterise the damage caused to both wheat and chickpea seedlings.
          4. Tillage trials that examine the impact of timing and implement (disc vs chisel/sweep) on larval survival. Planned for winter – spring 2015.

Acknowledgements for scarab research in DAQ00196

GRDC and DAF provide financial support for this research (DAQ00196). We are grateful to the assistance provided to undertake this research by agronomists and growers on the Downs, in particular Trevor Philp (PacSeeds), James Ryder, William Speed, John and Paul Griffiths.  Trevor Philp (PacSeeds) sowed the sorghum seed treatment and in-furrow trial.

Contact details

Melina Miles
DAF
Mb: 0407 113 306
Email: melina.miles@daff.qld.gov.au

Hugh Brier
DAFF
Mb: 0428 188 069
Email: hugh.brier@daff.qld.gov.au

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GRDC Project code: DAQ00196, DAQ00153