Mungbean and soybean insects

Author: Hugh Brier and Liz Williams, DAF Qld, Kingaroy | Date: 21 Jun 2016

Take home message

  • Mirids at late podfill are not nearly as damaging as podsucking bugs, but watch this space for confirmatory research
  • Preliminary results suggest that 0.5 m row spacing is a compromise between yield, pest abundance accuracy and crop health
  • Delta traps with a combination lure of etiella pheromone and Magnet plant pheromone appear to be the superior trap/lure combination for the early detection of etiella moths
  • Don’t forget pests that haven’t occurred recently

Introduction

Once again the past season has shown that no two seasons are alike.  In other words, what was a major pest issue in previous seasons was not necessarily a problem this past season.  Conversely, some pests not regularly present in damaging numbers on the Downs made their presence felt this summer.

Significant pest issues raised by consultants and growers include sampling in narrow rows, very high mirid numbers with rapid re-invasion post spraying, and late mirid damage in mungbeans.  Etiella was also a concern but in most crops, populations were far lower than in previous years and were too low to be of economic concern.  Conversely, soybean aphids, not regularly present in damaging numbers on the Downs, were present in significant numbers in some crops on the central Downs (Branch View and Pampas).  The other issue raised in this and other regions was lucerne crown borer in soybeans. Finally mysterious crop greening phytoplasma-like symptoms were reported on the Central Downs. At the time of writing, the exact causative agent is not determined, but if a phyoplasma, it could be vectored by the common brown leafhopper Orosius orientalis.

Note: some of the comments in this article refer to a half label rate of dimethoate for mirid control in mung beans.  Always check and follow the label before using insecticides.

Row spacing background

It is increasingly recommended that narrow row spacing in various crops can improve yield output.  This is often dependent on multiple factors, including crop type, variety, soil moisture availability and other environmental variables (Verrell and Jenkins 2014, McKenzie et al. 2015).  However, narrow rows can increase the risk of negative issues, such as a higher incidence of pathogens (e.g. Andrabi et al. 2011) and cause difficulties in pest sampling with traditional soft beatsheets (personal communications with agronomists). 

Research trials at two locations were conducted to determine pest population dynamics in different row spacings, including whether alternative sampling methods may alleviate pest sampling difficulties.

Row spacing RD&E

Trials were conducted on black soil (Warra) and red soil (Kingaroy) in mungbean crops with row spacings of 0.25 m, 0.5m and 1m.  Sampling was conducted on two occasions at both locations, during flowering and podding.  Additionally, two types of beatsheets were evaluated at the second time sampling – the traditional soft cloth beatsheet and a rigid aluminium beatsheet (Figure 1).  This rigid beatsheet was designed to be more easily inserted between narrow rows and retain insects more effectively than the soft beatsheet.

soft beat sheet, the main tool used to sample row crops for pests and beneficial insects.   rigid beat sheet, the main tool used to sample row crops for pests and beneficial insects.

Figure 1.  The traditional soft cloth beatsheet (left) and a rigid aluminium beatsheet (right) with collecting trough at the base, as used in 0.25 m rows of mungbeans.

The numbers of pests, beneficials and total insects varied considerably between row spacings, whether examined at linear row length or by the square metre.  Specifically, on a linear row length basis, lower pest numbers were detected in the 0.25 m rows compared to 0.5 m and 1 m rows.  In contrast, at the one square metre scale, 0.25 m rows had much higher values compared to the other row spacings.  As economic threshold values were developed on 1 m wide row spacing, these differences in pest numbers detected in these trials may mean that these thresholds need to be adjusted depending on the row spacing in the crop being sampled.  Further modelling is currently being undertaken to determine the influence of row spacing on pest and total insect sampling numbers, specifically in relation to the accuracy of these threshold values.  Preliminary results suggest that 0.5 m rows collected similar pest numbers to the 1 m rows and were, in practice, easier to sample than 0.25 m.  As there is some evidence that yield in 0.5 m row spacing is comparable or minimally less than narrower rows (e.g. Raymond et al. 2016), we tentatively recommend this as a compromise between yield, pest accuracy and crop health.

Further support for this comes from the evaluation of soft and rigid beatsheets, with comparable abundance and richness values between 0.5 m and 1 m rows.  At 0.25 m rows, abundance was reduced with the soft beatsheet compared to the rigid, likely from insects being dislodged during beatsheet insertion (more difficult with a soft structure in tight rows) or not being retained at the base of the beatsheet (sliding off due to no flat area or a collecting lip).  Thus, the soft beatsheet, which consultants and growers routinely use for counting pests and for determining whether to use pesticides, did not collect an accurate representation of pest presence (or underestimated the number of pests present, in the case of mirids by »20%)

Etiella background

The threat of etiella (Etiella behrii) damage was a concern for many consultants and growers this summer given the pest’s very high incidence in the summer of 2014-2015.  While populations were much lower in the summer of 2015-2016, the issue of early etiella detection was frequently raised.  This was prompted in part by DAQ00196 research showing that once inside pods, the larvae are ‘out of reach’ of insecticide sprays.  In response, trials were initiated to determine an appropriate monitoring method for the early detection of moth activity.  Specifically, if moths can be detected before eggs hatch or larvae enter pods, then insecticides with residual activity may have the potential to control the pest. 

Generally, the early detection of etiella moths is performed by either light traps (typically by researchers) or delta traps with female etiella pheromone capsules (by agronomists and growers); however, both methods have their limitations.  Light traps are very effective at collecting E.behrii moths, but require a power source and regular sample collection and maintenance, meaning they are less user-friendly for growers and agronomists, therefore making their widespread use impractical.  In contrast, delta traps with pheromone attractants are an inexpensive and easy to use option. However, the effectiveness of pheromone capsules attracting etiella is questionable (based on preliminary field trials and user communications). 

Etiella RD&E

To evaluate the typical grower etiella monitoring method (delta trap with etiella pheromone lure) with other potential moth monitoring methods, trials at two locations near Kingaroy were implemented in late peanut crops.  Twelve combinations of traps + lures were assessed, with three different trap types (delta, ball, funnel) and four different lure configurations – 1) single etiella pheromone capsule, 2) two etiella pheromone capsules, 3) Magnet® (a plant pheromone mimic) and 4) Magnet + a single etiella pheromone capsule.

Over the two week operational period, 20 of the 88 traps collected a total of 30 individuals.  Delta traps were the superior trap type, capturing 24 moths in 14 traps (Figure 2).  Although delta traps with the combination lure of Magnet + pheromone capsule captured etiella more frequently than the other lure types, low capture rates overall mean that statistical inferences cannot be calculated.  However, considering Magnet attracts both male and female moths (in contrast to the pheromone capsules which only attract males), it is logical that the use of this chemical would increase the probability of collecting flying etiella.  Interestingly, ball and funnel traps only collected etiella when the lure contained Magnet only, yet pheromone only traps (single or two capsules) were the only type to collect multiple etiella per trap.  Magnet lures did result in increased numbers of bycatch, including many beneficial invertebrates such as pollinators and parasitoids, which leads us to recommend that its use (volume of chemical or number of traps/vesicles) be limited to what is necessary. 

Bar chart showing number of traps that collected etiella moths.

Figure 2. Number of traps that collected etiella moths.  For lures, M = Magnet only, MP = Magnet + single etiella Pheromone, P = single Pheromone capsule and PP = two Pheromone capsules.

A proposed Stage 2 of this research for next summer includes the installation of a subset of the above traps (that collected the most etiella moths) in a number of growers’ paddocks throughout the Northern Region that were affected by etiella in 2015-2016.  This would further refine which trap and lure type is the most effective for the early detection of etiella moth activity at a spatial density (i.e. number of traps per paddock) that is more similar to real practise.  It should also be noted that due to their small size, etiella thresholds in mungbeans and soybeans are reasonably high, being in the order of 16 and 36 larvae per square metre, respectively.  Although these thresholds are not exceeded in many seasons, by the time larvae are detected, they are inside pods and stems and cannot be controlled.  An effective moth trapping programme will facilitate the early detection of the pest before larvae infest pods or stems, and allow for pre-emptive (more timely) action in years with major etiella outbreaks, as occurred in 2014.

Mirid background

Mirids (Creontiades sp.) are a regular mungbean pest, with most crops being sprayed at least once for this pest.  In soybeans they are also quite common, but the thresholds are much higher (»6/m2), which is considerably higher than the mungbean threshold of » 0.3 to 0.5/m2.  However this year, mirid populations of ≥ 10/m2 were common in both crops at the podfill stage.  Most mungbean crops were sprayed for mirids by early flowering, but rapid mirid re-invasions were widely reported, with many consultants switching to synthetic pyrethroids (SP’s) such as deltamethrin (registered in the crop but not against mirids), claiming better residual control.  The residual activity of the half label rate dimethoate (250mL/ha + salt) recommendation was also questioned by some consultants.

To add to the challenge, podsucking bug (PSB) populations were very low in many crops.  Consequently, the usual PSB sprays of deltamethrinÒ were not applied, and late mirid populations were not co-incidentally controlled during the podfill stage.  Growers/consultants were therefore also left with the question as to whether they should apply a late mirid spray to protect seed quality.

In response to these two issues, trials were initiated to compare the residual activity of dimethoate @ 250 mL/ha + 0.5% w/v salt, with Ballistic (deltamethrin) @ 500mL/ha + salt, as well as with a potential new but not yet registered bug insecticide (Product X).  Trials were also initiated to evaluate mirid damage to mungbean seeds during late podfill (the R4 stage).

Mirid control trials 2016

In 2016, treatments were applied at the crop’s R3 stage (beginning seed) at 48 DAE.  The results below (Figs.3-4) show that at 3 DAT (the 1st post-spray check), mirid numbers for all insecticide treatments were significantly reduced.  However, by 10 DAE, mirids had recovered to pre-spray levels, although still significantly lower than the untreated control, which showed a temporary increase post spray. Note that differences in the rate of mirid resurgence between insecticides were not significant. 

Line chart showing efficacy of Dimethoate and Ballistic and potential new mirid insecticide.

Figure 3. Efficacy of Dimethoate and Ballistic and potential new mirid insecticide (Product X) against a high mirid population in mungbeans.

The data in Figure 4 below quantifies the post-spray mirid pressure in cumulative mirid days for 0-7 days post spray (CUM0-7) and for 0-10 days (CUM 0-10), and shows no significant differences between the pesticides trialled.  So in summary, in a high mirid pressure scenario, the SP deltamethrin gave no advantage over dimethoate and mirids resurged rapidly for all treatments.  The data also shows that new product X has potential under high mirid pressure.  Note that in this trial, sprayed plots were re-infested from adjacent non-sprayed bulk crop, which may not occur to the same degree in a commercial crop. 

Regarding the rate of dimethoate, a previous trial in 2015 (see Fig. 5) showed that dimethoate applied at the IPM-recommended rate of 250mL/ha rate (+0.5 salt) was  as effective as the full registered rate of 500 mL/ha.  Finally, this half rate dimethoate + salt is (currently) the IPM preferred option as it has less impact on beneficials than the full dimethoate rate or SPs such as deltamethrin. 

Bar chart showing mirid activity post spray relative to the uninfested control.

Figure 4. Mirid activity post spray relative to the uninfested control. 3DAT = @ 3 days after treating, and CUM = cumulative mirid days from 0-7 days & 0-10 DAT.

Line chart showing efficacy of dimethoate.

Figure 5. Efficacy of dimethoate at 250 mL/ha + salt (0.5% w/v) (D250 +S) and dimethoate at 500 mL/ha against mirids (Creontiades sp.) in mungbeans.

Late mirid damage to mungbean seeds

Two trials were initiated to determine whether mirids damage mungbean seeds during late podfill.  In the first, caged mungbeans (2x 1.5 m of row = 3m) were infested with brown mirid adults (Creontiades pacificus) at a density of 30 per cage (equivalent to 10 per row metre for 12 days).  Uninfested control cages were also set up, and were disinfested with a deltamethrin spray for podsucking bugs and mirids.  Mungbeans were at the R6+ stage with >50 % black pods.  All black pods were removed prior to infestation to allow mirids to concentrate on the remaining green and turning (green to black) pods.  After 12 days, all pods were hand harvested and assessment for potential mirid damage, including discoloured stained seeds, and/or visible sting marks.

In the second trial, field grown mungbeans were assessed at the R6+ stage.  Six (6) racemes with 4-5 green pods were cupped and infested with 4 brown mirid adults per cup for 5 days.  Six uninfested racemes were also cupped for 5 days.  After 5 days, racemes with cups attached were cut from the plants and the number of live mirids counted, and pods assessed for damage. Mirid survival in the infested cups averaged 92%.

In both trials, podsucking bugs (green vegetable bug GVB and brown bean bugs) and mirids (mostly brown) were present in low numbers prior to infestation.  For this reason, there were unavoidable levels of background damage in the un-infested controls in both trials.  However, it was anticipated that if mirids were as damaging to seeds as podsucking bugs, any background damage would be surpassed by a large margin.

Mirid damage results and conclusions

The results for both trials (see table 1) show very low rates (not significant) of damage at only 4% and 10% of that expected from a GVB in the field cage and cupped pod trial respectively.  This suggests mirids are far less damaging to mungbean seeds than podsucking bugs.  However, further trials with more replications are required to determine if the low rates of damage recorded are accurate or significant.  Certainly none of the post-damage spotting, as observed by Melina Miles in faba beans, has been observed in this and other mungbean trials. 

Theoretically, if the low rates of damage is accurate, and hypothetically only 5% of that for GVB damage, then the corresponding mirid threshold at late podfill would be 20 times that for a GVB, and would be approximately 7 mirids/m2 for a 1t/ha crop.

Table 1. Seed damage in mungbeans infested with brown mirids (Creontiades pacificus)

Field cage trial
10 mirids/m for 12 days

Damaged seeds per cage

Damaged seeds per mirid day

Equivalent damaged seeds per GVB day

Trial damage as % of GVB

Control

60.9

Mirids

90.4

Difference

29.5

0.08

2

4%

Cupped pod trial. 4 mirids per 5 pods for 5 days

Damaged seeds per cup

Damaged seeds per mirid day

Equivalent damaged seeds per GVB day

Trial damage as % of GVB

Control

9.7

Mirids

13.2

Difference

3.5

0.19

2

10%

LSD (cup trial)

11.5 NS

Lucerne crown borer progress report

Lucerne crown borer (LCB - Zygrita diva) has been active in soybeans in many regions.  While not reported specifically from the Downs, it is always a potential threat, particularly in early plantings and in very hot summers.  In some recent NSW trials, over 80% of plants were infested with LCB.  Current recommendations are to strategically till soybean stubble to kill overwintering larvae and to avoid very early soybean plantings.  Also, if significant infestation levels are detected by splitting stems open, harvesting should be carried out as soon as possible.  Project DAQ00196 is currently trialling seed dressings to combat this pest, with initial results showing significant reductions in infestation and plant girdling levels.  However, registration for the product is still some time away with residue trials required to establish maximum residue levels (MRLs). 

Mystery soybean greening on Downs – soybean aphids, virus or phytoplasma?

Brown leafhopper

Figure 6. Common brown leafhopper (Orosius orientalis)

In early May, large areas of soybeans (300+ ha) on the central Downs were reported with a mystery plant greening and phytoplasma-like pod symptoms (see Figure 7a).  Some plants had witches broom (phytoplasma) like symptoms (masses of immature pods) while other less-affected plants showed well-developed green pods that , given the crop’s age, should have been harvest-ready. The magnitude of the symptoms only added to the confusion as typically, phytoplasma only affects isolated plants. 

Some of the affected crops had significant soybean aphid (Aphis glycines) infestations that were not controlled until pod fill; however, although soybean aphids can delay maturity (see Figure 7b), pods on affected plants do not normally exhibit the extreme symptoms shown in Figure 7a.  Additionally, soybean aphids do not vector phytoplasma, which is usually transmitted in Australia by the common brown leafhopper (Orosius orientalis; Figure 6). Although no such leafhoppers were observed in the crops in early May, it is possible that a wave of brown leafhoppers moved through the affected crops between scoutings; however, this is purely speculative.  At the time of writing, experts are leaning towards a phytoplasma being responsible, but the exact cause is yet to be confirmed.

Finally, in regards to soybean aphids, to avoid aphid-induced delayed crop maturity, spray aphids if they exceed the threshold (250 aphids per plant) by flowering. As a rule of thumb, aphids are above threshold if they are visible on the main stem.

Collage of showing sprayed and unsprayed plants

Figure 7a (left).  Green plants from downs with phytoplasma-like symptoms.  Unlike the green aphid-infested plants, these plants had bunched immature pods, as well as green fully developed pods.

Figure 7b (right).  Unsprayed plants at plot ends in trial heavily infested with soybean aphids (Aphis glycines) showing greatly delayed maturity in comparison the harvest-ready main trial area.

Final observations

  • Report any unusual or extreme pest activity
  • Don’t forget pests that haven’t occurred recently
  • Scout regularly to detect major pest outbreaks before they inflict commensurate damage
  • Mirids at late podfill no nearly as damaging as podsucking bugs, but watch this space for confirmatory research
  • From preliminary results, it is suggested that 0.5 m row spacing is a good compromise between yield, pest abundance accuracy and crop health
  • Delta traps with a combination lure of etiella pheromone and Magnet plant pheromone appear to be the superior trap/lure combination tested in trials for the early detection of flying etiella moths

Acknowledgments

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC.  The authors would like to thank them for their continued support.  Hugh Brier also thanks the GRDC for their funding of new grains entomologist Dr Liz Williams.

References

Andrabi, M., A. Vaid & V. Razdan (2011) Evaluation of different measures to control wilt causing pathogens in chickpea, Journal of Plant Protection Research, 51: 55-59.

McKenzie, K., H. Cox, R. Rachaputi, B. Raymond & N. Seymour (2015) Summer pulse agronomy including plant population row spacing varieties yields and nitrogen fixation, Jondaryan GRDC Update August 2015

Raymond, R., K. McKenzie & R. Rachaputi (2016) Faba bean agronomy – ideal row spacing and time of sowing, Goondiwindi GRDC Update March 2016.

Trebicki, P.; Harding, R. M.; Rodoni, B.; Baxter, G.; Powell, K. S. (2010) Seasonal activity and abundance of Orosius orientalis (Hemiptera: Cicadellidae) at agricultural sites in South Eastern Australia.  Journal of Applied Entomology Volume: 134  Issue: 2. pp: 91-97.

Verrell, A. & L. Jenkins (2014) Effect of row spacing on yield in chickpea under high yield potential, Mungindi GRDC Update March 2014

Contact details

Hugh Brier
Qld Department of Agriculture and Fisheries
PO Box 23 Kingaroy Qld
Ph: 07 4160 0740
Mb: 0428 188 069
Email: hugh.brier@daf.qld.gov.au

Dr Elizabeth (Liz) Williams
Department of Agriculture and Fisheries
PO Box 23 Kingaroy Qld. 
Ph: 07 4160 0718
Mb: 0476 850 415
Email: liz.williams@daf.qld.gov.au

GRDC Project code: DAQ00196