Proceedings of the Australian Society of Sugar Cane Technologists, volume 41, 363–370, 2019
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Peer-reviewed paper
Effect of neonicotinoid, pyrethroid and spirotetramat insecticides and a miticide on incidence and severity of Yellow Canopy Syndrome
DJ Olsen and AL Ward
Sugar Research Australia Limited, Indooroopilly, Qld 4068; [email protected]
Abstract
Yellow Canopy Syndrome (YCS) is a condition affecting Australian sugarcane that can lead to yield losses in excess of 30% in severely affected crops. The causal agent of this condition is unknown. Insect pests are well known causal agents of a wide variety of yield-limiting crop conditions, either as vectors of pathogens, directly through their feeding damage, or as transmitters of toxins, but little has been done to evaluate insects as a possible causal agent of YCS. This paper presents the findings of a one-year field trial in which insecticides from different chemical groups and an acaricide were tested to evaluate their effect on YCS incidence and severity. Results showed a delay in the onset of symptoms and a significant reduction in the severity of symptom expression following the application of neonicotinoid and pyrethroid treatments. These treatments also resulted in a significant yield improvement relative to cane in the untreated control. The acaricide treatment was ineffective. These findings suggest further work is warranted to determine which insects are being controlled and to identify the mechanism for the positive yield response.Key words
Yellow canopy syndrome, insects, neonicotinoids, imidacloprid, bifenthrinINTRODUCTION
Yellow Canopy Syndrome (YCS) is a condition affecting Australian sugarcane that can lead to yield losses in excess of 30% in severely affected crops (Olsen unpublished data). It has been seen in all areas of the Australian sugar industry except in New South Wales. The condition presents as yellowing of leaves, typically beginning in the mid to lower canopy around leaf +5 or +6 and may progress rapidly into the upper canopy as high as leaf +1 (the top visible dewlap leaf). Symptom onset begins with entire leaves turning from green to a yellow-green mottle. This mottling appears as small variable-sized islands of green leaf tissue scattered across an otherwise lightly-yellowed leaf. The mottling quickly transitions to a solid, even yellow colouring. Leaf colour is not a pure yellow; the hue includes a hint of orange (Olsen et al. 2015). Physiologically, the condition is characterised by an accumulation of starch and sucrose in the leaf, a disruption of the photosynthetic electron- transport chain, and significant reductions in photosynthesis and stomatal conductance (Joyce et al. 2016;
Marquardt et al. 2016).
There are many reasons sugarcane leaves may turn yellow including nutrient deficiencies, heavy metal toxicities, herbicide phytotoxicity, water stress, disease, insect damage, or simply natural senescence, and much work has been undertaken to determine the causal agent of YCS. Nutrient deficiency and heavy metal toxicity were found to be unlikely causal agents (Olsen et al. 2019). Similarly, a range of generic tests for viruses, bacteria and fungi, as well as for pathogens known to cause yellowing symptoms, such as luteoviruses and phytoplasmas, found no consistent organism associated with YCS (Braithwaite et al. 2017). Differential gene expression found YCS to be distinct from water stress and natural senescence (Marquardt et al. 2017).
Imidacloprid is used extensively as a soil-applied insecticide to manage canegrubs in the Australian sugar industry. It is applied either in a liquid solution (e.g. Confidor® Guard, Nuprid®, Senator®, etc) to plant and ratoon crops, or as a controlled-release granular formulation applied at or shortly after planting (suSCon® maxi Intel). Anecdotal observations have reported reductions in the expression of YCS in fields where imidacloprid
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insecticides have been applied to manage canegrubs. However, the apparent level of control tends to reduce as the season progresses and, overall, these observations are inconsistent throughout the industry.
Imidacloprid is a highly systemic insecticide. It is readily taken up by the roots of plants and is distributed to the apical growing points (Kundoo et al. 2018). The systemic nature of imidacloprid means it has the potential to control a range of foliar-feeding insects in the upper canopy and apical region of the sugarcane plants. It is registered in a variety of use patterns (in crops other than sugarcane) including as a soil drench and as a foliar- applied insecticide to control a range of sucking insects that feed on the foliar region of plants (Bayer Crop Science 2019). Given the well-documented role some insect species have in vectoring disease and causing plant disorders, it is possible that the apparent control of YCS provided by imidacloprid is the result of insecticidal activity targeting foliar-feeding or root-feeding insects.
In addition to its insecticidal properties, imidacloprid and other neonicotinoid compounds such as chlothianidin and thiamethoxam have properties that alter the physiology of plants, enabling them to better manage abiotic stress. Some of the beneficial effects reported include increases in the initial growth of plants, total weight, height and root mass (Endres et al. 2016; Thielert 2006; Gonias et al. 2008; Macedo and Castro 2011).
In addition, neonicotinoids can increase the tolerance of plants to water stress (Geissler and Wessjohann 2011).
It is therefore possible that the improvements to plant health described by growers using imidacloprid is due to stress buffering rather than any insecticidal control.
Imidacloprid has a half-life in the soil environment of between 27 and 229 days depending on the prevailing environmental conditions (Miles Inc. 1993). Hence, the apparent reduction in the ability of imidacloprid to manage YCS over time could be a result of degradation of the applied product, both as a result of plant uptake and metabolism or through natural degradation in the soil environment.
There has been no research conducted to establish the validity of these anecdotal claims. We established a trial to determine the potential role of insects and mites in the expression of YCS and to determine whether there is any basis to the anecdotal observation that imidacloprid may have the potential to provide a management solution to YCS.
METHODS AND MATERIALS
DesignThe field trial was established on a commercial sugarcane farm in Stone River, Ingham. Soil type was a low pH (5.0) sandy loam and the trial was rain-fed. Sugarcane cultivar Q200A was planted on 10 September 2015. Nine insecticidal treatments (Table 1) were applied in a randomised complete-block design each with three replicates.
Plot size was four rows by 10 m with a row spacing of 1.6 m. The experiment was monitored over 6 months from January until June 2016.
Table 1. Chemicals, rates applied at each application and application method.
Treatment Active compound Rate (g ai/ha) Application method and frequency
1.Confidor® Guard Imidacloprid 250 Soil-applied at plant
2.Confidor® Guard Imidacloprid 500 Soil-applied at plant and monthly via drip tape
3.Senator® Imidacloprid 500 Soil-applied at plant
4.Actara® Thiamethoxam 460 Soil-applied at plant
5.Astral® Bifenthrin 80 Foliar-applied monthly
6.Pegasus® Diafenthiuron 375 Foliar-applied monthly
7.Movento® Spirotetramat 96 Foliar-applied (Nov, Jan, Apr)
8.Movento® Energy Spirotetramat + imidacloprid
480 spirotetramat +
480 imidacloprid Soil-applied at plant
9.Untreated control - - -
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Treatment applicationsTreatment 1: Confidor® Guard (imidacloprid low rate) was applied to the soil at plant on 10 September 2015. No further applications were made.
Treatment 2: Confidor® Guard (imidacloprid monthly) was initially applied at plant on 10 September 2015. Drip tape was then installed at plant fill-in stage on 11 December 2015. The tape was installed at a depth of 20 cm in the plant row. Thereafter, imidacloprid was applied via the drip tape on a monthly basis from 11 December 2015 until 20 June 2016.
Treatment 3: Senator® (imidacloprid) was applied to the soil at plant on 10 September 2015. No further applications were made.
Treatment 4: Actara® (thiamethoxam, neonicotinoid) was applied to the soil at plant on 10 September 2015. No further applications were made.
Treatments 5, 6, 7 were applied using an inter-row tractor utilising a high-coverage spray applicator with vertically and horizontally mounted nozzles to achieve effective over-the-top and within-canopy coverage.
Treatment 5: Astral® (bifenthrin, pyrethroid) was foliar-applied monthly commencing on 23 November 2015 and concluding on 18 May 2016.
Treatment 6: Pegasus® (diafenthiuron, acaricide/insecticide) was foliar-applied monthly commencing on 23 November 2015 and concluding on 18 May 2016.
Treatment 7: Movento® (spirotetramat) was foliar-applied on three occasions: 26 November 2015, 23 January 2016 and 12 April 2016.
Treatment 8: (Movento® Energy (imidacloprid + spirotetramat)) was applied to the soil at plant on 10 September 2015. No further applications were made.
Agronomy
Following soil testing, fertiliser was applied according to the SIX EASY STEPS nutrient recommendations (N: 130 kg/ha, P: 20 kg/ha, K: 80 kg/ha, S: 10 kg/ha). Weeds were managed in a timely manner and according to standard industry practice. Soil from the site was tested for root-lesion nematodes, root-knot nematodes and Pachymetra. All three soil pathogens were found to be below critical thresholds for yield impact.
Sampling and monitoring YCS assessment
Six stalks were randomly selected in each plot and barcode-labelled. Monitoring was conducted weekly on these stalks from 1 February 2016 to 1 July 2016. YCS severity was scored for leaves +1, +2, +3, +4, +5, +6, and +7, where leaf+1 was the first fully expanded leaf corresponding to the top visible dewlap. Ratings were assigned according to the severity rating key shown (Table 2). Stalk height, measured from the base of the stalk to the top visible dewlap, and number of leaves per stalk were also recorded at this time.
Table 2. YCS severity rating key.
Severity rating Description
0 No YCS symptoms evident.
1
Yellow present in approximately 25-50% of the leaf. It may be present in a mottling either along the leaf edges or tips or on one side of the midrib only.
2
Yellowing is present in approximately 50% of the leaf in either solid yellow or mottling form. Yellow colour exhibits a stronger orange hue than rating 1. Typically found on both leaf margins and the leaf tip, although symptoms can occur on one side of the midrib only.
3 Yellow is present in at least 75% of the leaf. Advanced yellowing across the entire leaf blade, with mottling now developed into solid colouring.
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Data were analysed using an ANOVA at a 95% confidence level using a repeated-measures design with Treatment the between-subject factor and Date the within-subject factor.
Mite sampling
Sampling was conducted visually on 10 May 2016 on all treatments except Treatment 3. We collected three stalks from each plot and counted the mites on all leaves +1 to +6.
Canegrubs
Sampling was conducted on 2 May 2016. All plots were sampled by removing two stools per plot and the number of grubs recovered from each stool recorded. Each stool was dug out and all roots and soil were placed on a yellow sheet where they were inspected carefully for the presence of canegrubs.
Chemical residue analysis
On 9 May 2016, samples of leaf and stalk were collected from the treatments imidacloprid low rate, imidacloprid monthly, imidacloprid, spirotetramat and imidacloprid + spirotetramat and sent to Bayer Crop Science (Melbourne) for chemical residue analysis using their standard residue testing protocols.
Biomass measurements
Biomass was measured in all plots at 6, 9, and 12 months after planting. Plots were hand harvested, partitioned into stalk and leaf/cabbage, and weighed. For each plot, we cut 60 stalks from two 5 m sections in the middle two rows of the plot. Stalk height was also measured at this time. Biomass data were analysed as a randomised completed-block design, with between-treatment significance determined by LSD in all-pairwise comparisons.
RESULTS
Expression of Yellow Canopy Syndrome
In general, all treatments followed the same trend. Symptom onset occurred in mid-late March, with YCS peaking in mid-late May. YCS in the untreated control was clearly more severe than in many of the insecticide treatments (Figures 1).
Figure 1. Expression of Yellow Canopy Syndrome in the untreated control and the insecticide/acaricide treatments. Each data point is the mean of 18 stalks ± standard error.
Jan Feb Mar Apr May Jun Jul Aug
# YCS leaves
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Untreated Control Spirotetramat
Imidacloprid + Spirotetramat Bifenthrin
Diafenthiuron
Jan Feb Mar Apr May Jun Jul Aug
# YCS leaves
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Untreated Control Imidacloprid Half Rate Imidacloprid Imidacloprid Monthly Thiamethoxam
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Neonicotinoid treatments showed significantly less YCS (P<0.05) than the untreated control over the duration of the trial (Figure 1). There was no difference among these treatments until mid-late May when the imidacloprid low rate treatment showed greater YCS than the higher rates.
The bifenthrin treatment also reduced the expression of YCS considerably throughout the monitoring period. The remaining treatments, although reducing YCS expression when compared to the untreated control, were largely inferior to the bifenthrin and best imidacloprid treatments (Figure 1).
Biomass
Plant biomass did not differ significantly in any treatments at 6 months but did differ at 9 and 12 months with the pattern of difference being similar on each occasion (Table 3). Final biomass yields were highest in the bifenthrin and thiamethoxam treatments at 108 t/ha followed by the high-rate imidacloprid treatments which were about 100 t/ha (Table 3, Figure 2C). The lowest yielding treatments were the untreated control and low-rate imidacloprid, diafenthiuron and spirotetramat treatments that yielded 76-84 t/ha.
Table 3. Biomass yield (t/ha) at 6, 9, and 12 months after planting.
Treatment 6 months 9 months 12 months
Untreated control 57.1a 65.6ab 79.7d
Thiamethoxam 67.0a 78.5a 108.9a
Bifenthrin 58.8a 83.9a 108.0a
Imidacloprid monthly 60.3a 72.7ab 99.5abc
Imidacloprid low rate 55.9a 69.8ab 80.7d
Spirotetramat 54.8a 55.9b 83.9cd
Imidacloprid + spirotetramat 52.4a 70.0ab 91.1bcd Imidacloprid (Senator®) 57.6a 74.9ab 101.9ab
Diafenthiuron 54.3a 69.4ab 76.0d
Means within columns followed by the same letter are not significantly different (P = 0.05)
Figure 2. Yield components at 12 months after planting: stalks per m2, stalk weight (kg), total biomass (t/ha) and CCS. All results are expressed as Mean ± SE. Columns in each component marked with the same letter do not
differ at P<0.05.
D (CCS)
Untreated Control Thiamethoxam
Pyrethroid
Imidacloprid Monthly Imidacloprid Half Rate
Spirotetramat
Imidacloprid + Spirotetramat Imidacloprid
Diafenthiuron (m iticide)
units
0 5 10 15 20 p=0.9057
a c
ab bc
abc ab abc abc abc
C (Biomass t/ha)
Untreated Control Thiamethoxam
Pyrethroid
Imidacloprid Monthly Imidacloprid Half Rate
Spirotetramat
Imidacloprid + Spirotetramat Imidacloprid
Diafenthiuron (m iticide)
t/ha
0 20 40 60 80 100 120 140
p=0.0038
a ab
abc ab
bcd d cd
d d
B (Stalk weight)
kg
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
ab abc p=0.0013
a ab
bc bcd de cde
e A (Stalks m2)
stalks per m2
0 1 2 3 4 5 6 7 8 9 10 11 12
p=0.0290
a a
a
b b
ab ab ab ab
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Mirroring the yield results were significant differences in the stalk counts observed at 12 months, with the lowest counts from the diafenthiuron and spirotetramat treatments (Figure 2A). However, there were no differences in the stalk populations among neonicotinoid treatments and the untreated control. Average stalk weights also differed significantly among treatments (P<0.05) (Figure 2B). Stalks were 26%, 23%, 18%, 16% and 15%
heavier than in the untreated control for imidacloprid monthly, thiomethoxam, imidacloprid half-rate, imidacloprid high-rate, and imidacloprid + spirotetramat treatments, respectively. Water content was similar among treatments and within the range of 79-83% and 74-76% for stalk and leaf/cabbage respectively (data not shown).
CCS did not differ significantly among treatments (Figure 2D).
Canegrubs and mites
Only one greyback canegrub (Dermolepida albohirtum) was found in the trial suggesting that canegrubs were not a problem at this site.
The number of mites observed on the leaves differed between treatments (P<0.02) with the highest mite populations in the bifenthrin treatment at an average of 11.7 mites per stalk (Figure 3). However, given the low numbers of mites and absence of a pattern corresponding to YCS expression, it is unlikely mites are the causal agent.
Figure 3. Mean (±SE) number of mites per stalk in each treatment. Columns marked with the same letter do not differ at P<0.05.
Table 4. Insecticide residues detected*.
Sample type
Active ingredient and treatment
Active ingredient detected (mg/kg of)
Leaf
Imidacloprid monthly 0.58 Imidacloprid low rate 0.02 Imidacloprid high rate 0.05
Spirotetramat <0.01
Imdacloprid + spirotetramat 0.04 + <0.01
Stalk
Imidacloprid monthly 0.19 Imidacloprid low rate <0.01 Imidacloprid high rate <0.01
Spirotetramat <0.01
Imdacloprid + spirotetramat 0.01 + <0.01
*Limit of quantification is 0.01 mg/kg. Levels less than this expressed as <0.01.
0 5 10 15
Mean (SE) mites per stalk
a
ab b b
ab
b ab
ab
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Chemical residue analysisOnly residue from the imidacloprid monthly treatment was present in detectable levels in the stalk samples, whereas all treatments containing imidacloprid were detected in leaf tissue (Table 4). In leaf tissue, the imidacloprid monthly treatment had a concentration 11 times that of the single-applied treatment. The imidacloprid low-rate treatment was still present at a detectable level.
DISCUSSION
Our experiment was established to determine the potential role of insects in the expression of YCS and to test whether there is any basis to the anecdotal observations that imidacloprid applied to control cane grubs provides some control of YCS. Our results support the observation that YCS expression can be reduced by applying imidacloprid-based insecticides in the root zone of sugarcane plants at rates registered for canegrub control. In addition, Senator®, another insecticide containing imidacloprid, gave similar suppression to Confidor® Guard, indicating that the suppressive impact is not peculiar to one product but is related to the active ingredient.
The low-rate imidacloprid treatment was chosen to test whether the impact of imidacloprid was the result of its
‘stress shield’ properties as reported by Thielert (2006). Under situations where stress shield is apparent, only low concentrations of imidacloprid are required (Bayer Crop Science (pers. comm)). Our results suggest that YCS expression was reduced by the low rate early in the season but, as the season progressed, YCS expression approached that in the untreated control and eventual yield was also similar to the control treatment. Residue analysis demonstrated that imidacloprid was still detectable in the leaf tissue (taken from the cabbage), albeit at low levels, suggesting that in this trial stress shield was unlikely to have impacted either YCS expression or yield.
The monthly application of imidacloprid in the root zone reduced YCS expression and increased yield to a similar extent to the high rate applied only once. This suggests that the application of additional imidacloprid over and above the high rate commonly used for canegrub management provided no additional benefit.
The neonicotinoid thiamethoxam was the highest yielding treatment and suppressed the expression of YCS to a similar extent to the treatments containing high rates of imidacloprid. These observations suggest that the positive benefit observed from imidacloprid may extend to other neonicotinoid compounds.
Like thiamethoxam, the broad-spectrum insecticide bifenthrin also limited the expression of YCS and had a similar yield benefit. Bifenthrin is not systemic and has no known direct plant physiological impact, strongly suggesting that the benefit obtained from it may be the result of direct insecticidal activity.
The acaricide diafenthiuron did not reduce the expression of YCS symptoms and resulted in a yield below that of the control. The highest number of mites occurred in the bifenthrin treatment, one of the highest yielding treatments, suggesting that mites were not a significant impactor on yield. Although not a comprehensive trial of miticides and acknowledging the single sampling date, we consider this is sufficient evidence to conclude that mites are unlikely to be the primary causal agent of YCS.
Spirotetramat is also known to have some activity on mites (Bayer Crop Science pers. comm.) and to have some positive physiological impacts on plant growth similar to that observed with imidacloprid. Although mites were reduced over the control, this reduction was not significant and YCS was still evident at high levels. Yield was also relatively low, although it should be noted that high levels of phytotoxicity were evident following application of the insecticide that may have impacted on yield.
The treatments that had the greatest impact on YCS expression and the most positive impact on yield were those that likely have significant impacts on foliar-feeding sucking insects. We applied the neonicotinoids to the soil.
Neonicotinoids are readily absorbed and translocated by root systems and leaves alike, making these compounds highly systemic (Kundoo et al. 2018). Our residue studies confirmed the presence of imidacloprid in the leaf tissue and it is reasonable to assume that thiamethoxam would have also been present in the leaves.
Bifenthrin also provided a positive impact; it is a broad-spectrum insecticide which we chose to eliminate as many insects as possible from the surface of the leaf.
Due to resource limitations, the experimental design did not include comprehensive insect sampling throughout the season for foliar insects. As a result, we cannot draw any conclusions as to what might be managed by the neonicotinoid or the pyrethroid treatments. However, it is well known that neonicotinoids are particularly effective on sucking insects. This group would similarly be controlled by bifenthrin.
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Our findings suggest further work is warranted to determine which insects are being controlled and to identify the mechanism for the positive yield response that we observed.
ACKNOWLEDGEMENTS
We acknowledge the technical assistance provided by Megan Zahmel in establishing and maintaining the field trials and the assistance provided by Karel Lindsay in sampling for mites. Funding was provided by Sugar Research Australia Limited through project 2014/049 and by the Queensland Department of Agriculture and Fisheries
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