Introduction
As
orchid pests, melon thrips (Thrips palmi Karny)
in Thailand have a significant economic impact (Maketon et al.
2014). When
no insecticide is applied, they attack over 74% of blooms, and being
quarantined insects, prevent the export of infested orchids. The farmers must
prevent the infestation of melon thrips on
the plants to meet international phytosanitary standards and acceptance by the
exporters and importers (MOAC 2009).
Insecticide spraying is the most popular method
to keep melon thrips away from orchids because it is more convenient, faster,
and easier to implement than other preventative methods. However,
orchid growers in Thailand continue to face problems with spraying applications
in the nursery, mainly due to a shortage of labour and increasing wages (Punyawattoe
et al. 2016). With the traditional spray lance technique, Droplet deposition and uniformity
in the orchid canopy is poor, and resulting in a lack of efficacy to control
insect pests especially melon thrips. The nursery structure also makes it
difficult to import the necessary mechanisation (Punyawattoe
et al. 2019). In addition, the agricultural sector is raising concerns
about the risk of operator exposure and
contamination to the environment by agrochemicals. To
address these issues, the Government of Thailand is establishing a project
under the Plant
Protection Research and Development Office, Department of Agriculture, to
develop pesticide application technology in the orchid nursery. The objective
of the project is to develop innovative spray equipment or spraying techniques
for orchid growers. The
expectations are to find very simple, cheap and suitable equipment for
practical use in the field.
This research is also
consistent with Thailand’s policies on agricultural product quality and market competition. The
main objectives of this study are to assess and compare the efficacy of boom
sprayers as an innovative spray equipment and traditional spray lances in terms
of droplet deposition, spray loss and controlling the melon thrips population
on orchids including analysing the spraying time, amount and cost of insecticide. The
data collected will form the basis of an innovative spray application to
Thailand’s precision plant protection system.
Experiment site
The field trials were conducted at
a commercial Dendrobium orchid farm in Samphran
District, Nakhonpathom Province, Thailand. The experiments were performed on orchids measuring 0.4 m in height
from the orchid bench, plots of 20 m in length and 7 m in width, covering a
total area of 140 m2/ plot at a distance of 7 m from the edge of
each plot to create a buffer zone and avoid cross-contamination between
treatments (Fig. 1).
Spraying equipment
and spray volume
Three different items of spraying equipment were
designed for testing (Table 1). A conventional spray lance fitted with a traditional
hollow cone nozzle had an orifice of 2.0 mm in diameter. This was attached to a
high-pressure pump sprayer and applied with a pressure of 4 bar to deliver a
spray volume of 1000 L/ha according to the farmer’s practice.
The new vertical boom was fixed with a hose via an external pump and tank,
consisting of three flat fans (TeeJet XR8004) with a
nozzle spacing of 0.4 m installed at an offset angle of 10 degrees. The self-propelled boom consisted of six
flat fan nozzles (TeeJet XR8004) with a nozzle
spacing of 0.4 m. The spray pressure was set at 4 bar to deliver a spray
volume of 750 L/ha which is the recommended rate.
Spraying swath width
The spray lance was covered a swath, 0.5 m wide on both sides of each
orchid bench. The vertical boom was covered a swath one-metre-wide
along one side of the orchid bench before being turned to spray the other bench. The self-propelled boom was covered a swath two
metres wide along two sides of the orchid bench, and the operator moved the
boom along every other row (Fig. 2).
Droplet deposition and spray loss
To evaluate droplet
deposition and spray loss, a 1% solution of tartrazine (C16H9N4Na3O9S2)
was sprayed. Orchid samples were collected from the plots, each containing 120
flowers. For spray loss evaluation, the samples were placed in petri dishes,
five per row of orchids and kept in a UV-proof container. The samples were
washed in 10 mL of distilled water and tested for optical density using a
microplate reader, set to a 470 nm light.
The
amount of tracer contained in the sample was calculated according to each
target measurement. Correction factors like measurement range, dilution, and
the volume of absorbent liquid were considered. The droplet deposition and
spray loss results are provided in µL/flower
and µL/cm2, respectively.
Efficacy in the control
of melon thrips
To evaluate the
effectiveness of boom sprayers and spray lance against melon thrips, trials
were conducted using four treatments, including untreated control from May to July 2019
(Table 2). To assess the bio-efficacy
of spraying techniques, spinetoram 12% SC (Exalt) was sprayed at the
recommended rate of 10 mL/20L of water. Prior to spraying, the density of melon
thrips was assessed by collecting 20 randomly selected inflorescences per plot.
Four melon thrips per inflorescence (economic threshold level) indicated the
need for spraying (Srijuntra et al. 2019).
Treatments were evaluated three, five and seven days after spraying (PPRD 2012).
Analysis
of spraying time and cost of insecticide
Data on
spraying time and the cost of insecticide were collected at three different time intervals
between September to October
2019, covering an area of 1 Rai (0.16 ha) which is the official measurement
unit in Thailand. The cost of insecticide was determined after spraying by
weighing the quantity of insecticide used. The quantity of insecticide was
converted into US dollars to compare the average product cost per area, using
the prices charged by local agricultural chemical suppliers.
Statistical analysis
The experimental layout for the three treatments consisted of the
Randomized Complete Block Design. Seven replications were used for spray
deposition and spray loss. Four treatments with five replications were used to
assess the efficacy of the experiments on melon thrips. The means of total
spray deposition, spray loss and melon thrips population were compared using
analysis of variance (ANOVA) and Tukey’s test with SPSS v. 22.0 software (SPSS,
Inc., IBM, Chicago, IL, USA).
Results
Droplet deposition
and spray loss
The spray
lance at 1.40 µL/flower exhibited the
highest droplet disposition, followed by the self-propelled boom, and vertical
boom at 1.32 and 1.14 µL/flower,
respectively. However, they were not significantly
different, although the
%20315-320_files/image002.png)
Fig. 1: Experimental layout: (a)
spacing of orchid benches in the orchid nursery and (b) field plot layout for testing the plot and buffer zone
%20315-320_files/image004.png)
Fig. 2: Swath spraying width: (a) a
0.5-metre-wide swath using the spray lance; (b) a
one-meter-wide swath using the vertical boom and (c) a
two-meter-wide swath using the self-propelled boom
Table 1: Details of application parameters for spraying techniques in the
experiments
|
Treatment |
Nozzle type |
Number of
nozzles |
Flow rate
(L/min)a/ |
Spray
volume (L/ha) |
Swath width
(m) |
Number of
swath per tested plot |
|
Spray lance |
Hollow cone
nozzle ∅ 2.0 mm |
1 |
4 |
1,000 |
0.5 |
8 |
|
Vertical boom |
Fan XR8004 |
3 |
5.7 |
750 |
1.0 |
4 |
|
Self-propelled boom |
Fan XR8004 |
6 |
11.4 |
750 |
2.0 |
2 |
a/ At spray pressure of 4 bar
Table 2: Details of treatment on the
bio-efficacy test
|
Treatment |
Spray volume (L/ha) |
Insecticide |
Recommendation rate mL/20 L
of water |
|
Spray lance |
1,000 |
Spinetoram
12% SC (Exalt) |
10 |
|
Vertical boom |
750 |
10 |
|
|
Self-propelled boom |
750 |
10 |
|
|
Untreated control |
- |
- |
- |
Table 3: Means of
droplet deposition and spray losses among spray application techniques at Samphran district, Nakhonpathom
Province, Thailand during May, 2019
|
Treatment |
Spray volume (L/ha) |
Droplet deposition on
orchid flower (µL/flower) |
Spray losses
to the ground (µL/cm2) |
|
Spray lance |
1,000 |
1.40 |
0.47 a |
|
Vertical boom |
750 |
1.14 |
0.34 b |
|
Self-propelled boom |
750 |
1.32 |
0.33 b |
According to Tukey's Honest Significant Difference (HSD)
at P ≤ 0.05 means in the same
column followed by the same letter are not significantly different
Table 4: Efficacy of
spinetoram (Exalt 12 % SC) for controlling melon thrips; Thrips palmi Karny with different spray application techniques at Samphran district, Nakhonpathom Province, Thailand,
during May 2019 (1st trial)
|
Treatment |
Spray volume (L/ha) |
Insecticide usage
(mL/ha) |
Means of
thrips/inflorescences |
|||
|
Before spraying |
3 DASa/ |
5 DAS |
7 DAS |
|||
|
Spray lance |
1,000 |
500 |
4.13 |
0.45 b |
0.48
b |
0.35
b |
|
Vertical boom |
750 |
375 |
4.28 |
0.50 b |
0.43
b |
0.33
b |
|
Self-propelled boom |
750 |
375 |
4.15 |
0.35 b |
0.28 b |
0.25 b |
|
Untreated
control |
- |
- |
4.30 |
3.88 a |
4.18 a |
4.23 a |
a/DAS = Day after spraying: According to Tukey's
Honest Significant Difference (HSD) at P
≤ 0.05 means in the same column followed by the same letter are not
significantly different
Table 5: Efficacy of
spinetoram (Exalt 12 % SC) for controlling melon thrips; Thrips palmi Karny with different
spray application techniques at Samphran district, Nakhonpathom Province, Thailand, during
July 2019 (2nd Trial)
|
Treatment |
Spray volume (L/ha) |
Insecticide usage
(mL/ha) |
Means of
thrips/inflorescences |
|||
|
Before spraying |
3 DASa/ |
5 DAS |
7 DAS |
|||
|
Spray lance |
1,000 |
500 |
4.93 |
0.53 b |
0.52 b |
0.32
b |
|
Vertical boom |
750 |
375 |
4.85 |
0.63 b |
0.40 b |
0.35 b |
|
Self-propelled boom |
750 |
375 |
4.88 |
0.48 b |
0.38 b |
0.25 b |
|
Untreated
control |
- |
- |
5.00 |
4.28 a |
3.89 a |
3.95 a |
a/DAS = Day after spraying: According to Tukey's
Honest Significant Difference (HSD) at P
≤ 0.05 means in the same column followed by the same letter are not
significantly different
spray lance delivered 33% more spray volume
than the new spray booms. It was evident from
the observed data that the spray lance produced the highest spray loss at 0.47 µL/cm2, which was
significantly more than the self-propelled and vertical booms, which exhibited
spray losses of 0.34 and 0.33 µL/cm2,
respectively (Table 3).
Efficacy of melon
thrips
The recorded melon thrips infestation
was based on the number of melon thrips/inflorescence (Table 4–5).
The first trial
Although the spray lance delivered 33% more volume than the new spray
boom, none of the applications was found to be significantly effective in the
reduction of melon thrips population at the 5% level of significance. The melon thrips population varied from 0.35–0.50,
0.28–0.48 and 0.25–0.35 thrips/inflorescences at 3 DAS, 5 DAS and 7 DAS,
respectively. The means of melon thrips after all spray applications were not
significantly different from each other (Table 4).
The second trial
Similar trends were observed at 3 DAS, 5 DAS and 7 DAS for the spray
lance, vertical boom and self-propelled boom, none of which were significantly
effective in the reduction of the melon thrips population at the 5% level of significance.
The melon thrips population varied from 0.48–0.63, 0.38–0.52 and 0.25–0.35
thrips/inflorescences at 3 DAS, 5 DAS and 7 DAS, respectively. Moreover, all spray applications in both fields were
found to be better in comparison to the untreated control group.
Table 6: Summary of means of spraying time in minutes, amount
and cost of insecticide
|
Parameters |
Spraying equipment |
||
|
Spray lance |
Vertical boom |
Self-propelled
boom |
|
|
1. Spraying time
(min: sec) |
|
|
|
|
1st
spraying |
41:24 |
22:16 |
9:33 |
|
2nd
spraying |
42:18 |
21:38 |
11:06 |
|
3rd
spraying |
40:42 |
20:54 |
10:48 |
|
Means |
41:28 |
21:36 |
10:29 |
|
- |
47.90% |
74.71% |
|
|
2. Quantity of
insecticide (mL/0.16 ha) |
|
|
|
|
1st
spraying |
83 |
64 |
55 |
|
2nd
spraying |
85 |
62 |
64 |
|
3rd
spraying |
82 |
60 |
62 |
|
Means |
83.33 |
63 |
60.33 |
|
Decreasing
quantity of insecticide vs spray lance |
- |
25.00% |
27.38% |
|
3. Cost of
insecticide (US dollar/0.16 ha) a/ |
|
|
|
|
1st
spraying |
3.48 |
2.68 |
2.30 |
|
2nd
spraying |
3.56 |
2.60 |
2.68 |
|
3rd
spraying |
3.44 |
2.51 |
2.60 |
|
Means |
3.49 |
2.64 |
2.53 |
|
- |
24.35% |
27.50% |
|
a/ 1 US dollar = 31.03 Thai Baht and Spinetoram package
250 mL = 41.89 US dollar (1,300 Thai Baht)
Analysis of spraying time and amount and cost of insecticide
It is evident that the spraying
time of the spray lance was the slowest on average at 41:28 min/0.16 ha. Use of
the vertical and
the self-propelled booms was found to reduce the spraying time by approximately
20 to 30 min or 47.90 to 74.71% of time consumption.
The vertical and self-propelled
booms were able to reduce the amount of insecticide usage in the orchid
nursery by 25 and 27.38%, respectively (Table 6). When comparing the cost of
insecticide, the self-propelled boom was the cheapest, followed by the vertical boom, with the spray lance being the most
expensive. The vertical and self-propelled booms were found to reduce the cost
of insecticide by 24.35 and 27.50%, respectively (Table 6).
The droplet
deposition and effectiveness of the vertical and self-propelled booms in
controlling the melon thrips population were not significantly different,
although the spray lance delivered a higher spray volume. The results suggest
that since the droplets produced by the two boom sprayers were uniform, they
could penetrate the orchid flower equally. Furthermore, the boom sprayer has a
high-quality nozzle, enabling it to provide smaller droplets for better
coverage inside the inflorescences. The spray can therefore be transported
through the air more easily, as demonstrated by Sánchez-Hermosilla
et al. (2011), Balsari et al. (2012). When operating the spray lance, the
effectiveness of the process also depends on the skill of the worker involved.
In contrast, when operating the boom sprayers, the workers merely need to hold
or maintain the boom level over the target area (Nuyttens
et al. 2009; Braekman et al. 2010). Consequently, boom spraying, and lance
spraying were equally good when using the same volume of spray or less.
The results indicated trend similar to previous
studies, where high-volume spraying leads to greater spray loss. A higher spray
volume tends to result in more insecticide landing on ground inside the field
in comparison to when less spray is used (Braekman et
al. 2009;
Sánchez-Hermosilla et
al. 2012; Rincón et
al. 2017; Failla and Romano 2020).
The results of our experiments indicated that
the boom sprayer offers a new innovative alternative to the current technique
used in Thailand’s orchid nurseries since it has the potential to provide a
better droplet deposition. Furthermore, it is less time-consuming and may reduce insecticide
usage and spraying cost.
Conclusion
The boom sprayer is
an innovative application technique for reducing spray time, spray loss,
quantity and cost of insecticide to control the field population of melon
thrips. Boom sprayers offer an
efficient alternative to lance sprayer and may help to mitigate
issues relating to the lack of manpower and increasing cost of imported spray equipment. Further studies are required to
identify the effects of vertical nozzle spacing and distance to crop on the
spray deposition to improve the spray efficacy in an orchid nursery.
Acknowledgements
The researchers would like to thank the staff from the Pesticide
Application Research Section, Entomology and Zoology Group, Plant Protection
Research and Development Office, Department of
Agriculture, Thailand, for their invaluable assistance in this research.
Author Contributions
Punyawattoe P and Sampaothong S conceptualised part
of the research, performed the experiments, data collection, analysis and
compiling and editing the manuscript. Sutjaritthammajariyangkun
W, Thirawut S, Chaiyasing
N, and Supornsin S performed the experiments and were
involved in data collection.
Conflict of Interest
All authors declare no
conflict of interest
Data Availability
Data presented in this study
will be available on a fair request to the corresponding author.
Ethics Approval
Not applicable to this paper.
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