Khawaja Ghulam Rasool1, Mureed
Husain1*, Shehzad Salman1, Naeem Abbas2,
Khalid Mehmood1,3, Koko Dwi Sutanto1
and Abdulrahman Saad Aldawood1
1Economic Entomology Research
Unit, Plant Protection Department, College of
Food and Agriculture Sciences, King Saud University, Riyadh 11451, Saudi Arabia
2Plant
Protection Department, College of Food and Agriculture Sciences, King Saud University,
Riyadh 11451, Saudi Arabia
3Institute
of Plant Protection, Faculty of Agriculture and Environmental Sciences,
MNS-University of Agriculture, Multan, Pakistan
*For correspondence: mbukhsh@ksu.edu.sa
Received 22 July 2020; Accepted 12
February 2021; Published 16 April 2021
Invasions of the red palm weevil
(RPW) Rhynchophorus ferrugineus (Olivier) has
become a well-known problem for several palm species in date palm-growing
states, including Saudi Arabia. Although several control measures have been
implemented in efforts against RPW, yet, no approach has proven consistently
effective against this invasive pest. Therefore, further efforts are still
needed to develop an effective and efficient control method for RPW management
and mitigation. The objectives of the present study were to determine emamectin benzoate (ARETOR) efficacy against RPW larval
stages, in the laboratory using a diet incorporation bioassay, along with field
tests of the Syngenta Tree Micro-Injection Technique. Our laboratory bioassay
showed that the median lethal concentration of emamectin
benzoate (ARETOR) was 0.30 μg/mL−1 against wild
populations of RPW larvae. In the field studies, emamectin benzoate (ARETOR) showed
promising effects on RPW mortality for all stages. We
found 100% mortality for both larval and pupal stages for all exposure times,
except after 2 months, where 86% mortality was observed for larvae. Thus, we conclude that emamectin benzoate (ARETOR)
is an effective biopesticide against RPW. Our results
show that it killed RPW all stages, also prevented further damage to the
healthy tissues of date palms, in addition, provided protection against RPW
spread to other neighboring healthy date palms. © 2021 Friends
Science Publishers
Keywords: Date palm; Chemical
control; Emamectin benzoate (ARETOR); Red palm weevil; Infestation
The date palm Phoenix dactylifera (Linnaeus) is an oldest
important crop cultivated in the Arabian Peninsula and in temperate regions of
the world (Chao and Krueger 2007). Many
insect pests are known to attack and damage date palms, however the red palm
weevil (RPW), Rhynchophorus ferrugineus (Olivier) (Coleoptera: Curculionidae) is
known as a primary pest. The RPW has emerged as one of the most destructive
pests of date palm worldwide and young date palms are more vulnerable to RPW
attack (Faleiro 2006). Both larvae and
adults of RPW feed inside the tree trunk and complete many generations inside a
single palm tree (Faghih 1996; Salama et al.
2009). The life cycle of red palm weevil may range from 3 months and
above depending upon the rearing medium. In the date palm trunk red palm weevil
population stays as long as the entire tissues of the trunk are
available to feed.
The RPW infestation of date palm requires intensive investigation for
proper management. Several non-chemical approaches have been applied, but still
the problem persists, and chemical use cannot be ignored as a curative measure.
Many previous studies have investigated the efficacy of different insecticides,
such as imidacloprid, deltamethrin, and fipronil, under laboratory and field conditions
against RPW (Azam and Razvi 2001; Kaakeh 2006;
Dembilio and Jacas Miret 2012). Imidacloprid was found to be an
excellent insecticide, at concentrations of 3.5 mL/L and 1000 mL/L of water in
the laboratory and field, respectively (Kaakeh
2006). Similarly, imidacloprid applied at a rate of 1 mL/L to 2nd
and 4th instar RPW larvae resulted in 84 and 79% mortality,
respectively, after 20 days of exposure (Malik et
al. 2016). Deltamethrin was shown to be highly toxic against
20-day-old RPW larvae and adults after 24 h of exposure, followed by the
application of emamectin benzoate and imidacloprid (Shawir et al. 2014). Al–Shawaf et al. (2010) found that
fipronil was the most toxic insecticide against both larval and adult stages of
the RPW and date palm offshoots treated with fipronil (0.004%) were prevented
from attack by RPW (Al-Shawaf et al.
2013). Organophosphate insecticides, pirimiphos-methyl
and oxydemeton-methyl, also exhibited strong effects
against adult and larval stages of RPW, respectively (Ajlan et al. 2000).
Trunk injection with a mixture of endosulfan and
dimethoate resulted in 100% recovery of infested date palms (Azam and Rizvi
2001).
Emamectin benzoate, a
biopesticide, has a mode of action involving glutamate-gated chloride channel
allosteric modulators (Sparks and Nauen
2015). It is toxic to foliar insects and borers, and has been
demonstrated to protect trees from pest infestation (Mashal and
Obeidat 2019). For example,
a population of armyworm Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae),
in a rice field was reduced up to 92% following the application of emamectin benzoate at 0.3 g/L (Mainali
et al. 2014; Burkhard et
al. 2015). Similarly, emamectin
benzoate injected into date palms and ornamental palms killed 100% of RPW
larvae, and cured 95% trees of infestation after one year in Spain and Jordan.
Moreover, no residues of emamectin benzoate were
detected in the fruit of injected trees (Gomes and Ferry 2019; Mashal and Obeidat
2019).
Emamectin benzoate (ARETOR) is currently in use for the control of RPW in Saudi
Arabia. However, there have been no reports on the toxicity of emamectin benzoate (ARETOR) against RPW under laboratory
and field conditions in Riyadh, Saudi Arabia. Therefore, we evaluated the
toxicity/efficacy of emamectin benzoate (ARETOR) on
RPW under laboratory as well as field conditions to determine the effectiveness
of this insecticide in Riyadh, Saudi Arabia.
A sample of 150–200 RPW larvae, pupae, and adults were collected from a
date palm field located in Dirab, Riyadh, Saudi
Arabia (24.4164°N, 46.5765°E). Insects were collected manually and kept in 1 kg
plastic boxes. After collection, insects were transferred on the same day to
the laboratory for rearing and were maintained to get the F1
progeny. The larvae were reared on an artificial diet consisting of ground date
palm petioles, corn flour, wheat flour, and distillate water as primary
components, with potassium benzoate, sorbic acid, ascorbic acid, and agar added
as protectants from contamination. All the ingredients were mixed together with
a specific composition as follows: 500 g of ground petiole, 250 g wheat flour,
250 g corn flour, and 2 g ascorbic, 1.6 g potassium benzoate, 1.6 g sorbic
acid, 20 g agar and 2 litres distilled water) (Mehmood et al. 2018). The
population was maintained in a growth chamber (Steridium,
Australia) at 27 ± 2°C, 70 ± 5% RH, on a 12:12 h (light: dark) photoperiod. The diet was refreshed
every 3 days.
Emamectin benzoate (Revive/Aretor 4EC, Syngenta, Switzerland) was used to assess its toxicity in RPW larvae. A bioassay was
performed on fourth instar larvae using the diet incorporation method (Abbas et
al. 2012). Briefly, five serial diluted concentrations of emamectin benzoate were incorporated into a semi-synthetic
diet. Each concentration was repeated 3 times. After mixing thoroughly, the treated diet was transferred into 50 cm diameter plastic cups.
Four plastic cups were prepared for each replicate, and an individual larva was
placed in the plastic cup. Twelve larvae were used for each concentration, for
a total of 60 larvae for the bioassay. The bioassay was kept in a growth
chamber at 27 ± 2°C, 70 ± 5% RH and 12:12 h (light:dark) photoperiod. Larval mortality was
documented at 72 h post-treatment. All larvae that did not make a coordinated
movement when touched with a fine hair camel brush were considered dead.
To test the field efficacy of Syngenta Tree Micro-Injection Technique, a
formulation of Revive/(ARETOR) insecticide
containing 4% emamectin benzoate micro-emulsion was
injected into date palms naturally infested by RPW. Experiments were
carried out using a complete randomized block design at Alraiyanh
date palm farm, Al-Kharj, Riyadh region, Saudi Arabia (24.14.84°N, 15.182°E).
Each Revive/Aretor insecticide treatment was coupled
with a control (water injection) and observed at three time points:
after 2, 6 and 12 months. Each treatment contained 10 replications, with one
individual palm tree in each replicate. In total, 60 date palms were used in
this experiment.
Medium-sized date palms of similar age approximately 12–15 years old,
with medium RPW infestation levels, were selected randomly based on externally
visual symptoms by experienced personnel of the Ministry of Environment, Water,
and Agriculture, Riyadh, Saudi Arabia. These external symptoms included tunnels
on the trunks of trees and at the bases of date palm fronds, the presence of
frass consisting of chewed plant tissue with fermenting odour, the oozing of
thick brown liquid material, and the remains of weevil cocoons around the tree.
Selected date palms were treated with 48 mL Revive/ ARETOR
insecticide per tree, dispersed around four directions (12 mL/direction). Four
injection points were marked at the lower level of trunk bases (above the
roots) in four directions (East, North, West, and South) and holes were made
using a drilling machine equipped with a brad point drill-bit (diameter, 8 mm)
at an angle of 15–20 degrees, to a depth of 1/3 trunk diameter. The insecticide
was delivered undiluted into the trunk immediately after drilling using the
Tree Micro Injector (TMI) device (Fig. 1). After injection, a biodegradable
micro-injection plug was inserted into the drilled hole to act as a barrier
restricting any backflow of the insecticide.
According to the experimental plan, insecticide efficacy data
were recorded at three time points as mentioned above after treatment. To
record observations, each tree was cut down at the base, and then the trunks were cut
longitudinally into one-meter lengths to make the logs. All logs were
inspected carefully from top to base, and any sign of RPW infestation was
recorded. In addition, each part was split first into two equal halves and then
into quarters for detailed interior observations. Each quarter was thoroughly
observed (peripheral side, inner side, top, and base) to record any signs of
RPW infestation (galleries) and record counts of live and dead RPW individuals (Table
3). The method for
cutting the trees is shown in Fig. 2 and the log coding in Table 1. The
levels of RPW infestation were classified as either good, moderate, medium,
heavy or dead, according to defined RPW infestation index rates (Fig. 3).
The data were analyzed and means were
compared using Analysis of Variance and Least Significant Difference test (P ≤ 0.05) using SAS 9.2 (SAS 2008). Bioassay data were analyzed by probit analysis using
POLO PLUS software (LeOra Software 2002) to determine
the median lethal concentration of emamectin
benzoate.
Due to rapid pest-killing action, chemicals are
recommended for use against various pests worldwide, including in Saudi Arabia.
Knowledge of the toxicity of different synthetic chemicals is crucial for selection
of the most potent
Fig. 1: Tree Micro
injector (TMI) Device
Fig. 2: Dissection of date palm for
detailed observation and data collection
Fig. 3: Established levels of red
palm weevil infestation index
compounds for the management of insect pests. The biopesticide emamectin benzoate (ARETOR) is a powerful weapon for
integrated pest management due to its high selectivity and its comparative
safety for the environment and non-targeted fauna. In the present study, the LC50
of emamectin benzoate (ARETOR) was 0.30 µg/mL,
suggesting higher toxicity of the insecticide against RPW in the laboratory
(Table 2).
After cutting of the date palms it
was found that some of the date palms in all treatments
were heavily infested, but these infestations were old. In contrast, some trees
were completely healthy. Mortality rates of all RPW stages after 2, 6 and 12
months of pesticide application were statistically compared (Table 3).
The results revealed significant difference between the
treatment and control for both larval and pupal RPW stages after 2 months of
pesticide application according to the found and collected individuals (although
surprisingly few RPW individuals (live or dead) were found).
Mortality rates were 86 and 100% for larvae and pupae, respectively. Date palms
observed after 6 months of pesticide application revealed 100% mortality, with
12 dead RPW larvae. Date palms observed after 12 months of pesticide
application revealed 100% larval mortality and revealed significant difference
in comparison with the control. Moreover, no live or dead RPW pupae were
recorded.
Discussion
Table
1: General coding of experimental logs
used in field study
Date palm |
Log code |
Log quarter designation |
Final code |
|||||
Quarter |
Position |
Up/down final code |
Direction |
Side |
Position |
|
||
Tree No… |
Log X first meter from bottom |
1 |
Upper side |
1X1U |
East |
A |
left side of log quarter |
1X1A |
Down side |
1X1D |
B |
right side of log quarter |
1X1B |
||||
2 |
Upper side |
1X2U |
South |
A |
left side of log quarter |
1X2A |
||
Down side |
1X2D |
B |
right side of log quarter |
1X2B |
||||
3 |
Upper side |
1X3U |
North |
A |
left side of log quarter |
1X3A |
||
Down side |
1X3D |
B |
right side of log quarter |
1X3B |
||||
4 |
Upper side |
1X4U |
West |
A |
left side of log quarter |
1X4A |
||
Down side |
1X4D |
B |
right side of log quarter |
1X4B |
||||
Log Y Second meter from bottom |
1 |
Upper side |
1Y1U |
East |
A |
left side of log quarter |
1Y1A |
|
Down side |
1Y1D |
B |
right side of log quarter |
1Y1B |
||||
2 |
Upper side |
1Y2U |
South |
A |
left side of log quarter |
1Y2A |
||
Down side |
1Y2D |
B |
right side of log quarter |
1Y2B |
||||
3 |
Upper side |
1Y3U |
North |
A |
left side of log quarter |
1Y3A |
||
Down side |
1Y3D |
B |
right side of log quarter |
1Y3B |
||||
4 |
Upper side |
1Y4U |
West |
A |
left side of log quarter |
1Y4A |
||
Down side |
1Y4D |
B |
right side of log quarter |
1Y4B |
||||
Log Z Third meter from bottom |
1 |
Upper side |
1Z1U |
East |
A |
left side of log quarter |
1Z1A |
|
Down side |
1Z1D |
B |
right side of log quarter |
1Z1B |
||||
2 |
Upper side |
1Z2U |
South |
A |
left side of log quarter |
1Z2A |
||
Down side |
1Z2D |
B |
right side of log quarter |
1Z2B |
||||
3 |
Upper side |
1Z3U |
North |
A |
left side of log quarter |
1Z3A |
||
Down side |
1Z3D |
B |
right side of log quarter |
1Z3B |
||||
4 |
Upper side |
1Z4U |
West |
A |
left side of log quarter |
1Z4A |
||
Down side |
1Z4D |
B |
right side of log quarter |
1Z4B |
Table 2: Toxicity of emamectin benzoate in field population of red palm weevil
Population |
Number of larvae |
LC50 (FL 95%) (μg mL−1) |
Slope ± SE |
df |
𝜒2 |
P |
Field (G1) |
72 |
0.30 (0.23-0.38) |
4.45 ±1.07 |
4 |
7.65 |
0.10 |
Table 3: Field efficacy of Syngenta Tree Micro-injection Technique using Revive/ARETOR insecticide against red palm weevil
Stage |
Exposure (months) |
Treatment |
No. of trees |
Average alive |
Average dead |
Total average |
Mortality (%) |
Larvae |
2 |
Emamectin benzoate |
5 |
0.4 |
2.4 |
2.8 |
86a |
2 |
Control |
3 |
1.3 |
0 |
1.3 |
0b |
|
Pupae |
2 |
Emamectin benzoate |
5 |
0 |
1 |
1 |
100a |
2 |
Control |
3 |
0 |
0 |
0 |
0b |
|
Larvae |
6 |
Emamectin benzoate |
3 |
0 |
12 |
12 |
100a |
6 |
Control |
3 |
0 |
0 |
0 |
0b |
|
Pupae |
6 |
Emamectin benzoate |
3 |
0 |
0 |
0 |
0 |
6 |
Control |
3 |
0 |
0 |
0 |
0 |
|
Larvae |
12 |
Emamectin benzoate |
2 |
0 |
2.5 |
2.5 |
100a |
12 |
Control |
2 |
4 |
2.5 |
6.5 |
38b |
|
Pupae |
12 |
Emamectin benzoate |
2 |
0 |
0 |
0 |
0 |
12 |
Control |
2 |
0 |
0 |
0 |
0 |
Similar
letters within each category are not significantly different, α = 0.05
In our study, emamectin benzoate (ARETOR) LC50
value was 0.30 µg/mL, suggesting higher toxicity against RPW under the
laboratory conditions. Previously, toxicities of different insecticides have
been investigated in RPW (Ajlan et
al. 2000; Kaakeh 2006; Al–Shawaf
et al. 2010; Shawir et al. 2014; Malik et
al. 2016). A laboratory evaluation of abamectin at
500 ppm resulted in 100% larval mortality of RPW within 48 h of application (Abo-El-Saad
et al. 2013). These findings are in line with our results.
To date, very few studies reported on the emamectin
efficacy for RPW infestation control in date palms. In lab experiments with RPW
larvae, the same effects of emamectin have been
observed as when applied with a micro injector to the date palms. Larval
mortality rates of 97 and 100% were recorded after 90 days, when 50 or 100 mL/tree
emamectin were injected, respectively (Gomes and Ferry 2019). In another
recent study, Mashal and Obeidat injected Revive 4 and 9% into date palms infested
with RPW for periods of 3, 6 and 12 months, and found all-individual mortality
after 6 months and up (Mashal and Obeidat 2019). These results
support our findings, and the concept of long-term persistence of emamectin via a sophisticated translocation mechanism in
the trees that can kill borers and other insects (Burkhard
et al. 2015).
Unfortunately, limited numbers of RPW individuals were
recorded in the date palms dissected after 2, 6, and 12 months of pesticide
application. However, we believe that the 2-month period may have
been too long to leave enough individuals to be counted. It is possible that
larvae died inside the date palm trunk as a result of the insecticide
application and disintegrated by the time we dissected the trees. Supporting
this, we observed some slimy material in date palm trunks dissected after
2-months of insecticide application, likely resulting from larval degradation
in the galleries created by larvae. Therefore, dissection of date palms after
one and two weeks of insecticide application is recommended in future studies
to avoid the disintegration of RPW larvae, and resulting complications in
calculating the real efficacy of the insecticide. Our findings showed significant
differences in mortality percentage for all RPW stages between
the treatments and control.
The Syngenta Tree Micro-Injection Technique using emamectin
benzoate (ARETOR) is a promising solution for control of RPW, which in our
experiments resulted in 100% mortality of both larval
and pupal stages at all exposure times, except for the larvae collected from
date palms after 2 months, which showed 86% mortality. Moreover, no live RPW at any stage were recovered from trees treated with Revive
insecticide, except for one tree. This reflects that the insecticide killed all
RPW stages, while and stopping further damage to the healthy tissues of trees
and also providing protection against the spread of RPW to the other
neighboring healthy trees. Additional comprehensive studies are
recommended using date palms with active infestations before and during
treatment to obtain more detailed mortality data and draw further concrete
conclusions.
This study was supported by Deanship of Scientific Research at King
Saud University (RGP-1438-009). We also acknowledge Syngenta AG, Switzerland,
for technical assistance and partial funding.
Author Contributions
KGR, MH, and ASA participated in the planning, design and
coordination of the study. SS, KM, KDS participated in the red palm weevil
rearing, conducted practical work, KGR and MH, and NA collected and analysed
data, and write up. ASA and KGR supervised the work. All authors have read the
final version of the manuscript carefully and approved it.
Conflicts of Interest
The authors declare that they have no conflicting interests.
Data Availaility
All the data is present in the manuscript.
Ethics Approval
The work is original, Moreover, no legal permission was
required to conduct the experiments in the laboratory as well as in the field.
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