Laboratory bioassay of fall armyworm (Spodoptera frugiperda) larva using various insecticides

Abstract

Fall armyworm (FAW), Spodoptera frugiperda J.E. Smith, is a polyphagous invasive pest that seriously affects the maize crop. Various insecticides such as Spinosad, Chlorantraniliprole, Imidacloprid, Emamectin benzoate, Spinetoram and Neem-based insecticides are recommended to control this pest. However, their efficacy is not well studied in Nepal. Hence, a study was performed to evaluate the efficacy of the above-mentioned insecticides for FAW management in laboratory experiments. Seven different treatments (six insecticides such as Spinosad 45% SC, Chlorantraniliprole 18.5% SC, Imidacloprid 17.8% SL, Emamectin benzoate 5% SC, Spinetoram 11.7% SC, Neem-based pesticide (Azadirachtin 1500 ppm) and control (water spray) were evaluated in three replicated CRD design. Spinosad and Spinetoram were found effective for the FAW mortality in which > 50% mortality of the larva was obtained in twelve hours and > 90 % mortality in twenty-four hours. Likewise, Emamectin benzoate and Chlorantraniliprole also caused > 90% mortality within twenty-four hours. Azadirachtin and Imidacloprid were not as effective as other pesticides that caused only 17% larval mortality in the first twelve hours and 68% mortality in sixty hours. Similarly, Imidacloprid caused 8% larval mortality in twelve hours and 59% mortality in sixty hours. There was no mortality in water spray (control). This information gives an idea of all pesticides are not equally effective and efficient. Such information’s are important to the farmers to select the right insecticides for the control of FAW in maize crops.

Full text

Journal of Agriculture and Forestry University (2022), Vol. 5 133
Research Article
LABORATORY BIOASSAY OF FALL ARMYWORM (Spodoptera frugiperda)
LARVA USING VARIOUS INSECTICIDES
S. Sharma1, S. Tiwari*1, R. B. Thapa1, S. Pokhrel1 and S. Neupane2
1Agriculture and Forestry University, Rampur, Chitwan, Nepal
2Nepal Agriculture Research Council, Rampur, Chitwan
*Corresponding author: stiwari@afu.edu.np
Received date: 12 December 2021, Accepted date: 23 March 2022
ABSTRACT
Fall armyworm (FAW), Spodoptera frugiperda J.E. Smith, is a polyphagous invasive pest that seriously aects
the maize crop. Various insecticides such as Spinosad, Chlorantraniliprole, Imidacloprid, Emamectin benzoate,
Spinetoram and Neem-based insecticides are recommended to control this pest. However, their ecacy is not
well studied in Nepal. Hence, a study was performed to evaluate the ecacy of the above-mentioned insecticides
for FAW management in laboratory experiments. Seven dierent treatments (six insecticides such as Spinosad
45% SC, Chlorantraniliprole 18.5% SC, Imidacloprid 17.8% SL, Emamectin benzoate 5% SC, Spinetoram 11.7%
SC, Neem-based pesticide (Azadirachtin 1500 ppm) and control (water spray) were evaluated in three replicated
CRD design. Spinosad and Spinetoram were found eective for the FAW mortality in which > 50% mortality of
the larva was obtained in twelve hours and > 90 % mortality in twenty-four hours. Likewise, Emamectin benzoate
and Chlorantraniliprole also caused > 90% mortality within twenty-four hours. Azadirachtin and Imidacloprid
were not as eective as other pesticides that caused only 17% larval mortality in the rst twelve hours and 68%
mortality in sixty hours. Similarly, Imidacloprid caused 8% larval mortality in twelve hours and 59% mortality in
sixty hours. There was no mortality in water spray (control). This information gives an idea of all pesticides are
not equally eective and ecient. Such information’s are important to the farmers to select the right insecticides
for the control of FAW in maize crops.
Keywords: Fall armyworm, bioassay, pesticides, ecacy, mortality, time interval
INTRODUCTION
Fall armyworm (Spodoptera frugiperda J.E. Smith (Lepidoptera: Noctuidae) is a major pest of maize
crop in all maize growing districts of Nepal. It is a polyphagous and voracious pest. Fall armyworm infestation
has been reported on more than 300 species of plants (Montezano, 2018). The larval stage of FAW feeds on
maize whorls and cobs. Adults are active iers and can y about 150 km in a day (Johnson, 1987). It has been
reported that fall armyworm causes more than $13 billion (USD) in crop losses per annum in Sub-Saharan
Africa, resulting in food insecurity for millions of marginalized and poor farmers (Day et al., 2017). FAW
has spread to 47 countries in Africa, 17 countries in Asia and 105 countries in the world (Hruska, 2019). On
May 9, 2019, this pest was observed for the rst time in Nawalpur district of Nepal (Bajracharya et al., 2019),
causing 20-35% maize losses. The Plant Quarantine and Pesticide Management Centre of the Ministry of
Agriculture and Livestock Development reported that out of 77 districts in Nepal, FAW infestation occurred
in 65 districts (Onlinekhaber, 2021). In Nepal, a total of 956,477 ha of land is covered by maize area with the
production of 2,713,635 tons (MOAD, 2020). The loss of maize caused by FAW is very high accounting for
30 - 70% yield loss (MOALD, 2019).
Pesticide application is one of the major FAW management practices in developing countries.
However, these practices are not sustainable and have the potential to cause deleterious eects on human
health, the environment, and biodiversity. Furthermore, the use of chemical insecticides results in pesticide
resistance development in many insect pests and eects on non-target organisms such as pollinators and natural
enemies. Additionally, pesticide costs are greater than other management approaches and are expensive for
small-scale farmers. Various pesticides such as Spinetoram Spinosad, Chlorantraniliprole, Azadirachtin etc
are recommended for the control of this pest (Bhusal & Bhattarai, 2019; Hardke et al., 2014). Their eciency
has not been studied well before the application in open eld conditions. These practices further accelerate
the unintentional loss of pesticides and money which increases the input cost of farmers. Hence, a laboratory
bioassay was conducted to select an eective and ecient insecticide for the FAW management, which is
necessary to select an appropriate and ecient pesticide and save pesticide costs for farmers.
: 133-138

134 Sharma et. al
MATERIALS AND METHODS
A bioassay/experiment on the fall armyworm (FAW) was conducted in the Entomology laboratory
of Agriculture and Forestry University (AFU) in July 2021. A total of seven dierent treatments including
control (water spray) with three replications were taken for the study and observed the mortality of fth instar
FAW larva at dierent time intervals.
Each treatment (pesticide) was prepared according to the required dose (Table 1). Fifteen individual
Petri dishes (9 cm diameter, 1.5 cm height) were taken for each treatment and the Petri dishes were coded
from 1-15 and randomized into three replications with ve Petri dishes in each replication. This was done for
all the treatments. Uniform sized maize leaves (7*4 cm2) (4th to 5th leaf of knee height stage plant) from the
pesticide-free eld were used to feed larvae. The leaves were dipped in the respective insecticide solution
for 15 seconds. For the control treatment, tap water was used, gently dipped in a beaker and kept for about a
minute for air drying. Then the leaves were transferred to the Petri dishes of sizes 9 cm in diameter, 1.5 cm in
height.
Table 1. Treatments used in bioassay experiments
S.N. Treatments Trade name Level Dose/lit water
1 Azadirachtin 1500 ppm Gorkha Bio-Neem Green 4.0 ml
2 Emamectin benzoate 5% SG G- Super Yellow 0.4 gm
3 Spinosad 45% SC Tracer Blue 0.3 ml
4 Chlorantraniliprole 18.5% SC Allcora Green 0.4 ml
5 Spinetoram 11.7% SC Delegate Green 0.3 ml
6Imidacloprid 17.8% SL Rajmida Yellow 1.0 ml
7 Water spray Tap Water - Gently dipped
The second-generation fth instar larvae were taken from the laboratory rearing FAW colony and kept
individually in the Petri dish (9 cm diameter, 1.5 cm height) and kept for 24 hours for starvation. Insecticide
dipped leaves were kept inside Petri dish and covered by a lid (9.4 cm diameter and 1 cm height). The
mortality of the larva was observed at 12, 24, 36, 48, and 60 hours and this was continued for four days. The
experiment was arranged in a completely randomized design (CRD) and a one-way analysis of variance was
performed by using GenDisc4 (GenStat). The means were separated by using Tukey’s signicant dierence
test (Gomez and Gomez, 1984).
RESULTS AND DISCUSSION
There was a signicant dierence among the synthetic pesticides in the larval mortality of FAW
(Table 2). More than 50% mortality of FAW larva was observed in twelve hours by Spinosad and Spinetoram
pesticides whereas only 8% mortality was observed by Imidacloprid and 17% mortality by Azadirachtin.
Spinosad has been found as the most eective pesticide with the highest mortality of FAW larva within a very
short time as explained by Cook et al. (2004).

Journal of Agriculture and Forestry University (2022), Vol. 5 135
Table 2. Mean percentage (± SEM) of mortality of FAW larvae at 12, 24, 36, 48 and 60 hours after
application of pesticides in laboratory bioassay
Treatments Percent larval mortality ± SEM at dierent time intervals
12h 24h 36h 48h 60h
Azadirachtin1500 ppm 17.71±8.86abc 30.79±4.22b 63.85±13.07b 63.85±13.07ab 68.07±11.56ab
Emamectin benzoate 5% SG 38.86±6.99abc 90.00±0.00a 90.00±0.00a 90.00±0.00a 90.00±0.00a
Spinosad 45% SC 51.14±6.99ab 90.00±0.00a 90.00±0.00a 90.00±0.00a 90.00±0.00a
Chlorantraniliprole 18.5% SC 30.79±4.22abc 90.00±0.00a 90.00±0.00a 90.00±0.00a 90.00±0.00a
Spinetoram 11.7% SC 56.15±16.92a 90.00±0.00a 90±4.22 a 90.00±0.00a 90.00±0.00a
Imidacloprid 17.8% SL 8.86±8.86bc 38.86±6.98b 59.21±4.22 b 59.21±4.22b 59.21±4.22b
Water spray 0.00±0.00d 0.00±0.00c 0.00±0.00c 0.00±0.00c 0.00±0.00c
Grand Mean 29.10 61.40 69.00 69.00 69.60
CV % 52.00 8.70 12.70 12.70 11.10
LSD (5%) 26.89 3.30 5.60 5.60 5.30
P-value 0.004* ˂0.001** ˂0.001** ˂0.001** ˂0.001**
Note: CV: Coecient of Variation; **: Signicance at 1% (p<0.001); *: Signicance at 5% (p<0.05); LSD: Least Signicant
Dierence; Values with the same letters in a column are not signicantly dierent at 5% by Tukey’s signicant dierence test;
Values are in percentage, Sem (±) indicates standard error of mean percentage (% value ± Standard error of mean percentage);
h= hour
Similar studies have been conducted around the world including China, Africa, Brazil and India to
test the ecacy of pesticides in eld and laboratory conditions to control FAW, and reported that FAW is
susceptible to synthetic pesticides, however, Spinosad, Chlorantraniliprole and Emamectin benzoate causes
the highest mortality compared to other synthetic pesticides (Idrees et al., 2022). Continuous application of
such pesticides in the eld may increase resistance against many categories of pests including FAW (Osae
et al., 2022). The study conducted by Zhao et al. (2020) suggested that S. frugiperda are resistant to Lamda-
cyhalothrin due to the continuous application of pesticide in the maize eld. Similarly, there are other insects
such as diamondback moth and two-spotted spider mite which are resistant to abamectin (Kwan et al., 2010).
The time of exposure, as well as concentration, aects the mortality rate as well as toxicity of the insects
including FAW (Cook, Leonard & Gore, 2004).
0
10
20
30
40
50
60
70
80
90
100
Mortality % of FAW
Treatments
12h 24h 36h 48h 60h
Figure 1. Percentage mortality of FAW at dierent time intervals caused by various bioassay materials

136 Sharma et. al
The Chlorantraniliprole and Emamectin benzoate showed a mortality of 31% and 39%, respectively
within 12 hours of FAW larval release. Sisay et al. (2019) reported that Imidacloprid caused 40% mortality in 24
hours and 70% mortality in 72 hours. In twenty-four hours of the treatment application, all four pesticides, i.e.
Spinosad, Spinetoram, Chlorantraniliprole and Emamectin benzoate caused more than 90% larval mortality,
showing their high eectiveness for the FAW. Similar results were also reported by Sisay et al. (2019) and
revealed that Spinosad and Spinetoram caused more than 90% mortality in a laboratory experiment.
A similar pattern of higher mortality of FAW larvae within a very short time by Spinosad and
Spinetoram was observed in an experiment conducted by Belay et al. (2012). Similarly, Chlorantraniliprole
caused mortality of 30% in the rst twelve hours and more than 90% within twenty-four hours in this
experiment which is similar to the results obtained by Sisay et al. (2019), Hardke et al. (2011) and Thrash et
al. (2013).
Azadirachtin and Imidacloprid caused only 31% and 39% larval mortality within twenty-four hours.
There was no mortality of larvae in control (water spray). There were no signicant dierences in twenty-
four, thirty-six and seventy-two hours of treatment application in mortality of the FAW larvae in all the
treatments. After sixty hours of the experiment setup, 68% larval mortality was observed in Azadirachtin and
59% mortality of larvae in Imidacloprid. In some parts of the world, farmers have used Imidacloprid which is
less eective than the Azadirachtin. Hence, in such a situation, neem-based pesticides can be recommended
to keep the pest below the damage threshold level. Neem-based pesticides are safer and more eco-friendly
compared to Imidacloprid (Mordue et al., 2010). But dose and frequency of neem-based pesticides can
inuence the percentage of FAW mortality. Likewise, Emamectin benzoate and Chlorantraniliprole also
caused more than 90% mortality of the FAW within 24 hours though their eectiveness in the early twelve
hours was quite lower as compared to Spinosad and Spinetoram.
This study was conducted in a laboratory experiment in Chitwan condition but the eectiveness of such
pesticides in other ecological zones may dier. However, a similar study conducted in similar agroecological
zones by Bajracharya, Bhat and Sharma (2020) revealed similar results. According to them, the eectiveness
of Spinosad, Chlorantraniliprole, and Emamectin benzoate was more promising as compared to Azadirachtin
and Imidacloprid in open eld conditions. The stage of FAW larvae used in the bioassay experiment also
aects the mortality rate. Early instars can be more susceptible to pesticides compared to the later stages.
Chemical managements are popular for the FAW but the use of these synthetic pesticides involves high
cost, potential environmental and human health issues, and pest related problems (Tudi et al., 2021; Muratet
et al., 2015; Choudhary et al., 2018). In Nepal, there are ve common pesticides registered by the Plant
Quarantine and Pesticide Management Centre (PQPMC), such pesticides are Azadirachtin 1500 ppm @ 5
ml/liter, Spinetoram 11.7 SC @ 0.5 ml/liter of water, Chlorantraniliprole 18.5% SC @ 0.4 ml/liter of water,
Spinosad 45%SC @ 0.3 ml/liter of water, Emamectin benzoate 5% SG @ 0.4 g/liter of water (MoALD,
2019). However, the current use practices of pesticides are not safe for human health and the environment
(Bateman et al., 2021). This study provides preliminary information on eciency of pesticides before testing
them in eld conditions. Accordingly, eective, alternative and safe management strategies are recommended
for sustainable FAW management.
CONCLUSION
Fall armyworm (S. frugiperda) is an invasive lepidopteran pest of maize crops. This pest was formally
noticed in Africa in 2016 and in Asia in 2018. For the rst time, this pest was identied in Nawalpur, Nepal in
2019. The invasive status of this pest was rst ocially declared by NPPO Nepal on 12 August 2019. Being
an invasive status, the Government of Nepal immediately recommended ve insecticides for immediate
control and to prevent further spreading. These pesticides are Azadirachtin 1500 ppm, Spinetoram 11.7 SC,
Chlorantraniliprole 18.5% SC, Spinosad 45% SC, Emamectin benzoate 5% SG. The eectiveness of such
pesticides in 2019 was not tested in the laboratory and eld in Nepal. Later, these pesticides were tested in
the maize eld by the researchers in research stations and students in eld crops. The laboratory bioassay
was not carried out to see the laboratory response of these common pesticides for the FAW larvae. Hence,
this study aimed to see the eect of such common pesticides on FAW management in laboratory conditions.

Journal of Agriculture and Forestry University (2022), Vol. 5 137
Results showed that all these pesticides are not equally eective for FAW. Based on the laboratory bioassay,
Spinosad, Spinetoram, Chlorantraniliprole, and Emamectine benzoate are categorized in the eective group
and the other two Imidacloprid and Azadirachtin are grouped into non-eective category. Among the rst
group, Spinosad and Spinetoram caused more than 50% mortality in twelve hours and > 90% mortality in
twenty-four hours. Chlorantraniliprole and Emamectin benzoate caused < 50 % larval mortality in 12 hours
and > 90% mortality in twenty-four hours. Similarly, only 17% of mortality was caused by Azadirachtin in 12
hours and 70% in sixty hours. The least eective synthetic pesticide Imidacloprid caused only 8% mortality
in 12 hours and less than 60% mortality in sixty hours. Such research ndings indicate that all insecticides
are not equally eective. First, it is necessary to test their ecacy in laboratory conditions before using them
in eld conditions. The research ndings suggest that farmers should be serious when buying pesticides from
agro-vets in order to save cost and amount of pesticide spray in maize farms.
ACKNOWLDGEMENTS
The authors are grateful to the Directorate of Research and Extension (DOREX) of Agriculture and
Forestry University, the University Grants Commission Nepal, Department of Entomology/AFU and the
National Maize Research Programme (NMRP) for the nancial and technical supports.
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