Egg Cluster Characteristics of Fall Armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidea) in Sri Lanka under Laboratory Conditions

Abstract

Purpose: Spodoptera frugiperda is a recently introduced alien invasive pest in Sri Lanka which can be a threat to many crops including maize, paddy, sugarcane, green gram, vegetables and fruits. Identification of this pest at the egg stage is important in controlling the larval stages of development. Therefore, the present study was conducted to record the morphological and morphometric characters of the eggs of S. frugiperda for accurate identification at the initial stage. Research Method: Thirty-six egg masses of laboratory reared S. frugiperda were observed and measured under a dissecting microscope. Position of the egg clusters on plant, characters of the egg cluster and eggs, number of eggs per egg cluster and diameter of eggs were recorded. Findings: The study revealed that the female S. frugiperda laid eggs mainly on the underside of the leaf blade as clusters containing 9-200 (73 ± 7) eggs. Eggs were mostly arranged as single layers and when arranged in multiple layers, the bottom layer had a larger number of eggs than the upper layers. Most of the egg clusters were irregular in shape and some were covered with hairs. Eggs were spherical and initially creamy white in colour that gradually turned light grey when about to hatch. The diameter of eggs ranged from 0.38 mm to 0.55 mm. Originality/value: The study documents the morphological and morphometric characters of S. frugiperda eggs for accurate identification. Early detection of the insect pest facilitates the management of their population before causing an economic damage.

Full text

200
1* Department of Zoolog y and Environment Sciences, Universit y
of Colombo, Sri Lanka.
cddangalle@zoology.cmb.ac.lk
2 Plant Quarantine Unit, Gannoruwa, Peradeniya, Sri Lanka.
https://orcid.org/0000-0002-0882-7147
Open Access Article
Egg Cluster Characteristics of Fall Armyworm, Spodoptera frugiperda
(Lepidoptera: Noctuidea) in Sri Lanka under Laboratory Conditions
R.H. Kasige1, C.D. Dangalle1*, N. Pallewatta1 and M.T.M.D.R. Perera2
Received: 09th December 2020 / Accepted: 10th November 2021
ABSTRACT
Purpose: Spodoptera frugiperda is a recently introduced alien invasive pest in Sri Lanka which can be a
threat to many crops including maize, paddy, sugarcane, green gram, vegetables and fruits. Identication of
this pest at the egg stage is important in controlling the larval stages of development. Therefore, the present
study was conducted to record the morphological and morphometric characters of the eggs of S. frugiperda
for accurate identication at the initial stage.
Research Method: Thirty-six egg masses of laboratory reared S. frugiperda were observed and measured
under a dissecting microscope. Position of the egg clusters on plant, characters of the egg cluster and eggs,
number of eggs per egg cluster and diameter of eggs were recorded.
Findings: The study revealed that the female S. frugiperda laid eggs mainly on the underside of the leaf
blade as clusters containing 9 – 200 (73 ± 7) eggs. Eggs were mostly arranged as single layers and when
arranged in multiple layers, the bottom layer had a larger number of eggs than the upper layers. Most of the
egg clusters were irregular in shape and some were covered with hairs. Eggs were spherical and initially
creamy white in colour that gradually turned light grey when about to hatch. The diameter of eggs ranged
from 0.38 mm to 0.55 mm.
Originality/value: The study documents the morphological and morphometric characters of S. frugiperda
eggs for accurate identication. Early detection of the insect pest facilitates the management of their
population before causing an economic damage.
Keywords: Egg cluster, Morphology, Morphometrics, Spodoptera frugiperda, Sri Lanka
The Journal of Agricultural Sciences - Sri Lanka
Vol. 17, No 1, January 2022. Pp 200-210
ht tp://doi.org/10.4038/jas.v17i1.9620
INTRODUCTION
The Fall Armyworm, Spodoptera frugiperda
(J.E. Smith) is a lepidopteran polyphagous pest
native to tropical and subtropical America (De
Groote et al., 2020). It is a serious pest of over
80 crop plant species including maize, sorghum,
beans, cotton, wheat, potato, soybean, cowpea,
peanuts, sugarcane and vegetables (Sisay et
al., 2018; Assefa and Ayalew, 2019; Tambo et
al., 2020). The Fall Armyworm destroys young
crop plants by attacking their growing points,
and burrows into cobs of older plants adversely
aecting the yield quality and quantity (Tambo et
al., 2020). Due to its broad host range and long-
distance migration capability, S. frugiperda has
spread from the Western part of the world to the
Eastern part, and has caused large infestations in
southern China, Myanmar and India threatening
the rice and millet crop cultivations (Nagoshi et
al., 2020).
Spodoptera frugiperda has been a problematic
species in Sri Lanka since 2018, causing a
signicant damage to maize, cereals, grass species
and other important agricultural crops (Early et

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The Journal of Agricultural Sciences - Sri Lanka, 2022, Vol. 17 No 1
al., 2018). The insect is currently considered as an
alien invasive pest, and has caused considerable
damage to maize in the Uva province which
has the highest land extent used for maize
cultivation. 11.3% of the land area used for maize
cultivation in the Ratnapura district has also been
infested with S. frugiperda, and eld surveys
reveal that the pest can be devastating to paddy
cultivations in the future (Perera et al., 2019). S.
frugiperda attacks on sugarcane plantations have
also been reported from Sevanagala, Pelwatte,
Udawalawe, Siyambalanduwa and Hingurana
areas of Sri Lanka (Wanasinghe et al., 2019), and
according to Wijerathna et al. (2021), although S.
frugiperda displays the highest host preference to
maize, it can use vegeTable 0crops as alternative
hosts for oviposition in the absence of maize.
Thus, S. frugiperda has directly aected the
agriculture sector of the country resulting in a
huge economic loss mainly in maize production
and has threatened the country’s food and
nutritional security, and livelihood of farmers
and corn vendors causing a detrimental eect on
their social well-being. It has also aected the
livestock sector due to transport ban hold on corn
plants to prevent S. frugiperda dispersal.
One of the most important factors that help the
spread of S. frugiperda is its ability to produce
a large number of eggs (De Groote et al., 2020).
The adult females deposit a cluster of eggs with
a few hundred eggs, usually on the underside of
the leaves (Harrison et al., 2019). The abaxial
surface of the leaves are considered as the site
preferred by S. frugiperda for oviposition, and
egg masses are usually not observed in other
plant parts such as the bracts, ower buds,
owers and bolls (Junior et al., 2013). Egg
mass cluster frequency per plant, egg numbers
per cluster, morphological and morphometric
features of eggs have not revealed to be dierent
amongst S. frugiperda occupying dierent
geographical regions. However, changes in the
average number of egg masses per plant and
egg numbers per cluster have shown small but
signicant changes according to the host plant.
According to Sotelo-Cardona et al. (2021), S.
frugiperda egg mass clusters and eggs per cluster
is the highest on maize plants while infestations
are not found on tomato plants. This host plant
preference has also been observed by Murua et
al. (2008), irrespective of the fact that the records
have been collected from dierent geographical
regions in Argentina.
Control of insect pest species by destruction
of the egg stage is considered as a rst line-
of-defense in insect pest management as this
lessens the development of the harmful larval
and adult stages and prevents the infestation of
parasitic insects that maybe be attracted to the
eggs. Species of Hymenoptera are known to be
eective as egg parasitoids for the biological
control of S. frugiperda (Tefera et al., 2019), and
there are many predators that attack the eggs of
this species (Hardke et al., 2015). However, for
successful management, the eggs of S. frugiperda
must be distinctly identied from the eggs of the
benecial insects, and the locations that the eggs
are deposited accurately identied (Campbell et
al., 2016).
In most studies of S. frugiperda, egg stages
have been examined mainly on the basis of
morphological characters such as colour and
structural characteristics (texture, chorion
architecture) (Santillán–Guayasamín et al.,
2017). Use of morphometric characters are
limited, but essential, as the egg stages in the
species that belong to genus Spodoptera have
many similar characteristics which require both
morphological and morphometric variables for
identication.
Therefore, the present study was undertaken to
characterize the egg stage of S. frugiperda of Sri
Lanka using morphological and morphometric
analysis.
MATERIALS AND METHODS
Sample collection and laboratory rearing
Spodoptera frugiperda larvae were collected
during January to October 2019 from maize
elds in four locations – Mahailuppallama,
Anuradhapura district, North-Central province;
Polpithigama, Kurunegala district, North-
Western province; Meegahakiula, Badulla
district, Uva province; Rideemaliyadde, Badulla
district, Uva province. The larvae were cultured
in the insectary facilities of the Department of

202
R.H. Kasige, C.D. Dangalle, N. Pallewatta and M.T.M.D.R. Perera
Zoology and Environment Sciences, University
of Colombo, under 28 ± 2°C temperature, 80
± 10% relative humidity and a 12 h:12 h light:
dark natural photoperiod. The cultures were
maintained on 3 – 4 week age maize plants in
plastic pots. The emerged adult moths were
moved to standard insect rearing cages (66 x 66
x 78 cm3) and were given a food source of 50%
bee-honey and 50% water (bee-honey: water
1:1) and were provided with 30 cm height potted
maize plants for oviposition. The moths were
introduced to cages in 1: 3 male to female ratio.
Egg clusters were collected from plants after 3-4
days of introducing the adults to the cage.
Morphological and morphometric
characterization of S. frugiperda eggs
A total of 36 egg clusters obtained from the adults
of the larvae collected from the four locations
were used collectively for the study. The rst
egg cluster was collected in early June 2019,
while the last or 36th egg cluster was collected in
mid-September 2019. Observations were made
using the dissecting microscope / Zoom Stereo
Microscope (OLYMPUS SZ51, Japan).
Several attributes of S. frugiperda eggs including
the position of the egg cluster on the plant, shape
of the egg cluster, number of eggs per egg cluster
(clutch size), colour of eggs (using a Munsell
colour chart), shape and arrangement of eggs
within the egg cluster and other special observable
features were recorded. The average diameter of
an egg was measured using a graticule placed in
the eye piece of the dissecting microscope. The
characteristics of eggs were recorded using digital
photographs, drawings and written descriptions.
Eggs were checked daily to record newly emerged
larvae, colour changes of egg and incubation
period.
RESULTS AND DISCUSSION
Position of egg cluster on the plant
Mature and gravid adult S. frugiperda female
moths laid eggs in clusters/egg masses usually at
night. They laid eggs more than once per life time
at dierent positions of the plants. Egg clusters
were mostly found on the underside of the leaf
blade (abaxial) of maize (61.11%) but there were
instances where egg clusters were present on the
upper side of the leaf blade (adaxial) (13.89%),
near the base of the plant close to the leaf nodes
(5.56%) and rarely on the wall of the plastic moth
rearing cages (2.78%). Most egg clusters were
found near the mid rib and the leaf margin on
the underside of the maize leaf blade (61.11%)
(Figure 01).
The eggs are the rst life stage of most insects
that are directly exposed to the environment
and to predators, parasitoids and abiotic stresses
(Hilker and Fatouros, 2015). Further, egg
laying site selection is important in providing
an adequate place for the subsequent success of
larval development (Storey-Palma et al., 2014).
Therefore, many Lepidopteran insects such as
the leaf miner, Angelabella tecomae, the Bertha
armyworm, Mamestra congurata and the Beet
armyworm, Spodoptera exigua, lay eggs on
the underside of host plant leaves, where eggs
and the emerging larvae are protected from
high temperatures, sunlight, wind, predators
and parasitoids (Ulmer et al., 2003; Azidah
and Soan-Azirun, 2006; Storey-Palma et al.,
2014) . However, the majority of Lepidoptera
lay eggs singly and the larvae develop solitarily
(Stamp, 1980; Hebert, 1983; Ulmer et al., 2003).
Laying eggs singly is considered the ancestral
state and the proportion of Lepidoptera retaining
this strategy suggests that laying eggs singly is
generally advantageous because of the reduced
competition from conspecics for food (Ulmer
et al., 2003). However, certain Lepidoptera lay
their eggs in clusters indicating that under certain
circumstances this may be more benecial.

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The Journal of Agricultural Sciences - Sri Lanka, 2022, Vol. 17 No 1
Characters of the egg clusters
The female S. frugiperda laid eggs as clusters that
were mostly irregular in shape but some were
circular and rectangular. The eggs were tightly
attached to the substrate, packed and arranged
in rows resembling a chain of pearls /string of
beads. The eggs were sometimes deposited in
multiple layers but most eggs were spread over
a single layer attached to foliage. The highest
percentage; 72.22 %, of total observed egg
clusters were single layered while 27.77 % were
multiple layered. The egg masses were at when
arranged in a single layer. When having two or
more layers, the bottom layer contained a larger
number of eggs compared to the top layer / layers
(Figure 02).
Figure 01: Dierent positions where egg masses were laid. (a) Leaf nodes of the plant (b) Underside
of the leaf blade (c) Upper side of the leaf blade (d) Wall of the plastic moth rearing cage
Figure 02: Shape and arrangement of egg masses (a) – (f) Single layered; (g) Multiple layered

204
R.H. Kasige, C.D. Dangalle, N. Pallewatta and M.T.M.D.R. Perera
The strategy of egg clustering may hold
advantages for the eggs by protecting them from
desiccation and other environmental factors
as well as parasites and predators (Ulmer et
al., 2003). Single eggs are more likely to be
attacked by predators who are more strongly
deterred from attacking clusters than single
eggs (Agarwala and Dixon, 1993). Bessera and
Parra (2005) have demonstrated that having
multiple layers of eggs tend to reduce the level
of parasitism. The Hymenopteran parasitoid,
Trichogramma atopovirila, parasitizing S.
frugiperda eggs, showed reduced percentages of
parasitism when the number of layers in the egg
cluster increased. Insect eggs require exchange of
gases for respiration and this is achieved via tiny
pores located on the egg shell (Campbell et al.,
2016). Exchange of gases will be more ecient
if eggs are exposed to environment and may
explain the strategy for single layering of eggs
in S. frugiperda. Hair-like strands covering the
egg clusters of S. frugiperda and the ridge-like
structures present on their eggs may compensate
for their exposure to the environment. Detailed
sculpturing on the outside of the eggshell is
known to protect insect eggs from environmental
stress and provides a barrier against insecticide
penetration (Campbell et al., 2016).
In the present study, some egg clusters were
covered with white/ grey hair-like strands
secreted by the anal region of the female moths.
In Lepidoptera, most species of Noctuidae
cover their egg masses with secretions, hair-like
strands or discarded scales (Peterson, 1964),
and many species of Spodoptera appear to have
morphologically similar egg masses covered
with hair strands (Korycinska, 2012). According
to a study conducted by Temerak (2006), egg
masses of the Cotton Leaf Worm, Spodoptera
littoralis, are either naked or partially covered or
fully covered with a ratio of 10:2:1 respectively,
with only a low percentage of egg masses being
fully covered. A similar observation was also
revealed in the present study, in which the egg
masses that were fully covered represented the
lowest percentage (11.11%). However, according
to the current study, in S. frugiperda, the naked to
partially covered egg masses were more or less
similar in frequency accounting for 41.66% and
47.22% (1:1) of the total egg mass respectively
(Figure 03). S. frugiperda may cover their egg
masses with hairs for various reasons such as
unfavourable environmental conditions and
protection from natural enemies such as predators
and parasitoids. Dong et al. (2021) revealed that
the proportion of parasitism by egg parasitoids of
S. frugiperda was lower on covered egg masses
when compared with naked egg masses. Further,
according to Temerak (2006), the performance
of an insecticide on naked egg masses of
Spodoptera littoralis was faster and greater
than on full covered egg masses. Therefore,
when conducting pest control strategies against
S. frugiperda, chemically or biologically, it is
exceedingly important to consider the appearance
of egg masses and use chemicals that will suce
eective control or use parasitoid species that
will not be aected by the coverings of the egg
masses.
Figure 03: Appearance of egg masses (a) Naked (b) Partially covered with hairs (c) Fully covered
with hairs

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The Journal of Agricultural Sciences - Sri Lanka, 2022, Vol. 17 No 1
Figure 04: Sculpturing of eggs (a) the ridges present in eggs (b) lateral view showing the ridges from
apex to base of egg (c) dorsal view showing the horizontal connections between egg ridges
Morphology of eggs
The eggs were small with a shiny egg shell and
had a uniform colour with no pigmentation. The
colour varied from cream colour / pale yellow
/ pearl-white to light grey with egg maturity.
They were whitish creamy colour as soon as they
were laid and turned to light grey colour when
they were about to hatch. The larval head was
observable in greatly enlarged eggs which were
about to hatch.
When considering the shape of a single egg, it
was spherical in shape and had a rounded shape
in dorsal view. Eggs were heavily sculptured and
distinct ridges were observed from the apex to
the base of the egg shell when observed under
higher magnication (x40). The ridges were
horizontally connected to each other (Figure 04).
Morphometry of eggs
A total of 53 female insects collectively produced
203 egg clusters during the study period. The
duration of the egg stage varied from 3-4 (± 0.09)
days.
The number of eggs per egg cluster (clutch size)
varied under laboratory conditions. The clutch
size ranged from 9 – 200 eggs per cluster, with
an average of 73 (± 7) eggs. The diameter of the
eggs in a given egg cluster varied with an average
ranging from 0.38 mm to 0.55 mm. The mean
diameter of an egg was recorded as 0.43 ± 0.04
mm (Figure 05).
The average diameter of an egg was similar to
the diameter reported by Capinera (2000), who
observed a value of 0.43 mm. The diameter
of the egg stage depends predominantly on
the temperature and humidity of the external
environment (Luginbill, 1928). In the present
study, eggs hatched in 3-4 days under laboratory
settings, while other studies have reported it as
3 days (Hinds and Dew, 1915; Luginbill, 1928;
Murúa and Virla, 2004) and as a range of 3-4
days (when temperature is 70-80 oF) by Sparks
(1979). These variations could be due to dierent
plant varieties, dierences in phytochemicals
produced by the plant varieties and external
environmental conditions (Vargas-Madríz et al.,
2013).
The varietal dierences may have aected the
size of the eggs and contributed to individual
variations in egg morphometrics.

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R.H. Kasige, C.D. Dangalle, N. Pallewatta and M.T.M.D.R. Perera
Undergoing colour change during the incubation
period, sculpture on the egg shell, hair-
like covering of egg clusters are characters
unique to Lepidopterans of family Noctuidae
(Peterson, 1964). Such characters are known
to have developed insecticide resistance in
eggs (Campbell et al., 2016), and resistance to
chemicals in S. frugiperda have often resulted
in pest resurgence (Assefa and Ayalew, 2019).
However, control of S. frugiperda when in the
egg stage is more eective as larvae migrate into
the plant whorl immediately after its emergence
and remain sheltered during the whole larval
phase inicting a serious downside to insecticide
spraying (Bialozor et al., 2020). Small-holder
farmers of many countries carry out handpicking
and crushing of egg masses of S. frugiperda,
especially as the method is cheap and it only
requires readily available material (Ahissou et al.,
2021; Njuguna et al., 2021). Chemical extracts of
Neem (Azadirachta indica) have been found to
have high ovicidal activity on S. frugiperda egg
masses less than 48 hours of age (Paredes-Sánchez
et al., 2021). Further, many biological control
practices have been conducted using various
species of egg parasitoids (Beserra and Parra,
2005; Temerak, 2006; Liao et al., 2019; Jaraleño-
Teniente et al., 2020; Dong et al., 2021; Paredes-
Sánchez et al., 2021). However, for the success
of these control practices, accurate information
on S. frugiperda eggs is essential and knowledge
on their locations on plants, characters, strengths
and weaknesses is required. The present study
intends to provide the much needed knowledge
on S. frugiperda egg characters which may be
useful in developing control strategies. However,
studies conducted using eld conditions are also
required in order to determine any changes in egg
characters inuenced by environmental factors of
the sampling site and diet of female adult moths.
CONCLUSIONS
Spodoptera frugiperda may remain a signicant
challenge for the foreseeable future and eective
management strategies are required. Physical,
chemical and biological control of egg masses of
the pest is widely used by farmers as a rst line
of defense as this lessens the development of the
more harmful larval stages. However, eective
control of egg masses rely upon the accurate
detection, identication and understanding of
the defensive characteristics of the Spodoptera
frugiperda eggs. The current study intended to
enhance the knowledge on this aspect. According
to the study, the female S. frugiperda laid eggs
as clusters, mainly on abaxial surface of the leaf
blade near the mid-rib and leaf margin. An egg
cluster consisted of an average of 73 ± 7 eggs
arranged irregularly mainly as a single layer. Egg
colour varied from creamy white to light grey
Figure 05: Histogram showing the average egg diameter per egg mass for the 36 egg clusters
observed in the study

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The Journal of Agricultural Sciences - Sri Lanka, 2022, Vol. 17 No 1
depending upon egg maturity and the diameter
of eggs ranged from 0.38 mm to 0.55 mm.
Spodoptera frugiperda eggs were spherical and
resembled a chain of pearls. They were heavily
sculptured with vertical and horizontal ridges,
and naked, partially covered or fully covered
with hairs. Sculpturing of eggs and coverings
by hairs should be considered with signicance
when implementing pest control strategies. The
study provides information that is eective for
S. frugiperda management and may be practical
from a farmers’ point of view.
ACKNOWLEDGEMENTS
The authors express their gratitude for the
nancial assistance, laboratory facilities and
technical support provided by the Department of
Zoology and Environment Sciences of Faculty of
Science, University of Colombo.
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