INTRODUCTION
Background
Green onion (Allium fistulosum), also known as green bunching onion, is a widely cultivated perennial crop, particularly in Asian countries where it is often grown as an annual In Vietnam, it serves as a vital seasoning vegetable, flavoring herb, and medicinal plant, holding significant value The cultivation of green onions is on the rise in northern regions of Vietnam, including Hanoi, Hung Yen, Thai Binh, and Hoa Binh However, the challenge of ensuring safe production over large areas, particularly in terms of pest management, remains a significant concern.
Spodoptera exigua, originally from southern Asia, is a widespread pest affecting various crops, particularly green onions This pest targets around 90 plant species across 18 families, including cotton, tomato, and cabbage In Vietnam, it has been reported on 25 crops, significantly damaging green onions in Tien Giang, with 78% of farmers noting its impact S exigua poses a serious threat to onion production nationwide, causing up to 30% yield loss and reduced farmer income While chemical control methods have been initially effective, the pest has developed resistance over time, raising environmental concerns and increasing the risk of further resistance.
In Vietnam, research on S exigua, commonly known as the beet armyworm, is limited While some studies have documented its presence on crops like soybean, cotton, green beans, and onions (Pham Van Lam et al 2010), there is a notable lack of comprehensive research focusing on its biological and ecological characteristics, particularly concerning green onions.
Further research is essential to clarify the levels and components of various factors influencing outbreaks Accurate identification of the morphological, ecological, and biological characteristics of Spodoptera exigua is crucial for establishing effective preventive measures.
We investigated: “Study on biological and ecological characteristics of beet
The objects and requirement
To effectively control the pest Spodoptera exigua in agricultural production, it is essential to comprehend its biological and morphological characteristics This understanding will serve as a basis for creating environmentally friendly and human-safe pest management strategies.
Mass rearing S exigua in the laboratory condition
Investigate the biological and eclology characteristics of Spodoptera exigua including life cycle, reproduction capacity, characteristics of each stage.
LITERATURE REVIEW
The situation of Spodoptera exigua research in the world
2.1.1 The distribution of Spodoptera exigua
The beet armyworm, scientifically known as Spodoptera exigua, originated in South America and has since spread to various regions, including North America, Mexico, Africa, Europe, India, and Australia Recognized as one of the most prevalent pests affecting tropical and subtropical crops, it has established a global presence, particularly in Africa, Asia, Australia, and the Americas, with significant populations in both the Americas and Asia.
S exigua was reported by the Commonwealth Agricultural Bureaux (1972) to be present in 67 countries around the world However, according to many subsequent investigation documents, this species has been distributed in 101 countries in many regions of the world Its distribution is extended to Northern Europe (Norway, Finland, Sweden ) and South America (Brazil, Bolivia, Chile ) (Huffman, 1996) However, S exigua has been recorded by light trapping for many years at Sotkamo (64.13°N) in Finland (Mikkola, 1970) As a result, the geographical distribution of the beet armyworm S exigua is shifted back from
The beet armyworm has emerged as a significant secondary pest of cotton in the United States over the past two decades, causing extensive damage In 1995, an epidemic in Texas affected over 1,200,000 hectares of cotton, leading to control costs exceeding 31 million dollars for approximately 480,000 hectares By 1998, the infestation spread to over 2,040,000 hectares, resulting in substantial damage and losses nearing 1.2 million dollars Additionally, the beet armyworm has also impacted citrus fruit plants, destroying around 200 hectares in greenhouses.
California during the summers of 1957, 1958, and 1959, generating significant economic losses (Atkins, 1960)
In Florida, the beet armyworm is a significant defoliator of flower crops and cotton, with much of the damage exacerbated by insecticide use that disrupts natural enemy activity Young larvae feed in groups, skeletonizing cotton foliage, while mature larvae become solitary and create large, irregular holes in the leaves They also burrow into the crown of lettuce and the buds of cole crops Although the beet armyworm consumes more cabbage tissue than the diamondback moth, it is less damaging than the cabbage looper, which grows into a larger caterpillar.
Tomato fruit is highly vulnerable to damage, particularly as it approaches maturity While the beet armyworm poses some risk, it is not as significant a threat to tomatoes as the corn earworm, Helicoverpa zea Early infestations during the growing season are more detrimental than those that occur later, with just one caterpillar per 20 tomato plants potentially leading to economic losses.
The beet armyworm is a highly destructive polyphagous pest affecting various field crops across East Asia First identified as a threat to onion crops in Japan in 1980, it has since emerged as a significant pest in southwestern Japan (Takai, 1991) In Malaysia, this pest targets crops such as cabbage, beans, spinach, and celery, with adult moths being present year-round in Cameron Highlands, Pahang (Azidah, 2008).
Hibernation and migration are the primary winter survival strategies of S exigua (Shankar et al., 2014) The beet armyworm is a highly destructive pest that feeds on over 90 plant species across 18 families in North America, including key crops like cotton, tomato, and cabbage (Pearson, 1983) Additionally, certain weed species, such as Chenopodium album and Amaranthus spp., provide suitable environments for larval development.
Polyphagous insects exhibit a wide potential host range, yet they demonstrate specific food preferences, with varying suitability among different food plants (Greenberg et al., 2001; Farahani et al., 2012).
A study conducted in 2001 examined the growth of beet armyworm larvae on three crops—cabbage, cotton, and pepper—and two weeds, redroot pigweed and sunflower The findings revealed that the highest relative consumption occurred on cabbage, followed by pepper, sunflower, cotton, and pigweed In terms of relative growth, as measured by pupal weights, pigweed supported the greatest growth, with cotton, cabbage, and pepper showing equal growth rates, while sunflower yielded the least Additionally, larvae matured fastest on pigweed The authors concluded that pigweed is nutritionally superior, whereas cabbage serves as the poorest host.
2.1.2 Biological characteristic of beet armyworm Spodoptera exigua
Adult moths have a wingspan of 25 to 30 mm, characterized by mottled gray and brown forewings with a light-colored bean-shaped area and irregular banding, while their hind wings are more uniformly gray or white with a black border After emerging, these moths mate and lay eggs over a period of three days, with oviposition lasting between three to seven days, and they typically die nine to ten days post-emergence Under summer conditions, the beet armyworm's life cycle spans 24 days, allowing for five or more generations annually Research indicates that in cotton plants, the development from egg to adult takes 20-30 days at summer temperatures, potentially resulting in over four generations each year.
Adult moths are nocturnal, seeking shelter during the day under onion stalks, dry trees, and shrubs (Capinera, 2002) They typically begin mating and laying eggs on the second night, with a male capable of mating with up to five females, while females may mate with multiple males Mating activity is highest during the first three nights and declines after the fourth night (Heppner, 1998) The duration of mating can vary between 30 to 180 minutes (Luo et al., 2003).
Host plants significantly influence the fecundity of adult female moths of the beet armyworm Research indicates that the fecundity capacity varies based on the host plant consumed during the larval stage (Capinera, 2002) Specifically, Sappington et al (2001) found that female moths that developed from caterpillars fed on cotton leaves laid 26% fewer eggs than those that fed on amaranth leaves On average, adult female moths of the beet armyworm can produce approximately 630.8 eggs each.
- 1,631 eggs/ female) when the larvae were reared on castor beans (Afify et al.,
Research indicates varying egg production rates based on the larvae's diet When reared on citrus trees, females produced 520 eggs, while sweet pepper plants yielded 1,019.8 eggs Amaranth resulted in the highest production at 1,310.3 eggs per female Additionally, larvae fed on pepper plants produced between 282 and 1,307 eggs, and those on soybean plants averaged 472.5 eggs Notably, onion leaves led to a production rate of 629.5 eggs per female, surpassing the 320.8 eggs produced when larvae were fed okra leaves.
Adults of Spodoptera exigua show a preference for laying eggs on nitrogen-rich hosts like onions and chives, with females depositing eggs on the leaves and stems of onions (Chen et al., 2008) In contrast, when it comes to okra, peppers, and cabbage, they tend to choose the tops and leaves for oviposition rather than the stems and petioles, with a notable egg count of up to 89.04.
% of their eggs on the leaves (Azidah & Sofian-Azirun, 2006b)
Common pigweed (Amaranthus hybridus L.) serves as a preferred host for the beet armyworm (Spodoptera exigua) Research indicates that chemicals extracted from pigweed using distilled water, ethanol, or dichloromethane can deter oviposition (Mitchell & Heath, 1985) Additionally, a study examined the impact of white mold infection (Sclerotium rolfsii) on peanut plants (Arachis hypogaea L.) and found that adult moths favored ovipositing on infected plants over healthy ones when both were damaged, indicating a preference for white mold-infected peanut plants (Cardoza et al., 2003).
The situation of Spodoptera exigua research in Vietnam
VIETNAM 2.2.1 The distribution of Spodoptera exigua
The beet armyworm, known scientifically as Laphygma exigua, has been present in Vietnam since 1967, affecting crops such as tomato, green bean, and potato in Hai Hung province A survey conducted between 1977 and 1979 revealed that this pest is significant in southern provinces, impacting various crops including cotton, grape onion, and beans (Pham Van Lam et al., 2010).
In 1998, the beet armyworm caused extensive damage to grape farmers by infesting hundreds of hectares and attacking leaves, flowers, and young fruit Between 2000 and 2004, this pest was also reported on various upland crops in Ninh Thuan, including cotton, maize, beans, onions, garlic, vegetables, tomatoes, melons, and flowers (Nguyen Thi Huong, 2018) Affected provinces included Bac Ninh, Bac Giang, Hanoi, Hai Duong, Hai Phong, Ho Chi Minh, Lam Dong, and Nghe.
The beet armyworm poses a significant threat to a wide range of crops, frequently causing severe damage in regions such as Thai Nguyen, Vinh Phuc, Ninh Thuan, and Tien Giang This pest affects industrial crops like cotton, as well as various fruit and vegetable crops, including lychee, tomatoes, carrots, cabbage, mustard greens, broccoli, celery, cucumbers, watermelons, okra, cowpeas, soybeans, green beans, onions, groundnuts, spinach, bitter melon, corn, grapes, chili, kohlrabi, cauliflower, and garlic (Pham Van Lam et al., 2010; Pham Van Lam, 2013).
The beet armyworm thrives in dry, hot, and sunny conditions with minimal rainfall, leading to significant crop damage when food sources are plentiful (Nguyen Huu Binh & Pham Huu Nhuong, 1997) However, their population declines notably during the rainy season, as temperatures and humidity levels between 67-76% are optimal for their growth (Pham Huu Nhuong, 1998) In the spring-summer crop cycle, beet armyworm density is initially high but decreases after the flowering stage (Nguyen Thi Thu Cuc, 1995a).
According to Pham Van Bien et al (2000), the primary pests affecting spices and vegetables include Spodoptera litura, Spodoptera exigua, Plusia chalcites, and Hydrellia phylippina, which are known to damage a variety of crops Additionally, Pham Huu Nhuong (1998) provides further insights into these pest species.
Spodoptera exigua is a pretty dangerous pest, causing damage to many crops in the provinces from May 20, 1998, to May 30, 1998 However, the density of
The population of Spodoptera exigua during the posterior peak decreased compared to the previous period, primarily due to the impact of the season's first rains in late May and early June.
The blue worm's density is notably lower and significantly decreases during the rainy season, as reported by the Plant Protection Department in various suburban growing areas of Hanoi.
Spodoptera exigua often emerges strongly in the dry season, causing the heaviest damage on onions in January, this is a very serious insect resistance drugs (Pham Huu Nhuong, 1998)
In the Mekong Delta, the beet armyworm is a seasonal pest that primarily affects crops during the winter-spring and spring-summer seasons Initially, during the winter-spring crop, the beet armyworm population is low, causing significant damage mainly after the bean plants begin to flower Notably, late-planted soybean fields experience more severe damage compared to those planted earlier Conversely, in the spring-summer crop, the density of beet armyworms is high at the start but decreases as the flowering period progresses (Nguyen Thi Thu Cuc, 1995a).
2.2.2 Biological characteristics of the beet armyworm Spodoptera exigua
Nguyen Van Dinh et al (2012) investigated the morphological properties of a specific stage in the life cycle of larvae, which exhibit five instars The first three instars are characterized by an olive green color, while the fourth instar displays a green hue The fifth instar presents two body colors: rose-brown and pale green, both featuring glossy skin The eggs are spheroidal, initially green and later becoming opaque white Pupae start as yellow-green and transition to reddish-brown, with mature pupae appearing brown-black Adult specimens are noted for their gray hair, compound eyes, and filiform antennae.
Adult beet armyworms are primarily active at night They begin breeding two nights after reaching maturity, with females laying eggs just hours after mating.
The egg stage lasts between 2 to 5 days, depending on temperature (Nguyen Thi Thu Cuc, 1999) After hatching, larvae remain near the egg cluster, feeding on the chlorophyll-rich parts of the leaves and shedding their white cuticle As they enter the end of the first instar, larvae begin to disperse to nearby leaves In the second instar, they create small holes in the leaves and start dropping silk to the ground The third instar is the most destructive, causing significant damage by biting large holes in the leaves During the fourth and fifth instars, larvae become less active, often hiding on the ground or beneath the leaves When pupating, they are encased in soil cocoons (Nguyen Van Dinh et al., 2012).
The larva is in the fifth instar, according to Nguyen Thi Thu Cuc (1999) The larvae stage carries 10 to 19 days The time pupate stage develops ranges from 5-
The life cycle of the beet armyworm lasts an average of 19 to 30 days, with adult oviposition primarily occurring at night Adults begin mating on the second night after reaching maturity, and females start laying eggs just a few hours later, following a pre-oviposition period of two days.
In a study by Nguyen Thi Huong (2018), it was found that healthy onion plants supported a range of 0-132 eggs laid per female per day, with an average of 44.75 eggs, while damaged plants had a lower range of 0-86 eggs and an average of 40.58 eggs per female per day The removal of defoliated leaves on healthy plants resulted in a higher number of egg clusters, averaging 3 eggs per female per day In comparison, the average egg counts on uninjured and damaged plants were 62 and 33.08 eggs per female per day, respectively, highlighting the impact of plant health on egg-laying behavior.
2.2.3 Ecological characteristics of beet armyworm Spodoptera exigua
Research on beet armyworm rearing at four temperature levels (20, 25, 28, and 30°C) with 65% humidity reveals that the species undergoes six instar larval stages The findings indicate that lower temperatures, particularly 20°C, prolong the larval stage, while higher temperatures accelerate larval development, oviposition, and the overall life cycle Notably, oviposition rates peak at specific temperature conditions.
At 25 degrees Celsius, females produce an average of 402,334 eggs, while at 30 degrees, the average drops to 206.67 eggs Additionally, male lifespan is shorter than that of females, with males living between 17.12 to 20.54 days at 30 degrees Celsius, compared to females, which can live up to 49.47 to 57.36 days at 20 degrees Celsius.
MATERIAL AND METHODOLOGY
Material
Research subject: Beet armyworm (Spodoptera exigua) Research material: Green onion (Allium fistulosum) plant in a pot (15x20cm) with 4-5 clusters per pot
The experimental setup includes a magnifying glass, a Leica M165C stereo microscope, and a Sanyo MIR 154 incubator for monitoring temperature and humidity Daily records of temperature and humidity are maintained using a thermometer and a water tray The materials used in the experiment consist of pure longan flower honey, along with various equipment such as plastic bags, blotting paper, absorbent cotton, sample containers, trays, markers, pens, pencils, laboratory notebooks, net cages, and a plastic cup Additionally, a Kern technical scale (Model: PNS 600-3) is utilized for precise measurements.
Location and time research
Location: Room 07 of the Department of Entomology, Faculty of Agronomy, Vietnam National University of Agriculture
Time for study: 6 months, from 8/2021 to 2/2022.
Research Content
Study on morphological characteristics of beet armyworm
Research on biological and ecological characteristics of beet armyworm Research on the food consumption rate of larvae beet armyworm
Research on the effected of pesticides on oviposition behaviour of beet armyworm
Methodology
Green onions were cultivated in a greenhouse, planted in pots measuring 15x20 cm, with 4-5 clusters per pot After 30 days of growth, the green onions reached a length of 25 cm and were utilized as feed for larvae.
Figure 3.2 The pot planted green onion in net house
Beet armyworm larvae, ranging from the second to the sixth instar, were collected from the fields in Van Duc commune, Gia Lam district, Hanoi, and Nghia Lien commune, Van Giang district, Hung Yen These larvae were then mass-reared under controlled room conditions at temperatures between 23°C and 30°C.
The larvae were placed in a plastic box measuring 12x17x5 cm, equipped with an open-hole lip, along with 10 fresh green onions approximately 7 cm in length and a moist tissue Each box contained varying numbers of larvae for different instars: 30 for the second, 20 for the third, 10 for the fourth, 5 for the fifth, and 6 for the sixth instar The leaves were replaced daily, while the plastic box was changed twice a week until the larvae developed into pupae.
Figure 3.3 The plastic box (12x17x5 cm) rearing population larvae
Pupae that were enclosed on the same day were individually placed in plastic boxes (2x2x3 cm) with moisture tissue and checked daily for adult emergence Using a stereomicroscope to examine the pupae of S exigua, distinct differences between males and females were observed on the posterior ventral surface of the abdomen The genital slit of the female is positioned at the anterior edge of the 8th abdominal segment, while in males, it is located in the mid-part of the abdomen.
In male specimens, the 9th abdominal segment is located on the ventral surface, while the genital slit in females is approximately twice as long as in males A darkening of the pupa indicates its transition to adulthood, at which point the pupae should be transferred to a 25x25x25 cm cage containing a pot of green onions It is recommended to release three pairs of adults per cage and provide a cotton ball soaked in a 10% honey solution for nourishment The pot of green onions and honey solution should be replaced twice weekly, and any old pots containing eggs should be moved to a new cage of the same dimensions Eggs and larvae should be maintained until they reach the third instar, after which they can be transferred to a 15x17x5 cm plastic box for feeding.
Figure 3.4 The cage (25x25x25 cm) mass rearing
3.4.1.2 Morphological and biological characteristics of beet armyworm Spodoptera exigua
Observation to describe the morphological characteristics of separate stages with the numeral of individuals observed n = 30
During the egg stage, clusters of eggs are carefully cut from the mass-rearing environment and placed into a plastic box measuring 2x3x3 This box is lined with moist tissue and contains two fresh green onions, each measuring 3 cm Observations should focus on the shape and structure of the egg arrangement, as well as the color transformation of the eggs from the moment they are laid until they hatch.
Figure 3.5 Plastic box(2x3x3cm) rearing larvae
During the larvae stage, the larvae are transferred into a plastic box (2x3x3 cm) containing 3 cm of fresh green onion and a moist tissue, with daily replacements of both The duration of each instar is monitored until the larvae molt into the next stage, with increased food consumption noted during this transition Moulting is identified by the presence of shed skin The first and second instar larvae are observed twice daily at 8:00 AM and 4:00 PM, while other instars are checked once daily at 8:00 AM to assess development time and mortality rates for each instar.
After 24 hours when they changed new instar, we investigated morphology of larvae Put it on the freezer for 5 minutes to motionless and observed, described, photographed, measured the width of the head capsule and the length, the width of the body by Stereomicroscope (Leica M165C) and cross-section as a calibrator n0
During the pupae stage, monitor the color changes from day one until the emergence of the adult moth, distinguishing between male and female pupae After 48 hours of pupation, measure the length, width, and weight of the pupae using cross-sectioning as a calibration method The duration of pupal development is determined from the day the larva is enclosed until it transforms into an adult.
In the adult stage, the darkening of the pupa indicates the transition to adulthood One male and one female pupa are transferred into a 10x5x8 cm cage, accompanied by five 10 cm green onion leaves, which are placed on a moisture-covered cotton base To prevent water loss, adults are provided with a cotton ball soaked in a 10% honey solution for feeding, with both the green onion and honey solution replaced daily Key metrics such as adult longevity, pre-oviposition time, oviposition time, and the number of eggs laid are recorded until the last adult dies The fertility and hatchability of the eggs laid by the female moth are assessed daily, and the adult moths are dried and examined under a stereo magnifying glass to analyze their color, vein pattern, and size.
Figure 3.6 Plastic cage (10x5x8cm) pair adult moth 3.4.2 Methods of temperature effected on ecology characteristic of beet armyworm Spodoptera exigua
We investigated the biological characteristic of the beet armyworm S exigua at 25°C and 30°C, humidity 65% and 12L: 12D light conditions
The larvae were put on the Sanyo MIR incubator, the mothod feed larvae was similar to 3.4.1.2
3.4.3 Methods of food consumption rate of larvae beet armyworm
We investigated the food consumption capacity of each instar larvae on green onion leaves at 30 o C humidity 65% and 12L: 12D light conditions
First instar larvae were individually placed in plastic boxes measuring 2x3x3 cm, along with green onion leaves and a moist tissue The mass of green onion leaves provided was approximately 200 mg for the first instar larvae, 500 mg for the second, and around 700 mg, 800 mg, 800 mg, and 800 mg for the third, fourth, fifth, and sixth instars, respectively.
Leaves were cut from undamaged plants at the midpoint of the petiole and weighed After 24 hours, excreta were removed, and the leaves were weighed again Clean plastic boxes were used to supply new weighed leaves daily until the larvae reached the prepupal stage To assess weight loss due to evaporation, leaves of known weight were exposed to the same conditions without larvae and weighed afterward Daily food consumption per larva was calculated by subtracting the weight of remaining leaf tissue from the initial weight of the leaves provided, with adjustments made for evaporation.
The food consumption rate of worms is calculated according to the formula of as (Waldbauer, 1968) follows:
I: initial weight of the leaf
F: weight of uninfested leaves α = w/ Ic, β = w/ Fc Ic: initial weight of leaves (control)
Fc: final leaf weight after 24 hours w: the amount of water lost; w = Ic – Fc
3.4.4 Methods of the oviposition behaviour of Spodoptera exigua
The experiment involved diluting the insecticide Emaben 2.0EC (Emamectin Benzoate) at a concentration of 0.1ml per 100ml of water, and preparing neem oil with azadirachtin using 1ml of essential oil, 5ml of soapy water, and 1L of water Healthy green onion plants, consisting of 5 clusters in pots measuring 20x30cm with 10-15 leaves each at a height of 15cm, were used Three formulations were tested: water spray on three pots, and separate pots for Emaben 2.0EC and neem oil sprays After 24 hours of treatment, each green onion pot was positioned in a 10% honey solution, and a pair of male and female moths was released The pre-oviposition period lasted three days at 20°C, with the experiment conducted at an ambient temperature of 19 ± 2°C and relative humidity of 70–80% The number of egg masses laid on treated and control leaves was recorded after 24 hours, and egg clusters were collected and placed in separate boxes (2x3cm) with moisture paper for hatching monitoring The number of hatched eggs and first instar larvae were recorded for both control and treatment groups, with ten replicates maintained for each Additionally, the mass of eggs, emergence ratio, and mortality rate of the first instar larvae were documented.
Statistical analysis
Statistical analysis was conducted using IBM SPSS Statistics (Ver 20) to evaluate the impact of temperature on S exigua The study assessed various parameters, including larval length, pupal weight, and the fecundity of adult female moths A Shapiro-Wilk test confirmed that the data were normally distributed, allowing for the use of an Independent Samples t-Test for analysis However, parameters such as head capsule width, pupal dimensions, adult moth wing span, larval development time, longevity, life span, female moth oviposition, and egg emergence rates were not normally distributed, necessitating the use of a nonparametric Mann–Whitney U test P-values of 0.05 or less were deemed statistically significant.
RESULTS AND DISCUSSION
Morphological and biological characteristics of beet armyworm
Eggs are circular when viewed from above and slightly peaked from the side, tapering to a point They range in color from greenish to white and are covered with a whitish scale layer, giving them a fuzzy appearance Typically laid on onion leaf stalks, the newly laid eggs are milky white and darken before hatching Each nest contains 3-4 rows of eggs arranged in a row, adhering to the onion's surface.
Figure 4.1 Egg mass of beet armyworm Spodoptera exigua
Similar observations have earlier been reported by Atkins (1960) and Singh
Research indicates that the number of eggs per egg mass varies significantly, with Bichoo (1976) reporting a range of 5-161 eggs, peaking in winter (23-161) and dropping in summer (5-110) Vora et al (1984) and Ahmed et al (1997) found even higher counts of 20-280 and 25-300 eggs per egg mass on linseed and chilli, respectively Yash Pal et al (2005) noted 17-102 eggs on cotton, while Abdullah et al (2000) observed 50-150 eggs in insects fed artificial diets These findings suggest that S exigua shows a preference for laying eggs on linseed and chilli over cotton and chickpea.
Table 4.1 The width of head capsule and the width, the length (Mean±SE) of larvae beet armyworm
Table 4.2 presents the length and width measurements of beet armyworm larvae at two different temperatures At 25°C, the first instar larvae measured 2.04 mm in length and 0.24 mm in width, while at 30°C, they were slightly larger at 2.05 mm and 0.26 mm, respectively For the second instar larvae, the dimensions at 20°C were 4.74 mm and 0.54 mm, and at 30°C, they were 4.84 mm and 0.53 mm The third instar larvae measured 9.39 mm and 0.98 mm at 25°C, with no significant difference observed at 30°C, where they measured 9.64 mm and 0.96 mm Lastly, the fourth instar larvae showed similar dimensions at both temperatures, measuring 13.54 mm and 1.65 mm at 25°C, and 13.62 mm and 1.67 mm at 30°C.
At 25 degrees Celsius, the fifth instar larvae of the beet armyworm measured 15.96 mm in length and 2.1 mm in width, while at 30 degrees Celsius, these measurements increased to 16.1 mm and 2.17 mm, respectively For the sixth instar larvae, the length and width at 25 degrees were 20.03 mm and 2.71 mm, and at 30 degrees, they were 20.54 mm and 2.74 mm.
S exigua larvae undergo six instar stages, characterized by distinct physical features Their bodies display five prominent stripes: two large dark stripes on each side extending from the first to the last abdominal segment, and a black stripe alternating with a white one along the same length Additionally, two short, light-spaced stripes run from the thorax to the end of the abdomen, though they are not continuous As the larvae progress through each instar, these stripes become increasingly pronounced The abdomen itself is typically pink or light blue, adorned with two white stripes on each side, setting it apart from the dorsal surface.
The first instar larva measures 0.27-0.29 mm in head capsule size and 2.04-2.05 mm in body length Newly hatched larvae exhibit a gray color with a glossy black head, while their back features matte stripes and numerous tiny hairs and dots The body stripes are not distinctly visible, and large light brown dots are present near the head During this stage, the larvae create tiny holes in onion leaves and typically gather around the egg cluster to feed They are often found on the exterior of the onion, consuming the cuticle at the top and leaving behind a white membrane.
The second instar larva measures 0.56-0.57 mm in head capsule size and 4.74-4.84 mm in body length During moulting, the head capsule appears yellow, and the head is larger than the abdomen By the end of this stage, the body turns green, with a yellow-green abdomen and distinct body segments Notably, three translucent white stripes are present: one along the middle of the back and two on the sides, extending from the first to the last abdominal segment The second instar larva resides within the stalk, consuming chlorophyll parenchyma, leaving behind a white cuticle, and feeding on the leaf tips.
The third instar larva measures 0.93-0.94 mm in head capsule size and 9.39-9.64 mm in body length, featuring a light brown head capsule adorned with fine hairs above smaller dots on its back Notably, yellowish circular rings appear between body segments, accompanied by two sub-dorsal longitudinal stripes on each side As moulting begins, the larva develops a pale stripe and its body becomes noticeably fatter.
The fourth instar larva measures 1.52-1.54 mm in head capsule size and 13.58-13.62 mm in body length Characterized by a green coloration, the larvae have a smaller head compared to their body, with a darker dorsal side and a prominent dark lateral stripe The dorsal stripes are distinctly visible, while the upper body exhibits a light green hue Notably, the body lengthens and grows rapidly during this stage.
The fifth instar larva measures 1.81-1.85 mm in head capsule size and 15.96-16.1 mm in body length It is characterized by a green dorsal coloration with a lateral white stripe, and features a series of black patches or dashes on its dorsal and dorsolateral surfaces The spiracles are white with a narrow black border During early morning or evening, these larvae are observed moving to the top of the onion until midday.
The sixth instar larva measures 2.10-2.14 mm in head capsule size and 20.03-20.54 mm in body length It features a dark stripe along its back that extends from the first to the last abdominal segment As it approaches pupation, the dorsal body color changes to dark green or dark brown.
(d) Figure 4.2 Damage symptom on green onion plants caused by S exigua larvae
(a), (b) Symptoms of the first instar to second instar larva;
(c), (d) Symptoms of the third instar to sixth instar larva
Figure 4.3 Larvae of beet armyworm S exigua (a) First instar larvae;
Figure 4.4 Larvae of beet armyworm S exigua (a) Third instar larvae; (b)
Figure 4.5 Larvae of beet armyworm S exigua (a) Fifth instar larvae; (b)
The instar number of the larva was consistent with the findings of Nguyen Thi Huong (2018) when reared on onions at temperatures of 25°C and 30°C Additionally, during an experiment involving cotton leaves at temperatures ranging from 25°C to 29°C in the dry season, observations were made regarding the larvae's development.
Research by Nguyen Huu Binh and Pham Huu Nhuong (1997) indicates that larvae on soybean plants predominantly reach the 5th instar stage under unstable temperature and humidity conditions, specifically between 26-33°C and 80-100% humidity Additionally, some larvae develop into the 6th instar stage, as noted by Nguyen Thi Thu Cuc (1999).
In their research, Idris and Emelia (2001) observed that silky acacia worms undergo five instar larval stages Similarly, Azidah and Sofian-Azirun (2006b) reported that these worms can have between five to eight instar larvae, while Chen et al (2008) typically noted five to six instar larvae in their studies.
1932) claimed that the normal worm was five years old Therefore, the instar larvae stage was due to the impact of temperature, humidity, food
Previous studies by Atkins (1960) and Singh & Bichoo (1976) have noted similar observations However, my findings differ from those reported by Shankar et al (2014), which indicated significant color variation in the fourth and fifth instar larvae, ranging from green to light brown, dark brown, or even completely black, with a greenish-brown head capsule.
Table 4.2 The length, width (cm) and wight (mg) (Mean± SE) of pupae
Spodoptera exigua at two different
Width (cm) Length (cm) Weight(mg) Mean ± SE Mean ± SE Mean ± SE
The effect of temperature on the ecology characteristic of beet
The development time of beet armyworm larvae varies significantly with temperature, as shown in Table 4.4 Specifically, larvae cultured at 30°C experience shorter immature stages compared to those at 25°C The first and second instar larvae take 2-3 days to develop at both temperatures In detail, the average durations for the third, fourth, fifth, and sixth instar larvae at 25°C are 3.09, 2.77, 3.49, and 3.86 days, respectively, while at 30°C, the durations are 2.74, 1.86, 3.03, and 3.29 days.
Table 4.4 The mean duration (days) of the development stage of beet armyworm Spodoptera exigua when reared on two different temperatures
Stage 25°C 30°C P - value Min-Max Mean ± SE Min-Max Mean ± SE
6 th instar larvae 3-5 3.86±0.09 a 3-4 3.29±0.08 b