Morphology and life cycle of Spodoptera frugiperda(J.E. Smith) (Lepidoptera: Noctuidae) with soybean leaf (Glycine max[L.] Merr.) in the laboratory

Soybean (Glycine max[L.] Merr. ) is an important vegetable source of vegetable protein for the whole world. Wilis is one of the superior varieties in great demand among farmers. However, soybean production in recent years in Indonesia has declined. One of the causes is the presence of parasites which can cause losses both in quantity and quality. The fall armyworm (FAW), Spodoptera frugiperda(J. E. Smith) (Lepidoptera: Noctuidae) is a serious pest that can attack 353 host plants from 76 plant families and one of them is the Fabaceae family. This research was conducted at the laboratory and experimental farm of Hasanuddin University Faculty of Agriculture and took place from August to December 2020. The larvae were obtained from Sidenreng Rappang regency, Watang Pulu district. Insect was reared in the laboratory to produce eggs which are used to observe the life cycle of S. frugiperda with soybean leaves as food. Biological observations were made, including the duration of each stadium, and the shape and size of the egg until it becomes an adults. The results of this study showed that the larvae of S. frugiperda consisted of six instars. The mean duration of eggs to hatching was 1.8 days, the duration of the larval was 18.78 days, the duration of the pre-pupa was 1.61 days, the duration of the pupal was 10 days and the longevityis s 9.28 days for males adults and 13.73 days for females adults.


Introduction Soybean (Glycine max[L.]
Merr.) is one of the important crops of vegetable protein sources around the world.In Indonesia alone, soybeans are widely used as raw material for making tempeh, tofu, and soy sauce [1].So the demand for soybeans is increasing every year.The increase in soybean demand is not proportional to the soybean production produced.So that the national demand for soybeans is not met.This can be caused by the lack of quality of soybeans produced by farmers and the presence of plantdisrupting organisms that attack so that productivity is reduced [2].
The fall armyworm (FAW), Spodoptera frugiperda(J.E. Smith) (Lepidoptera: Noctuidae) is an important pest that can feed on 353 plants from 76 host plants [3].The S. frugiperda can move to various plants in the field, and exposure to the host range makes this insect population form large colonies even though the host is not the main host [4].The S. frugiperdacan complete its life cycle on any host plant, so this pest can feed on other plants, especially plants with high C/N content [5].The presence of other host plants, including soybean plants, will affect the development of insect populations.This pest can also attack soybean plants, which can cause damage to around 37.13-58.70%[6] Soybean plants can be potential hosts for S. frugiperda, which causes the spread of this pest to be more widespread.Damage caused by S. frugiperda to soybean plants varies, ranging from defoliation, feeding on leaf shoots and flower clusters, to cutting pods near the base in early attacks [7].Soybeans contain high protein ranging from 36g/100g of dry matter.The protein contained in soybeans is one of the macronutrients for insects for the formation of enzymes and hormones, such as ecdysone which induces insect growth and cell molting activity [8].The development process of S. frugiperda in soybean plants is longer than that of corn plants so the damage caused increases in the vegetative and generative phases [9].
Currently, information related to population development and infestation in Indonesia is still focused on maize crops.Controlling this pest is quite difficult in some countries because this pest is resistant to many insecticides.Based on the description above, it is necessary to conduct research on the life cycle of S. frugiperda pests on soybean leaves in the laboratory as a reference for determining control techniques and reproductive potential.

Place and time
This research was conducted in the experimental farm and laboratory of the Department of Plant Pests and Diseases, Faculty of Agriculture, Hasanuddin University, Makassar, which took place from August 2020 to December 2020.

Feed crop preparation
The preparation of the food plants begins with preparing the planting media to be used in the form of soil and manure in a ratio of 1:1.The soil that has been mixed with manure is transferred to a polybag measuring 30 × 40cm.Soybean seeds are planted and maintained so that the leaves grow healthily as larval feed.

Maintenance and propagationof Spodoptera frugiperda
Larvae obtained in corn fields were reared in the laboratory by separating the larvae with baby corn feed, larvae that became pupae were transferred to plastic containers with moist and loose soil to make it easier for the pupae to enter the soil and develop.Pupae that become adults are transferred to a cage containing cotton wool that has been applied with 10% honey to produce eggs.The eggs that hatch into larvae are fed with soybean leaves.

Life cycle observations of Spodoptera frugiperda
Life cycle observations were conducted in a controlled room with a temperature of 24-27°C and humidity of 49-53%.

Egg stage observation.
Eggs laid by female were collected.Observations were made from the time the eggs were laid until they developed and hatched into first larvae.Before the eggs hatched, each group of eggs was counted for number and diameters.Successfully hatched larvae are counted again to see the mortality rate of the hatching process.

Larval stage observations.
Observations were made in 1×24 h from larval hatching until entering the puparium phase, the number of instars in each larval development was calculated based on the 1230 (2023) 012108 IOP Publishing doi:10.1088/1755-1315/1230/1/0121083 occurrence of molting by looking at the exuvia formed, larval color, and larval size under a digital microscopes with a magnification of 50 to 1600× 2.4.3.Observation of pre-pupal and pupa stages Observation of pre-pupae was carried out when the larvae entered the puparium formation, with the parameters observed were puparium shape, the size of the puparium, the color of the pupae, the duration of larvae to prepupae, duration of prepupae to pupae, and duration of pupae to adults.

2.4.4.Observation of adult stage and fecundity
The adult longevity is observed after pupae become adults until dies.In the development of the adult, the sex ratio of S. frugiperdawas observed.Fecundity observations were made by placed the male and female (1:1).Observations were made in 1×24 h with parameters such as the duration the female produced eggs (oviposition), the number of laid eggs, and the number of cluster egg produced per female.After the aduls died, morphometric observations of insects were made, namely wing span, adult body length, and observation of adult antennae.

Mortality of insects
Mortality was observed at each stadium of the insect using the following formula: Description: P = % mortality, n = number of dead larvae, N = initial number of larvae

Data analysis
The data obtained from the observations were recorded and the Ms. Excel software program, the data were processed by calculating the average insects in each parameter and observation.The range of time for eggs to hatch is 1−3 days.A female can produce 1−4 clusters of eggs in one group of eggs containing 56-244 eggs with an average of 113.20.So that in her life cycle, a female can produce 86−358 eggs.Female did not lay eggs on soybean leaves due to the morphology of soybean leaves has dense trichomes making it difficult for eggs to adhere to the leaves.

3.2.Larvae
Figure 2 shows that the larvae consist of six instars that can be characterized by larval molting.Hatching larvae entering first instar will be pale green with a head that is larger than the abdomen and black.However, on the second day of observation with the same larval instar, the head size will appear smaller to match the size of the larval abdomen.Larvae in this instar have an average length of 1.667mm (Table 1).Larval 1st instar about 2−3 days with an average of 2.32 days (Tabel 4).
In second or 2nd instar larvae, pinacula spots with a single seta are visible.The larvae are green in color with the head appearing the same size or even smaller than the abdomen.The average larval length is 6.568mm (Tabel 1).The 2nd Instar about 2-4 days with an average of 2.82 days (Tabel 4).At Instar 3, the morphological characteristics of S. frugiperda larvae will begin to be seen and easily identified.Figure 3 shows that the morphological characteristics of S. frugiperda are a dark-colored head with a pale inverted "Y" letter, a dark-colored pinacula on the dorsal part with one seta, a line on the lateral part, four pinacula forming a quadrilateral on abdominal segment 8. The larval color becomes dark green and has an average length of 11.962 mm (Tabel 1).Instar 3 lasts about 2-4 days with an average of 3.10 days (Tabel 4).In the 4th instar larvae, all the morphological characteristics of S. frugiperda larvae are evident Figure 4 shows that this larva has four pairs of false limbs (prolegs) on the abdomen and another pair at the rear end of the body.It has 3 stripes on the upper body, one on the dorsal, and one on each sub-dorsal, and has an average length of 17.458 cm (Tabel 1).Instar 4 lasts about 3-4 days with an average of 3.45 days (Tabel 4).The 5th instar larvae last about 3−5 days with an average of 3.60 days(Tabel 4).Instar 6 larvae last until entering the pre-pupal phase with a duration of 2-5 days with an average of 3.67 days (Tabel 4).In instars 5 and 6, the morphological characteristics of the larvae will be the same as those of instar 4 larvae, except that the size of the larvae will be larger and darker brown than before.Instar 5 has an average larval length of 20.268 mm and instar 6 has an average larval length of 21.997mm (Tabel 1).The overall duration of life from larval stage instar 1 to instar 6 ranged from 15-25 days with an average of 18.78 days.The larval body is brownish-green with a "Y" pattern on the head and thick bands on the laterals 5th instar larva 20.268 1.713 The larval body will appear browner in color with a "Y" pattern on the head and thick bands on the lateral parts, as well as a square pinacula on the abdominal segment 8 which is visible.
6th instar larva 21.997 1.218 The larval body will appear browner in color with a "Y" pattern on the head and thick bands on the lateral parts, as well as square pinacula on the abdominal segment 8 that are visible and larger larval size.
pre-pupae 14.680 0.993 Larvae will be less active in moving and feeding, and their bodies will appear shrunken.

Pupa 13.239 1.185
The pupa is oval with a green color at first and will turn reddish brown in color

Pupae
Larvae that have reached their maximum development will begin to move less actively and stop eating.the size of the larvae will shrink and reduce.this indicates that the larvae have entered the pre-pupal phase.The length of time it takes for the larvae to pupate (pre-pupal process) ranges from 1-3 days with an average of 1.61 days and a length of 14.46 mm.When the pupa has just formed, it will appear greenish which will then change color to brownish red when the pupa has been completely touched (Figure 5).In the field, larvae entering the pre-pupal phase will fall to the ground and bind to soil particles.However, in this study, the pupal formation process can be carried out without the larvae being in the soil.The larvae will bind the dirt and food residues around them with silk to cover their bodies and form pupal houses.Figure 6 shows that the pupa is oval-shaped, and reddish brown, and the color will get darker when approaching the turn of the stadia into the imago, and has an average length of 13.32 mm.The time required for the pupa to become imago ranges from 8-14 days with an average of 10 days.Figure 7 shows that male and female pupae can be distinguished by looking at the sex opening near the pupa cremaster.Male pupae will have a smaller distance between the genital opening and the anal opening.In female pupae, the distance between the genital opening and the anal opening is greater.S. frugiperda adult has differences between males and females.In Figure 8, males have gray forewings with a prominent pattern of black stripes,, oval white spots, and cream-colored kidney-like patterns and the hindwings are silver with lines on the wing margins.In female imago, the forewings are gray-brown and the pattern is indistinct.The hind wings are silver with lines on the wing margins.2 shows that the size of the imago between males and females is also different where the size of the male adult is smaller than the female adult.Generally, the front wings are longer than the rear wings but the rear wings have a larger width.The front wings are longer because they front wings function to protect the thin rear wings.During the nesting period, S. frugiperda imago is active from night to morning.Table 3 shows that the pre-nesting period ranges from 3-6 days with an average of 4.63 days,the nesting period ranged from 3-4 days with an average of 3.4 days, while post-nesting ranged from 2-6 days with an average of 3.97 days.Table 5 shows that male adult can survive around 8-12 days with an average of 9.28 days, while female adult can survive around 8-14 days with an average of 13.73 days.Based on the length of their life cycle, female insects live longer, ranging from 39-47 days with an average of 42.13 days compared to male insects whose life cycle is shorter, namely 38-45 days with an average of 40.44 days during their life cycle (Tabel 5). Figure 9 shows that the antenna shape of S. frugiperda is Filiform which is shaped like a thread with each segment that forms the antenna having the same flagellomer size.The male imago has an antenna size of 6.22mm while the size of the female imago has a length of 5.50mm.The shape of antennae of the genus Spodoptera has the same shape.The difference between S. frugiperdaadult and other Spodopetra imago can be seen from the wing pattern and size of the adult.Table 6 shows that the mortality rate at the egg stage is quite high.The eggs produced by the imago are numerous but several factors cause the eggs to be damaged and not hatch.This is because the hatched larvae eat the eggshells as a source of nutrition, which damages the eggshells of the larvae.cannibalism in larvae can reduce their population.

Mortality Spodoptera frugiperda
The larvae instar 4 and 6 experienced increased mortality, the larvae were stressed because soybean is not their favorite host.Other larvae were able to adapt to the diet, although this affected their size and life span.

Discussion
Spodoptera frugiperda is an invasive pest that originated in South America and spread to various countries [3][10].his pest was reported to enter and spread in Indonesia on March 26, 2019, in West Sumatra and spread in various provinces in Indonesia [11].In July 2019, the presence of S. frugiperda was reported in Takalar and Gowa districts by looking at the morphological characteristics of the larvae that attacked.The larvae of Spodoptera frugiperda are polyphagous [12]and can feed on 343 plant taxa from 76 plant families [3].Soybean is a plant that can be a host for S. frugiperda so that its spread can be widespread [10].
The morphology of eggs in this study is not different from the morphology of S. frugiperda eggs in general.Where this egg is round with white or yellow color and will become darker when the egg will hatch indicating the embryo of the egg has matured.Eggs are covered by fine hairs and accumulate [13].Eggs measure 0.48 mm and will hatch in about 2-3 days [12].Female imago can produce 1-4 groups of eggs.Imago cannot lay eggs on soybean leaves.This can be caused by the morphology of soybean leaves which have tight trichomes making it difficult for female imago to lay their eggs.In addition, soybean plants are C3 plants, which when compared to corn plants which are the main crop and C4 plants have more complete nutrition.So that can be one reason the female imago does not lay eggs on soybean leaves.[12].
Spodoptera frugiperda larvae consist of 6 instars [12].The size of larvae in instar 1 is 1.67mm, instar 2 larvae are 6.57mm, instar 3 larvae is 11.96 mm, instar 4 larvae is 17.45mm, instar 5 larvae is 20.26mm, instar 6 larvae is 21.99 mm [13] [14].hen compared to previous studies, S. frugiperda larvae fed with soybean feed have a smaller size when compared to previous studies with corn feed which is the main host [15].This may be due to the insect's diet.Nutritional factors of the feed given to the larvae can affect the behavior, physiology, and evolution of insects [12].S. frugiperda larvae have morphological characteristics with a pinacula on the 8th segment that is square and a "Y" shaped pattern on the head that is white [16][17] [18].
In this study, the pupa has an oval shape with a reddish-brown color and becomes darker when entering the imago stage [14].The pupa measures 13.32 mm and Pupae are 13.32 mm in size and last about 8-14 days [19] [18].The difference in the duration and length of the pupa can be influenced by environmental factors and feeding factors for larvae that are not their favorite host plants, thus affecting the development of the pupa as well as the previous larvae where the type of feed factor will affect the initial development to the final development in the insect cycle [12].Differentiating the sex of the pupa can be done by looking at the genital opening.The distance between the genital opening and the anal opening can be used to distinguish between female and male pupae.The distance between females is greater than males [9] [20].
Adult male forewings are grayish brown with indistinct reniform patches, faintly black-striped, with small v-shaped markings; light brown rounded patches, slightly oval and oblique shapes, and white patches at the apical edge of the wings [17] [21].The forewings of adult females are mottled gray and brown, with brown markings and without white patches near the apical edge of the wings as seen in males.The hind wings of both sexes of S. frugiperda imago are whitish silver in color with a dark-colored stripe on the edges [17] [22].
The size of the male imago is slightly smaller than the female imago [21].Generally, the front wings are longer than the hind wings but the hind wings have a greater width [22].The front wing is longer because it serves to protect the thin rear wing [21].The antennae of S.frugiperda are Filiform and shaped like a thread with each segment that makes up the antennae having the same flagellomer size.The male imago has an antenna size of 6.22mm while the size of the female imago has a length of 5.50mm.
The pre-nesting period ranges from 3-6 days with an average of 4.63 days, the nesting period ranges from 3-4 days with an average of 3.4 days, while the post-nesting period ranges from 2-6 days with an average of 3.97 days.Male imago can survive around 8-12 days with an average of 9.28 days, while female imago can survive around 8-14 days with an average of 13.73 days [23].When compared to previous research [15], the nesting time is longer.This can be caused by the factor that the food given does not meet the nutritional needs that can affect the fertility and age of adult insects [10][12].
The life cycle of Spodoptera frugiperda lasts for 40.44 days in male insects and 42.13 days in female insects.The egg stage until hatching is 1.8 days, the larval stage is 18.78 days, the pre-pupal stage is 1.61 days, the pupal stage is 10 days and the imago stage of male insects lasts 9.28 days, while in females it is 13.73 days.The quality and quantity of feed given at the larval stage can affect the growth rate, development time, larval weight, and survival which causes changes in the life cycle time of S. frugiperda [12].The use of soybean plants as larval feed can extend the life of the larval stage because soybeans contain a large number of amino acids that can affect the hormones of juvenile insects to carry out the molting process [8].
Changes in feed can affect the mortality of S. frugiperda.Feeding soybeans as larval food can maintain larval survival even though it is not the main host plant because soybeans contain vitamin C which plays a role in the insect diet, thus increasing larval survival [8].At instars 4 and 6, the larvae are stressed, causing growth retardation and death [24].In addition, the larvae of S. frugiperda are also cannibalistic.Cannibalistic behavior in S. frugiperda occurs at the larval stage, where larger larvae feed on smaller larvae.Cannibalism occurs partly due to the lack or mismatch of available food sources.Cannibalism is a common behavior of S. frugiperda larvae [25].

Conclusion
Soybean plants have the potential to be an alternative host for Spodoptera frugiperda if the main host plant is not available.this can be seen from the survival and development of Spodoptera frugiperda from the egg phase to becoming imago.In addition, the use of soybeans as food can affect the size of each larval instar, pupa, and imago.

Figure 3 .
Figure 3. Morphological characters of S. frugiperda, (a) Dark-colored head with a pale inverted Y, (b) Dark-colored pinacula on the dorsal part with a single seta, (c) Thick band on the lateral part, (d) Four pinacula forming a square on abdominal segment 8.

Figure 4 .
Figure 4. Four pairs of pseudolegs (prolegs) on the abdomen and another pair on the posterior end of the body (a), 3 stripes on the upper part of the body (b).

Table 1 .
Length measures of egg stadia, each instar of pre-pupal larvae and pupae in soybean plants.

Table 2 .
Morphological observations of male and female of Spodpterafrugiperda.

Table 3 .
Length of pre-nesting, nesting, and post-nesting period of S. frugiperda.

Table 4 .
Length of life of egg stadia, each larval instar, pre-pupae and pupae.

Table 5 .
Adults stadia and life cycle duration of S. frugiperda.

Table 6 .
Mortality rate of S. frugiperda.