Feeding assimilation of fall armyworm (Spodoptera frugiperda J.E Smith) (Lepidoptera: Noctuidae) larvae on several host types

Spodoptera frugiperda (Noctuidae: Lepidoptera) is a pest that is easy to adapt and quickly develops. S. frugiperda can switch to crops other than maize as the primary host. Food assimilation must be tested to determine which plants S . frugiperda larvae prefer. This research was conducted to determine the feeding assimilation of S. frugiperda larvae and determine plants that can be alternative hostsinstead of maize . This study was arranged using a completely randomized design (CRD) with three treatments and five replications to obtain 15 experimental units. Larvae feed consists of rice, maize, and Napier grass. Feed is given during their 3rd instar stage until it becomes a prepupa. The suitability of plants as a food source for insects can be determined by a food assimilation test calculated using the Gravimetric method. The findings from the observations of food assimilation of S. frugiperda larvae in larvae-fed maize had higher consumption rate, growth rate, and digestibility than larvae-fed Napier grass. The conversion efficiency of digested food in grass-consuming larvae is 24.173%, higher than maize, 10.227%, and rice 0.00%. In compliance, the efficiency of food utilization was higher for larvae-fed Napier grass compared to those fed maize and rice.


Introduction
Fall armyworm (Spodoptera frugiperda J.E. Smith) (Lepidoptera: Noctuidae) is an insect native to tropical region.This pest was initially discovered in the regions of Central West Africa in early 2016, where it was reported the attack of maize crops with heavy infestation.By September 2017, the prevalence of this pest had spread to Egypt and other parts of Africa (Heinrichs et al., 2018) [1].(Sharanabasappa et al.,et al. [2] conducted a study on the biology of FAW in India.[3].A study conducted in Cameroon, Africa, investigated the impact and spread of this pest, as well as the reactions of farmers [4].The behavior and distribution of FAW were observed and widely attacked on two categories of hosts in the state of Florida, USA [5].In 2019, fall armyworm was identified in West Pasaman Regency, Indonesia, and has since spread to all districts with maize crops in West Sumatra [6].This pest can attack more than 80 plant species, causing a significant reduction in production if not adequately controlled [7].S. frugiperda can attack all parts of the plant and has an extensive host range.
S. frugiperda is also considered one of the invasive pests because it is not originally from Indonesia.If its population increases, it will endanger and harm cultivated plants, especially in the tropics [8].Control measures to date have generally relied on the use of synthetic pesticides.Using synthetic pesticides on S. frugiperda can have a negative impact because it causes resistance to occur quickly.The emergence of resistance will cause insecticides to no longer be effective so this pest will develop further [9].To avoid the disadvantages of using these synthetic insecticides, it is necessary to establish the concept of integrated pest control.Applying the concept of integrated pest control is a viable option for decreasing the reliance on synthetic insecticides.The success of this concept requires knowledge of insect biology, ecology, and physiology.Knowing insect physiology can determine how much insects can utilize host plants for their growth [10].The nutritional content of the host plant determines insects in choosing their hosts.In plants that do not meet the nutrients for S. frugiperda, this pest takes a long time to develop, and the weight of the pupae is reduced.The amount and quality of food affect growth rate, development time, body weight, and survival [11].The preferred crop for this pest is maize, but the 4 this pest is polyphagous, so it will quickly move to other plants, such as soybeans and cotton [12].S. frugiperda is an invasive pest that easily adapts to new environments, and its development is swift.Several plants, such as maize, Napier grass, and rice, often exist in the same ecosystem.Host selection by S. frugiperda can be based on its needs.Food assimilation will determine its preference for a type of plant.In this regardS.frugiperda can move and attack other plants besides the primary host.Napier grass and rice belong to the Poaceae family, the same family as maize as the primary host.This study aims to identify the feeding assimilation of S. frugiperda larvae and determine plants that can be alternative hosts instead of maize, designing effective S. frugiperda control strategies.

Methods
The research occured at the Insect Bioecology Laboratory of the Department of Plant Protection, Faculty of Agriculture, Universitas Andalas, Padang.Dry weight analysis was conducted at the Laboratory of Pest Disease Observation and Development of Biological Agents, Bukittinggi.The research was conducted from August to November 2022.
This study was organized using a completely randomized design (CRD) with three treatments and five replicates.Each replicate consisted of 15 larvae.The treatments included three types of larvae food plants: maize (Zea mays), Napier grass (Pennisetum purpereum), and rice (Oryza sativa).

Provision of crops as feed
Maize plants were planted in polybags measuring 15 x 15 x 15 cm 3 .Soil mixed with manure in a 1:1 ratio was put into the polybags.Each polybag was produced with 3-5 seeds at a 1-3 cm depth.Plants were planted in 5 polybags every 15 days during the study to maintain food availability.Rice plants were sown in trays.Rice seeds were soaked with water to cover the surface of the seeds (± 2 mm high).Rice plants were sown every 15 days during the study to maintain availability.Napier grass was obtained from the plantations around the cattle pens and rice fields.

Rearing of S. frugiperda
Larvae of S. frugiperda were gathered from maize fields in the Kuranji area, Padang City.Larvae were directly gathered and put into plastic containers with a diameter of 6 cm and a height of 4 cm containing maize leaves as food.One S. frugiperda larva was placed in each plastic container.S. frugiperda larvae collected from the field were reared in the laboratory on maize leaves until reaching the immature stage.Male and female imago were allowed to copulate and produce eggs.The eggs are reared until they hatch.The newly hatched eggs are the first generation (F1) of the parents collected from the field.S. frugiperda was reared until the third generation (F3) was obtained.The third instar larvae produced from the third generation (F3) will be tested in this study.

Feeding of S. frugiperda larvae
Larvae of S. frugiperda in the third instar were weighed one by one, 15 larvae per replicate.They were then put into the insect-rearing container.Feed was given according to the treatment:maize leaves, rice, and Napier grass.Maize leaves and Napier grass were cut into 3 cm x 3 cm pieces, with one leaf per larva.Rice leaves were given as one leaf per larvae.
The leaves given as food were weighed individually to determine their weight.Following the treatment, the test larvae, remaining feed, and feces were weighed and then wrapped using aluminum foil one by one and subjected to drying in an oven at 100 o C until a constant weight was achieved (6 hours).Upon reaching a stable weight, a subsequent weighing was conducted to ascertain the dry weight.To calculate the initial dry weight, the larvae and leaves were weighed to establish their wet weight.Each larva and leaf were individually enveloped in aluminum foil and subjected to dry until a consistent weight was achieved.The larvae and leaves were weighed again to determine their dry weight.The number of larvae used, leaf size, and larvae instar were the same as those used for the treatments.

Observation variable 2.4.1 Amount of feed consumed
The amount of feed consumed was obtained by calculating the variance between the wet weight of the treatment leaves and the wet weight of the remaining feed.Larvae weight gain was obtained by weighing the weight before and after treatment.The difference between the larvae's wet weight after feeding an the larvae's wet weight before feeding was calculated according to the treatment.Initial larvae dry weight; Third instar larvae were wrapped in aluminum foil and oven-dried until constant weight.After the larvae were dry, their dry weight was assessed through weighing.This weight was used as the initial larvae dry weight.Final larvae dry weight: Third instar larvae fed according to the treatment were wrapped in aluminum foil and oven-dried until the weight was constant.The weight of the larvae was measured to ascertain their dry weight.This weight was used as the final larvae dry weight.

Feed dry weight
Maize leaves and Napier grass were weighed to establish their wet weight.The leaves were wrapped one by one using aluminum foil and dried using an oven until the weight was constant.it was weighed to determine the dry weight.This weight is used as the dry weight of the feed.

Dry weight of feed residue
The larvae feed leaves during the treatment were weighed to determine their wet weight.The leftover feed was wrapped in aluminum foil one by one and then oven-dried. it was weighed after the weight was constant to determine the dry weight.

Dry weight of secretion
Feces of the larvae produced during the treatment were collected and weighed to determine the wet weight.Then, they were wrapped using 14 aluminum foils and oven-dried. it was weighed afterthe weight was constant to determine the dry weight.
Growth parameters,feeding consumption, and feeding efficiency of larvae were measured using the gravimetric method [13].The acquired data was utilized to calculate the parameters of food utilization efficiency.The parameters assessed in this experiment included consumption rate (LK), relative consumption rate (LKR), growth rate (LP), relative growth rate (LPR), digestibility (DC), food conversion efficiency consumed (EMK), and food conversion efficiency digested (EMC).Parameter calculations were based on dry weight to avoid deviations by variations in water content.

Data Analysis
Data was analyzed employing analysis of variance, followed by an LSD test at a 5% absolute level (software for Windows Stat 8.0).

Result 3.1 Amount of food consumed by S. frugiperda larvae
The observation of the amount of food consumed by S. frugiperda larvae (grams) is presented in Table 1.Based on the analysis of variance and LSD test at the 5% level, the amount of food consumed by S. frugiperda larvae significantly differed between maize and Napier grass with rice (P=0.000).Table 1 shows that the maize leaves consumed by S. frugiperda larvae were 0.2010 g; Napier grass consumed was 0.2242 g; and no rice leaves were consumed with a value of P = 0.0000.The amount of food consumed by S. frugiperda larvae was not significantly different between maize and Napier grass.However, there was a significant difference between rice, maize, and Napier grass.

Weight gain of S. frugiperda larvae
Larvae were provided with maize leaves, Napier grass, and rice showed different weight gain from S. frugiperda larvae (Table 2).Table 2 shows that the larvae that consumed maize had a weight gain of 0.0313 g, and the larvae that consumed Napier grass had a weight gain of 0.0283 g with a P=0.0000 value the feeding periode of S. frugiperda larvae consuming maize lasted for two days, while that of larvae consuming napier grass lasted for three days.Larvae weight gain was not significantly different between maize and napier grass.However, there was a significant difference between rice, maize, and napier grass.

Dry weight of early larvae of S. frugiperda
The initial dry weight of S. frugiperda larvae before feeding the treatments can be seen in Table 3.There was a significant difference between the larvae fed with maize, Napier grass, and rice.Table 3 shows that the dry weight of early larvae consuming maize is 0.0051 g, and Napier grass is 0.0039 g (P = 0.0000).The dry weight of early larvae of S. frugiperda on maize, napier grass, and rice feed significantly differs between treatments.

Final larvae dry weight of S. frugiperda
The final dry weight of S. frugiperda larvae after different feeding treatments is shown in Table 4. Maizefed S. frugiperda larvae had the highest values, while rice-fed larvae had the lowest ones.Table 4 shows that the highest final larvae dry weight of S. frugiperda was found in the larvae fed with maize, which was 0.0077g.The lowest final larvae dry weight was found in the larvae that consumed rice.The final dry weight of larvae that consumed napier grass was 0.0044 g with P=0.0000.

Feed dry weight
The dry weight of maize leaves, napier grass, and rice as food consumed by S. frugiperda larvae was found to be significantly different among treatments (Table 5).In Table 5, the examination of the dry weight of the feed shows asignificant difference between treatments.The highest feed dry weight is found in maize, which is 0.0601.The dry weight of napier grass was 0.0261 g.The lowest dry weight was found in rice, 0.0133 g, with P = 0.0000.

Dry weight of feed residue
Table 6 displays the dry weight of feed residues consumed by S. frugiperda larvae following the treatment.The dry weight of feed residue significantly differed between maize, rice, and Napier grass.Table 6.Dry weight of feed residues consumed by S. frugiperda larvae Treatment Dry Weight of leftover feed (g) ± SD Napier grass 0.0195 ± 0.0042 a Maize 0.0080 ± 0.0022 b Rice 0.0000 ± 0.0000 b Note: Numbers followed by the same lowercase letter in the same row are not significantly different based on the results of the LSD test at the 5% level.
Table 6 indicates no significant difference in the dry weight of leftover feed from maize and rice leaves.The highest dry weight of feed residue consumed by S. frugiperda larvae was found in Napier grass.The dry weight of the remaining Napier grass feed amounted to 0.0195 g, while the dry weight of the remaining maize leaf feed had a value of 0.0080 g, and the dry weight of the remaining rice leaf feed was 0.0000 g.The lowest dry weight of the remaining feed was found in rice with a value of P = 0.0195.The lowest feed residue dry weight was found in rice with a value of P = 0.0000.

Dry weight of S. frugiperda larvae secretion
The dry weight of S. frugiperda larvae secretions differed significantly between treatments.Dry secretion weight was highest for larvae consuming napier grass and lowest for larvae consuming rice (Table 7).Table 7 shows that the dry weight of S. frugiperda larvae secretion significantly differed between treatments.The highest dry weight of S. frugiperda larvae secretion was 0.0112 g in larvae fed with Napier grass.Maize-fed larvae had a dry weight of 0.0147 g of secretion.The lowest secretion dry weight was found in the rice treatment (P=0.0000).Table 8 shows the observation of feeding assimilation of S. frugiperda larvae.Maize-fed larvae had a higher consumption rate, growth rate, and digestibility than Napier grass-and rice-fed larvae.Regarding food utilization efficiency, larvae fed with Napier grass were higher than larvae fed with maize and rice.
Based on the analysis of variance and LSD test at the 5% level on the assimilation of food of S. frugiperda larvae on several types of hosts, further tests showed significant differences between treatments.

Discussion
Larvae exhibited more weight gain when fed maize than those fed Napier grass and rice.The amount of food consumed was higher for larvae fed Napier grass than for larvae fed maize and rice.Napier grass was eaten more by S. frugiperda larvae but resulted in lower larvae weight gain than maize and rice.This is related to decrease larvae's efficiency in utilizing food.The large amount of food consumed causes the food to be processed immediately, decreasing food utilization efficiency [14].The growth of insect larvae is also primarily determined by the nutrient content of their feed.The composition of napier grass's nutrients includes 19.9% dry matter, 18.2% crude protein, 1.6% fat, 34% crude fiber, 11.7% ash, and 42.3% extract material without nitrogen.Different types of food affect the consumption rate (LK), relative consumption rate (LKR), growth rate (LP), relative growth rate (LPR), digestibility (DC), efficiency of food consumed (EMK), and efficiency of food digested (EMC).These parameters can be used to see the quality of plants that are favored or not by larvae.This is due to differences in leaf surface structure, feed plant nutrients, and feed moisture content.
Maize-fed larvae had the highest values in observations of consumption rate, relative consumption rate, growth rate, relative growth rate, and digestibility.The consumption rate is in line with the larvae growth rate, where more food consumed will increase the weight gain of the insect, so the growth rate will also increase [15].In observing food conversion efficiency consumed and food conversion efficiency digested, larvae fed with napier grass had the highest value.The difference in conversion efficiency of food eaten is also determined by the amount of consumption used for growth and development of larvae.The larvae that consumed maize had high digestibility but decreased food utilization efficiency.The conversion efficiency of food consumed and digested decreases, causing digestibility to increase [16].
Maize-fed S. frugiperda larvae have a high consumption rate, but the efficiency of food consumed and digested is low.According to Lina et al. [17], an increased consumption rate will cause food to be processed immediately, resulting in a potential decline in food consumption and digestion efficiency.The larvae growth rate is determined by how much food is consumed and how much food can be converted to body building.The more food eaten is converted into bodybuilding substances, the higher the larvae weight gain.
Digestibility was highest for larvae-fed maize.The dry weight of feed and dry weight of secretion, which had the highest value, was found in larvae fed with maize.This is by research conducted by [18],who found that digestibility is influenced by the dry weight of food consumed and the dry weight of secretion s excreted.Not all food that undergoes digestion is transformed into body weight, but the amount of digested food is used to maintain larvae survival [18].
The efficiency of food consumed and digested was higher in larvae fed napier grass than in maize and rice.High moisture content helps in the amount of food assimilated as more food is consumed.Insect growth is better on feed that has a higher moisture content.Table 1 illustrates that the moisture content of maize is higher than the moisture content of napier grass and rice.
In maize-fed larvae food consumption increases, so food must be processed immediately and not stored for long in the digestive tract.This is done by increasing the consumption rate in Lina's study [17] Increasing the consumption rate causes the conversion efficiency of digested food to decrease.
Host plant quality affects insect growth and development.Good quality feed will increase insect growth and development.Conversely, low-quality feed will reduce the level of insect growth and development.The criteria for good feed is to produce a survival percentage of more than 70% of insects, shorter larvae stadia, fewer instars, and higher larvae and pupal weights [19].The growth and consumption rates were higher in larvae fed with maize than those provided with napier grass and rice.This follows the nutritional content in Table 1, which shows that maize has higher protein levels, carbohydrates, and water than napier grass and rice.Nutrient content that is suitable for larvae can accelerate the growth rate.In addition, it also affects survival to be better.The better the quality of the plants consumed, the stronger the larvae's immune system and the better their survival in the environment.Nutrient content that is not optimal will reduce the growth rate and result in low metabolism [20].
The nutrient content required by insects is related to the nutrient content of the host plant.Insect nutritional needs are requirements for growth and development, tissue maintenance, reproduction, and energy for an organism.Nutritional balance is essential for insects; in the order Lepidoptera, the need for protein, amino acids, and carbohydrates is generally balanced.The nutritional content of maize and Napier grass can be seen in Table 1.Plant nutrients that affect larvae growth and development include protein and carbohydrates.The protein, carbohydrate, and water content of maize are higher than those of Napier grass and rice.Protein content is needed for the formation of hormones that trigger growth and molting in insects.Carbohydrate content plays more of a role as a source of nutrition.Carbohydrate content at a certain amount can produce more optimal growth [21].
The lifespan of S. frugiperda larvae was shorter when nourished with maize than those fed on napier grass and rice.This is related to the protein content of the feed plants.A higher protein content in the feed resulted in a shorter average life span of one generation and higher reproductive potential [21].Larvae utilize the protein content to form body tissues that transcend the instar stage during development In rice-fed larvae, S. frugiperda larvae had the lowest value in each observation compared to maize and napier grass.This is thought to be due to the low water content of the rice leaves so that at the time of treatment, the rice leaves curled up.In addition, the nutrient content of rice does not match the nutritional needs of S. frugiperda in its growth and development.The silica content contained in rice is also one of the factors that S. frugiperda does not consume.

Conclusion
Feeding assimilation of S. frugiperda larvae resulted in different consumption rates, growth rates, and food utilization efficiency.Consumption rate, growth rate, and digestibility were highest for larvae fed with maize.Regarding food conversion efficiency, the highest consumption and digestibility were found in larvae fed with napier grass.S. frugiperda larvae favored napier grass, while the rice did not.Rice does not have the potential as an alternative host in S. frugiperda.

Table 1 .
Amount of food consumed by S. frugiperda larvae

Table 2 .
Weight gain of S. frugiperda larvae after feeding Note: Numbers followed by different lowercase letters in the table are significantly different based on the results of the LSD test at the 5% level.

Table 3 .
Dry weight of early larvae of S. frugiperda on different host species Note: Numbers followed by different lowercase letters in the table are significantly different based on the results of the LSD test at the 5% level.

Table 4 .
Final larvae dry weight of S. frugiperda after treatment on different host species Note: Numbers followed by different lowercase letters in the table are significantly different based on the results of the LSD test at the 5% level.

Table 5
Note: Numbers followed by different lowercase letters in the table are significantly different based on the results of the LSD test at the 5% level.

Table 7 .
Dry weight of S. frugiperda larvae secretion after treatment Numbers followed by different lowercase letters in the table are significantly different based on the results of the LSD test at the 5% level. Note:

Table 8 .
Feeding assimilation of S. frugiperda larvae on different host types Note: Numbers followed by different lowercase letters in the table are significantly different based on the results of the LSD test at the 5% level.LK= Consumption rate, LKR = Relative consumption rate, LPR= Relative growth rate, DC= Digestibility, EMK= Food efficiency of consumption, EMC= Food efficiency of digestion.