Introduction

The Egyptian cotton leaf worm Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae) is found in Africa, Mediterranean Europe, and Middle East countries (CABI 2022). In Egypt, it is one of the most important pests of cotton Gossypium herbaceum; the key economic field crop in this country. Furthermore, about 132 host plant species belonging to 45 families, including economically important field crops (e.g., corn Zea mays and wheat Triticum aestivum) and vegetable crops (e.g., lettuce Lactuna sativa, tomato Solanum lycopersicum, potato Solanum tuberosum, and sweet potato Ipomoea batatas), besides ornamental trees (e.g., cotton rose Hibiscus mutabilis and roses Rosa) have been recorded as primary host plants of S. littoralis (CABI 2022).

Several studies have been carried out to demonstrate the effects of host plant types on the different life-history traits of S. littoralis (Al-Shannaf 2011; Mohamed et al. 2019; Ismail 2020; Hemmati et al. 2022; Mousavi et al. 2023). Population performance and growth of herbivorous insects are affected by the nutritional contents/ biochemical attributes of host plants (Ismail 2020; Hemmati et al. 2022; Shirinbeik Mohajer et al. 2022). The identification of insect biology, host preference, and behavior are crucial to find economically and ecologically sustainable solutions to the problems caused by herbivorous insects (Behmer 2009). From the applied point of view, biological studies help in the application of the suitable insecticide at the right time (Xue et al. 2010; Nandhini et al. 2023). Moreover, understanding the diversity of insect responses to different host species represents a key challenge for the development of durable pest control strategies (Després et al. 2007; Hemmati et al. 2012; Cabezas et al. 2013; Kianpour et al. 2014). The management of polyphagous and mobile pests requires pest management systems that focus not only on one major seasonal crop on a single field or farm, but also on wide-area cropping systems (Abel et al. 2007; Wu 2007; Herde 2009). From the basic point of view, biological studies aid in the development of detailed simulation models and may help in the construction of life tables and pest forecasting (Naranjo and Ellsworth 2005; He et al. 2021).

Survival, development, and reproduction of phytophagous insects are considerably affected by the primary and secondary chemical compositions of host plants; hence, food consumption and utilization depend on both plant quality and insect nutritional performance (Scriber and Slansky 1981; Singh and Mullick 1997). The factors determining nutrient availability for growth and maintenance over a given period of development are the amount and type of food consumed and the efficiencies of its utilization (Browne and Raubenheimer 2003). Like other insect orders, the balance of nutrients in many lepidopterans is important. Lepidopteran insects respond to unsuitable diets in diverse ways, such as altering the amount of ingested food, switching from one food source to another, and/or regulating the efficiency of the nutrients (Genc 2006). Food utilization efficiency reflects the quality and the quantity of food consumed (Naseri et al. 2010; Baghery et al. 2013), which may greatly affect insect fitness, such as development, survivorship, reproduction, and life table parameters (Scriber and Slansky 1981; Tsai and Wang 2001; Kim and Lee 2002). Study of insect nutrition is significant in providing critical information for economic exploitation, management of pest insects, and clarifying the relationship of energy among the communities (Awmack and Leather 2002; Babic et al. 2008). Moreover, studies on the consumption, digestion and utilization of food plants by insects are important both from basic and applied points of view. They provide information on the quantitative loss brought about by the pests (Jooyandeh et al. 2018). Analysis of the nutritional indices can provide an understanding of the behavioral and physiological bases of insect-plant interactions (Lazarevic and Peric-Mataruga 2003). A few studies have been carried out concerning the effects of different host plants on food consumption and utilization of S. littoralis (Gacemi et al. 2019; Ismail 2020; Mousavi et al. 2023).

Life table parameters are important in the measurement of population growth capacity of species under specified conditions. These parameters are also used as indices of population growth rates responding to selected conditions and as bioclimatic indices in assessing the potential of a pest population growth in a new area (Southwood and Henderson 2000). Fertility life tables are appropriate to study the dynamics of insect populations (Maia et al. 2000). Life table studies have several applications, including analyzing population stability and structure, estimating extinction probabilities, predicting life history evolution, predicting outbreaks in pest species, and examining the dynamics of colonizing or invading species (Haghani et al. 2006). Life table information may also be useful in constructing population models (Carey 2001) and understanding interactions with other insect pests and natural enemies (Omer et al. 1996). A few studies have been conducted to elucidate the effects of host plants on the life tables of S. liitoralis (Makkar et al. 2015; Abd-Allah and Ahmed 2022; Hemmati et al. 2022). Based on the pest management perspective, the life tables are essential to determine the most vulnerable pest stage (i.e., the stage which suffers the highest mortality) to make time-based application of insecticides (Singh and Singh 2022).

The extensive use of conventional insecticides for the management of S. littoralis has resulted in development of resistance to major classes of pesticides (Smagghe et al. 1999) and can have negative impacts on the environment (Pathak et al. 2022). Given the emergence of resistance and environmental hazards, it is necessary to investigate alternative pest management approaches that are more cost-effective and sustainable than conventional insecticides. Identifying the host preference is an eco-friendly approach within a framework of integrated pest management (La Rossa et al. 2013).

In the current study, we investigated the different biological parameters and nutritional indices of S. littoralis fed on four host plants, viz., castor bean, tomato, potato, and cucumber. We also quantified the concentrations of nitrogen, phosphorus, and potassium in these host plants. The current study could help in identifying the host preference of S. littoralis; thus, the development of sustainable control strategy against this pest.

Materials and methods

Insects

A stock colony of S. littoralis was reared in the laboratories of the Plant Protection Research Institute, Dokki, Giza at 27 ± 2 °C, 65 ± 5% relative humidity and a 16-h light: 8-h dark photoperiod, without exposure to insecticides or pathogens, according to Shaurub et al. (2023). Larvae were fed on fresh leaves of castor bean Ricinus communis, while adults were fed on a 10% sucrose solution.

Host plants

Four host plants were tested in this study, castor bean (R. communis; Family: Euphorbiaceae), tomato (S. lycopersicum; Family: Solanaceae), potato (S. tuberosum; Family: Solanaceae) and cucumber (Cucumis sativus; Family: Cucurbitaceae). These plants were selected because (i) they are primary host plants of S. littoralis (CABI 2022), (ii) there is a paucity of data on their effects on the life-history traits and nutritional indices of S. littoralis, (iii) although castor bean is a wild plant, it is the authenticated host plant for rearing S. littoralis in the laboratory, and (iv) tomato, potato, and cucumber are the most important vegetable crops in Egypt as they are involved in many food industries (Egyptian Chamber of Food Industries, Personal Communication). Fresh and fully expanded leaves of these plants were used as the diet for S. littoralis larvae.

All agricultural operations for the cultivation of tomato, potato, and cucumber were performed according to the traditional local agricultural management practices and the recommendations of the Ministry of Agriculture. They were cultivated without exposure to insecticides. Briefly, tomato seedlings were transplanted on ridges of 70 cm width with a spacing of 30 cm in the row. At soil preparation, the compost was applied two weeks before sowing at a rate of 20 m3 per feddan (fed) (1 fed = 4200 m2). The plants were irrigated every week during the growing season. Potato tubers were divided into pieces (∼ 40.0 g weight). Before planting, all plots received calcium superphosphate (15% P2O5) at a rate of 100 kg per fed and plant compost at a rate of 15.0 m3 per fed. Urea (46.5% nitrogen) was used for nitrogen fertilization at a rate of 150 kg per fed in two equal doses where the first and second doses were added 30 and 60 days after planting, respectively. Also, the traditional potassium fertilization was conducted using potassium sulfate (48% K2O) at a rate of 50.0 kg per fed 60 days after planting. Summer potatoes required 10–12 irrigations, while winter potatoes required 6–8 irrigations. The initial irrigation of summer crop was administered 18–21 days after planting, and subsequent irrigations were applied as needed in response to prevailing weather conditions and soil type. At soil preparation for cucumber planting, the compost was applied two weeks before sowing at a rate of 0.2 m3 per plot (15 m2). The plants were irrigated every week during the growing season. As castor is a wild plant and naturally grows, no information is available concerning its planting. Castor bean is a wild plant that naturally grows near the streams without exposure to insecticides.

Biological studies

Newly hatched larvae of S. littoralis were collected from the stock culture and divided into four groups, with 50 larvae each. Each group was fed on one type only of the tested four host plants (castor bean, tomato, potato, and cucumber). Larvae were released into a clean container (20 × 15 × 15 cm) covered with a piece of muslin cloth for ventilation. Larvae were fed daily on fresh leaves until pupation. Four replicates were conducted per each treatment. Two-day-old pupae were sexed and kept in rearing jars (15 cm in diameter, 20 cm in depth) till adult emergence. Newly emerged unmated moths were divided into 10 pairs, with 1 ♂ × 1 ♀ each. Each pair was transferred to a glass jar (7 cm in diameter, 10 cm in depth) covered with a piece of muslin cloth and provided with a small branch of oleander Nerium oleander as an oviposition medium (Shaurub et al. 2023). Moths were fed on a 10% fresh sucrose solution. The adults were monitored daily for mortality and oviposition, and number of egg masses laid by each female were collected and counted twice daily until the female died. Egg masses obtained from each treatment were observed daily for hatching to estimate the hatch percent.

Nutritional indices

Nutritional indices were estimated using 4th, 5th, and 6th -instar larvae of S. littoralis as they were easier to measure than the primary instars (Shaurub et al. 2020). Under the above-mentioned laboratory conditions, the developmental times of these instars are 2, 2, and 3 days, respectively (Shaurub et al. 2020).

Newly molted weighed 4th -instar larvae were starved for 10 h and released into a clean container (20 × 15 × 15 cm) covered with a piece of muslin cloth for ventilation and supplied with weighed leaves of each host plant. Rearing containers were cleaned daily and consumed leaves were replaced by fresh weighed leaves. To minimize experimental errors often associated in calculating the nutritional indices, enough food was supplied so that at least 80% of the available food was consumed during the experiment (Schmidt and Reese 1986). The weights of larvae (4th, 5th, and 6th instars) were recorded daily before and after feeding until they finished feeding and reached the pre-pupal stage. The initial fresh food and the food and feces remaining at the end of each experiment were weighed daily. The quantity of food ingested was determined by subtracting the diet remaining at the end of each experiment from the total weight of diet provided. To estimate the actual loss of moisture, which was used for calculating the corrected weight of consumed leaves, fresh leaves were kept in a similar rearing cup under the same experimental conditions. Each treatment was replicated four times, with 50 larvae each.

Food consumption and utilization were calculated according to the equations of Waldbauer (1968) as follows:

$${\rm{Consumption}}\,{\rm{index}}\,\left( {{\rm{CI}}} \right)\,{\rm{ = }}F/TA$$
$${\rm{Relative}}\,{\rm{growth}}\,{\rm{rate}}\,\left( {{\rm{RGR}}} \right)\, = G/TA$$
$${\rm{Approximate}}\,{\rm{digestibility}}\,\left( {{\rm{AD}}} \right)\, = \,\left( {F-E/F} \right)\, \times \,100$$
$$\begin{array}{l}{\rm{Efficiency}}\,{\rm{of}}\,{\rm{conversion}}\,{\rm{of}}\,{\rm{ingested}}\,{\rm{food}}\,{\rm{to}}\,{\rm{biomass}}\,\left( {{\rm{ECI}}} \right)\,\\= \,\left( {RGR/CI} \right)\, \times \,100\end{array}$$
$$\begin{array}{l}{\rm{Efficiency}}\,{\rm{of}}\,{\rm{conversion}}\,{\rm{of}}\,{\rm{digested}}\,{\rm{food}}\,{\rm{to}}\,{\rm{biomass}}\,\left( {{\rm{ECD}}} \right)\,\\= \,\left( {G/F - E} \right)\, \times \,100\end{array}$$

where:

A = fresh mean weight of larvae during the feeding period (mg)

E = fresh mass of feces (mg)

F = fresh weight of food ingested (mg)

G = fresh weight gain of larvae at the end of the feeding period (mg)

T = duration of the feeding period (days)

Phytochemical analysis

The concentrations of nitrogen (Muñoz-Huerta et al. 2013), potassium (Liu et al. 2018), and phosphorous (Liu et al. 2018) were estimated in castor bean, tomato, potato, and cucumber leaves. Each nutrient was replicated four times. Percentage concentration of each nutrient was calculated.

Statistical analysis

All datasets were first assessed for normality using the Shapiro-Wilk test (Shapiro and Wilk 1965), and subsequently expressed as the mean ± standard error (SE) for analysis. Data of biological, nutritional, and phytochemical studies were analyzed using one-way analysis of variance (ANOVA) for each variable among the four tested host plants. When the ANOVA statistics were significant, the means of each variable among the four tested host plants were separated by Tuckey’s test. Pearson’s correlation coefficient test between the larval weight and the amount of nutrients in each host plant was conducted. Significance level was set at α = 0.05. All statistical calculations were conducted using IBM-SPSS Statistics, v. 25 (IBM, Armonk, New York, NY, USA).

Results

Life-history traits

Table 1 shows the effects of castor bean, tomato, potato, and cucumber on certain life-history traits of S. littoralis. Developmental times of larvae fed on castor bean, tomato, potato, and cucumber were not significantly affected, with the developmental time in case of tomato was shorter than that in case of castor bean, potato, and cucumber by 10.93, 12.83, and 12.37%, respectively. Similarly, the tested host plants did not affect the developmental times of surviving pupae, with the shortest pupal developmental time (13.0 days) in case of castor bean. Developmental times of pupae that survived larvae reared on tomato, potato, and cucumber were approximately similar to each other (15.2–15.4 days). Weights of full-grown larvae were significantly affected by the host plants (P < 0.05), with the largest weight in case of larvae fed on castor bean (650.8 mg) and the smallest weight in case of cucumber (300.7 mg). Weight of full-grown larvae in case of tomato (450.5 mg) and potato (450.9 mg) were approximately similar to each other. Adult emergence was dramatically decreased when larvae were reared on tomato, potato, and cucumber (46.7, 40.2 and 23.8%, respectively) compared to that in case of rearing on castor bean (95.2%). Treatment with castor bean, tomato, potato, and cucumber significantly affected adult longevity of both sexes (P < 0.05), with the longest female longevity (7.1 days) and male longevity (6.1 days) in case of castor bean and potato, respectively. The number of eggs deposited per female (fecundity) were also significantly affected by the types of the tested host plants (P < 0.05). The highest number of eggs (480 eggs/female) and the lowest number of eggs (170 eggs/female) were deposited by females that survived larvae fed on castor bean and tomato, respectively. Fertility (% egg-hatch) was also drastically affected by the host plants. The highest fertility (98.1%) and the lowest fertility (40.0%) were attained in case of females that survived larvae fed on castor bean and cucumber, respectively.

Table 1 Life-history traits of S. littoralis fed on castor bean, tomato, potato and cucumber
Full size table

Food consumption and utilization

The data of nutritional indices of 4th, 5th, and 6th -instar larvae of S. littoralis were not consistent with each other (Tables 2, 3, 4, 5 and 6). The 6th -instar larvae fed on castor bean consumed higher food compared to those fed on tomato, potato, and cucumber. Overall, during the development of 4th, 5th, and 6th -instar larvae, the lowest food consumed was attained in case of feeding on tomato (Table 2). During the development of the 6th -instar larvae, the highest RGR was obtained in case of feeding on castor bean, whereas the lowest RGR was obtained in case of feeding on tomato, potato, and cucumber. The RGR values of 1-day-old, 2-day-old, and 3-day-old 6th -instars fed on tomato, potato, and cucumber did not change among these host plants for each age, separately (Table 3). The highest AD values were observed in 6th instars fed on potato, followed by castor bean, tomato, and finally cucumber. In contrast, the lowest AD values were recorded in the 4th instars fed on castor bean (Table 4). In case of late 4th and 5th instars (2-day-old instars), the highest ECI values were found on castor bean. In contrast, early 4th and 5th instars (1-day-old instars) exerted the lowest AD values in case of feeding on castor bean and cucumber, respectively. Full-grown larvae (6th -instars) showed the highest ECI values versus the four tested host plants. In a descending order, ECI on castor bean > ECI on tomato > ECI on potato > ECI on cucumber (Table 5). The highest ECD values of full-grown larvae were obtained in case of feeding on castor bean, followed by tomato, potato, and finally cucumber; a pattern which was similar to that of the ECI of full-grown larvae. During the development of 4th and 5th -instar larvae, the highest ECD values were in case of 2-day-old 4th -instars and 1-day-old 5th -instars fed on castor bean (Table 6).

Table 2 Consumption index (CI) of S. littoralis larvae fed on castor bean, tomato, potato and cucumber
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Table 3 Relative growth rate (RGR) of S. littoralis larvae fed on castor bean, tomato, potato and cucumber
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Table 4 Approximate digestibility (AD) of S. littoralis larvae fed on castor bean, tomato, potato, and cucumber
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Table 5 Efficiency of conversion of ingested food to biomass (ECI) of S. littoralis larvae fed on castor bean, tomato, potato and cucumber
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Table 6 Efficiency of conversion of digested food to biomass (ECD) of S. littoralis larvae fed on castor bean, tomato, potato and cucumber
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Host plant nutrients

Overall, castor bean was considered the most nutritive host plant as it contained the highest concentrations of nitrogen and phosphorous. In contrast, tomato was the least nutritive host as it contained the lowest concentrations of nitrogen, phosphorous, and potassium (Fig. 1).

Fig. 1
figure 1

Concentration of nitrogen (N), phosphorous (P) and potassium (K) in castor bean, tomato, potato and cucumber. Data are presented as the mean ± SE. Means followed by different letters per each variable are significantly different (P < 0.05) based on Tuckey’s test

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Pearson’s correlation coefficient revealed a positive correlation between nitrogen concentration and weight of S. littoralis larvae (r = 0.603, P = 0.013), followed by phosphorous concentration (r = 0.595, P = 0.015). In contrast, there was a negative correlation between potassium concentration and larval weight (r =– 0.526, P = 0.037). The most effective variables were nitrogen concentration with larval weight (R2 = 0.364, F = 7.998, P = 0.013). By the equation, larval weight (mg) = 255.99 + 0.6 nitrogen concentration.

Discussion

Quality and quantity of food consumed by insect species directly influence their host preferences and affect their biological, physiological, and behavioral characteristics (Nation 2002; Golizadeh et al. 2009; Cabezas et al. 2013). The current study clearly shows that S. littoralis performed differently in larval and pupal developmental times, larval weight, pupal survival, adult longevity, fecundity and fertility when castor bean, tomato, potato, and cucumber were offered as the food plants for larvae. In accord with the obtained findings, Mohamed et al. (2019) reported that the shortest larval and pupal developmental times, and the highest number of deposited eggs were obtained for S. littoralis fed on clover and broad bean. However, Ismail (2020) showed that the highest larval weight, adult emergence, number of deposited eggs per female, and egg-hatch percentage of S. littoralis were recorded in case of feeding on cabbage compared to clover and broad bean. da Silva et al. (2017a) showed that soybean and cotton were the most suitable hosts for development and oviposition of Spodoptera eridania and Spodoptera cosmioides compared to oat, wheat, and maize. Xu et al. (2010) found that cowpea was the most suitable host plant for development of Spodoptera litura, based on the shortest larval and pupal developmental times, compared to tobacco, Chinese cabbage, and sweet potato. Although the lowest pupation rate was recorded in case of feeding on sweet potato, the highest number of eggs deposited were recorded at this host plant. Zhang et al. (2021) demonstrated that rearing Spodoptera exigua on asparagus lettuce resulted in the highest survival rate and the shortest larval developmental time. The vice versa for larvae reared on sweet peppers. Effects of host plants on the various biological features of Spodoptera frugiperda have been reported by several authors (da Silva et al. 2017b; Diédhiou et al. 2021; Maharani1 et al. 2021; Al-Ayat et al. 2022; Altaf et al. 2022; Gopalakrishnan 2022; Nandhini et al. 2023).

Fecundity and fertility of S. littoralis appear to be dependent on the larval weight. Greenberg et al. (2001) and Syed and Abro (2003) found a significant relationship between fecundity and pupal weight of herbivore lepidopterans surviving larvae fed on different host plants. Shahout et al. (2011) elucidated that weight and length of ovaries of S. litura were affected by the host plants. The development of the lepidopteran reproductive system is dependent on nutrients acquired during their lifetime (Johansson 1964). Taylor and Sands (2009) reported that the number of deposited eggs by the herbivore lepidopteran species Samea multiplicalis were directly influenced by the nitrogen concentration in the larval host plant, Salvinia molesta. These findings could explain the highest number of deposited eggs by S. littoralis females surviving larvae fed on castor bean, which contained the highest concentration of nitrogen, compared to the remaining tested host plants. It is well known that the maturation of insect eggs is dependent basically on the materials taken up from the surrounding hemolymph and by materials synthesized by the ovary in situ (Indrasith et al. 1988). These materials include proteins, lipids and carbohydrates, all of which are required for embryogenesis (Kanost et al. 1990).

The tested host plants exerted significant effects on the growth and nutritional indices of S. littoralis larvae. In agreement with the obtained results, Gacemi et al. (2019) elucidated that 5th -instar larvae of S. littoralis reared on artichoke showed the highest ECI and ECD, and the lowest of both values on cabbage. The highest CI, RGR, and AD were on cabbage. Ismail (2020) recorded that the highest CI of S. littoralis larvae was on cabbage, followed by broad bean, and clover, with the highest RGR and AD on broad bean and cabbage, respectively. Mousavi et al. (2023) showed that the lowest and the highest AD values of S. littoralis larvae were on basil and purslane, respectively. The highest ECI and ECD were on chives and coriander, respectively, whereas the lowest ECI, ECD, and RGR were on purslane. Jooyandeh (2018) showed that Sivand and Super Queen tomato cultivars were unsuitable hosts for rearing Helicoverpa armigera based on the nutritional performance of larvae. Mehrkhou et al. (2013) showed the highest RGR, ECI, and ECD of Pieris brassicae larvae were on white cabbage cultivar. Xue et al. (2010) reported that the highest CI of S. litura larvae was on sweet potato, whereas the lowest CI and AD were on tobacco. Although, larvae fed on tobacco showed the highest ECI and ECD. Zhu et al. (2005) found that S. litura larvae had lower RGR, CI, and AD on banana, although they had a significantly higher ECI and ECD. The effects of host plant types on the nutritional indices of S. frugiperda have been reported by several authors (da Silva et al. 2017b; Bavisa et al. 2021; Al-Ayat et al. 2022; Nandhini et al. 2023).

The nutritional indices of 4th, 5th, and 6th -instar larvae of S. littoralis were not consistent with each other. They were not only varied based on the type of host plant and instar, but also on the age of the same instar. This because the nutritional requirements of the insect change through development (Browne 1995) and thus differences typically result in changes of food consumption and utilization. Analysis of the nutritional indices can provide an understanding of the behavioral and physiological bases of insect-plant interactions (Lazarevic and Peric-Mataruga 2003). The factors determining nutrient availability for growth and maintenance over a given period of development are the amount and type of food consumed and the efficiency with which it is utilized (Browne and Raubenheimer 2003). The current investigation showed that the tested host plants had significant effects but with varying degrees on the nutritional and growth indices of S. littoralis larvae. The significant obtained differences may be attributed to the variations of nutritive values of host plants. The ECI and ECD are important parameters of nutritional responses of an insect (Parra et al. 2012). The 6th -instar larvae fed on castor bean showed the highest values of the ECI and ECD. This is suggestive of higher efficiency to convert ingested and digested castor bean to biomass. In contrast, larvae fed on cucumber showed the lowest values of the ECI and ECD. Timmins and Reynolds (1992) attributed reduction in the efficiency of food utilization to increased energetic costs arising from a reduced ability to utilize dietary nitrogen, which would not necessarily interfere with absorption from the gut. The ECD value indicates the allocation of assimilated food to growth, hence a decreased ECD proves as an indicator of higher metabolic maintenance costs (Slansky and Scriber 1985). It is well known that the degree of food utilization depends on the digestibility of food and the efficiency with which digested food is converted into biomass (Batista Pereira et al. 2002).

The CI can be considered as indirect measurement of the relative susceptibilities of crops to pest infestation (Praveen and Dhandapani 2001).The highest CI value of 6th -instar larvae of S. littoralis fed on castor bean indicated the highest rate of intake relative to the mean larval weight during the feeding on this host plant. The highest value of CI of larvae fed on castor bean was concomitant with the highest values of ECI and ECD, leading to the highest weight of full-grown larvae fed on castor bean. The lowest values of ECI and ECD of larvae fed on cucumber revealed the lowest weight of full-grown larvae fed on cucumber.

It is known that the lepidopteran larvae fed on high-nutrient food obtained a faster growth rate and possessed a short life cycle when compared with those fed on low-nutrient food (Hwang et al. 2008). In the present study, the importance of nitrogen and phosphorous in the tested host plants to S. littoralis larvae have been proven as there was a positive correlation between their concentrations and larval weight, with the highest concentrations in castor bean and the lowest concentrations in tomato. This reflects the high fitness of S. littoralis fed on castor bean and the low fitness on tomato. Nitrogen is the major nutrient required by insects and in most cases the main limiting factor for optimal growth of insects (Rostami et al. 2012). Application of nitrogen fertilizer normally increases herbivore feeding preference, food consumption, survival, growth, reproduction, and population density (Taylor and Sands 2009; Bala et al. 2018). Phosphorus had a positive effect on several parameters of aphid performance (Bala et al. 2018). Estiarte et al. (1994) reported that nitrogen limitation produced lower nutritional quality of leaves, with lower relative growth rates and lower efficiency of conversion of ingested biomass on the polyphagous herbivore H. armigera.

Moreover, the negative impacts of secondary metabolites in the host plants on herbivore insects are also considered. For example, the antifeedant effects of phenolic content in tomato leaves (Selvanarayanan and Muthukumaran 2005) and glycoalkaloids in the family Solanaceae (e.g., tomato and potato) (Chowański et al. 2016) have been reported. Furthermore, glycoalkaloids extracted from potato leaves exerted negative effects on the hatching success of eggs of S. exigua (Thawabteh et al. 2019).

It appears that castor bean was the most preferred host plant of S. littoralis larvae. With insects of the order Lepidoptera, host plant preference for larvae is commonly associated with adult female choice of the site of oviposition (Singer 1984; Leal and Zucoloto 2008). Accounting for this behavioral pattern, many studies have investigated the relationship between host preference of adult females and performance of their offspring (Damman and Feeney 1988; Nylin and Janz 1993; Singer et al. 1994), known as the ‘preference-performance hypothesis’ or as the ‘mother-knows-best hypothesis’ (Jaenike 1978; Gripenberg et al. 2010). Host preference of herbivore insects seems to be triggered by plant characteristics that affect the insects’ performance, including nutritional composition, allelochemicals, and even physical characteristics such as hardness, size, shape, and texture (Renwich 1983; Tabashinik and Slansky 1987; Bruce et al. 2005). These factors may determine host recognition (Scriber and Slansky 1985; Thompson and Pellmyr 1991; Dodds et al. 1996) and may play an important role for the biology and nutritional ecology of S. littoralis.

Conclusion

The present study emphasized that castor bean was the most preferred host plant of S. littoralis larvae. This finding is suggestive of implementing a mixed cropping system, not a monoculture system, in which castor bean should be cultivated with the candidate economic plant, leading to less infestation by S. littoralis. The obtained laboratory findings need to be confirmed with some field trials before reaching a definite conclusion.