Introduction
Sexual size dimorphism (SSD), defined as the systematic difference in body size between males and females, is a widespread phenomenon observed across numerous species. In insects, females are typically larger than males, primarily due to a strong positive correlation between body size and fecundity (Honek, Reference Honek1993). Within the order Lepidoptera, analyses of 48 species indicate that 73% exhibit female-biased SSD (i.e. females larger than males), while only 6% display male-biased SSD (i.e. males larger than females) (Stillwell et al., Reference Stillwell, Blanckenhorn, Teder, Davidowitz and Fox2010). Furthermore, Teder (Reference Teder2014) estimated that approximately 16% of insect species overall exhibit male-biased SSD. Differences in adult body size between sexes may arise during development through three non-mutually exclusive mechanisms: differences in hatchling or neonatal size, divergent growth rates, and/or variation in developmental timing – either individually or in combination (Badyaev, Reference Badyaev2002; Blanckenhorn et al., Reference Blanckenhorn, Dixon, Fairbairn, Foellmer, Gibert and van der Linde2007; Esperk et al., Reference Esperk, Tammaru, Nylin and Teder2007; Stillwell et al., Reference Stillwell, Blanckenhorn, Teder, Davidowitz and Fox2010). A growing body of empirical evidence indicates that SSD in holometabolous insects can be influenced by natural selection acting during juvenile developmental stages, as demonstrated across multiple taxa (Badyaev, Reference Badyaev2002; Tammaru et al., Reference Tammaru, Esperk, Ivanov and Teder2010; Stillwell et al., Reference Stillwell, Daws and Davidowitz2014; Vendl et al., Reference Vendl, Šípek, Kouklík and Kratochvíl2018; Sõber et al., Reference Sõber, Sandre, Esperk, Teder and Tammaru2019). Nevertheless, despite the substantial volume of experimental research conducted, relatively few studies have systematically documented the ontogenetic trajectories of males and females throughout the larval stage Stillwell et al. (Reference Stillwell, Daws and Davidowitz2014) for the hawkmoth, Manduca sexta (Linnaeus); Vendl et al. (Reference Vendl, Kratochvíl and Šípek2016) for the rose chafer, Pachnoda marginata (Drury); Sõber et al. (Reference Sõber, Sandre, Esperk, Teder and Tammaru2019) for the common heath moth, Ematurga atomaria (Linnaeus); Šet et al. (Reference Šet, Eva turk, Golobinek, Lokovšek, Gregorič, Lebrón and Kuntner2021); Liu et al. (Reference Liu, Song, Huang, Ge, Xue and He2025) for the fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith). Consequently, the developmental mechanisms underlying the emergence of SSD during ontogeny remain insufficiently understood.
The FAW, S. frugiperda (Lepidoptera: Noctuidae), is a polyphagous migratory pest native to tropical and subtropical regions of the Americas and is widely recognised as a major threat to corn (Zea mays L.; Poaceae) production systems (Barfield et al., Reference Barfield, Mitchell and Poe1978; Sparks, Reference Sparks1979; Garcia et al., Reference Garcia, Godoy, Thomas, Nagosh and Meagher2018). Since its first detection in China in 2019, the FAW has become permanently established, with continuous monitoring confirming its presence across multiple provinces. Over the past 6 years, we have conducted comprehensive and systematic investigations into the life history traits of this species within the suburban agroecosystem of Nanchang (28°46′ N, 115°50′ E), focusing on the effects of temperature (Huang et al., Reference Huang, Xue, Chen, Guo, Tang and He2021), host plant species (Huang et al., Reference Huang, Xue, Wan, Tang, Liang and He2023), and seasonal variation (Huang et al., Reference Huang, Xue, Tang, Wu and He2024). Collectively, these studies indicate that the species exhibits male-biased SSD, with larval development in both sexes comprising five instars (Liu et al., Reference Liu, Song, Huang, Ge, Xue and He2025). Notably, larvae reared on corn leaves at 25°C exhibited significantly shorter developmental durations compared to those fed on sugarcane leaves. In addition, pupal weights were significantly greater when larvae developed on corn leaves than on sugarcane leaves (Huang et al., Reference Huang, Xue, Wan, Tang, Liang and He2023), suggesting that corn foliage provides superior nutritional quality relative to sugarcane (Saccharum officinarum L.; Poaceae) foliage. To further investigate the ontogenetic trajectory of SSD, larval body mass was measured daily under controlled thermal conditions (22°C and 28°C). Results show that the emergence of significant SSD at 22°C was primarily due to the prolonged developmental period of males, whereas the absence of SSD at 28°C resulted from a combination of extended female larval development and a higher growth rate in males (Liu et al., Reference Liu, Song, Huang, Ge, Xue and He2025). According to Liu et al. (Reference Liu, Song, Huang, Ge, Xue and He2025), larval development in both sexes comprises five instars. At 22°C, the first to fourth moults occurred at 3–4, 6–7, 11–12, and 15–16 days post-hatching, respectively; at 28°C, they occurred at 1–2, 3–4, 6–7, and 8–9 days post-hatching, respectively. Moreover, the FAW displays a relatively short life cycle, high survival rate, and substantial fecundity, making it a suitable model organism for studying the ontogeny of SSD. These findings collectively provide a robust foundation for ongoing research into the developmental mechanisms underlying SSD in this species.
Growth trajectories of male and female larvae of Spodoptera frugiperda. The body mass of larvae was compared between the sexes during the first 11 days before pupation when reared on sugarcane leaves at 25°C, and during the first 7 days before pupation when reared on corn leaves at 25°C. * indicates significant differences between the sexes (GLMM, P < 0.05).
Figure 1 Long description
A) Line graph titled, Corn. Horizontal axis label, Day before pupation (d). Horizontal axis values, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1. Vertical axis label, Body mass (mg). Vertical axis range, 0 to 500. Two plotted series with point markers. Readable coordinate pairs for series 1: (11, 20), (10, 30), (9, 40), (8, 60), (7, 120), (6, 170), (5, 300), (4, 440), (3, 410), (2, 230), (1, 220). Readable coordinate pairs for series 2: (11, 20), (10, 30), (9, 40), (8, 60), (7, 110), (6, 170), (5, 300), (4, 440), (3, 400), (2, 230), (1, 220). Asterisk symbols appear near the upper part of the plot around days 3, 2 and 1. B) Line graph titled, Sugarcane. Horizontal axis label, Day before pupation (d). Horizontal axis values, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1. Vertical axis label, Body mass (mg). Vertical axis range, 0 to 500. Two plotted series with point markers. Readable coordinate pairs for series 1: (12, 30), (11, 40), (10, 60), (9, 90), (8, 140), (7, 180), (6, 220), (5, 320), (4, 430), (3, 360), (2, 210), (1, 200). Readable coordinate pairs for series 2: (12, 30), (11, 40), (10, 50), (9, 80), (8, 130), (7, 170), (6, 210), (5, 300), (4, 400), (3, 340), (2, 200), (1, 190). Multiple asterisk symbols appear above points across the middle to right portion of the plot. C) Line graph. Horizontal axis has tick marks without a visible axis label. Vertical axis label, Sexual size dimorphism index. Vertical axis range, negative 0.15 to 0.25. One plotted line. Insufficient visual information to describe this element accurately. D) Line graph. Horizontal axis has tick marks without a visible axis label. Vertical axis label, Sexual size dimorphism index. Vertical axis range, negative 0.15 to 0.25. One plotted line. Insufficient visual information to describe this element accurately. E) Line graph. Horizontal axis label, Day before pupation (d). Horizontal axis values, 9, 8, 7, 6, 5, 4, 3, 2, 1, Pupa. Vertical axis label, Relative growth rate (ln mg per mg per d). Vertical axis range, negative 0.6 to 1.0. Two plotted series with point markers and vertical error bars. Readable coordinate pairs for series 1: (9, 0.6), (8, 0.7), (7, 0.6), (6, 0.8), (5, 0.4), (4, 0.5), (3, 0.4), (2, 0.0), (1, negative 0.5), (Pupa, 0.3). Readable coordinate pairs for series 2: (9, 0.6), (8, 0.7), (7, 0.6), (6, 0.8), (5, 0.4), (4, 0.6), (3, 0.4), (2, 0.0), (1, negative 0.6), (Pupa, 0.3). Asterisk symbols appear near some points, including around day 6 and near day 1. F) Line graph. Horizontal axis label, Day before pupation (d). Horizontal axis values, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, Pupa. Vertical axis label, Relative growth rate (ln mg per mg per d). Vertical axis range, negative 0.6 to 1.0. Two plotted series with point markers and vertical error bars. Readable coordinate pairs for series 1: (11, 0.4), (10, 0.5), (9, 0.4), (8, 0.3), (7, 0.5), (6, 0.2), (5, 0.3), (4, 0.5), (3, 0.2), (2, 0.0), (1, negative 0.5), (Pupa, 0.2). Readable coordinate pairs for series 2: (11, 0.4), (10, 0.5), (9, 0.4), (8, 0.3), (7, 0.4), (6, 0.2), (5, 0.2), (4, 0.4), (3, 0.2), (2, 0.0), (1, negative 0.6), (Pupa, 0.2). Asterisk symbols appear near some points, including around days 7 and 4 and near day 1.
In this study, a sufficiently large sample size was adopted to ensure robust statistical power for systematically examining the effects of host plant species – corn versus sugarcane – on the life history traits of the FAW, as well as the developmental growth trajectories of male and female larvae. The primary objective was to elucidate the underlying mechanisms contributing to male-biased SSD.
Materials and methods
Methodology for live insect handling and experimental design
In late June 2024, over 50 mature larvae were collected from corn fields located at 28°46′ N, 115°50′ E. Each larva was individually reared in Petri dishes (9.0 cm diameter × 2.0 cm height) and supplied with fresh corn leaves under natural environmental conditions until adult emergence. Upon reaching adulthood, the moths were sexed. Each pair was transferred to a sealed polyethylene bag (20 × 30 cm) to facilitate mating and oviposition. The bags were equipped with cotton balls soaked in a 10% honey–water solution to provide nutritional support. Egg masses from four randomly selected pairs were retained for experimental use. Following hatching, more than 300 neonates per parental pair were systematically sampled, resulting in a total cohort of over 1,200 individuals from a mixture of broods. The newly hatched larvae were allocated to eight transparent plastic containers (19 × 12.5 × 7 cm), each containing either corn leaves or sugarcane leaves. These containers were placed in a controlled rearing room maintained at 25°C ± 2°C, 70% ± 15% relative humidity, and under natural photoperiod conditions. Each container initially held more than 150 larvae. Due to technical challenges associated with handling early instar larvae before, it is only on the 7th day for larvae fed on corn leaves and the 11th day for those fed on sugarcane leaves, that larvae were transferred individually to Petri dishes for isolated rearing. Larval body weight was recorded daily until pupation. In this study, the developmental progress of 600 larvae reared on corn leaves and 600 larvae reared on sugarcane leaves was monitored individually. Petri dishes were inspected daily, and fresh plant material was provided as required.
Moulting data were collected for larvae reared in communal conditions prior to weighing, as well as for those maintained individually during the weighing process. Daily weight measurements began on day 7 for larvae fed on corn leaves and on day 11 for those fed on sugarcane leaves, using an analytical balance (AUY120, Shimadzu Corporation, Kyoto, Japan). Weighing continued until feeding cessation and the onset of the prepupal stage, defined by burrowing into moist filter paper in preparation for pupation, followed by transition into the pupal stage. To avoid confounding effects from developmental timing differences on SSD, we compared larval mass between males and females: 1–9 days before pupation for those reared on corn, and 1–12 days before pupation for those reared on sugarcane (fig. 1). Adult body mass was measured immediately after meconium expulsion. Furthermore, developmental durations and survival rates for both larval and pupal stages were systematically recorded.
The equations employed in this study are presented in table 1. In this study, the growth rate and proportional weight loss from pupae to adults were computed according to the formula proposed by Gotthard et al. (Reference Gotthard, Nylin and Wiklund1994). Relative growth rate (RGR) was computed according to the formula proposed by Tammaru et al. (Reference Tammaru, Nylin, Ruohoma¨ki and Gotthard2004). The SSD index was computed according to the formula proposed by Lovich and Gibbons (Reference Lovich and Gibbons1992). A positive index value indicated that males were the larger sex, whereas a negative value indicated that females were the larger sex. In this study, males were considered the larger sex. Body mass was measured in 120 adult specimens, comprising 60 males and 60 females.
Statistical analysis
Statistical analyses were conducted using SPSS software (Version 23.0; IBM Corp., Armonk, NY, USA). A generalized linear mixed model (GLMM) was applied to assess the effects of food, sex, and their interaction on life history traits, with food and sex specified as fixed effects and replicate experimental group included as a random effect. For proportional data approaching boundary values (0 or 1), an arcsine square root transformation was performed to satisfy the assumptions of normality and homoscedasticity. Differences in larval and pupal survival rates across food plant types were assessed using chi-square tests. Variations in body mass and growth rate across developmental stages among individuals reared on different food plants were analysed via a GLMM, with developmental stage and sex modelled as fixed effects and feeding group incorporated as a random effect. Post hoc pairwise comparisons among developmental stages were conducted using Tukey’s Honestly Significant Difference test. Linear regression analyses (LR) were performed to examine the relationship between developmental duration and pupal mass. All statistical analyses were evaluated at a significance level of α = 0.05.
Results
Viability rate
As shown in table 2, the larval survival rates to the pupal stage were 96.67% in the corn-fed group and 97.00% in the sugarcane-fed group. The pupal survival rates to adulthood were 97.41% and 96.22% for the corn and sugarcane diet groups, respectively. Statistical analysis indicated no significant differences in survival rates between the two dietary treatment groups (Chi-square tests: larvae: X 2 = 0.109, P = 0.742; pupae: X 2 = 1.343, P = 0.246).
Equations mentioned in this study
Table 1 Long description
Four numbered equations used to calculate biological metrics from body mass, development time, and sex-specific size are listed. The first metric defines growth rate using the natural log of pupal weight divided by larval development duration. The second defines relative growth rate using the natural log of the ratio of final to initial weight divided by time in days. The third defines proportional weight loss as one minus the ratio of adult weight to pupal weight, so larger adult-to-pupal ratios imply less loss. The fourth defines sexual size dimorphism as the ratio of the larger sex to the smaller sex, then reduced by one, so values near zero indicate similar sizes and larger values indicate greater dimorphism. All four metrics are derived measures rather than direct observations, and results depend on consistent units and comparable weight or size measurements across individuals.
Note: Gotthard et al. (Reference Gotthard, Nylin and Wiklund1994) for Eqs 1 3; Tammaru et al. (Reference Tammaru, Nylin, Ruohoma¨ki and Gotthard2004) for Eq. 2; Lovich and Gibbons (Reference Lovich and Gibbons1992) for Eq. 4.
Larval and pupal survival rates of Spodoptera frugiperda feeding on corn and sugarcane at 25°C
Table 2 Long description
Survival and death counts and the resulting survival rates for Spodoptera frugiperda at the larval and pupal stages when fed corn or sugarcane at 25 degrees Celsius are reported. On corn, larvae had 580 survivors and 20 deaths, for a survival rate of 96.67 percent, while pupae had 565 survivors and 15 deaths, for 97.41 percent. On sugarcane, larvae had 582 survivors and 18 deaths, for 97.00 percent, while pupae had 560 survivors and 22 deaths, for 96.22 percent. Survival rates are all close to 96 to 97 percent, indicating only small differences between diets and stages. The highest survival rate occurs in pupae fed corn, and the lowest occurs in pupae fed sugarcane. Differences are modest, so conclusions about diet effects should be cautious without additional context such as replication or statistical testing.
Developmental duration
Food, sex, and their interaction (food × sex) had statistically significant effects on larval developmental duration (table 3). Female larvae reared on corn exhibited a 0.14-day shorter developmental duration than males under the same diet (GLMM: P = 0.040; table 4). In contrast, male larvae fed on sugarcane displayed a significantly prolonged larval development, exceeding that of females by 1.15 days, with this difference showing strong statistical significance (P < 0.001; table 4). Moreover, both female and male larvae reared on corn completed larval development significantly faster than those fed on sugarcane (females: 14.41 vs. 20.41 days, P < 0.001; males: 14.55 vs. 21.56 days, P < 0.001). Similarly, pupal developmental duration was significantly shorter in corn-fed individuals compared to sugarcane-fed counterparts for both sexes (female pupae: 8.19 vs. 9.08 days, P < 0.001; male pupae: 9.50 vs. 10.36 days, P < 0.001).
Results from a linear mixed-model analysis of fixed effects on larval time, pupal time, prepupal weight, pupal weight, growth rate, adult weight, and weight loss rate in Spodoptera frugiperda in relation to food and sex
Table 3 Long description
Linear mixed-model fixed effects are reported for larval time, pupal time, prepupal weight, pupal weight, growth rate, adult weight, and weight loss rate in Spodoptera frugiperda, testing food, sex, and their interaction. Food shows very strong effects for every trait, with the largest F values for growth rate and larval time and all food p values below 0.001. Sex is also significant for every trait, with especially large effects on pupal time and weight loss rate, and p values below 0.001 except adult weight at 0.002. The food by sex interaction is significant for larval time, prepupal weight, pupal weight, and growth rate, all with p values below 0.001. The interaction is not significant for pupal time and weight loss rate, and it is also not significant for adult weight. Overall, food is the dominant predictor across traits, sex contributes additional differences, and interaction effects are trait dependent. The table reports statistical significance and relative effect strength but does not provide direction of change or group means.
Life history data (mean ± standard error) for male and female Spodoptera frugiperda feeding on corn and sugarcane at 25°C
Table 4 Long description
Life history means with standard errors are reported for male and female fall armyworm reared on corn or sugarcane at the same temperature, with separate sample sizes for immature traits and adult traits. Development was slower on sugarcane: larval time was about 21.6 days for males and 20.4 for females on sugarcane versus about 14.6 and 14.4 on corn, and pupal time was about 10.4 and 9.1 on sugarcane versus about 9.5 and 8.2 on corn. Pupal mass was higher on corn than sugarcane for both sexes, and males were heavier than females in both diets, about 225 versus 221 milligrams on corn and about 202 versus 189 on sugarcane. Growth rate was similar between sexes on corn, but on sugarcane females had a higher growth rate than males, about 0.257 versus 0.247 in natural log milligrams per day. Adult mass was higher on corn than sugarcane, and on corn females were heavier than males, about 113 versus 106 milligrams, while on sugarcane adult masses were close, about 93 for females and 91 for males. Weight loss from pupa to adult was lower in females than males on both diets, about 47 versus 51 percent on corn and about 50 versus 54 percent on sugarcane. Sex size dimorphism indices indicate small male bias in pupae on both diets and a small female bias in adults, with the adult bias stronger on corn than sugarcane. Some sex differences are flagged as statistically meaningful in the source, but the table does not provide effect sizes beyond the reported means and standard errors.
* means significantly different between the sexes (P < 0.05).
Pupal mass and growth rate
Pupal masses were significantly influenced by food type and sex, and the interaction between these factors (table 3). The SSD index of the pupae developing on corn leaves 0.019 was lower than that of the pupae developing on sugarcane leaves 0.070 (table 4). Male pupae exhibited significantly greater mass compared to female pupae across both food plant types (P < 0.001). Furthermore, pupal mass for both sexes was significantly higher when larvae were reared on corn leaves than on sugarcane leaves (female pupae: 220.65 mg vs. 188.72 mg, P < 0.001; male pupae: 224.88 mg vs. 201.97 mg, P < 0.001) (table 4).
The growth rate was significantly influenced by food type, sex, and the interaction between these factors (table 3). Specifically, both female and male larvae exhibited significantly higher growth rates when fed on corn leaves compared to those fed on sugarcane leaves (female pupae: 0.38 vs. 0.26 mg/day, P < 0.001; male pupae: 0.37 vs. 0.25 mg/day, P < 0.001) (table 4).
Adult mass and weight loss
Adult mass was significantly influenced by both food source and sex, with no significant interaction effect between these two factors (table 3). The SSD index of adults reared on corn leaves (−0.067) was higher than that of those reared on sugarcane leaves (−0.022; table 4). Adults of both sexes that fed on corn leaves exhibited significantly greater mass compared to those that fed on sugarcane leaves (females: 113.23 mg vs. 92.81 mg, P < 0.001; males: 106.11 mg vs. 90.79 mg, P < 0.001) (table 4). Notably, the higher adult mass observed in females relative to males can be attributed to significantly greater mass loss in male pupae during development compared to female pupae (table 4).
Weight loss was significantly influenced by food source and sex, but not by their interaction (table 3). Both females and males feeding on corn leaves exhibited significantly lower weight loss compared to those feeding on sugarcane leaves (females: 47.43% vs. 50.26%, P < 0.001; males: 51.40% vs. 53.57%, P < 0.001) (table 4).
Growth trajectories of male and female larvae
When larvae were fed on corn leaves, their body mass was significantly influenced by developmental age and by the interaction between developmental age and sex, but not by sex alone (table 5; table S1). SSD in larvae reared on corn leaves first emerged 2 days prior to pupation (i.e. in fifth-instar larvae), with males exhibiting significantly greater body mass than females (GLMM, P < 0.001; fig. 1A). This significant difference persisted through the prepupal and pupal stages (fig. 1A; table S1; P < 0.001). In contrast, for larvae fed on sugarcane leaves, body mass was significantly affected by food type, sex, and the interaction between these factors (table 5). SSD became evident earlier – 10 days before pupation (i.e. in third-instar larvae) – with male larvae consistently showing greater mass than females (fig. 1B; GLMM, P < 0.001), and this difference remained stable through to the pupal stage. The observed differences in the ontogenetic timing of SSD development between the two diets can be attributed to divergent patterns in male developmental duration.
Results from a linear mixed-model analysis of fixed effects on body mass and growth rate in Spodoptera frugiperda in relation to developmental age and sex feeding on corn and sugarcane at 25°C
Table 5 Long description
Linear mixed-model fixed-effect tests report degrees of freedom, F values, and p values for body mass and growth rate by food plant. On corn, developmental age is highly significant for body mass (df 9, F 4816.049, p less than 0.001) and growth rate (df 9, F 2009.461, p less than 0.001). On corn, sex is not significant for body mass (df 1, F 2.711, p 0.100) or growth rate (df 1, F 0.984, p 0.321), while the developmental age by sex interaction is significant for body mass (df 9, F 2.185, p 0.020) but not for growth rate (df 9, F 1.457, p 0.158). On sugarcane, developmental age is highly significant for body mass (df 12, F 3020.861, p less than 0.001) and growth rate (df 12, F 808.869, p less than 0.001). On sugarcane, sex is significant for both body mass (df 1, F 81.400, p less than 0.001) and growth rate (df 1, F 262.994, p less than 0.001), and the developmental age by sex interaction is also significant for body mass (df 12, F 3.806, p less than 0.001) and growth rate (df 12, F 2.237, p 0.008). Overall, developmental age shows the strongest and most consistent effects across traits and plants, with sex-related differences more evident on sugarcane; significance reflects statistical evidence and does not indicate effect size direction.
When larvae were fed corn leaves, their growth rate was significantly influenced by developmental age, but not by sex or the interaction between developmental age and sex (table 5). Sex-based differences in RGR were observed only on the second, fourth, and sixth days prior to pupation (fig. 1E; table S2), with female growth rates being significantly higher than those of males (GLMM, P < 0.001). In contrast, when larvae were reared on sugarcane leaves, growth rate was significantly affected by developmental age, sex, and the interaction between these two factors (table 5). Males exhibited significantly higher growth rates than females exclusively on the fifth and eighth days before pupation, whereas females displayed significantly higher growth rates than males on the first, third, fourth, sixth, and seventh days prior to pupation (GLMM, P < 0.001) (fig. 1F; table S2). Notably, the higher growth rate in female larvae is consistent with their significantly shorter larval development period compared to males (see table 4).
Correlation analysis between the pupal mass and larval developmental time
A significant positive correlation was observed between pupal mass and larval developmental time when larvae were reared on corn leaves (fig. 2A, P < 0.001). In contrast, when larvae were reared on sugarcane leaves, female pupal mass showed a significant positive correlation with larval developmental time, whereas male pupal mass displayed a slight negative correlation, although this relationship was not statistically significant (fig. 2B, P = 0.707).
Correlation between pupal mass and larval developmental time of Spodoptera frugiperda reared on sugarcane and corn leaves at 25°C. ** indicates a significant correlation (LR, P ≤ 0.01).
Figure 2 Long description
A) Scatter plot with two point series and two fitted straight lines. The y axis label is, Pupal mass, milligram. The y axis ranges from 100 to 350 with tick marks at 100, 150, 200, 250, 300 and 350. The x axis label is, Larval developmental time, day. The x axis ranges from 10 to 30 with tick marks at 10, 15, 20, 25 and 30. Legend text reads, male r equals 0.180 asterisk asterisk and female r equals 0.239 asterisk asterisk. Text reads, Corn. The male series uses diamond markers and the female series uses circular markers. Both fitted lines slope upward from lower x values to higher x values. B) Scatter plot with two point series and two fitted straight lines. The y axis label is, Pupal mass, milligram. The y axis ranges from 100 to 350 with tick marks at 100, 150, 200, 250, 300 and 350. The x axis label is, Larval developmental time, day. The x axis ranges from 10 to 30 with tick marks at 10, 15, 20, 25 and 30. Legend text reads, male r equals minus 0.023 and female r equals 0.202 asterisk asterisk. Text reads, Sugarcane. The male series uses diamond markers and the female series uses circular markers. One fitted line is nearly flat to slightly downward and the other fitted line slopes upward from lower x values to higher x values.
Discussion
Based on the comparison of life history traits derived from a large dataset of the FAW individuals reared on corn and sugarcane leaves, this study reveals that sex-specific differences in life-history traits vary significantly depending on the host plant species. Significant differences were observed in life-history traits between FAW larvae raised on corn leaves and those raised on sugarcane leaves. Individuals reared on corn exhibited significantly shorter larval and pupal development durations, greater pupal mass, higher growth rates, reduced SSD, and lower weight loss compared to those reared on sugarcane. These findings confirm that host plant quality exerts a comprehensive influence on the life-history traits of FAW, consistent with previous studies (Nylin and Gotthard, Reference Nylin and Gotthard1998; Awmack and Leather, Reference Awmack and Leather2002; Huang et al., Reference Huang, Xiao, He and Xue2018; Ajmal et al., Reference Ajmal, Ali, Jamal, Saeed, Radicetti and Civolani2024; Tanaka et al., Reference Tanaka, Mika and Murata2024).
Our study systematically examined the influence of environmental variables, with a particular focus on host plant species, on the temporal dynamics and persistence of SSD in larvae. Male-biased SSD became apparent during the fifth-instar stage in larvae reared on corn leaves and persisted consistently through to the pupal stage. In contrast, male-biased SSD emerged earlier – during the third instar stage – in individuals reared on sugarcane leaves and remained stable throughout subsequent development. The observed sexual divergence in larval mass can be primarily attributed to the prolonged developmental duration of males relative to females. As documented across various insect taxa, SSD is typically associated with differential development times between the sexes (Teder, Reference Teder2014). Furthermore, females exhibited significantly higher growth rates, consistent with their shorter overall developmental period compared to males – a finding corroborated by Stillwell et al. (Reference Stillwell, Daws and Davidowitz2014). In their comprehensive analysis of SSD development in Manduca sexta across multiple stages and dietary conditions, Stillwell et al. (Reference Stillwell, Daws and Davidowitz2014) demonstrated that SSD arises early in larval development when high-quality food is provided. Conversely, under medium-quality nutritional conditions, the onset of SSD is delayed, while no discernible SSD was observed under the lowest food quality treatment, resulting in adults of comparable size between sexes.
In insects, the developmental growth rate is influenced by environmental factors, particularly nutrient availability (Jarrett and Miller, Reference Jarrett and Miller2024). In Drosophila, nutrients predominantly regulate growth rate and body size through modulation of insulin-like peptide secretion. Genetic impairments in components of the insulin signalling pathway lead to extended developmental periods and reduced body size (Ikeya et al., Reference Ikeya, Galic Belawat, Nairz and Hafen2002). Recent studies have shown that the insulin/insulin-like growth factor signalling pathway in Drosophila plays a pivotal role in sex-specific regulation of developmental processes affecting body size (Millington et al., Reference Millington, Brownrigg, Chao, Sun, Basner-Collins and Wat2021). These findings suggest that similar mechanisms may underlie the phenotypic differences observed between larvae reared on corn leaves and those reared on sugarcane leaves.
We further observed that male-biased SSD in the FAW was associated with greater phenotypic plasticity in pupal mass when larvae were reared on sugarcane leaves. Specifically, male larvae exhibited a significantly higher increase in body weight compared to female larvae under these conditions (males: 21.56 ± 0.09 mg vs. females: 20.41 ± 0.08 mg), in contrast to those reared on corn leaves (males: 14.55 ± 0.05 mg vs. females: 14.41 ± 0.05 mg). These findings are consistent with those reported by Rohner et al. (Reference Rohner, Teder, Esperk, Lüpold and Blanckenhorn2018), who demonstrated that male-biased SSD is associated with enhanced phenotypic plasticity in males within higher Diptera species. Collectively, the results indicate that males exhibit greater sensitivity to environmental variation than females.
In most female-biased insect species, female body size exhibits greater sensitivity to environmental conditions compared to male body size: under improved environmental conditions, females show a proportionally larger increase in size than males. Consequently, SSD with female bias intensifies as overall body size increases (Teder and Tammaru, Reference Teder and Tammaru2005). However, contrary to the well-documented pattern for female-biased SSD, we observed that male-biased SSD in the FAW became more pronounced under poor food conditions (feeding on sugarcane leaves). This pattern is statistically supported by a significant sex × food quality interaction. This finding highlights the nuanced and direction-dependent nature of condition dependence in SSD, demonstrating sex-specific responses to environmental variation.
A positive correlation between development time and body size has long been assumed in the literature. However, accumulating evidence suggests that the relationship between these two traits can vary – ranging from positive to zero or even negative – depending on the nature of selection pressures, whether antagonistic or synergistic (Davidowitz et al., Reference Davidowitz, Roff and Nijhout2016). In this study, we observed a slightly negative correlation between male pupal mass and larval developmental time when larvae were reared on sugarcane leaves. Furthermore, larvae reared on low-quality sugarcane leaves exhibited a significantly prolonged larval development period and a significantly lower pupal weight compared with those reared on high-quality corn leaves, demonstrating a robust negative phenotypic correlation between environmentally induced variation in body size and developmental duration (Teder et al., Reference Teder, Vellau and Tammaru2014). A comparable negative correlation between development time and body size has also been reported in the rice stem borer, Chilo suppressalis (Walker), under laboratory settings when larvae were fed on rice stems (Huang et al., Reference Huang, Xiao, He and Xue2018).
Conclusion
This study represents the first systematic investigation into the effects of corn and sugarcane as host plants on life-history traits and the ontogeny of SSD in the FAW. Host plant quality was found to significantly influence developmental duration, pupal mass, growth rate, SSD, and weight loss during development. Males exhibited greater sensitivity to environmental variation, demonstrating higher phenotypic plasticity in pupal mass and a negative correlation between developmental period and body mass. The onset of SSD differed between host plants: SSD emerged in the third instar on sugarcane and in the fifth instar on corn, primarily due to prolonged male development on corn. Female individuals displayed faster growth rates, attributable to accelerated larval development. These findings provide foundational insights into SSD in polyphagous insect species, thereby informing future research.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S0007485326101473.
Author contributions
H.-M.H.: writing – review and editing, visualisation, software, project administration, methodology, investigation, formal analysis, data curation, and funding acquisition; L.-L.H.: writing – review and editing, supervision, resources, methodology, and funding acquisition; W.L.: writing – review and editing, validation, methodology, and investigation; J.-J.T.: validation, methodology, and investigation; F.-S.X.: writing – review and editing, writing – original draft, visualisation, supervision, resources, project administration, methodology, formal analysis, and conceptualisation.
Financial support
This study was supported by the National Natural Science Foundation of China (Grant No. 31560608) and the High-level Scientific Research and Innovation Team of Yuzhang Normal University (Project No. YZTD202305).
Competing interests
None.
