Temperature and relative humidity mediated life processes of Spodoptera species (Lepidoptera: Noctuidae)

Abstract

Anthropogenic-mediated climate change is expected to negatively affect pest management in agriculture. Hence, we investigated the oviposition, immature mortality, and developmental processes of Spodoptera species (Spodoptera exigua (Hübner) and Spodoptera litura (Fabricius)) under different temperatures (20, 25, and 30°C) and relative humidity (RH) (30–35, 50–55, 70–75, and 90–95%) conditions. For fecundity, mouths of each Spodoptera species were released into a rectangular box whose inner walls were covered with a sheet of white paper for each combination of temperature and RH. The mouths were kept inside the box to deposit eggs for 72 h. Temperature and RH significantly affected the fecundity, with the maximum number of eggs laid in 70–75% at 30°C. The highest egg and larval mortalities were recorded in 30–35 and 90–95% RH, respectively. Temperature and RH greatly affected the developmental period (egg–adult) and adult emergence rate. The rapid development was recorded in 70–75% RH at 30°C. Higher number of adults was found with an increase in temperature and RH. Adult longevity was significantly higher in 70–75% RH at 20°C. Based on the present study's findings, temperature and RH had an individual apparent effect on the developmental processes of Spodoptera species instead of an interactive effect. Therefore, there is need for an in-depth study of the influence of several climatic factors, including CO₂, on the developmental modality and demographic changes of Spodoptera species to assess the impacts of climatic components and the sustainable development of management strategies.

Introduction

Two Spodoptera species (Lepidoptera: Noctuidae), the beet armyworm (Spodoptera exigua) and the tobacco cutworm (Spodoptera litura), are economic polyphagous pests that damage a wide range of field crops, vegetables, and ornamental plants in temperate and sub-tropical regions of Asia, Australasia, and the Pacific islands (Zheng et al., 2011; EPPO, 2013). Spodoptera species are a cosmopolitan species that feed on numerous plant species, from several families of plants (Luo and Cao, 2000; Greenberg et al., 2001; Xue et al., 2010).

Spodoptera species generally have several instars of larvae (Ali and Gaylor, 1992); starting from the fourth instar, larvae start to cause significant damage by consuming plants (Dhir et al., 1992; Sharma et al., 2014; Dai et al., 2017). Thus, plant damage from larvae is the most serious problem due to their heavy feeding tendency and insecticide-resistance development (Brewer and Trumble, 1991; Ali and Gaylor, 1992). Conventional insecticides are normally applied to mitigate the problem given by Spodoptera species. However, management of these pests has failed due to its rapidity in developing resistance to conventional chemical insecticides and its wide host range, higher mobility, and higher reproduction capacity (Brewer and Trumble, 1991; Ali and Gaylor, 1992), and a reduction in the haphazard use of broad-spectrum insecticides due to the substantial environmental concerns involved (Mascarenhas et al., 1998; Osorio et al., 2008; Ahmad and Arif, 2010; Lai and Su, 2011). In Korea, Spodoptera species are widespread in almost all provinces and cause significant damage to various field crops and vegetables (Kim and Shin, 1987; Jung et al., 2019). Further, there is a significant threat of a potential distribution expansion of Spodoptera species on the Korean peninsula, as the species is found in a hot spot for climate change, especially global warming (Jung et al., 2019). The most foreseeable impacts of climate change, in particular, will have a profound effect on herbivore insect behaviour and population dynamics (Hunter, 2001). The severity of damage caused by any pest species depends on the rate of population increase for that species, which is influenced by temperature, humidity, and nutritional quality of the host species (Howe, 1965; Tshiala et al., 2012). It has been reported that the oviposition and development of Spodoptera species can be influenced by the physical/chemical attributes of the hosts as well as abiotic factors (temperature, humidity, and light) (Bae et al., 1997; Bae, 1999; Saeed et al., 2010; Maharjan et al., 2022; Maharjan et al., 2023). Several studies have been conducted on different species of Spodoptera addressing the effects of temperature (Fand et al., 2015; Dai et al., 2017; Maharjan et al., 2022, 2023). Majority of developmental the studies of Spodoptera species have done at a single RH level. Research on the combined effects of temperature and RH on life variables of Spodoptera species is scarce. Thus, we conducted this study to investigate the interactive effect of the temperature and RH conditions on oviposition, egg and larval mortality, development, adult emergence, and adult longevity of S. exigua and S. litura based on an artificial diet (Goh et al., 1990).

Materials and methods

Laboratory insect-rearing colony

To maintain test insect colony, Spodoptera species were gathered from a soybean field at the Department of Southern Area Crop Science, NICS, RDA, Miryang, South Gyeongnam Province; 35°49′40″N, 128°74′01″E, Republic of Korea in 2020. Field larval populations were raised on an artificial medium (Goh et al., 1990). Fresh mouths of each species were placed in different acryl cages (40L × 40W × 40H cm³, with side ventilation). Inside cage, soybean plants planted in the plastic house were provided for oviposition, and a solution of honey (10%) and water were provided as a food source. Rearing cages were placed under the room conditions (26 ± 1°C, 60 ± 5% RH, and a 16:8 h L:D photoperiod). Moreover, the test insects (Spodoptera species) used in this experiment were originally collected from a soybean field, and reared on an artificial medium for successive generations (approx. >5 generations) under the room conditions.

Experimental procedure

One pair of each Spodoptera species (sex was determined based on their morphology) (EPPO, 2015; Bandoly and Steppuhn, 2016) collected from the laboratory-rearing colony was released into a rectangular box (7L × 7W × 9.5H cm³, with topside ventilation) whose inner walls were covered with a sheet of white paper with food sources. Then, the rectangular box with adults was placed inside humidity chambers (27L × 20W × 17H cm³) set at one of the four different relative humidity (RH) levels (30–35, 50–55, 70–75, and 90–95%) (Winston and Bates, 1960). Different salt compositions of MgCl₂, Mg(NO₃)₂⋅6H₂O, NaCl, and K₂SO₄ (Duksan Pure Chemicals, Ansan-si, Gyeonggi Province, Republic of Korea) were employed for maintaining 30–35, 50–55, 70–75, and 90–95% RH levels, respectively. Each RH-maintained chamber was then placed inside incubators (Eyela, model MTI-202B, Tokyo, Japan), and temperatures were fixed at 20, 25, and 30°C, and a 16:8 h L:D photoperiod. To measure the actual temperature and RH within the humidity chambers, a HOBO data logger (Huato Log-USB, Huato Electronic Co., Ltd, Shenzhen, China) was used. To measure fecundity, mouths were kept inside the cage for 72 h, then adult Spodoptera species were removed from the humidity chamber, and the eggs laid on the sheet of white paper were counted. Once eggs were hatched, new larvae were transferred into individual Petri dishes (5 cm dia. × 1.5 cm height, with topside ventilation) (SPL Lifesciences Co. Ltd, Gyeonggi Province, Republic of Korea) individually and fed an artificial diet. Then, the Petri dishes were wrapped with para-film, and placed inside the same temperature and RH chamber for recording the life variables of the Spodoptera species. The developmental periods from egg to adult emergence were measured, and the percentage of mouth emerged was estimated according to the overall emerged mouths from the entire number of eggs laid. Seven sets of female and male mouths were utilised for each combination of temperature and RH, and the experiment was replicated ten times for each temperature and RH condition. Once mouths developed, longevity of mouths was measured by keeping each mouth separately in Petri dishes (5 cm dia. × 1.5 cm height, with topside ventilation) (SPL Lifesciences Co. Ltd, Gyeonggi Province, Republic of Korea) at the corresponding temperatures and RH with 10% honey and water solution.

Statistical analysis

Considering temperature and RH as core factors, a two-way analysis of variance (ANOVA) was performed for analysing the life variables (number of eggs laid, egg mortality, larval mortality, developmental period, mouth emergence, and longevity). Tukey's test was applied as a post-hoc analysis at the P < 0.05 level of significance for the effect of temperature and RH, and its interaction (PROC GLM; SAS Institute Inc., 2002).

Results

Fecundity

For S. exigua, mouth fecundity was varied with temperature and RH (model, P < 0.0001). Temperature (P < 0.0001) and RH (P < 0.0001) significantly impacted the mouth fecundity. Similarly, combined effects of temperature and RH were also significant (P = 0.0017). Fecundity increased with an increase in temperature. The highest fecundity 402.73 (±29.9 SE) was at 30°C and 70–75% RH (fig. 1A).

Figure 1. Temperature (20, 25, and 30°C) and relative humidity (30–35, 50–55, 70–75, and 90–95%) mediated fecundity of S. exigua (A) and S. litura (B) fed on an artificial diet over a period of 72 h. Bars represent standard error. ANOVA for S. exigua: model: F _{11, 348} = 84.75, P < 0.0001, temperature: F _{2, 348} = 369.55, P < 0.0001, RH: F _{3, 348} = 57.16, P < 0.0001, and interaction: F _{6, 348} = 3.62, P = 0.0017. For S. litura: model: F _{11, 348} = 138.49, P < 0.0001, temperature: F _{2, 348} = 619.04, P < 0.0001, RH: F _{3, 348} = 76.05, P < 0.0001, and interaction: F _{6, 348} = 9.52, P = 0.0017. Means followed by the same letters are not significantly different among relative humidity levels (ANOVA, Tukey's [HSD] test, P < 0.05).

For S. litura, mouth fecundity was varied with temperature and RH (model, P < 0.0001). Temperature (P < 0.0001) and RH (P < 0.0001) significantly impacted the mouth fecundity. Combined effects of temperature and RH were also significant (P = 0.0017). Fecundity increased with an increase in temperature. The highest fecundity 438.97 (±11.8 SE) was at 30°C and 70–75% RH (fig. 1B).

Egg mortality

For S. exigua, the percentage of egg mortality was varied with temperature and RH (model, P < 0.0001). Temperature (P < 0.0001) and RH (P < 0.0001) significantly impacted on the egg mortality. Combined effects of temperature and RH were also significant (P = 0.0400). Egg mortality increased with decreasing temperature. The highest number of eggs that died was 61.91% (±5.8 SE) at 20°C and 30–35% RH (table 1).

Table 1. Temperature (20, 25, and 30°C) and relative humidity (30–35, 50–55, 70–75, and 90–95%) mediated egg and larva mortality (%), developmental period (days), and adult longevity (days) of S. exigua fed on an artificial diet

ANOVA for egg mortality: model: F _{11, 2508} = 9.58; P < 0.0001, temperature: F _{2, 2508} = 14.08; P < 0.0001, RH: F _{3, 2508} = 21.74, P < 0.0001, and interaction: F _{6, 2508} = 2.97, P = 0.0400. Larva mortality: model: F _{11, 1617} = 17.11, P < 0.0001, temperature: F _{2, 1617} = 2.19, P = 0.1122, RH: F _{3, 1617} = 59.03, P < 0.0001, and interaction: F _{6, 1617} = 1.12, P = 0.3453. Developmental period: model: F _{11, 1068} = 4896.15, P < 0.0001, temperature: F _{2, 1068} =  26,788.0, P < 0.0001, RH: F _{3, 1068} = 10.92, P < 0.0001, and interaction: F _{6, 1068} = 41.48, P < 0.0001. Adult longevity: model: F _{11, 1063} = 135.70, P < 0.0001, temperature: F _{2, 1063} = 657.45, P < 0.0001, RH: F _{3, 1063} = 61.15, P < 0.0001, and interaction: F _{6, 1063} = 0.00, P = 0.0000. Means (±SE) followed by the same capital letters in a row and same small letters in a column are not significantly different among temperature regimes and relative humidity levels, respectively (ANOVA, Tukey's [HSD] test, P < 0.05).

For S. litura, the percentage of egg mortality was varied with temperature and RH (model, P < 0.0001). Temperature (P < 0.0001) and RH (P < 0.0001) significantly impacted the egg morality. However, combined effects of temperature and RH were not significant (P = 0.1262). Egg mortality increased with decreasing temperature. The highest number of eggs that died was 58.10% (±5.8 SE) at 20°C and 30–35% RH (table 2).

Table 2. Temperature (20, 25, and 30°C) and relative humidity (30–35, 50–55, 70–75, and 90–95%) mediated egg and larva mortality (%), developmental period (days), and adult longevity (days) of S. litura fed on an artificial diet

ANOVA for egg mortality: model: F _{11, 2508} = 8.97, P < 0.0001, temperature: F _{2, 2508} = 17.44, P < 0.0001, RH: F _{3, 2508} = 17.95, P < 0.0001, and interaction: F _{6, 2508} = 1.66, P = 0.1262. Larva mortality: model: F _{11, 1737} = 12.54, P < 0.0001, temperature: F _{2, 1737} = 0.69, P = 0.5036, RH: F _{3, 1737} = 38.80, P < 0.0001, and interaction: F _{6, 1737} = 3.37, P = 0.0026. Developmental period: model: F _{11, 1222} = 1264.16, P < 0.0001, temperature: F _{2, 1222} = 6842.65, P < 0.0001, RH: F _{3, 1222} = 32.33, P < 0.0001, and interaction: F _{6, 1222} = 20.58, P < 0.0001. Adult longevity: model: F _{11, 1222} = 290.08, P < 0.0001, temperature: F _{2, 1222} = 1460.40, P < 0.0001, RH: F _{3, 1222} = 90.54, P < 0.0001, and interaction: F _{6, 1222} = 0.00, P = 1.0000. Means (±SE) followed by the same capital letters in a row and same small letters in a column are not significantly different among temperature regimes and relative humidity levels, respectively (ANOVA, Tukey's [HSD] test, P < 0.05).

Larval mortality

For S. exigua, the percentage of larval mortality was varied with temperature and RH (model, P < 0.0001). RH (P < 0.0001) significantly impacted the larval mortality. However, temperature (P = 0.1122) and the combined effects of temperature and RH did not significantly impacted the larval mortality (P = 0.3453). The highest percentage of larvae that died was 60.13% (±6.9 SE) at 30°C and 90–95% RH (table 1).

For S. litura, the percentage of larval mortality was varied with temperature and RH (model, P < 0.0001). RH (P < 0.0001) and interactions between temperature and RH significantly impacted the larval mortality (P = 0.0026). However, temperature did not significantly impacted the larval morality (P = 0.5036). The highest number of larvae that died was 53.46% (±10.7 SE) at 30°C and 90–95% RH (table 2).

Developmental period

For S. exigua, the developmental period of adult mouths varied with temperature and RH (model, P < 0.0001). Temperature (P < 0.0001) and RH (P < 0.0001) significantly impacted the developmental period of mouths. Similarly, the combined effects of temperature and RH were also significant (P < 0.0001). Developmental period decreased with increasing temperature and RH. The developmental period was shortest (16.49 days ± 0.2 SE) at 30°C and 70–75% RH (table 1).

For S. litura, the developmental period of adult mouths varied with temperature and RH (model, P < 0.0001). Temperature (P < 0.0001) and RH (P < 0.0001) significantly impacted the developmental period of mouths. Similarly, the combined effects of temperature and RH were also significant (P < 0.0001). Developmental period decreased with increasing temperature and RH. The developmental period was shortest (25.60 days ± 0.4 SE) at 30°C and 70–75% RH (table 2).

Adult emergence

For S. exigua, the adult emergence rate varied with temperature and RH (model, P < 0.0001). Temperature (P < 0.0001) and RH (P < 0.0001) significantly impacted the adult emergence rate. However, the combined effects of temperature and RH were not significant (P = 0.1461). Adult emergence rate increased with increasing temperature and RH. The highest adult emergence rate was 61.52% (±5.8 SE) at 30°C and 70–75% RH (fig. 2A).

Figure 2. Temperature (20, 25, and 30°C) and relative humidity (30–35, 50–55, 70–75, and 90–95%) mediated adult emergence rate of S. exigua (A) and S. litura (B) fed on an artificial diet. Bars represent standard error. ANOVA for S. exigua: model: F _{11, 2508} = 12.54, P < 0.0001, temperature: F _{2, 2508} = 16.95, P < 0.0001, RH: F _{3, 2508} = 31.52, P < 0.0001, and interaction: F _{6, 2508} = 1.59, P = 0.1461. For S. litura: model: F _{11, 2508} = 13.15, P < 0.0001, temperature: F _{2, 2508} = 13.17, P < 0.0001, RH: F _{3, 2508} = 34.92, P < 0.0001, and interaction: F _{6, 2508} = 2.27, P = 0.0349. Means followed by the same letters are not significantly different among relative humidity levels (ANOVA, Tukey's [HSD] test, P < 0.05).

For S. litura, the adult emergence rate varied with temperature and RH (model, P < 0.0001). Temperature (P < 0.0001) and RH (P < 0.0001) significantly impacted the adult emergence. Similarly, the combined effects of temperature and RH were also significant (P = 0.0349). Adult emergence rate increased with increasing temperature and RH. The highest adult emergence rate was 71.91% (±5.4 SE) at 30°C and 70–75% RH (fig. 2B).

Adult longevity

For S. exigua, the adult longevity varied with temperature and RH (model, P < 0.0001). Temperature (P < 0.0001) and RH (P < 0.0001) significantly impacted the adult longevity. However, the combined effects of temperature and RH were not significant (P = 1.0000). The highest adult longevity was 12.86 days (±0.2 SE) at 20°C and 70–75% RH (table 1).

For S. litura, the adult longevity varied with temperature and RH (model, P < 0.0001). Temperature (P < 0.0001) and RH (P < 0.0001) significantly impacted the adult longevity. However, the combined effects of temperature and RH were not significant (P = 1.0000). The highest adult longevity was 11.10 days (±0.2 SE) at 20°C and 70–75% RH (table 2).

Discussion

The outcomes of this study suggest that both Spodoptera species could complete their life cycle on all tested temperatures and RH conditions. However, the influences of environmental conditions such as temperature and RH significantly varied on the life variables and adult performance of Spodoptera species. The present study shows that temperature emphatically affects all the life variables except for larval mortality, while RH affects all the life variables, such as oviposition, egg and larval survival, developmental period, adult emergence, and adult longevity, of both Spodoptera species. In addition to larval mortality and adult longevity for both Spodoptera species, combined effects of temperature and RH were recorded on the egg morality for S. exigua and adult emergence for S. litura.

Among the tested temperature and RH conditions, 30°C and 70–75% RH proved to be the furthermost optimal environmental conditions for fecundity and normal development of both Spodoptera species. We found that fecundity was emphatically decreased at minimal temperature. Previous studies reported that Spodoptera species laid a higher number of eggs at 25–30°C (Gupta et al., 2007; Mehrkhou et al., 2012; Jaba et al., 2020), similar to our results. In the present study, Spodoptera species successfully laid eggs at a lower temperature (20°C), though it was the lowest number of eggs among the temperatures. In line with our findings, Choi and Park (2000) reported a lower number of eggs laid by S. exigua at a lower temperature (16°C). Nevertheless, Lee et al. (1991) reported a maximum number of eggs laid by S. exigua at a lower temperature (20°C). This fecundity discrepancy may be associated with relative humidity along with temperature. Several studies reported that different temperatures and RH have significant effects on the oviposition of Spodoptera species (Shuqing, 2002; Jaba et al., 2020).

Being a poikilothermic organism, insects are physiologically sensitive to external environmental factors, and temperature is the most significant factor influencing the overall developmental processes of insects (Taylor, 1981; Bale et al., 2002). Previous studies reported that RH affects the success of the egg hatchability of insect species (Holmes et al., 2012; Norhisham et al., 2013). Norhisham et al. (2013) reported that egg mortality was highest at lower RH levels in Dinoderus minutus Fabricius (Coleoptera: Bostrichidae). In line with earlier studies, we report here that there is higher egg mortality at lower (30–35%) rather than higher (90–95%) RH levels. Wigglesworth (1984) reported that water loss through the egg and pupal membranes due to low RH can be responsible for inhibiting the survival and normal development of holometabolous insects, resulting in desiccation. We believe that the detrimental effects on egg eclosion success caused by lower RH could be associated with desiccation. This phenomenon is supported by Clark and Faeth (1997, 1998), where the authors noticed that eggs on the surface of egg clusters were desiccated, while eggs within the egg cluster were not. Previous studies indicated lower larval survival of Spodoptera species in higher temperature and RH (Jaba et al., 2020; Maharjan et al., 2023). In line with previous studies on Spodoptera species, we also found higher larval mortality in high humidity (90–95%) at all temperature regimes, and humidity levels (>90%) negatively affected larval survival. During microscopic observation, we noticed dead larvae with a higher amount of water within the larval body and fungal spores on the outer surface of the body. Thus, we believe that larval death could be caused by RH-assisted mould formation.

Earlier studies revealed that temperature and RH (Ranga Rao et al., 1989; Jaba et al., 2020; Rao and Prasad, 2020; Maharjan et al., 2022, 2023; Malekera et al., 2022) are potential factors influencing the life processes of Spodoptera species. A top of Spodoptera species, the influence of temperature and RH, and their interaction on the life variables of other insect species such as Spotted stem borer (Chilo partellus) (Lepidoptera: Crambidae) and rice weevil (Sitophilus oryzae L.) (Coleoptera: Curculionidae) was also reported by previous studies (Tamiru et al., 2012; Hasan et al., 2017). In line with the findings of these earlier studies, the findings of this study indicated that the developmental period of Spodoptera species was shorter with the increase in temperature. We also suggest here that the increment in RH from 30–35 to 70–75% reduced the developmental period and enhanced the adult emergence. However, extreme RH levels (lower (30–35%) and higher (>90%)) adversely impacted the emergence rate of adults, which is in agreement with Wigglesworth (1984), who reported that low RH results in water loss through egg cells and pupal membranes, which can lead to desiccation and be a detrimental means for the success of insect survival, which could also partly be linked to higher mortality of larvae driven by fungal infections. Further, the distinct effects of temperature and RH rather than the interactive effect might be responsible for the deleterious impact on the adult emergence. Earlier research indicated that temperature plays a more significant part than RH in the life processes of Spodoptera species (Ranga Rao et al., 1989; Bale et al., 2002). As with other life variables, temperature and RH also influenced the longevity of adult Spodoptera species (Ranga Rao et al., 1989; Rao and Prasad, 2020; Maharjan et al., 2022, 2023), which is in line with the findings of previous studies; we observed inverse effects of temperature on adult longevity, and we found higher adult longevity of Spodoptera species in 70–75% RH at 20°C. Significant factors confirming insects' longevity are still unknown. Among the significant factors, temperature is considered as one of the most influential factors regulating the lifespan of insect species. In addition to temperature, genetic background, physiology and metabolic processes, and nutrients and chemical compounds available on host plants may also influence the adult longevity of insect species (Kenyon, 2010; Maharjan et al., 2022, 2023).

Here, we reported that both the temperature and the RH are significant factors that have central roles in the developmental processes and adult performance of Spodoptera species, however, the way that temperature and relative humidity interact depends on certain life variables of Spodoptera species. This study showed that a temperature of 30°C and RH of 70–75% were optimal conditions for overall development of Spodoptera species. Temperature had the greatest effect on the life variables of Spodoptera species, though the effect of RH was minimal with occasional statistical significance for some variables. This study suggests that immature stages (especially egg and larva) are more sensitive to RH. Information on egg and larval mortality indicates that the effect of RH is important when an insect is in an immature stage and in a stressed state due to adverse temperature. Further, the outcomes of this study demonstration that Spodoptera species are supposed to cause significant damage to the field crops and vegetables grown under environmental conditions (temperature around 30°C and 70–75% RH). Lastly, information generated from this study can be integrated with other management strategies for the management of Spodoptera species.