Background

Sublethal effects of chemicals encompass biological, physiological, demographic, or behavioral changes in organisms or communities that endure exposure to a toxic substance at doses or concentrations that are either lethal or below lethal levels. When a population exposed to a toxicant survives without apparent mortality, it is termed a sublethal dose/concentration (Desneux et al. 2007). Generally, concentrations of insecticides falling below the median lethal dose/concentration (LD50/LC50) are categorized as sublethal. These sublethal effects can manifest as various alterations such as decreased lifespan, slower development, hindered population growth, reduced fertility, changes in reproductive capacity, shifts in gender ratios, physical abnormalities, and alterations in behaviors like feeding, searching, and egg-laying (Rimoldi et al. 2015).

The fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) is a highly destructive insect pest that causes extensive damage to crops in different regions of the Americas (Song et al. 2020). It has become a major threat to agricultural crops in Africa (Goergen et al. 2016), affecting a range of crops including corn, rice, sugarcane, and sorghum (Jamil et al. 2021). This pest can have multiple broods per year and is capable of traveling long distances (Abrahams et al. 2017). The immature stage of S. frugiperda feeds exclusively on leaves, flowers, and stalks of cultivated plants, resulting in significant economic losses (Boregas et al. 2013).

Chemical pesticides are often utilized to control S. frugiperda (with various types of insecticides working differently to eliminate the target pest (Casida and Durkin 2013). However, due to the adverse effects of insecticides, pest management through biological control shows significant potential (Agboyi et al. 2023). S. frugiperda has been effectively controlled using various entomopathogens; viruses, bacteria, fungi, protozoa, and nematodes as reported by Gómez-Valderrama et al. (2022). During severe pest outbreaks, these bioagents have been successfully utilized, as stated by Grijalba et al. (2018). Among them, entomopathogenic fungi (EPF) are noteworthy due to their wide range of hosts, pathogenicity, and capacity to manage insect pests.

Metarhizium spp., are insect-pathogenic fungi, prevalent to soil inhabitant worldwide, and have been widely studied and utilized for biological control of pests (Clifton et al. 2018)). The process of infection by Metarhizium spp. on insect hosts involves several stages (Aw and Hue 2017). Initially, the fungal conidia attach to the waxy cuticle of the host and germinate, producing germ tubes that differentiate into appressoria. This allows the fungus to penetrate the host via a penetration peg. The fungus employs cuticle-degrading enzymes and mechanical pressure to penetrate the insect cuticle and overcomes any stress encountered during the process (Leao et al. 2015). The fungus invades the insect hemolymph, causing insect mycosis. After the death of the host, the hyphae emerge from the cadaver and form a dense mycelial network and green spores, initiating a new round of infection (Abbas et al. 2022).

Although EPF are widely used to control pests, their potential applications are often underestimated when only considering their lethal effects. In reality, sublethal concentrations of EPF may occur in the field when the concentration of insecticides decreases gradually after the initial application, and the host is likely to be exposed to sublethal doses during that time (Abbas et al. 2022). Thus, when using M. anisopliae to control S. frugiperda, it can produce some sublethal effects. Despite numerous studies involving Metarhizium spp., there are limited documents about their sublethal effects on life table parameters of insects, particularly on S. frugiperda. The use of EPF can be a valuable and complementary approach to reduce the amount of insecticides and their drawbacks. Therefore, the present study aimed to investigate the sublethal effects of M. anisopliae on S. frugiperda using the age-stage, two-sex life table method.

Methods

Spodoptera frugiperda culture

S. frugiperda larvae were collected from a maize field located at (32° 08′ 03.1″ N 72° 41′ 23.6″ E). The larvae were provided with fresh maize leaves as food in Petri plates. The pupae were transferred to separate Petri plates. Once the adults emerged, they were placed in adult rearing cages (11 × 11 × 10 cm) and fed with a honey solution (10%). Further, maize seedlings were placed in the cages to facilitate oviposition. The insect culture was sustained under laboratory conditions of 26 ± 2 °C, 65.5 ± 5% relative humidity, and a 12:12-h light/dark cycle.

Entomopathogenic fungus

For this study, Metarhizium anisopliae strain Met F52 (Earth Bio-Sciences, New Haven, CT) was used which was available in the Biocontrol lab (Mubeen et al. 2022). The fungus was cultured on potato dextrose agar (PDA) plates and was incubated at 27 °C for 14 days. A spore suspension was prepared using distilled water, and a Tween® 80 solution was added at 0.1% (v/v). The spores were counted using a Neubauer chamber (Thermo Fisher Scientific, Waltham, Massachusetts, USA). The viability of the tested fungus was determined by spraying 100 µl aliquots of conidia suspension containing 109 conidia per ml on PDA plates. The viability of the fungus was over 90%. The suspension was adjusted to different concentrations (1 × 103, 1 × 104, 1 × 105; 1 × 106; 1 × 107; 1 × 108, and 1 × 109 conidia ml−1) and used immediately for lethal pathogenicity bioassays.

Lethal toxicity of EPF against S. frugiperda

For this experiment, 30 s-instar larvae of S. frugiperda were selected from a laboratory-cultured population and subjected to each concentration of M. anisopliae. Each larva was treated with approximately 4 µl from each concentration and then larvae were transferred into separate clean Petri plates containing fresh maize leaves. The mortality data were recorded after a period of 48 h. The probit analyses were performed, using POLO software, to determine the LC20, LC30, and LC50 values.

Life table study

The sublethal concentrations (LC20, LC30, and LC50) of M. anisopliae were examined for their impact on the life table parameters of S. frugiperda. The larvae were exposed to each concentration through the topical bioassay method as described earlier. Sixty larvae were tested for each concentration with each larva considered as one replication. The treated larvae were individually placed in clean Petri plates and given fresh maize leaves daily. The developmental periods of larval instars and the pupae were recorded. After adult emergence, female and male adults were paired and placed in adult cages with maize seedlings in plastic pots and muslin cloth stripes to facilitate oviposition. The adults’ longevity and fecundity rates were recorded, as well as the mortality rate of each stage.

Statistical analysis

Life table analysis

The computer program TWOSEX-MS Chart (Chi and Liu 1985; Chi 1988) was used to determine the fecundity rate, developmental time of each stage, ovipositional period, and survival rate based on an age-stage two-sex life table. The basic age-stage-specific parameters, including survival rate (sxj), reproductive value (vxj), life expectancy rate (exj), and population parameters such as intrinsic (r) and finite (λ) rate of increase, reproductive rate (R0), and mean generation time (T), were also analyzed using TWOSEX-MS Chart (Chi and SU 2006). To minimize variations in results, the quick paired bootstrapping technique (Paired 1 by 1) with 100,000 replications were used to estimate the mean and standard error of the population (Efron and Tibshirani 1993) with the TWOSEX-MS Chart program.

Results

The LC50, LC30, and LC20 values were obtained from the mortality concentration–response and were found as 3.6 × 107, 5.4 × 104, and 1.04 × 103 conidia ml−1, respectively, based on the lethal toxicity for seven serial dilutions. These sublethal concentrations were utilized to investigate their effects on life table parameters of S. frugiperda. Table 1 provides the developmental time of each larval instar of S. frugiperda, which was significantly (p < 0.05) different across treatments. In general, larvae developed faster in the control group (2.42–5.49 days) than when LC50 value was applied, which prolonged the period (3.25–6.45 days). Similarly, the pupal duration was prolonged after applying LC50 (8.24 days) than control (6.91 d). The adult longevity was shorter in LC50 (9.64 days), followed by LC30 (10.49 d), and LC20 (11.73 days), compared to control (11.7 d) (Table 1).

Table 1 Development period (days) of Spodoptera furgiperda after exposure to sublethal concentration of Metarhizium anisopliae
Full size table

APOP and TPOP periods were longer in LC30 (3.07 and 39.8 days, respectively) and LC50 treatments (2.79 and 41.4 days, respectively). The ovipositional period of the female was shortened (2.9 days) when LC50 and LC30 values were applied, compared to the control group (3.54 days). The females laid the least number of eggs (464.79 eggs), when LC50 value of M. anisopliae was applied than those in control (696.93 eggs). The r (0.114 d−1) and R0 (147.1 offspring) values of S. frugiperda reduced when LC50 of M. anisopliae was applied than of the control group (r = 0.156 d−1 and R0 = 325.2 offspring). On the other hand, the λ value increased to (1.120 d−1) in the LC50 treatment than control (λ = 0.169 d−1). The generation time lasted longer (43.7 days) when LC50 was applied than in the control (36.9 d) (Table 2).

Table 2 Life table and reproductive parameters (means ± SE) of Spodoptera frugiperda after exposure to sublethal concentration of Metarhizium anisopliae
Full size table

Figure 1 illustrates the sxj (age-specific survival rate) value of S. frugiperda after exposure to M. anisopliae. In the control group, male and female adults emerged at 24th and 25th day, respectively. In the LC50 treatment group, males and females emerged at 35th and 32nd day, respectively (Fig. 1). The exj curve indicated that the life expectancy of S. frugiperda treated with all sublethal concentrations of M. anisopliae was lower than the control group. At age zero (e01), exj of S. frugiperda was 42.9 days in the control group, 33.6 days in LC50, 40.4 days in LC30, and 41.2 days in LC20 (Fig. 2). The vxj showed the future population growth of individuals of age x and stage j. It showed that the highest reproductive value for females was observed in the control group at age 31st, whereas a low peak was recorded on 40th day in the LC50 treatment. In LC30 and LC20 treatments, high peaks were observed on 39th and 28th days, respectively (Fig. 3). The fecundity rate of S. frugiperda female appeared later in the treatment groups than in the control. The maternity rate of S. frugiperda peaked at later ages in the treatment groups as well (Fig. 4).

Fig. 1
figure 1

Age-stage-specific survival rate (sxj) of Spodoptera frugiperda after exposure to sublethal concentration of Metarhizium anisopliae. A: LC20, B: LC30, C: LC50, and D: Control

Full size image
Fig. 2
figure 2

Age-stage-specific life expectancy (exj) of Spodoptera frugiperda after exposure to sublethal concentration of Metarhizium anisopliae. A: LC20, B: LC30, C: LC50, and D: Control

Full size image
Fig. 3
figure 3

Age-stage-specific reproductive value (vxj) of Spodoptera frugiperda after exposure to sublethal concentration of Metarhizium anisopliae. A: LC20, B: LC30, C: LC50, and D: Control

Full size image
Fig. 4
figure 4

Age-stage-specific survival rate (lx), age-stage-specific fecundity (fxj), age-specific fecundity (mx) and age-specific maternity (lxmx) of Spodoptera frugiperda after exposure to sublethal concentration of Metarhizium anisopliae. A: LC20, B: LC30, C: LC50, and D: Control

Full size image

Discussion

Metarhizium anisopliae is a well-known entomopathogenic biological control agent that has been used to control various insect pests, including S. frugiperda (Mubeen et al. 2022). Although previous studies have determined the lethal effects of M. anisopliae on S. frugiperda (Mubeen et al. 2022), the sublethal effects of this fungus on the life table parameters of S. frugiperda remain unclear. In the present study, S. frugiperda were exposed to sublethal concentrations of M. anisopliae spores and monitored their survival, development, and reproductive parameters. The results showed that exposure to M. anisopliae at LC50 and LC30 had significant sublethal effects on the developmental period of S. frugiperda, leading to a longer period and decreased fecundity in female insects as compared to the control group. The prolongation in larval and pupal periods may occur because the energy that would typically support larval growth is instead directed toward detoxifying the toxin compounds generated by entomopathogenic fungi. This aligns with the study of Afriyanita et al. (2019), who highlighted that these toxic compounds can hinder larval development. Instead of being used into growth and development, the energy derived from food is redirected toward the detoxification process to counteract the effects of the toxic substances.

The present study’s findings align with previous research that has shown that the sublethal effects of M. anisopliae on the developmental period of S. frugiperda and other insect pests. For instance, Mwamburi (2021) reported that treatment with M. anisopliae resulted in a prolonged developmental period and reduced larval weight in S. frugiperda. Similarly, Jarrahi and Safavi (2016) found that sublethal concentrations of M. anisopliae extended the developmental phase of immature stages of the cotton bollworm, Helicoverpa armigera (Hb.). These findings suggest that M. anisopliae negatively affects insect growth and development, possibly by reducing feeding and growth rates caused by the infection (Idrees et al. 2021). However, it is worth noting that efficacy of M. anisopliae can vary depending on the timing and frequency of applications (Sánchez-Peña et al. 2011) and environmental factors such as temperature and humidity (Kamga et al. 2022). M. anisopliae employs various mechanisms to infect and kill insect pests, including direct infection, production of insecticidal toxins, and induction of immune responses in the host insect (Aw and Hue 2017). In addition, M. anisopliae produces a range of secondary metabolites that are toxic to insect pests, such as destruxins, which disrupt insect cell membranes and cause cell death (Sbaraini et al. 2016).

The present study demonstrated that the exposure of S. frugiperda to M. anisopliae resulted in a significant reduction in the egg-laying capacity of female insects, especially at LC50 and LC30 values. This finding is consistent with those reported by Zafar et al. (2020) on Plutella xylostella. Reduced reproductive fitness may be attributed to a variety of factors, including changes in hormone levels, reduced nutrient uptake, and the diversion of resources to immune responses (Schwenke et al. 2016). Additionally, increased conidial concentration may also contribute to this effect by redirecting host resources such as energy, to the pathogen (Roy et al. 2006). The findings are also supported by previous studies that reported sublethal effects of EPF on the fecundity rate of insect pests (Pelizza et al. 2013).

Examination of life table parameters is crucial in identifying the sublethal impacts of EPF on insect survival, development, and reproduction. The results indicated significant changes in the life table parameters r, R0, and T of S. frugiperda, with lower values of r and R0 observed in the LC30 and LC50 groups than the control. These results are similar to those obtained by De Souza et al. (2020) on H. armigera. Additionally, the present study showed that the sublethal concentrations of LC30 and LC50 had notable effects on sxj, exj, and vxj values. The mortality rate of insects was high in the treated groups, with high mortality rates observed in the latest developmental stages. The survival rate can serve as an indicator of insect population growth following exposure to fungal treatment (Huang and Chi 2012). Our findings revealed the negative impact of M. anisopliae on S. frugiperda as evidenced by the lowest sxj of each age stage relative to the control and the linear downward trend of lx of the population. Moreover, the reproductive values of female in the LC30 and LC50 groups decreased than the control, as indicated by the vxj and exj values. These results are consistent with those of Xia et al. (2023) who investigated the effects of B. bassiana against various insects.

The present study highlighted also the negative impact of M. anisopliae on the growth and survival of S. frugiperda, which is crucial information for developing effective pest management strategies (Zhang et al. 2015). It is important to note, however, that the efficacy of EPF in pest management program depends on their virulence toward target pests and their safety for non-target organisms. Therefore, it is important to estimate the impact of sublethal concentrations of EPF on natural enemies to ensure their reliable and applicable outcomes. Additionally, the use of chemical insecticides in the same environment may reduce the effectiveness of EPF by negatively affecting their conidial survival. Therefore, it is crucial to determine the compatibility of tested EPF with chemical insecticides used in an IPM program).

Conclusion

The findings revealed noteworthy alterations in all population and age-stage-specific parameters as a result of sublethal exposure to M. anisopliae at LC30 and LC50 values. This underscores the considerable influence of the fungal treatment on the survival, development, and reproductive dynamics of S. frugiperda populations. The outcomes of this study emphasize the significance of considering sublethal effects when assessing the impact of M. anisopliae as a potential biological control agent against S. frugiperda, shedding light on the broader implications of its employment in pest management strategies. Further investigations are needed to better understand the sublethal effects of M. anisopliae on other insect pests and to develop more effective and sustainable pest control approaches.