Introduction
Fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), is a global threat to food security and can damage over 350 plant species worldwide, including key staple crops such as maize (Kenis et al., Reference Kenis, Benelli, Biondi, Calatayud, Day, Desneux, Harrison, Kriticos, Rwomushana, van den Berg, Verheggen, Zhang, Agboyi, Ahissou, Ba, Bernal, Bueno, Carriere, Carvalho, Chen, Cicero, Plessis, Early, Fallet, Fiaboe, Firake, Goergen, Groot, Gupta, Hu, Huang, Jaber, Malo, Meagher, Mohamed, Sanchez, Nagoshi, Negre, Niassy, Noboru, Nyamukondiwa, Omoto, Palli, Pavela, Ramirez-Romero, Rojas, Subramanian, Tabashnik, Tay, Virla, Wang, Williams, Zang, Zhang and Wu2022). The FAW was first reported on maize in India in 2018 (Sharanabasappa et al., Reference Sharanabasappa, Kalleshwaraswamy, Asokan, Mahadevaswamy, Maruthi, Pavithra, Kavita, Shivaray, Prabhu and Georg2018a). As this pest is difficult to manage, farmers mostly employ traditional and novel chemical insecticides to control its outbreaks (Deshmukh et al., Reference Deshmukh, Kalleshwaraswamy, Prasanna, Sannathimmappa, Kavyashree, Sharath, Pradeep and Kiran Kumar2021a; Paredes-Sánchez et al., Reference Paredes-Sánchez, Rivera, Bocanegra-García, Martínez-Padrón, Berrones-Morales, Niño-García and Herrera-Mayorga2021). However, the continuous and indiscriminate spraying of chemical insecticides has led to the selection of resistant FAW populations (Ngegba et al., Reference Ngegba, Khalid, Jiang and Zhong2025), indicating the importance of searching for safer and more sustainable S. frugiperda management (Deshmukh et al., Reference Deshmukh, Prasanna, Kalleshwaraswamy, Jaba, Choudhary and Omkar2021b; Kenis et al., Reference Kenis, Benelli, Biondi, Calatayud, Day, Desneux, Harrison, Kriticos, Rwomushana, van den Berg, Verheggen, Zhang, Agboyi, Ahissou, Ba, Bernal, Bueno, Carriere, Carvalho, Chen, Cicero, Plessis, Early, Fallet, Fiaboe, Firake, Goergen, Groot, Gupta, Hu, Huang, Jaber, Malo, Meagher, Mohamed, Sanchez, Nagoshi, Negre, Niassy, Noboru, Nyamukondiwa, Omoto, Palli, Pavela, Ramirez-Romero, Rojas, Subramanian, Tabashnik, Tay, Virla, Wang, Williams, Zang, Zhang and Wu2022; Ngegba et al., Reference Ngegba, Khalid, Jiang and Zhong2025).
The release of egg parasitoids is recognised as a promising management strategy against FAW (Mahajan and Bhamare Reference Mahajan and Bhamare2023). Species from the genus Trichogramma (Hymenoptera: Trichogrammatidae) and Telenomus remus (Nixon) (Hymenoptera: Scelionidae) are among the most important and studied worldwide biocontrol agents of S. frugiperda (Li et al., Reference Li, Bueno, Desneux, Zhang, Wang, Dong and Zang2023; Veena et al., Reference Veena, Hosamani, Prabhuraj and Shivanand2023; Navik et al., Reference Navik, Dsilva, Patil and Sushil2024). Trichogrammatids have been released in over 32 million hectares of agricultural land, targeting more than 30 key pest species. This success is attributed to their ease of mass production and application, low production cost, and broad host range (Ayvaz and Karaborklu Reference Ayvaz and Karaborklu2008; Wajnberg and Hassan Reference Wajnberg and Hassan1994; Yuan et al., Reference Yuan, Guo and Li2024). Despite such parasitism potential, trichogrammatids have limited parasitism in the inner layers of FAW egg masses, which are frequently covered with scales from the wings of the S. frugiperda moth (Bueno et al., Reference Bueno, Sutil, Maciel, Roswadoski, Colmenarez and Colombo2023a). Differently, Te. remus has a narrower range of hosts and a more expensive rearing (Yuan et al., Reference Yuan, Guo and Li2024). However, it is considered more efficient against S. frugiperda eggs than Trichogramma spp. due to its capacity to parasitise the whole S. frugiperda egg masses, including the eggs within the inner layers (Colmenarez et al., Reference Colmenarez, Babendreier, Wurst, Vasquez and Bueno2022). Nevertheless, the success of any egg parasitoid species to control S. frugiperda depends on the bioecological characteristics of the parasitoid and its interaction not only with the specific host species but also with different temperatures, which can differ among the parasitoid and host species and must be studied in laboratory and field conditions (Bueno et al., Reference Bueno, Sutil, Maciel, Roswadoski, Colmenarez and Colombo2023a). Consequently, extensive laboratory studies are necessary prior to field deployment to fully understand their biological traits and performance against S. frugiperda eggs, especially in specific environmental conditions (Wajnberg and Hassan Reference Wajnberg and Hassan1994).
Within this framework, our research aimed to examine the biological traits of four well-known egg parasitoids – Trichogramma chilonis Ishii, Trichogramma pretiosum Riley, Trichogrammatoidea bactrae Nagaraja (Hymenoptera: Trichogrammatidae), and Telenomus remus (Nixon) (Hymenoptera: Scelionidae) – to further investigate how those egg parasitoids interact in carefully monitored laboratory settings with S. frugiperda eggs. Therefore, the present study contributed to gain knowledge about numerous important details regarding the characteristics of these egg parasitoids, including developmental stages, adult lifespan, life cycle, parasitism rate, emergence rate, and sex ratio. This understanding of their biological characteristics serves as a foundation for evaluating the possible efficacy of these parasitoids to manage S. frugiperda. These discoveries could play a pivotal role in shaping sustainable pest control tactics and offer a glimmer of hope in addressing the economic challenges faced by maize farmers.
Materials and methods
Study site
The laboratory studies and maintenance of insect rearing encompassing Corcyra cephalonica (Stainton) (Lepidoptera: Pyralidae), S. frugiperda, and the four species of egg parasitoids were meticulously carried out in controlled laboratory conditions [25 ± 1 °C, 70 ± 5 % RH, 14/10-h (L:D) photoperiod] and at field-caged trial carried out in maize field located at the Main Agricultural Research Station, UAS, Raichur, India (16° 20′ 15.26′′ E and 77° 32′ 56.46′′ N).
Rearing of the hosts and parasitoids
Four egg parasitoids (Tr. chilonis, Tr. pretiosum, Tr. bactrae, and Te. remus) and two host species (C. cephalonica and S. frugiperda) were tested. The protocols and methodologies established by Jalali and Singh (Reference Jalali and Singh1989) for C. cephalonica, Sharanabasappa et al. (Reference Sharanabasappa, Kalleshwaraswamy, Maruthi and Pavithra2018b) and Silva and Parra (Reference Silva and Parra2013) for S. frugiperda, Tefera et al. (Reference Tefera, Goftishu, Ba and Muniappan2019) for trichogrammatids, and Bueno et al. (Reference Bueno, Carneiro, Pratissoli, Bueno and Fernandes2008) for Te. remus were meticulously followed to rear and maintain those insect rearings under controlled laboratory conditions, ensuring the success of the experiments.
Rearing of C. cephalonica was maintained in wooden boxes (43 × 22 × 12 cm) equipped with ventilated lids featuring iron mesh (20-mesh). Each box contained a substrate consisting of sterilised broken jowar grains (2.5 kg per box), supplemented with groundnut kernel powder (100 g) and yeast powder (5 g) to support the growth of C. cephalonica larvae. To prevent bacterial and mite infestations, a mixture of 0.05% streptomycin sulphate (0.5 g/kg) and wettable sulphur 80 WP (5.0 g/kg) was incorporated into the rearing box. Each box was inoculated with 0.5 cc (equivalent to 8000 eggs) of C. cephalonica eggs and placed in a controlled rearing room (25 ± 1 °C and 70 ± 5% RH). Larvae hatched within 3–4 days, began constructing webbings, and grew undisturbed until the emergence of moths after about 35–40 days.
Moths were mechanically collected every morning using glass tubes (15 × 25 cm) and then transferred to a specially designed oviposition chamber (43 × 22 × 12 cm) featuring wire mesh at the bottom for easy egg collection. Within this chamber, adults were allowed to mate, lay eggs, and were provided with a honey solution (50%) fortified with vitamin E to enhance their vitality and ensure the production of high-quality eggs. The eggs laid were collected the following morning, sieved (40 mesh) to remove moth scales and other debris, and stored in a refrigerator. These eggs served as the foundation for sustaining the parasitoid rearing and perpetuating subsequent generations (Jalali and Singh Reference Jalali and Singh1989).
Field-collected S. frugiperda larvae (from a maize field located at 16° 20′ 15.26′′ E and 77° 32′ 56.46′′ N) were transferred to the laboratory and individualised in plastic vials within trays (38 × 26 × 6 cm) to prevent cannibalism. Early instars (up to 4–5 days) were fed with fresh maize leaves, while late instars (from the fifth day onwards) received a freshly prepared artificial diet (Jaba et al., Reference Jaba, Sathish and Mishra2020) daily until pupation (Silva and Parra Reference Silva and Parra2013). Healthy pupae were collected and sorted by sex based on genital structures, then kept separately in plastic containers within a growth chamber until moth emergence, typically occurring after 8–10 days. Subsequently, adult moths (5 couples per jar) were introduced into oviposition jars (3 L) containing paper strips (15 × 21 cm) as oviposition substrate, along with honey-soaked cotton swabs fortified with multivitamins, ascorbic acid, and methyl parahydroxybenzoate. Fresh honey swabs were replaced every other day. The jars were covered with black muslin cloth and secured with rubber bands. The obtained egg masses were used for experiments and colony maintenance. Neonate larvae hatched and were transferred to a breadbox (20 × 10 cm) containing fresh maize leaves for further rearing (Sharanabasappa et al., Reference Sharanabasappa, Kalleshwaraswamy, Maruthi and Pavithra2018b).
Trichogramma spp. was reared as described by Tefera et al. (Reference Tefera, Goftishu, Ba and Muniappan2019) and briefly summarised here. To prepare Trichogramma spp., freshly collected and cleaned C. cephalonica eggs were used. ‘Tricho-cards’ were prepared by applying 2% glue made from locally available Camlin gum (starch-based) onto a card measuring 15 × 7.5 cm. Subsequently, 1 cc of C. cephalonica eggs was evenly sprinkled onto the glued surface using a sieve. To arrest host embryo growth, these prepared cards were exposed to 15 W of UV rays for approximately 10–15 minutes. The cards were then placed in a 3-L jar with freshly emerged wasps for parasitisation over a 24-h period. A small piece of cotton swab soaked in a 50% honey solution with deionised water was provided as food for the adults. Every 24 h, the old cards were replaced with fresh egg cards, continuing for 3–4 days. The obtained cards were served both for the maintenance of the parasitoid colony and for experimental purposes.
Telenomus remus was reared as described by Bueno et al. (Reference Bueno, Carneiro, Pratissoli, Bueno and Fernandes2008) using freshly laid S. frugiperda egg masses (0–24 h old) affixed to a cardboard sheet (10 × 2 cm) by applying 2% glue. These egg-laden cards, along with eggs previously parasitised by Te. remus, were placed into glass tubes (15 × 2.5 cm). To nourish the emerging adult parasitoids, a small piece of cotton swab soaked in a 50% honey solution was stuck to the side of the glass tube. The tubes were sealed with cotton plugs, and the adults were allowed to parasitise for 24 h. The adults of Te. remus were provided with fresh egg cards for parasitisation by replacing the old ones every 24 h. Most parasitised eggs darkened within 4 to 5 days. The cards carrying only parasitised eggs were transferred into fresh, clean tubes for the emergence of the parasitoids by removing unparasitised eggs after scoring. Telenomus remus wasps emerged after 9 to 10 days, with males generally emerging before females (Colmenarez et al., Reference Colmenarez, Babendreier, Wurst, Vasquez and Bueno2022). The males are distinguishable by their filiform antennae and shorter abdomen, while females have clubbed antennae, longer and wider abdomens, and a visible ovipositor under a 10 × hand lens. Ten percent of the emerging parasitoids were retained to maintain the rearing, and the rest were used for experimental trials.
Laboratory biological traits of the egg parasitoids
To investigate the biology of the egg parasitoids on FAW eggs, adult females from the three studied trichogrammatid species (≤24 h old) and Te. remus (≤48 h old) were individually placed into glass vials (8 × 2 cm). These vials contained small cards (2 × 1 cm) of freshly collected S. frugiperda eggs (≤24 h old) that had been exposed to UV irradiation. Eggs were affixed to small white card strips coated with diluted gum. Daily until the death of the parasitoid female, the eggs were removed and replaced in the vials, and cards with parasitised eggs were transferred to a controlled environmental chamber [25 ± 1 °C, 70 ± 5 % RH, 14/10-h (L:D) photoperiod] until evaluation. The observed parameters included the total developmental time (from egg to adult in days), adult longevity (in days), lifespan parasitism, emergence rate (%), sex ratio [female/(female + male)], parasitism (number of parasitised eggs/total number of eggs), and percentage of adult emergence (number of parasitoids emerged/total number of parasitised eggs).
The entire parasitoid egg-to-adult period of parasitoids reared on S. frugiperda eggs until the emergence of the wasp was measured (parasitoid egg-to-adult period in days). This duration was carefully recorded to gain a comprehensive understanding of the time it takes for the egg parasitoid to complete its development in controlled conditions. Tracking the entire life cycle from the moment of parasitisation to adult emergence provided valuable data on the developmental timeline of these parasitoids.
To assess adult longevity, freshly emerged parasitoid males and females were individually placed into separate transparent glass vials (8 × 2 cm). Each vial was numbered for identification purposes, and observations of adult longevity were recorded at 12-h intervals. Adult longevity was calculated from the moment of adult emergence to the eventual death of the parasitoid. These observations were conducted under two distinct conditions: one with a food source (50% diluted honey) provided for the adults and the other without any food source.
As previously mentioned, S. frugiperda eggs were glued on the cards and exposed to female parasitoids, with 50 eggs for trichogrammatids and 100 eggs for Te. remus, respectively, until the end of their lifespan. These egg cards were placed inside glass vials, and fine streaks of diluted honey were applied to the inner walls of these vials to serve as an adult food source for the parasitoids. The host eggs parasitised were counted, which was used to determine the lifespan parasitism of the parasitoid. Lifespan parasitism was expressed as the number of eggs parasitised by a single female in her lifetime, providing valuable insights into the reproductive capacity of the different parasitoid species.
To calculate the percentage of adult emergence among egg parasitoids, the number of adults that successfully emerged from the total number of parasitised host eggs (recognised by their blackened appearance) was counted. This determination was based on the presence of emergence holes on the parasitised eggs, and the percentage of adult emergence was calculated using the following formula.
\begin{equation*}Emergence{\text{ }}\left( \% \right) = {\text{ }}\frac{{Number{\text{ }}of{\text{ }}eggs{\text{ }}with{\text{ }}emergence{\text{ }}holes}}{{Total{\text{ }}number{\text{ }}of{\text{ }}parasitized{\text{ }}eggs}}\text{x}{\text{ }}100\end{equation*}As described previously, the egg cards were prepared and exposed to freshly emerged parasitoid females for a 24-h period. The adults that emerged from these eggs were observed until their natural demise. Sex determination was carried out using a microscope (Nikon SMZ 745) based on antennal characteristics. Typically, males are smaller than females, with a rounded abdomen and filiform antennae, featuring more plumose and long black bristles (Laurentis et al., Reference Laurentis, Ramalho, Santos, Carvalho, Vacari, De Bortoli, Veneziani, da Costa Inacio and Dami2019; Oktaviani et al., Reference Oktaviani, Maryana and Pudjianto2021). In contrast, females are generally larger than males, have a tapering abdomen, and possess clubbed antennae with fewer plumose and shorter bristles. The sex ratio was calculated using the formula.
\begin{equation*}Sex{\text{ }}ratio = \frac{{Number{\text{ }}of{\text{ }}females}}{{\left( {Number{\text{ }}of{\text{ }}females + number{\text{ }}of{\text{ }}males} \right)}}\end{equation*}Cage experiments, set up in the field for Tr. chilonis parasitism
Tr. chilonis was chosen for field testing due to its overall best performance among the trichogrammatids and its easier rearing than Te. remus. A maize field was cultivated in the Main Agricultural Research Station, UAS, Raichur, India (16° 20′ 15.26′′ E and 77° 32′ 56.46′′ N). Maize hybrid Dhanvi-112 was sown with a spacing of 60 cm between lines and 20 cm between plants, following local recommendations. Twenty days after emerging, cages (1 × 1 m) were installed in the middle of each replication in a randomised block design with four treatments (50,000, 100,000, and 150,000 parasitoids/ha and control), each replicated five times.
Initially, the cages were positioned in the field to segregate the plants into groups. Subsequently, any S. frugiperda egg masses, larvae, or other organisms, including pests and natural enemies, were manually removed within the cages (Figueiredo et al., Reference Figueiredo, Lucia and Cruz2002). Each cage contained ten plants, arranged in two rows, with each plant hosting five sentinel egg masses. These sentinel masses, each containing 50 and 24-h-old eggs (250 S. frugiperda per cage), were placed on the midribs of leaves on five randomly selected plants. To ensure precision, the exact number of eggs on the sentinel masses was verified using a binocular microscope. Next, 5, 10, and 15 wasps per 250 eggs of S. frugiperda were released per cage. Then, after 48 h of exposure within the cage, the parasitised cards were removed and transferred to the laboratory, where they were individually placed in glass vials (2.5 × 15 cm) under room temperature conditions [25 ± 1 °C, 70 ± 5 % RH, 14/10-h (L:D) photoperiod]. The percentages of parasitism and adult emergence were determined as previously described.
Statistical analysis
This analysis allowed assessment of possible significant differences among the treatment groups. Subsequently, means were separated using Tukey’s HSD test values at a significance level of P = 0.01, to determine the significance of the differences among the treatment groups and to identify which groups exhibited statistically distinct outcomes. The statistical analysis was performed using SPSS software, version 16.0.
Results
Laboratory biological traits of the egg parasitoids
The egg-to-adult period (days) varied among the four egg parasitoid species from 7.8 to 10.0 days. On one hand, Te. remus showed the longest egg-to-adult period (10.00 ± 0.89 days), which did not significantly differ from Tr. chilonis and Tr. pretiosum (9.40 ± 0.75 and 9.00 ± 1.22 days, respectively). On the other hand, Tr. bactrae displayed the shortest egg-to-adult period (7.80 ± 0.66 days) (Table 1). These findings underscore the variability in developmental timelines among species, which is crucial for optimising mass rearing protocols and deployment timing in pest management programs.
Biological parameters of different egg parasitoids on Spodoptera frugiperda under laboratory conditions
Means sharing common letters in the same line are not significantly different by Tukey’s test (P ≤ 0.05).
Adult longevity (days) also varied among different egg parasitoid species and gender. Males displayed shorter lifespans than females across all species. With the offer of food, Te. remus males survived longer with a mean of 7.30 ± 0.62 days, followed by Tr. chilonis and Tr. pretiosum males with mean longevities of 5.80 ± 1.07 and 4.40 ± 0.68 days, respectively. Similarly, among fed females, Te. remus females had the highest mean longevity (11.80 ± 1.42 days), followed by Tr. pretiosum (8.50 ± 0.97 days) and Tr. chilonis (8.20 ± 1.16 days) (Table 1). These observations highlight the importance of adequate nutritional resources for the longevity and overall performance of egg parasitoid species, particularly in mass rearing facilities and field applications. The differences in adult longevity between species may also influence their efficacy in sustained pest suppression within agroecosystems.
The total life cycle of the studied egg parasitoid species ranged from 8 to 26 days. Te. remus had the longest total life cycle (22.20 ± 1.43 days), potentially due to its longer Parasitoid egg-to-adult period and adult longevity, followed by Tr. pretiosum and Tr. chilonis with mean durations of 14.60 ± 1.78 and 13.80 ± 1.74 days, respectively. Similarly, Tr. bactrae had a shorter life cycle with a mean of 13.20 ± 1.39 days (Table 1). Understanding this parameter is crucial for understanding population dynamics, predicting the timing of peak parasitism activity in biological control programs, and optimising release strategies to ensure continuous pest control throughout the cropping season.
Lifespan parasitism followed a similar trend as the egg-to-adult period, with Te. remus having the highest number of 73.60 ± 7.23 eggs per female, followed by Tr. chilonis, Tr. pretiosum, and Tr. bactrae with 45.40 ± 2.56, 42.00 ± 3.70, and 37.80 ± 2.44 eggs per female, respectively (Table 1). Higher lifespan parasitism is desirable as it directly influences the ability of egg parasitoids to suppress pest populations.
The sex ratio was female-biased in all cases, with no statistical differences among the parasitoid species (Table 1).
Cage tests for parasitism under field conditions
In cage tests, 75 fall armyworm egg masses were collected and exposed to Tr. chilonis exhibited significantly higher parasitism rates on fall armyworm egg masses (72.88% and 71.36%) when released at the rate of 15 and 10 adults per 250 eggs (5 egg masses) of S. frugiperda per cage (Table 2), which was equivalent to 150,000 and 100,000 parasitoids per hectare, respectively, in a single release. Parasitism rates were comparatively lower at 5 adults per cage (63.12%). Varying the release rate did not significantly affect the percentage of adult parasitoid emergence. The highest density level (150,000/ha) resulted in an adult emergence rate of 94.38%, which was similar to the release density of 100,000 parasitoids/ha (Table 2).
Influence of release rate of Trichogramma chilonis on per cent parasitism and adult emergence of Spodoptera frugiperda under caged condition during 2019–2020
Discussion
Pest management is a critical aspect of agricultural practices worldwide, as it directly impacts crop yield, quality, and overall food security (Bueno et al., Reference Bueno, Colmenarez, Carnevalli and Sutil2023b, Reference Bueno, Panizzi, Hunt, Dourado, Pitta and Gonçalves2021). Among the various methods employed for pest control, biological control strategies have gained considerable attention for their sustainable and environmentally friendly attributes (Hajek and Eilenberg Reference Hajek and Eilenberg2018), especially against FAW (Abbas et al., Reference Abbas, Ullah, Hafeez, Han, Dara, Gul and Zhao2022). In biocontrol strategies, egg parasitoids have emerged as valuable allies in the battle against FAW (Li et al., Reference Li, Bueno, Desneux, Zhang, Wang, Dong and Zang2023). In this context, egg parasitoids, such as Tr. chilonis, Tr. pretiosum, Tr. bactrae, and Te. remus, play a pivotal role in integrated pest management (IPM) programs to control FAW (Kenis et al., Reference Kenis, Benelli, Biondi, Calatayud, Day, Desneux, Harrison, Kriticos, Rwomushana, van den Berg, Verheggen, Zhang, Agboyi, Ahissou, Ba, Bernal, Bueno, Carriere, Carvalho, Chen, Cicero, Plessis, Early, Fallet, Fiaboe, Firake, Goergen, Groot, Gupta, Hu, Huang, Jaber, Malo, Meagher, Mohamed, Sanchez, Nagoshi, Negre, Niassy, Noboru, Nyamukondiwa, Omoto, Palli, Pavela, Ramirez-Romero, Rojas, Subramanian, Tabashnik, Tay, Virla, Wang, Williams, Zang, Zhang and Wu2022). Therefore, understanding the biology and impact of these parasitoids on FAW eggs is essential for designing IPM strategies for releasing those egg parasitoids, which could reduce the reliance on chemical pesticides, promote sustainable farming practices, and safeguard global food production (Bueno et al., Reference Bueno, Sutil, Maciel, Roswadoski, Colmenarez and Colombo2023a). Therefore, the findings reported here will significantly contribute to knowledge about the potential of these parasitoids and their role in sustainable FAW management. The biological parameters of the four studied egg parasitoid species varied significantly for some of those parameters, making them more or less favourable for adoption against this pest.
Overall, the shorter the egg-to-adult period (days), the better. Shorter egg-to-adult periods may enhance the effectiveness of parasitoids in biological control programs by allowing quicker population establishment and pest suppression under field conditions. When the parasitoids are exposed in the field, especially in the larval stage when egg parasitoids are immobile and unable to look for shelter, they are susceptible to biotic and abiotic mortality factors that can impair their efficacy (Bueno et al., Reference Bueno, Sutil, Cingolani and Colmenarez2024). The shorter the egg-to-adult period (days), the higher are the chances to scape from those mortality factors. These factors include predation (Parra Reference Parra2014) or climate change (Grande et al., Reference Grande, Queiroz, Gonçalves, Hayashida, Ventura and Bueno2021), for instance, an abrupt temperature change that can more easily kill the egg parasitoids than their hosts. Parasitoids differ from herbivorous insects by their inability to synthesise lipids as adults, making them more vulnerable to an increase in temperature than most of their hosts (Denis et al., Reference Denis, Pierre, van Baaren and van Alphen2011). Therefore, Tr. bactrae presented the most favourable parasitoid egg-to-adult period (days) of only 7.8 days. These findings bear some resemblance to the study conducted by Ramya (Reference Ramya2019) on Tr. bactrae and pink bollworm eggs, where they reported a parasitoid egg-to-adult period of 8.60 days.
Despite the shortest egg-to-adult development period, Tr. bactrae had the lowest number of parasitised FAW eggs during the wasp’s lifespan (lifespan parasitism), which is a great disadvantage for using this parasitoid for biological control. The choice of the best parasitoid species to be released is impacted by a myriad of different biotic and abiotic factors, including the egg parasitoid species or strains being released in the field (Bueno et al., Reference Bueno, Sutil, Cingolani and Colmenarez2024). For instance, laboratory tests that compared the potential parasitism of Tr. atopovirilia and Tr. pretiosum on S. frugiperda eggs found that Tr. atopovirilia obtained 70.14% parasitism while Tr. pretiosum only 29.23%, highlighting the importance of evaluating which Trichogramma spp. should be used for FAW management (Jaraleño-Teniente et al., Reference Jaraleño-Teniente, Lomeli-Flores, Rodríguez-Leyva, Bujanos-Muñiz and Rodríguez-Rodríguez2020).
Increased adult longevity can positively impact the parasitism of egg parasitoids. Usually, egg parasitoids have a short time frame to find and parasitise their hosts in the agroecosystem (Bueno et al., Reference Bueno, Sutil, Cingolani and Colmenarez2024). Therefore, a longer adult lifespan might increase parasitism in field conditions, especially during host scarcity (Benelli et al., Reference Benelli, Giunti, Tena, Desneux, Caselli and Canale2017). For all parasitoid species studied here, parasitoid feeding was highly influential, increasing both adult male and female longevity.
To meet their energetic and nutritional requirements, adult parasitoids usually consume carbohydrate-rich sources such as (extra-)floral nectar and honeydew (Geerinck et al., Reference Geerinck, Stockmans, Wackers, Cusumano, Jacquemyn and Lievens2025). However, these sugar sources are often scarce or even absent in field most of the staple crops (Segoli and Rosenheim Reference Segoli and Rosenheim2013), which can reduce the parasitism potential of biocontrol agents (Adjaoke et al., Reference Adjaoke, Yotto and Adandonon2023) as observed here for both males and females of the parasitoids of all studied species. Among the parasitoid species, Te. remus exhibited the greatest adult longevity, with males and females living for 7.30 ± 0.62 and 11.80 ± 1.42 days, respectively. Similar results were reported by Bueno et al. (Reference Bueno, Carneiro, Pratissoli, Bueno and Fernandes2008), who registered a mean adult longevity of 13.5 ± 0.01 days. This higher Te. remus adult longevity certainly contributed to the highest lifespan parasitism recorded for this egg parasitoid species.
Trichogramma chilonis adult longevity followed Te. remus closely with males and females living for 5.80 ± 1.07 and 8.20 ± 1.16 days, respectively. Unmole (Reference Unmole2010) reported that adult Tr. chilonis had a mean longevity of 8.5 ± 0.7 days. The same study found that the total life cycle of Tr. chilonis, when parasitising Maruca vitrata eggs, was 13 days, and the lifespan parasitism ranged from 42 to 48 eggs. The differences in parasitoid egg-to-adult period and adult longevity may be attributed to the different host species, because the host-parasitoid relationship is complex and dynamic (Cohen et al., Reference Cohen, Jonsson, Müller, Godfray and Savage2005). Bueno et al. (Reference Bueno, Bueno, Parra, Vieira and Oliveira2010), who also investigated Tr. pretiosum parasitising S. frugiperda eggs under laboratory conditions at 25 °C, reported a Parasitoid egg-to-adult period of 10.0 ± 0.00 days, adult longevity of 9.80 ± 0.24 days, a lifespan parasitism of 14.80 ± 0.72, and a female progeny ratio of 0.70 ± 0.01. Ramya (Reference Ramya2019), investigating Tr. bactrae on pink bollworm eggs, recorded a parasitoid egg-to-adult of 8.60 days and a 2.90-day adult female longevity, resulting in a mean total life cycle of 13.20 days. Similarly, the study by Nadeem and Hamed (Reference Nadeem and Hamed2008) on S. cerealella eggs noted a parasitoid egg-to-adult period of 12.6 days at 25 °C. The host’s biology and nutritional quality often influence these parameters in parasitoids, contributing to the observed differences in various studies (Consoli et al., Reference Consoli, Kitajima and Parra1999; Pak et al., Reference Pak, Van Dalen, Kaashoek and Dijkman1990; Renou et al., Reference Renou, Nagnan, Berthier and Durier1992; Zhang et al., Reference Zhang, Xue, Tariq, Li, Qian, Cui, Tian, Monticelli, Desneux and Zang2024).
In the cage test, Tr. chilonis wasps achieved a significantly higher parasitism rate at the released rate of 100,000 and 150,000 parasitoids per hectare and did not differ. Therefore, releasing 100,000 parasitoids per hectare is more viable due to its lower costs than releasing 150,000. Aggarwal and Jindal (Reference Aggarwal and Jindal2013) reported a significant reduction in dead hearts with a single release of 100,000 Tr. chilonis/ha on 12-day-old crops. Jalali and Singh (Reference Jalali and Singh2003) found 100,000/ha to be the most effective dosage against C. partellus, and Kaur and Brar (Reference Kaur and Brar2008) agreed for stem borer control in basmati rice. However, Kumar et al. (Reference Kumar, Shera, Sharma and Sangha2017) suggested that 125,000/ha led to substantial C. partellus damage reduction in fodder maize, while Rawat et al. (Reference Rawat, Pawar and Joshi1994) reported successful inundative Tr. chilonis releases against C. partellus in maize in Himachal Pradesh. These findings underscore the importance of choosing appropriate Tr. chilonis densities for effective biological control of S. frugiperda and promoting sustainable maize pest management.
Therefore, conducting host-specific investigations is important to gain a more precise understanding of the biology of parasitoids. As Tr. chilonis is considered a potential biocontrol agent for managing S. frugiperda, our findings provide essential baseline data. Future research could build upon these findings, potentially exploring environmental conditions, geographical locations, genetic diversity within the host and parasitoids populations, and other factors. In conclusion, our pioneering study significantly contributes to the understanding of the biology of Tr. chilonis, when parasitising the eggs of S. frugiperda, laying the groundwork for more effective IPM strategies in the context of agricultural ecosystems.
Conclusion
Tr. chilonis stands out as an effective agent in suppressing S. frugiperda, presenting comparatively high reproductive attributes such as lifespan parasitism and adult emergence rate. Meanwhile, our assessment of Tr. chilonis at different release densities (50,000, 100,000, and 150,000 per hectare) in maize ecosystems reveals that the release of 100,000 parasitoids per hectare is the optimal and highly effective dosage for pest control.
Acknowledgements
The authors are grateful to Dr. Chandish Ballal, Ex-Director; Dr. Y. Lalita, Chief Technical Officer; and Dr. Richa Varshney, Scientist, at ICAR-NBAIR, Bengaluru, for their support, guidance, and providing parasitoids culture.
Author contributions
AH, PA, and SH: conceptualisation and manuscript correction; VK: experimentation, original data collection, and draft writing; VK and AH: data curation; SSD, AFB, and DMG: manuscript reviewing and editing. All authors read and approved the final manuscript.
Competing interests
No conflict of interest was reported by the authors.
