Background

Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) is a pest native to tropical regions of the Americas (Kenis et al. 2023) that has recently spread to Africa, Asia, and Oceania. Since 2019, this pest has rapidly expanded over 1,538 counties in 26 Chinese provinces between 2019 and 2020 (Zhang et al. 2021). This resulted in a significant damage to thousands of hectares of maize (Zea mays L.) and other crops, representing a growing threat to agricultural productivity and food security (Wu et al. 2021). Previous reports of maize yield loss from S. frugiperda ranged from 40 to 72%, but losses can be as high as 100%, essentially eliminating a season’s crop (De Groote et al. 2020). The Centre for Agriculture and Bioscience International (CABI) has reported that if appropriate control measures are not taken, maize yield losses from S. frugiperda in 12 African countries could total 8.5 to 21 million tons annually. This could cost around $2.5 billion to $6.3 billion (Day et al. 2017). As a highly polyphagous agricultural pest, S. frugiperda can feed on over 185 different plant species across 42 families, with maize, rice (Oryza sativa L.), and sorghum (Sorghum bicolor (L.) Moench) being particularly preferred hosts (Wu et al. 2021). The reproduction ability and growth rate of S. frugiperda are remarkable—females can produce more than 1000 eggs within a life cycle of approximately 30 days (Firake and Behere 2020). Due to the significant risk this pest poses to crops, it was designated among the top 10 most dangerous plant pests in the world by the CABI in 2017 (https://www.cabi.org/isc/fallarmyworm).

Management of the S. frugiperda primarily relies on the application of chemical pesticides. However, there is a significant concern regarding the harmful impacts of these compounds on human health, non-target species, and the environment, and overuse/overreliance on chemical pesticides has a high probability of resulting in the selection of insecticide-resistant pest populations (Palma-Onetto et al. 2021). Therefore, alternative control strategies are increasingly needed, with natural enemies potentially being one key biocontrol agent since they have been broadly used both for preventive and curative control of a number of insect pests, including targeting S. frugiperda (Kenis et al. 2023). Campoletis chlorideae Uchida (Hymenoptera: Ichneumonidae) is an endoparasitic wasp that parasitizes early-stage larvae of more than 30 lepidopteran species, including Helicoverpa armigera (Hübner), H. assulta (Guenée), Spodoptera litura (Fabricius), and Mythimna separata (Walker) (Liu et al. 2012). C. chlorideae also plays an important role in limiting the population size of lepidopteran pests. Recently, it has been identified as a native parasitoid of FAW larvae in China (Niu et al. 2021) and India (Navik et al. 2021). Previous study has shown that C. chlorideae adults prefer to parasitize 2nd–3rd instars of S. frugiperda (Tian et al. 2021). Laboratory studies have also found that the parasitism ((number of parasitized hosts/total number of test hosts) × 100%) and eclosion rates ((number of adults after eclosion/total number of cocoons) × 100%) of C. chlorideae from S. frugiperda 2nd instar can reach 82 and 85%, respectively (Tian et al. 2021). Thus, C. chlorideae is a promising natural enemy resource that may be developed as a biological control agent for the control of FAW in the field. However, the biological control capacity of C. chlorideae against S. frugiperda using it as a host is still unclear. In addition, the use of parasitoids in the field requires reliable and economically feasible mass production (Zang et al. 2021). To improve practical applications, hosts must be mass-reared, and proper mass production techniques are essential to reduce production costs and improve production efficiency (Wang et al. 2020). S. frugiperda has a high reproductive capacity and a short life cycle, which suggests it may be an ideal host for mass-produced C. chlorideae. However, one potential limitation is that, similar to many other lepidopterans, 4–6th instars S. frugiperda exhibit cannibalism when reared in groups (Hou et al. 2022). To overcome this challenge, an artificial diet limiting cannibalism was recently developed (Chen et al. 2022), enabling the potential of group-reared S. frugiperda in C. chlorideae mass production to be further evaluated.

Here, parasitoid cocoons of C. chlorideae were collected from a S. frugiperda-infested maize field in Guizhou Province, China, and the species was identified by cytochrome c oxidase subunit I (COI) sequencing through sequence alignment and phylogeny evolution analysis. An age-stage, two-sex life table analysis was then performed on individually reared S. frugiperda to estimate the biological control capacity of C. chlorideae. In addition, the potential for mass rearing of C. chlorideae for release was assessed using an age-stage, two-sex life table when S. frugiperda were reared in group conditions. These results showed that C. chlorideae has considerable promise as a larval parasitoid of S. frugiperda, with favorable biocontrol performance and acceptable mass production potential when the FAW larvae are group-reared on an optimized artificial diet.

Methods

Rearing the host insect, S. frugiperda

A laboratory colony of the host insect S. frugiperda was started with larvae collected from maize fields in Qiannan Bouyei and Miao Autonomous Prefecture (105.4074° E, 25.0488° N), Guizhou Province, China, and maintained in a climate-controlled room (25 ± 1 °C, 70 ± 5% RH, and 14 L: 10 D). The larvae were maintained in polystyrene Petri dishes and fed an artificial diet (Pinto et al. 2019). To avoid cannibalism (He et al. 2021), all the 4th instar individuals were reared separately until pupation in 28-compartment boxes (17.5 × 11 × 2.5 cm), hereafter called rearing boxes, which is provided by Shenzhen Daji Plastic Packaging Co., Ltd. (Shenzhen, China). S. frugiperda pupae were collected and placed into transparent plastic boxes covered with white PP (polypropylene) non-woven fabric as an egg-laying substrate for the emerging adults. A 10% honey solution (v/v) was provided as a food source and replaced daily. After egg laying, substrates with egg masses were collected daily.

Collection and rearing of C. chlorideae

To establish C. chlorideae population, intact cocoons were collected from a S. frugiperda-infested maize field in Huaxi District (106.6776° E, 26.4490° N, 1119 m), Guiyang, Guizhou Province, China, and returned to the laboratory. A survey was conducted to investigate the emergence rate and sex ratio of parasitoid cocoons collected from the field. For rearing of C. chlorideae, a pair of newly emerged parasitoids (one female and one male) were placed into transparent plastic boxes (13.5 × 8.2 × 6.8 cm) with gauze (200-mesh). A cotton ball soaked in a 10% honey-water solution (v/v) was attached to the inner wall of the box and refreshed every 24 h. Seventy 2nd instar S. frugiperda with an artificial diet were transferred into the box for parasitization. After a 24-h exposure, the wasps were removed, and the hosts were separated into multiple rearing boxes until new wasps emerged from parasitized FAW larvae.

Morphology and molecular identification of C. chlorideae

For the morphological analysis, twenty 3-day-old C. chlorideae females that had mated but not laid eggs were placed into twenty separate glass tubes containing thirty 2nd instar S. frugiperda and allowed to lay eggs within 30 min. All these S. frugiperda larvae were examined one by one under a stereomicroscope. Briefly, the larvae were placed individually on the stage of the stereomicroscope. The lower light source lamp was turned on, and the body of the larva was gently pressed with a small brush and held in the center of the field of view of the stereomicroscope. The larval body was then gently rolled with a small brush to check for the presence of the light-brown, oval-shaped C. chlorideae eggs about 350–400 μm (Fig. A, B) inside the larval body. If eggs were found inside the larvae, parasitism was confirmed. From there, the successfully parasitized larvae were selected and transferred to rearing boxes reared separately. The parasitoid wasp completed the egg to larval stage in the host larvae and then completely replaces the host larvae, after which it transforms into a pupa and then emerges as an adult. In this experiment, 15–20 parasitized larvae were used to dissect for observing the developmental morphology of C. chlorideae under a stereomicroscope every 24 h as per the protocol of Pourian et al. (2015) after the parasitoid laid its eggs. The morphology of three-day-old adult females and males was also observed under a stereomicroscope, based on the original description (Uchida 1957).

For COI sequence analysis, the genome of adult parasitoids was extracted using a genomic DNA extraction kit according to manufacturer’s instructions (Tiangen, Beijing, China). The genomic DNA was used as a template for polymerase chain reaction (PCR) amplification with the COI primer pair (COI-F: 5′-GGTCAACAAATCATAAAGATATTGG-3′ and COI-R: 5′-TAAACTTCAGGGTGACCAAAAAATCA-3′) (Vrijenhoek 1994) with the following PCR conditions: 94 °C 3 min; 94 °C 30 s, 50 °C 30 s, 72 °C 30 s, 35 cycles; 72 °C 5 min. The PCR products were purified after agarose gel electrophoresis using a gel extraction kit (Tiangen, Beijing, China) before sequencing (Chengdu Sangon Biotech). The sequencing results were analyzed using DNAMAN8.0 and BLAST in National Center for Biotechnology Information (NCBI). A phylogenetic tree based on the partial COI sequences was constructed using MEGA X with Neighbor-Joining method for 1000 bootstrap (Kumar et al. 2018).

Developmental and parasitization parameters of C. chlorideae on S. frugiperda 2nd instar

In order to determine the pre-adult stage parameters of the two-sex life table of C. chlorideae on the individually reared S. frugiperda, 70 larvae (2nd instar) were introduced into a box used for parasitism and provided with an artificial diet. A pair (a mated female and a mated male) of 3-day-old C. chlorideae were released into the box, allowed to oviposit for 24 h, and then removed. Approximately twenty S. frugiperda successfully parasitized by stereoscopic observation without dissection were selected from the box and transferred to individual rearing boxes. Every 24 h, the immatures C. chlorideae were checked, and the developmental time was recorded until all C. chlorideae had either emerged or died. The experiment was repeated with a total of 10 pairs, resulting in a total of 209 parasitized larvae that were used for experimental observation to record developmental stages and survival rates.

The pre-adult parameters of the two-sex life table of C. chlorideae on group-reared S. frugiperda were investigated similarly using a separate cohort of S. frugiperda reared in group conditions, resulting in a total of 204 parasitized larvae that were investigated. Adult stage parameters, including fecundity, longevity, and parasitism rate of C. chlorideae on both individually reared and group-reared 2nd instar S. frugiperda, were investigated using a new cohort of FAW larvae. Specifically, a pair of newly emerged (< 6 h-old) female and male wasps was first allowed to mate in a glass tube (12 × 3 cm) and they placed into transparent plastic boxes (13.5 × 8.2 × 6.8 cm) with 70 2nd instar S. frugiperda available for subsequent parasitization. FAW larvae were removed from the tube every 24 h and replenished with 70 larvae until all C. chlorideae were dead. During parasitization, if the male wasp died prior to the female, it was replaced with another male. To measure lifetime fecundity, the number of C. chlorideae eggs on each S. frugiperda larvae was counted and recorded (all the eggs were counted when one larva had several eggs) before each transfer. For each replicate (a pair of C. chlorideae), all the parasitized were reared individually as previously described until the larvae died or continued to survive without parasitoid emergence or died with parasitoid emergence. A total of 17 replicates were performed to investigate these adult stage parameters of C. chlorideae on individually reared FAW larvae.

The fecundity, longevity, and parasitism rate of adult C. chlorideae on group-reared 2nd instar S. frugiperda were analyzed similarly, except that the FAW was group-reared after parasitism until the emergence of wasps. The lifetime fecundity data in group-rearing conditions were investigated with the number of parasitoid's pupae rather than the number of C. chlorideae eggs and this treatment was performed with 27 replicates.

Data analyses

A bootstrap-match technique developed by Amir-Maafi et al. (2022) for life tables was used to match the data of the pre-adult stage and the adult stage under both individual- and group-rearing conditions, respectively. Life table data (i.e., the developmental duration, survival rate, longevity, and total fecundity) of C. chlorideae parasitizing the 2nd instar of S. frugiperda were analyzed using the age-stage, two-sex life table method (Chi and Liu 1985; Chi and Su 2006; Tuan et al. 2014; Chi et al. 2020) according to the TWOSEX-MSChart (Chi 2022a). The age-stage-specific parasitism rate (cxj, where x is age and j is stage) was calculated based on the method of Chi and Yang (2003) by using the CONSUME-MSChart computer program (Chi 2022c). The population growth and parasitic capacity of C. chlorideae were projected using Timing-MSChart (Chi 2022b). All these formulas and definitions used in this study are shown in Table S1. Unless stated otherwise, some of these formulas and definitions are derived from other works (Goodman 1982; Chi 1990; Farhadi et al. 2011; Huang and Chi 2013; Yu et al. 2013; Mou et al. 2015). The variances and standard errors (SE) of these population parameters were estimated by the bootstrap resampling method with 100,000 re-samplings (B = 100,000) (Huang and Chi 2011; Li et al. 2022). The 0.025th and 0.975th percentiles of the finite rate of increase (λ) from the sorted 100,000 bootstrap samples were used to represent the variability of population growth and parasitic potential, according to Huang et al. (2018). All figures were created using Sigma Plot 14.0.

Results

Campoletis chlorideae identification, morphology, and field investigations

The ontogenetic development of C. chlorideae can be divided into four stages: the egg (embryonic period, Fig. 1A, B), larval (postembryonic development, Fig. 1C–E), pupal (Fig. 1F–H), and adult (Fig. 1O, P). Overall egg dimensions were found to be ~ 50–100 µm × 300–400 µm within the host larvae, and this stage lasted for ~ 2 days before further development was seen. The egg/embryo development proceeded with the emergence (still within the host larvae) of a larva with a light-brown head capsule, a smooth and transparent body about 600–900 µm, an elongated tail, and a pale green gut, occurring over the next 2–3 days (Fig. 1C, D). At the late larval stage (~ 3 days), the wasp morphology changed from caudate type to hymenopteriform (wasp-like) shape, and the intestine changed from pale yellow to pale green (Fig. 1E, G). At ~ 7 days postparasitization the mature larvae emerged from the host and appeared pale pink throughout the sexually dimorphic body. Mature female larvae (Fig. 1H) had three pairs of ova-like depressions on the surface of the ventral segments 10, 11, and 12, whereas mature male larvae had an unpaired ova-like depression near the posterior edge of segment 12. The entire larval stage lasted 6–8 days, with the body length reaching around 1–4 mm.

Fig. 1
figure 1

Morphological changes of Campoletis chlorideae. A, B 1–2-day-old eggs of C. chlorideae; C–G 3–7-day-old larvae of C. chlorideae; H the mature larvae-female emerged from the host body. I cocoon of C. chlorideae; J prepupae stage of C. chlorideae; K–N pupae stages of C. chlorideae; O adult female C. chlorideae; P adult male C. chlorideae

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The cocoon was weaved by the larvae after emergence and appeared white to off-white with small black dots in the cocoon silk, with overall measurements of ~ Ø 2 mm × 6 mm (Fig. 1I). The dissected pupae were long, ovate, and milky white, with colorless compound eyes that gradually turned red and then black, with the color of the emerging insect body also changing from light to dark over time (Fig. 1J–N). Three days into the pupal stage, the ovipositor of female pupa developed at the end of the abdomen (Fig. 1K). The pre-pupal and pupal stages lasted 5–7 d, reaching a body length of about 4–6 mm. Male C. chlorideae typically underwent eclosion earlier than females. The body length of male and female adults was around 5–6 mm, both with a pair of long antennae and a yellow, narrow abdomen. Adult males were usually smaller than females, and females displayed a long, pronounced ovipositor (Fig. 1O).

BLAST analysis of the DNA barcode segment of the COI gene using the NCBI database showed 100% query coverage and the sequence showed 99.85% identity to C. chlorideae voucher sample SCAU 3048973 (GenBank accession number: MW250777). The phylogenetic tree revealed that the COI sequence clustered with the previously identified COI sequences of C. chlorideae (Fig. 2). C. chlorideae cocoons were collected from a S. frugiperda-infested maize field in Huaxi District, a total of 20 cocoons. The emergence rate of C. chlorideae from cocoons was 85%, with ~ 30% females.

Fig. 2
figure 2

Phylogenetic analysis of the identified partial COI of Campoletis chlorideae and 10 homologies from other species, including nine homologies of C. chlorideae and one homology from Diadegma semiclausum

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Longevity and lifetime fecundity of C. chlorideae parasitizing S. frugiperda larvae

The developmental periods for each stage of C. chlorideae from individually reared and group-reared S. frugiperda larvae are summarized in Table 1. For individually reared insects, the total pre-adult developmental time was 17.11 ± 0.16 days with an egg–larva and pupa duration of 9.98 ± 0.12 and 7.28 ± 0.08 days, respectively. The proportion of males (Nf/N = 0.2057 ± 0.03 and Nm/N = 0.3733 ± 0.03) was significantly higher than the proportion of females (P < 0.05). Mean female adult longevity (11.02 ± 0.49 days) was slightly longer than males (9.04 ± 0.47 days), resulting in a slightly longer overall life span of females (28.3 ± 0.52 days) compared with males (26.05 ± 0.53 days). Females started to lay eggs as soon as they emerged, showing no obvious adult pre-ovipositional period (APOP). Therefore, the total pre-ovipositional period (TPOP) of 17.28 ± 0.26 days was similar to the total pre-adult developmental time. The duration of the ovipositional period was 9.95 ± 0.38 days, with a mean daily ovipositional rate of 30.3 eggs/female and a total fecundity (F) of 301.5 ± 16.4 eggs/female. For group-reared S. frugiperda, the developmental times for egg–larva, pupa, and pre-adult were 8.82 ± 0.07, 7.02 ± 0.14, and 15.72 ± 0.15 days, respectively. The proportion of males in the group-reared cohort was Nf/N = 0.0687 ± 0.02 and Nm/N = 0.2452 ± 0.03. The average adult longevity of C. chlorideae females was 8.79 ± 0.59 days, which was slightly shorter than males (9.54 ± 0.42 days), but the average overall life span between females and males was similar: 25.21 ± 0.79 and 25.06 ± 0.46 days, respectively. There was no obvious APOP for the C. chlorideae female adults (0.07 ± 0.07 days), and they demonstrated similar TPOP (16.5 ± 0.39 days) and total pre-adult development times (15.72 ± 0.15 days). The ovipositional period of C. chlorideae females was 7.21 ± 0.60 days, and the total fecundity (F) was 87.71 ± 6.32 pupae, with a mean daily ovipositional rate of 12.17 pupae/female.

Table 1 Means ± SE of developmental times, adult longevity, total longevity, adult pre-ovipositional period (APOP), total pre-ovipositional period (TPOP), ovipositional days, and total fecundity of Campoletis chlorideae parasitizing the 2nd instar of Spodoptera frugiperda (2nd Sf) under individual- and group-rearing conditions
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Age-stage and age-specific survival rate and fecundity of C. chlorideae parasitizing S. frugiperda larvae

The age-stage-specific survival rate (sxj) is the probability of a newborn offspring surviving to age x and stage j. Attributable to the developmental time variables among individuals, there were overlaps in the sxj among stages. For individually reared wasps, the sxj of C. chlorideae in the egg-larva stage was high (> 80%) in the first 8 days but decreased sharply as they pupated at 8–10 d (Fig. 3A). The highest number of wasps in the pupal stage (71%) was seen on day 12th. Within ~ 20 days after egg depositing, approximately 33% of the emerging population were males (19 d) and 20% females (20 d), with ~ 47% mortality (i.e., those that did not live to the adult stage) seen. Starting at ~ 24 d, gradual mortality of adult males and females was seen until the population expired at ~ 34–36 d.

Fig. 3
figure 3

Age-stage survival rate and age-specific survival rate of Campoletis chlorideae parasitizing the individually reared and group-reared Spodoptera frugiperda. Age-stage survival rate (sxj) of C. chlorideae parasitizing the 2nd instar of S. frugiperda in individually rearing (A) and group-rearing conditions (B). Age-specific survival rate (lx), female age-specific fecundity (fx3), population age-specific fecundity (mx), and age-specific net maternity (lxmx) of C. chlorideae parasitizing the 2nd instar of S. frugiperda in individually rearing (C) and group-rearing conditions (D)

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For group-reared wasps, egg-larva dynamics were similar to those of the individually reared ones; however, the pupae appeared to emerge faster, with the sxj of an adult male population of ~ 20–25% (16–20 d) and an adult female population of only 7–8%, indicating an overall survival to adult stage of only 28–33%, with 67–72% mortality (Fig. 3B).

The age-specific survival rate (lx) represents the survival rate from egg to age x, which ignores stage differentiation and is the sum of sxj of all stages at age x. The value of lx of C. chlorideae decreased steadily for individually reared wasps (Fig. 3C). The female age-specific fecundity (fx3) began at day 14, peaked at 19 days (42.41 offspring/female) gradually declined over the next 16 days. The population age-specific fecundity (mx) and age-specific net maternity (lxmx) reached their highest values on the 5th day after emergence (i.e., 19 d), and these peaks were 14.14 offspring/female and 7.91 offspring/female, as shown in (Fig. 3C).

In the group-reared condition, the value of lx decreased fastest at 9–15 days (Fig. 3D). The value of fx3 nearly reached its maximum value initially (19.33 offspring/female, 16 d), but it then decreased rapidly with minor fluctuations. The values of mx and lxmx displayed a similar trend as in the individual rearing condition, in which they reached their highest values at 19 days (3.03 offspring/female and 0.95 offspring/female, respectively).

Life expectancy and reproductive value of C. chlorideae parasitizing S. frugiperda larvae

The life expectancy value (exj) of a newly laid egg of C. chlorideae (age of 0 day) under individual rearing conditions was 20.36 days (Fig. 4A). The highest exj for the C. chlorideae pupae was 14.61 days at 8 days postparasitism, while the peak life expectancy of adult females was 13.29 days at 15 days postparasitism. This exj in female adults was longer than in male adults, which peaked at 11.64 days at 14 days postparasitism. In contrast, the exj was 14.61 days for newly laid eggs under the group-rearing condition, after which the value of the exj showed a downward trend (Fig. 4B). The highest exj for the pupae, adult females, and adult males was 10.11, 10.21, and 11.06 days at the ages of 15-, 15-, and 14-days postparasitism, respectively.

Fig. 4
figure 4

Life expectancy and reproductive values of Campoletis chlorideae parasitizing the individually reared and group-reared Spodoptera frugiperda. Life expectancy (exj) of C. chlorideae parasitizing the 2nd instar of S. frugiperda in individually rearing (A) and group-rearing (B). Reproductive values (vxj) of C. chlorideae parasitizing the 2nd instar of S. frugiperda in individual- (C) and group-rearing (D) conditions

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The female-specific reproductive value of (vxj) C. chlorideae was 169.07 d−1 at 16 days postparasitism when the female adults emerged immediately in the individually reared condition (Fig. 4C). The reproductive value at the age of zero (v0,1) was equal to the finite rate of increase (λ), which was 1.2148. On the other hand, the peak age-stage reproductive value (vxj) of C. chlorideae occurred at 15 days (87.28 d−1) after parasitization in the group-reared larvae and then gradually decreased (Fig. 4D). The reproductive value at the age of zero (v0,1) or finite rate of increase (λ) for this group-reared condition was 1.0961.

Population parameters of C. chlorideae parasitizing S. frugiperda larvae

The population demographic parameters of C. chlorideae parasitizing S. frugiperda 2nd instar under individual- and group-rearing conditions were examined (Table 2). Under individual rearing conditions, the R0 and T (the time required to achieve R0 when the population reached a steady growth rate (λ = 1.2148 ± 0.0092 and r = 0.1946 ± 0.0076) of the C. chlorideae population in S. frugiperda larvae were 62.03 ± 9.07 females and 21.21 ± 0.27 d. Under group-rearing conditions, the intrinsic rate of increase (r = 0.0918 ± 0.0148 d−1) and finite rate of increase of C. chlorideae populations (λ = 1.0961 ± 0.0162 d−1) were > 0 and > 1, respectively. The net reproduction rate (R0) was only 6.02 ± 1.61 females. The mean generation time (T) of C. chlorideae populations derived from group-reared hosts was 19.56 ± 0.50 days, whereas it was 21.21 ± 0.27 days in the individually reared hosts.

Table 2 Population parameters (means ± SE) of Campoletis chlorideae parasitizing the 2nd instar of Spodoptera frugiperda under both individual- and group-rearing conditions
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Parasitism rates, population, and parasitism projection of C. chlorideae parasitizing individually reared S. frugiperda larvae

Since only adult C. chlorideae females parasitize and kill S. frugiperda larvae, both the age-specific parasitism rate (kx) and age-specific net parasitism rate (qx) (representing the mean number of S. frugiperda 2nd instar parasitized by C. chlorideae at age x and kx taking the survival rate into consideration, respectively) were 0 before the adult stage (Fig. 5A). These values then peaked at day 19, indicating the parasitizing of 11.99 and 6.71 larvae/parasitoid (at 19 d), respectively, before decreasing over the next 16 d to zero (~ death of the wasp) at 36 d.

Fig. 5
figure 5

Population parameters of Campoletis chlorideae parasitizing individually reared Spodoptera frugiperda larvae. A Age-specific survival rate (lx), age-specific parasitism rate (kx), and age-specific net parasitism rate (qx) of C. chlorideae parasitizing the 2nd instar of S. frugiperda in individual rearing conditions. B Age-specific cumulative parasitism rates (Cx) and cumulative net reproductive rates (Rx) of C. chlorideae parasitizing the 2nd instar of S. frugiperda in individual rearing conditions. Simulated population growth of C. chlorideae parasitizing the 2nd instar of individually reared S. frugiperda over a period of 60 d. C Uncertainty of population projection based on the original, 0.025th percentile, and 0.975th percentile of life tables, starting with an initial population of 10 eggs. D Parasitic potential and uncertainty of population projection based on the original, 0.025th percentile, and 0.975th percentile of the life table, starting with an initial population of 10 eggs

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The total parasitism rate (i.e., number of host larvae parasitized, except the number of immunized larvae) per adult C. chlorideae female was 257.12 ± 12.72, indicating a net parasitism rate (C0) for C. chlorideae of ~ 53 S. frugiperda larvae during its lifespan (Table 3). Due to the low survival and parasitism rates after age 26 d, the age-specific cumulative net reproductive rates (Rx) and cumulative parasitism rates (Cx) increased only slightly during the remaining 10 days (Fig. 5B). The finite parasitism rate (ω) and stable parasitism rate (ψ) of C. chlorideae were 0.2422 ± 0.0282 d−1 and 0.1993 ± 0.0217 d−1 under individual rearing conditions, respectively (Table 3). The predicted total population size of C. chlorideae (N(t)) and parasitism potential (P(t)), beginning with an initial population of 10 eggs assuming an unlimited population of hosts available, including projections using life tables from the 0.025th and 0.975th percentiles of the net reproductive rate, over a time course of 60 days, was modeled (Fig. 5C, D). The projected N(t) and P(t) over the same parameters indicated no increase in initial population size and parasitism rates (0–14 d), but periodical bursts of growth in C. chlorideae population sizes and parasitic consumptions of S. frugiperda by C. chlorideae over time (Fig. 5C, D).

Table 3 Parasitism rate parameters (means ± SE) of Campoletis chlorideae parasitizing the 2nd instar of Spodoptera frugiperda under individual rearing conditions
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Discussion

The aim of the present study was to determine various parameters of the development and parasitization of the parasitoid wasp, C. chlorideae, important for determining the potential for its use for pest biocontrol. The parameters included the growth (lengths of time), development, reproduction, and survival of the different stages of C. chlorideae (egg, larva, pupa, and adult), including estimation of parasitism rates and population dynamics of C. chlorideae. In addition, these parameters were examined as functions of how the host (S. frugiperda) was reared, i.e., either individually or group-reared. In related studies, the potential for group rearing is important because this would facilitate mass production of the parasitoid (Wei et al. 2022); however, individual rearing likely better mimics field conditions, where only one larva usually appears in the leaf whorl of maize (Xie et al. 2021). Our data show that C. chlorideae targeting individually reared S. frugiperda results in a high fecundity (301.5 ± 16.4 eggs/female), a long longevity (~ 28 days), and a high finite rate of increase (λ = 1.2148), however these parameters of C. chlorideae on group-reared S. frugiperda (87.71 ± 6.32 pupae/female, ~ 25 days and λ = 1.0961 ± 0.0162) indicated a potential bottleneck for mass production.

A previous study found that temperatures < 12 and > 35 °C were detrimental to the survival and development of the larvae of C. chlorideae, indicating that temperature fluctuations had a significant effect on the development of parasitic insects (Dhillon and Sharma 2008). The present study was conducted at 25 °C under controlled temperature conditions, and life cycle parameters of C. chlorideae derived from individually reared S. frugiperda showed that the egg-larval and pupal durations were 9.98 ± 0.12 and 7.28 ± 0.08 days, respectively, slightly longer than C. chlorideae growth in Heliothis armigera at 27 °C (8.1 to 9.1 days and 6.5 days) (Dhillon and Sharma 2008). The likely changes in temperature would have a great bearing on the effectiveness of natural enemies for pest management, which in turn would affect insect host-natural enemy associations, crop production, and food security. At the same temperature (25 °C), the life cycle of C. chlorideae growth in larvae of S. frugiperda was about a half shorter than that of S. frugiperda (Xie et al. 2021), indicating that the endoparasitoid C. chlorideae had a competitive advantage in controlling S. frugiperda in the field. The combination of the morphological and developmental parameters established in this work can be used to determine optimal release conditions for C. chlorideae that can be coupled to the emergence/detection of S. frugiperda in the field.

The present data indicate that the total number of C. chlorideae offspring (yield) from individually reared S. frugiperda reached 301.5 eggs/female with an emergence rate of ~ 58%, a cumulative parasitism number of 257.12 ± 12.72 host larvae/female C. chlorideae, and a finite rate of increase (λ) (day−1) of 1.2148 ± 0.0092. The relatively small parasitism number compared with the offspring numbers could be attributed to the fact that C. chlorideae have laid more than one egg in some larvae and because some parasitized larvae can overcome the parasitoids (Poelman et al. 2022). The predicted population size from 10 eggs after 60 days (assuming unlimited hosts) was determined to be 386,764 (rounding causes the sum to differ by “1”), composed of 365,970 egg-larvae, 11,415 pupae, and 3,462 female and 5,916 male adults. Eclosion levels (58%) for C. chlorideae produced from individually reared S. frugiperda were lower than those reported for C. chlorideae in maize-reared S. frugiperda (~ 80%) (Tian et al. 2021). This could be a result of the components in the artificial diet, which may have affected the development of C. chloridea in the S. frugiperda larvae since food source and/or addition of supplements in artificial diets have been shown to affect herbivore performance and defense of host insects (Johnson 2008), impacting the emergence rate of parasitoids grown on various foods (Sarfraz et al. 2009). Similar studies have reported that maize genotypes can also influence the parasitism rate of the related C. sonorensis parasitoid wasp on maize-reared S. frugiperda (Barreto-Barriga et al. 2021). Although there is a room for improvement, a high fecundity (F = 301.5 ± 16.4 eggs/female) and a 58% eclosion rate should be sufficient to produce adequate populations of C. chlorideae for field application purposes over a given period of time under similar conditions.

For S. frugiperda hosts derived from group-rearing conditions, the C. chlorideae eclosion rate and sex ratios (female: male ratio of 1: 3.6) were low, with each female parasitoid producing about 87.71 pupae/female (~ 64% less). The proportion of N-type parasitic wasps (parasitoids dead before pupae stage) was as high as 69%. These data suggest that despite being fed the “low cannibalism rate” artificial diet, many parasitized larvae were still consumed by others. These consumed larvae were likely parasitized larvae, as they grow at a slower rate than non-parasitized larvae. Nutrition and larval density are not hypothesized to be the primary causes of such cannibalism, as more than enough food was provided. Additionally, high cannibalism rates have also been reported in larvae kept at very low density (2 per box) (He et al. 2021). Thus, the potential mechanisms underlying this cannibalism as well as optimal artificial diets to reduce it warrant further investigation. Despite this, the population parameter (λ = 1.0961 ± 0.0162) indicates this population size can still steadily increase. Overall, however, our data indicate that, under the current state of knowledge, the use of individual rearing techniques is likely a better option than group rearing for effective production of C. chlorideae for application purposes. This may not be significantly more costly, as large individual compartment trays can readily be obtained and potentially filled using semi-automated approaches. Parasitization of these cohorts is similar in both circumstances (but much more effective in terms of yield) for individually reared hosts.

Conclusions

Biological control agents are sustainable pest control methods beneficial for biodiversity. This work evaluates the mass production potential and the efficacy of the ichneumonid parasitoid C. chlorideae in parasitizing FAW that are reared in both individual and group settings. It highlights the promising biological control potential of C. chlorideae against FAW. The endemic status of C. chlorideae to China supports its mass production and release as a sustainable and eco-friendly approach for managing the FAW. The age-stage, two-sex life table can provide a description and prediction of parameters such as developmental time, survival, parasitism, and reproductive potential of the parasitoid under field conditions similar to those used in the laboratory. Future research should focus on developing methods to increase the production of C. chlorideae, such as using different insect hosts and artificial diets of parasitized S. frugiperda that are optimized for low cannibalism under group-rearing conditions or semi-automated approaches to reduce the cost under individually rearing conditions.