Parasitism success of Microplitis manilae (Hymenoptera: Braconidae) on different diet‐fed Spodoptera frugiperda (Lepidoptera: Noctuidae) larvae

Efficient rearing is necessary for the successful biological control of pests. The braconid wasp, Microplitis manilae, is a natural solitary parasitoid of larval Spodoptera frugiperda. However, the effect of the host diet on the parasitism success of M. manilae on S. frugiperda larvae remains unclear. Here, we determined the parasitism rate, oviposition, eclosion rate, developmental period and offspring sex ratio of M. manilae parasitizing S. frugiperda fed with fresh corn leaves or an artificial diet. M. manilae parasitizing the corn leaf‐fed S. frugiperda larvae exhibited a significantly higher parasitism rate (40%), oviposition (62.50 eggs/female) and eclosion rate (81.38%) than the wasp parasitizing the artificial diet‐fed S. frugiperda larvae (22%, 38.33 eggs/female, and 58.73%, respectively). Furthermore, M. manilae parasitizing the corn leaf‐fed S. frugiperda larvae exhibited a significantly shorter developmental period (16.35 days) and lower sex ratio (1.17) than the wasp parasitizing the artificial diet‐fed S. frugiperda larvae (18.00 and 2.15 days, respectively). Overall, M. manilae exhibited better parasitic success on corn leaf‐fed S. frugiperda larvae than on artificial diet‐fed S. frugiperda larvae. Therefore, using fresh corn leaf‐fed S. frugiperda is the most suitable strategy for mass‐rearing M. manilae for pest biocontrol. Our findings suggest M. manilae as an effective biocontrol candidate for controlling S. frugiperda in corn fields.

Therefore, developing eco-friendly, non-polluting and sustainable strategies is necessary for controlling S. frugiperda population.One potential solution involves biological control, which is the use of natural enemies to control pest species.Biological control strategies are seen as promising long-term pest management solutions due to their low cost, long-lasting effects and lack of harmful residues (Wang et al., 2019).These advantages result in improved agricultural product quality and safety.
Recent studies on M. manilae have primarily focused on its biological characteristics.However, whether the host diet affects the parasitism success of M. manilae on S. frugiperda and the mechanism involved remain unknown.Therefore, in this study, we determined whether diet influences the suitability of larval S. frugiperda as a host for M. manilae.We fed S. frugiperda larvae with fresh corn leaves or an artificial corn/soybean meal diet and evaluated the parasitism rate, eclosion rate, developmental period and oviposition of M. manilae in the laboratory.This study outlines an efficient method for using S. frugiperda as a host for the mass rearing of M. manilae as a biocontrol agent in corn fields.

| Diets
A laboratory colony of caterpillar larvae was established and fed an artificial diet comprising corn meal, soybean meal and agar (patent no.201921652702.2;Xing et al., 2022).Before feeding, the diet was autoclaved, steamed for 30 min and allowed to cool.
Corn was cultivated in a circular plastic pot (12 cm diameter, 11.3 cm height) in an artificial climate chamber at a temperature of 25 ± 1°C, relative humidity (RH) of 50 ± 5% and photoperiod of 14:10 h (L:D).No chemical pesticides were applied during corn cultivation, and the seedlings were grown to approximately 20 cm for the experiment.

| Insects
Caterpillar specimens were collected from a corn field in Ge-Chon Village, Xingren Town, Kaili City, Guizhou Province, China (26°3′3.6048″N, 108°5′51.7776″E).The insects were reared in circular plastic boxes (17.8 cm diameter, 13 cm height) in an artificial climate chamber at a temperature of 25 ± 1°C, RH of 50 ± 5% and photoperiod of 14:10 h (L:D).Multiple generations of S. frugiperda were bred and fed either fresh corn leaves or an artificial diet until the larvae reached the second-instar stage.
Larval parasitoid specimens were collected in August 2021 from an asparagus field in Xin-Meng Village, Mochong Town, Duyun City, Guizhou Province, China (25°55′53″ N, 107°55′14″ E).Two distinct wasp populations were reared for more than three generations to obtain laboratory-reared caterpillars that were fed either corn leaves or an artificial diet.The wasps were maintained in an artificial climate chamber at a temperature of 25 ± 1°C, RH of 50 ± 5% and photoperiod of 14:10 h (L:D).The parasitoid larvae were fed 10% honey water after fledging.In all experiments, the parasitoids were mated in a round plastic box (17.8 cm diameter, 13 cm height) with 10% honey water for 24 h.

| Determination of parasitism success
In each experiment, one mated pair of parasitoid larvae was added Parasitism rate ( % ) = (number of cocoons ∕ total number of larvae) × 100.
Offspring sex ratio = number of emerged females ∕ number of emerged males.

| Statistical analyses
After checking the assumptions of normal distribution and homogeneity of variances, the data were analysed using SPSS version 27.0 (IBM Corp., Armonk, NY, USA) and visualized using Origin 2021 software (OriginLab Corporation, Northampton, MA, USA).
The numbers of males and females in different caterpillar populations were determined using the Shapiro-Wilk test (Figures 4 and   5).Statistically significant differences in the parasitism rates, eclosion rates and developmental periods were determined using t-tests (p < 0.05).The effects of different diets on the daily parasitism rate and oviposition were analysed using a one-way analysis of variance with Tukey's honest significant difference test (p < 0.05).

| Parasitization and oviposition of M. manilae on S. frugiperda larvae fed with an artificial diet or corn leaves
M. manilae larvae exhibited a significantly (p < 0.05) higher parasitism rate and oviposition on caterpillar larvae fed with corn leaves (40% and 62.50 eggs/female, respectively) than on those fed an artificial diet (22% and 38.33 eggs/female, respectively; Figures 1   and 2).In addition, the daily parasitism rate and oviposition differed significantly from 1 to 3 days but not thereafter.

| Eclosion rate and sex ratio of M. manilae on S. frugiperda larvae fed an artificial diet or corn leaves
M. manilae larvae exhibited a significantly (p < 0.001) higher eclosion rate on caterpillar larvae fed with corn leaves (81.38%) than on those fed an artificial diet (58.73%; Figure 3).In addition, significant differences in the numbers of male and female offspring were observed from 1 to 3 days but not after that (Figures 4 and 5).

| Developmental period of M. manilae on S. frugiperda larvae fed with an artificial diet or corn leaves
M. manilae larvae exhibited a significantly (p < 0.001) longer egglarval developmental period on caterpillar larvae fed an artificial diet (11.26 days) than on those fed corn leaves (9.65 days; Figure 6).No significant differences were observed in the pupal developmental period of the parasitoid larvae parasitizing caterpillar larvae fed an artificial diet (6.87 days) or corn leaves (6.83 days).Furthermore, parasitoid larvae parasitizing the artificial diet-fed caterpillar larvae exhibited a significantly (p < 0.001) longer immature developmental period (18.00 days) than those parasitizing the corn leaf-fed caterpillar larvae (16.35 days).

| DISCUSS ION
M. manilae is an important larval parasitoid of Noctuidae pests, such as S. exigua and S. litura (Carvalho et al., 2013;Wang et al., 2019;Xing et al., 2022;Yan et al., 2023).Host plants significantly affect the fitness and parasitism success of the parasitoid by influencing the growth and development of herbivorous hosts (Turlings & Benrey, 1998).Additionally, the host diet may influence the life history and success of parasitic wasps (Xing et al., 2022).In this study, we found that the host diet affected the parasitization rate, eclosion rate, developmental period and oviposition of parasitoid wasps.
Parasitoid larvae emerging from artificial diet-fed caterpillar larvae exhibited longer developmental periods and lower parasitization rate, eclosion rate and oviposition than those emerging from corn leaf-fed fed larvae (Figures 1-3 and 6).These results suggest that fresh corn leaves are more suitable than artificial diets for S. frugiperda in the mass rearing of M. manilae.
Parasitism success of parasitoid wasps is affected by the host diet (Ju et al., 2021;Reis et al., 2003;Wu et al., 2016).Our results are similar to those of Ji ( 2012), who compared soybean-, taro-, cabbage-and brassica-based diets and reported that M. pallidipes that developed in S. exigua larvae fed soybeans have the highest parasitization rate (82.39%), whereas parasitoids that developed inside hosts those fed with brassica leaves showed low parasitization rates (66.39%).The consumption of certain phytochemicals by the host may be responsible for this difference.For example, volatile phytochemicals may help parasitoid wasps locate their hosts (Ji, 2012;  Ju et al., 2021).In a comparison of the four host plants of Plutella xylostella, the highest Diadegma insulare parasitism rates were observed in hosts feeding on dry goldenrod, whereas the lowest was observed in hosts feeding on Descurainia sophia (Sarfraz et al., 2009).
Similarly, the parasitism and eclosion rates of M. manilae were significantly higher in S. frugiperda larvae fed fresh corn leaves than in those fed an artificial diet in this study.In addition, the developmental period of M. manilae larvae was significantly longer on artificial diet-fed caterpillar larvae than on corn leaf-fed caterpillar larvae.These differences may be attributed to the nutritional differences between the two diets, the presence of plant secondary metabolites or a combination of these factors (Turlings & Benrey, 1998).
We also observed that parasitoid larvae had higher fecundity when they developed within the host larvae that fed on corn leaves.
As herbivory induces a chemical defence response in plants, the resulting volatile compounds may attract parasitic wasps (Zhang et al., 2010).For example, Zhang et al. (2022) reported that the functional plant volatile compound, β-ionone, enhances the parasitism success of M. pallidipes on host insects.This is consistent with the results of Wu et al. (2016), who studied the effects of host diet on parasitism success and reported that female M. manilae parasitizing kale-fed S. litura had significantly higher fecundity (278 eggs/female) than those whose host larvae fed on other plants or an artificial diet.The sex ratio of parasitic wasps is significantly affected by plant quality, and it affects females more than males (Harvey, 2005).In this study, parasitoid larvae exhibited a significantly higher sex ratio (2.15) on artificial diet-fed hosts than in corn leaf-fed hosts (1.17).
This may be because male larval wasps are more tolerant to plant secondary compounds or metabolize these compounds more effectively than females.However, whether plant secondary compounds affect male and female wasps and their action mechanisms remain unknown.Toxicity studies have revealed that male wasps are more vulnerable to certain pesticides than female wasps (Rathman et al., 1992).
In conclusion, we found that the host diet significantly affected the parasitoid's success.Notably, parasitoid larvae parasitizing the corn leaf-fed caterpillar larvae exhibited a significantly higher parasitization rate and oviposition, shorter developmental period and lower sex ratio than those parasitizing the artificial diet-fed caterpillar larvae.Our results suggest that a corn leaf diet is suitable for S. frugiperda in the context of mass rearing of M. manilae.However, an artificial diet should not be disregarded as it is more cost-effective, particularly in terms of labour requirements and quality control, than fresh corn leaves.Our results also suggest that M. manilae parasitoids are an effective biocontrol agent for S. frugiperda caterpillars in corn fields.
to a box 1 day after eclosion.Each treatment included 15 mated female parasitoids, and each female was provided 30 secondinstar caterpillar larvae for oviposition.The insects were placed in a round plastic box (17.8 cm in diameter and 13 cm in height) with different food treatments (corn leaves or artificial diet) and cotton balls containing 10% honey water to ensure adequate nutrition and hydration.The parasitoid wasps were removed daily and reintroduced into new boxes containing host larvae of the same age until the females died.Each day, caterpillar growth and development were recorded, the diet was replenished according to different host populations, and faeces were removed until the parasitoid larvae emerged from the caterpillars and spun cocoons.The cocoons were collected in test tubes, and sex, oviposition, development, parasitism rate, eclosion time, eclosion rate and adult longevity were recorded.Parasitism rate, eclosion rate and offspring sex ratio were calculated as follows: | 201 TIAN et al.

F
I G U R E 1 Daily parasitization of M. manilae on S. frugiperda larvae fed with an artificial diet or corn leaves.Bars (Mean ± standard error of mean [SEM]).Different letters indicate significant differences at p < 0.05.[Colour figure can be viewed at wileyonlinelibrary.com]

F
I G U R E 2 Daily oviposition of M. manilae on S. frugiperda larvae fed with an artificial diet or corn leaves.Bars (Mean ± SEM).Different letters indicate significant differences at p < 0.05.[Colour figure can be viewed at wileyonlinelibrary.com]Eclosion rate of M. manilae on S. frugiperda larvae fed with an artificial diet or corn leaves.Bars (Mean ± SEM).Different letters indicate significant differences at p < 0.001.[Colour figure can be viewed at wileyonlinelibrary.com]Daily mean number of female offspring of M. manilae parasitizing S. frugiperda larvae fed with an artificial diet or corn leaves.Bars (Mean ± SEM).Different letters indicate significant differences at p < 0.05 (Shapiro-Wilk test).[Colour figure can be viewed at wileyonlinelibrary.com]Daily mean number of male offspring of M. manilae parasitizing S. frugiperda larvae fed with an artificial diet or corn leaves.Bars (Mean ± SEM).Different letters indicate significant differences at p < 0.05 (Shapiro-Wilk test).[Colour figure can be viewed at wileyonlinelibrary.com] Data curation; writing -original draft; methodology.Xue-Yuan Di: Writing -review and editing; methodology; project administration.Bin Yan: Funding acquisition; investigation.Peng Ren: Writing -review and editing.Hui-Zi Wu: Writing -review and editing.Mao-Fa Yang: Methodology; funding acquisition; writingreview and editing; data curation; supervision; resources.ACK N O WLE D G E M ENTSWe would like to thank technical staff Li-Min Ye for their advice on drafting the manuscript.F I G U R E 6Developmental period of M. manilae on S. frugiperda larvae fed with an artificial diet or corn leaves.Bars (Mean ± SEM).Different letters indicate significant differences at p < 0.05 (t-test).[Colourfigure can be viewed at wileyonlinelibrary.com]