Simple SummaryCampoletis chlorideae, a solitary larval endoparasitoid, has a wide host range. It has great biocontrol potential against some major worldwide agricultural economic pests, which is valuable and shows potential for development. The ovariole is where the eggs develop and mature. Ovariole number variability is species-specific, and closely related to fecundity. However, the ovariole characteristics of the parasitoid, particularly the effect of rearing host on ovariole number and body size, as well as their correlation, have not been studied until now. Our study shows that the parasitoid has paired ovaries which contain multiple ovarioles, and the ovariole number displays great individual variability and asymmetry between bilateral ovaries. Moreover, suitable host species and larval instars could improve their ovariole number and body size. There is a strong positive correlation between ovariole number and body size, namely that larger body size represents more ovarioles. Overall, our findings indicate that the parasitoid has great reproductive plasticity by regulating ovariole number. Additionally, body size combined with ovariole number can be used as an important index to evaluate the quality of the parasitoid. Our works provide practical help for the development and application of biocontrol using the parasitoid.AbstractCampoletis chlorideaehas great biocontrol potential against some major noctuid pests. In order to achieve the commercial development and utilization ofC. chlorideae, this study focused on the effect of rearing host species and larval instars on the ovariole number and body size of this wasp. Firstly, the morphology of the reproductive system and ovarioles of female wasps were observed. The number of ovarioles displayed great variability and asymmetry between bilateral ovaries. Moreover, the effect of four host species on ovariole number and body size ofC. chlorideaewere studied. The wasps had a larger ovariole number and body size when reared inHelicoverpa armigera. Additionally, the ovariole number and body size were larger when reared in the third instar larvae than in the first or second instar larvae ofH. armigera. There was a strong positive correlation between the ovariole number and body size ofC. chlorideae. The ovariole number and body size of the wasp could be improved under optimized artificial rearing conditions. According to these results, body size combined with ovariole number can be used as an important index to evaluate the quality ofC. chlorideae. This study provides important clues for the development and application of biocontrol usingC. chlorideae.