The radiation dose is a critical factor in achieving the desired sterility level of insects in the SIT program (Proverbs and Newton, 1962). In this study, we demonstrated that the biological traits of male C. pomonella varied in their decline with increasing levels of irradiation. This observation is consistent with a previous study on Glossina palpalis tsetse flies, which showed that higher radiation doses reduced the fecundity of females (Pagabeleguem et al., 2023). Additionally, we observed a significant decrease in the mating rate of sterile male C. pomonella with normal females when exposed to radiation above 200 Gy, mirroring the findings in G. palpalis (Pagabeleguem et al., 2023). The OK SIR program, designed to control C. pomonella populations, initially utilized 250 Gy of 60Co-γ irradiation, resulting in male sterility rates of 85–92% (Holm, 1985). Since 2014, a consistent dose of 200 Gy has been employed to balance sterility levels and mating competitiveness (Thistlewood and Judd, 2019). To optimize the trade-off between male sterility and minimize the negative effects of radiation on the insects, it is recommended to use 200 Gy as the optimal dose for inducing sterility in males C. pomonella.
The mating competitiveness of sterile insects is crucial for their ability to effectively compete with wild males for mating opportunities with females (Vreysen et al., 2007; Vreysen and Robinson, 2011). Previous research has shown that inadequate dosages may result in incomplete sterilization, while excessive irradiation can reduce the mating competitiveness of released insects compared to wild counterparts, thus affecting the success of SIT programs (Parker et al., 2021). Therefore, maintaining a delicate balance between mating competitiveness and sterility is essential during copulatory interactions with wild female insects (Bakri et al., 2005). Evaluating the optimal irradiation dose is critical to assessing the mating competitiveness of male insects bred for this purpose (Parker et al., 2021). However, reduced mating competitiveness is commonly observed in irradiated C. pomonella populations (North, 1975). Jiang et al. (2023) also reported a significant decrease in the mating competitiveness of male Spodoptera frugiperda following exposure to 250 Gy of X-ray irradiation. In our recent studies, male C. pomonella pupae irradiated with 366 Gy of X-rays exhibited mating competitiveness indices of 0.001 in laboratory (Zhang et al., 2023a) and 0.0088 in orchards (Zhang et al., 2023b). In this study, when subjected to 200 Gy of X-ray irradiation, the mating competitiveness index of sterilized male C. pomonella decreased, indicating a decline in mating competitiveness post-irradiation. Nevertheless, compared to the impact of 366 Gy irradiation, the mating competitiveness index of C. pomonella males following 200 Gy irradiation increased to 0.17 in laboratory conditions and 0.096 in orchards, suggesting that an optimized irradiation dose can enhance the mating competitiveness of sterile insects to a certain degree. However, the mating competitiveness of C. pomonella males post-exposure to 200 Gy irradiation persists at a sub-optimal level. Therefore, it is imperative to prioritize efforts aimed at improving the mating competitiveness of sterile males in future studies.
The diminished mating competitiveness evident in irradiated insects may stem from the compromised olfactory system in male insects, leading to a reduced ability to perceive female sex pheromones. To investigate this hypothesis, linalool, a volatile compound found in host plants known to enhance the attraction of C. capitata moths (Casado et al., 2006), was utilized to fumigate male C. pomonella moths. The findings indicated that although linalool fumigation did not affect longevity, fecundity, hatching rates, or overall mating performance, it did enhance the mating competitiveness of male moths. Another investigation on C. capitata demonstrated that the mating competitiveness of normal male adults and mass-reared irradiated males increased subsequent to fumigation with ginger root oil (GRO), compared to non-fumigated adults. Notably, GRO fumigation did not have any notable impacts on the body weight or longevity of irradiated mass-reared males (Shelly and McInnis, 2001). These results suggest that linalool fumigation might potentially mitigate male sterility induced by X-ray irradiation, possibly due to enhanced male sex pheromone communication in the antennae of male C. pomonella moths, as observed in C. capitata post ginger root oil fumigation (Papadopoulos et al., 2006).
The olfactory system plays a vital role in various insect behaviors like feeding, orientation, host-seeking, mating, and oviposition (Li et al., 2020; Zhang et al., 2019). Chemoreception in insects is a complex process involving various chemosensory genes, such as odorant receptors (ORs), gustatory receptors (GRs), ionotropic receptors (IRs), odorant-binding proteins (OBPs), chemosensory proteins (CSPs), and sensory neuron membrane proteins (SNMPs) (Yang et al., 2024). In particular, the importance of chemosensory genes in recognizing pheromones in C. pomonella has been extensively researched. Tian et al. (2021) have identified male pheromone receptor genes in C. pomonella, including CpomOR1, CpomOR2a, CpomOR3, CpomOR5, CpomOR6a, and CpomOR7, through transcriptome analysis of the antenna. Knock-out the CpomOR1 gene using CRISPR/Cas9 technology has been shown to impact the fertility and fecundity of C. pomonella (Garczynski et al., 2017). Moreover, CpomOR3a and CpomOR3b genes in C. pomonella are involved in recognizing plant volatiles and the sex pheromone codlemone, thereby enhancing the moths' response to codlemone (Wan et al., 2019). In this study, we revealed a decrease in gene expression in C. pomonella males' antenna following 200 Gy of X-ray irradiation, suggesting a reduced sex pheromones response in male moths. Conversely, an increase in the expression of CpomOR3a, CpomOR3b, and CpomOR5 genes was observed after linalool fumigation, enhancing the recognition of sex pheromones.These findings support the efficacy of linalool fumigation in mitigating male sterility induced by X-ray irradiation, likely through the upregulation of essential chemosensory genes and the activation of pheromone recognition mechanisms in C. pomonella.
Previous studies have demonstrated that the release of C. pomonella males into the pear orchard following exposure to 366 Gy X-ray did not affect the wild populations or the fruit decay rate (Zhang et al., 2023). In this study, we carried out a small-scale trial in different fields utilizing the SI) to manage C. pomonella populations. Our study demonstrated that by releasing three successive batches of sterile male C. pomonella moths irradiated at 200 Gy, along with linalool fumigation, in the specified orchard area, there was a substantial decrease in C. pomonella infestations in the apple crop. This reduction was evident through significantly lower infestation rates compared to the control orchard. Additionally, field observations indicated a notable decrease in the recapture rate of wild C. pomonella moths in comparison to the control group. These results suggest that the utilization of sterile males effectively suppressed the wild C. pomonella population in the orchards, consequently reducing fruit damage. The findings highlight an enhancement in the field application of the SIT for C. pomonella control after optimizing the irradiation dose.
In summary, we have identified the optimal sterilizing dose of X-ray irradiation for C. pomonella and introduced a plant volatiles fumigation approach to enhance the mating competitiveness of sterilized males. Additionally, we investigated the effects of irradiation and fumigation on the expression of genes related to pheromone recognition in the male antennal. Notably, our study demonstrated the successful reduction of the C. pomonella population in the orchards through repeated releases of sterilized males in conjunction with linalool fumigation on a pilot scale. These findings offer valuable insights for the implementation of the SIT as a strategy of controlling C. pomonella.