Radiation dose-rate is a neglected critical parameter in dose-response of insects

Current status of the sterile insect technique for the suppression of mosquito populations on a global scale

BACKGROUND

The World Health Organization (WHO) has emphasized the urgent need for alternative strategies to chemical insecticides for controlling mosquito populations, particularly the invasive Aedes species, which are known vectors of arboviruses. Among these alternative approaches, the sterile insect technique (SIT) is experiencing rapid development, with numerous pilot trials being conducted worldwide.

MAIN TEXT

This review aims to elucidate the principles of SIT and highlight the significant recent advancements that have facilitated its scalability. I also employ a phased conditional approach to categorize the progression of 39 projects, drawing on peer reviewed studies, press releases and direct communication with project managers. This review indicates that a substantial number of projects illustrate the efficacy of SIT in suppressing Aedes populations, with one project even demonstrating a reduction in dengue incidence. I offer several recommendations to mitigate potential failures and address the challenges of compensation and overcompensation when implementing SIT field trials. Furthermore, I examine the potential implications of male mating harassment on the effectiveness of SIT in reducing disease transmission.

CONCLUSIONS

This comprehensive assessment underscores the promise of SIT as a viable strategy for mosquito control. The insights gained from these trials not only contribute to the understanding of SIT's effectiveness but also highlight the importance of careful project management and ecological considerations in the pursuit of public health objectives.

Comparing the biological effectiveness of low energy 100 kV and high energy 5 MeV X-rays against Oriental fruit fly (Diptera: Tephritidae)

Radioisotope irradiators (using cesium-137 or cobalt-60) are used as sources of ionizing radiation to control quarantine or phytosanitary insect pests in internationally traded fresh commodities and to sterilize insects used in sterile insect release programs. There are institutional initiatives to replace isotopic irradiators (producing γ-rays) with lower-energy X-ray machines due to concerns about radiological terrorism and increasingly stringent regulations on the movement of radioisotopes. Questions remain about whether the biological effects of low-energy X-rays are comparable to those of γ-rays since differences in energy levels and dose rates of X-rays may have different efficacies. We compared adult emergence, flight ability, and adult survival in the Oriental fruit fly, Bactrocera dorsalis (Hendel) (Diptera: Tephritdae), after irradiation of third instar larvae with 100 kV or 5 MeV (5,000 kV) X-rays at 20 and 40 Gy in replicated studies. At 20 Gy, the adult emergence rate was significantly lower after irradiation with 100 kV compared to 5 MeV X-rays, suggesting higher efficacy at the lower energy level. In a follow-up study using 100 kV X-rays, applying 20 Gy using a slow dose rate (0.24 Gy min-1) resulted in significantly higher adult emergence than did a fast dose rate (3.3 Gy min-1), suggesting lower efficacy. Although our study suggests higher efficacy of low energy 100 kV X-rays, there is uncertainty in measuring the dose from an X-ray tube operating at 100 kV using an ionization chamber; we discuss how this uncertainty may change the interpretation of the results. Using a 100 kV X-ray irradiator to develop a phytosanitary treatment may underestimate the dose required for insect control using commercial high-energy γ-ray or X-ray systems.

Analysis of X-ray irradiation effects on the mortality values and hemolymph immune cell composition of Apis mellifera and its parasite, Varroa destructor

Varroa destructor is one of the most destructive enemies of the honey bee, Apis mellifera all around the world. Several control methods are known to control V. destructor, but the efficacy of several alternative control methods remains unexplored. Irradiation can be one of these unknown solutions but before practical application, the effectiveness, and the physiological effects of ionizing radiation on the host and the parasite are waiting to be tested. Therefore, the objective of our study was to investigate the effects of different doses (15, 50, 100, and 150 Gy) of high-energy X-ray irradiation through mortality rates and hemocyte composition changes in A. mellifera workers and record the mortality rates of the parasite. The mortality rate was recorded during short-term (12, 24, and 48 h) and long-term periods (3, 6, 12, 18, and 24d). The sensitivity of the host and the parasite in case of the higher doses of radiation tested (50, 100, and 150 Gy) been demonstrated by total mortality of the host and 90 % of its parasite has been observed on the 18th day after the irradiation. V. destructor showed higher sensitivity (1.52-times higher than the adult honey bee workers) at the lowest dose (15 Gy). A. mellifera hemocytes were influenced significantly by radiation dosage and the elapsed time after treatment. The higher radiation doses increased plasmatocyte numbers in parallel with the decrease in prohemocyte numbers. On the contrary, the numbers of granulocytes and oencoytes increased in the treated samples, but the putative effects of the different dosages on the recorded number of these hemocyte types could not be statistically proven. In summary, based on the outcome of our study X-ray irradiation can be deemed an effective tool for controlling phoretic V. destructor. However, further research is needed to understand the physiological response of the affected organisms.

Upscaling irradiation protocols of Aedes albopictus pupae within an SIT program in Reunion Island

The implementation of the sterile insect technique against Aedes albopictus relies on many parameters, in particular on the success of the sterilization of males to be released into the target area in overflooding numbers to mate with wild females. Achieving consistent sterility levels requires efficient and standardized irradiation protocols. Here, we assessed the effects of exposure environment, density of pupae, irradiation dose, quantity of water and location in the canister on the induced sterility of male pupae. We found that the irradiation of 2000 pupae in 130 ml of water and with a dose of 40 Gy was the best combination of factors to reliably sterilize male pupae with the specific irradiator used in our control program, allowing the sterilization of 14000 pupae per exposure cycle. The location in the canister had no effect on induced sterility. The results reported here allowed the standardization and optimization of irradiation protocols for a Sterile Insect Technique program to control Ae. albopictus on Reunion Island, which required the production of more than 300,000 sterile males per week.

Verification of AKT and CDK5 Gene and RNA Interference Combined with Irradiation to Mediate Fertility Changes in Plutella xylostella (Linnaeus)

Plutella xylostella (Linnaeus) mainly damages cruciferous crops and causes huge economic losses. Presently, chemical pesticides dominate its control, but prolonged use has led to the development of high resistance. In contrast, the sterile insect technique provides a preventive and control method to avoid the development of resistance. We discovered two genes related to the reproduction of Plutella xylostella and investigated the efficacy of combining irradiation with RNA interference for pest management. The results demonstrate that after injecting PxAKT and PxCDK5, there was a significant decrease of 28.06% and 25.64% in egg production, and a decrease of 19.09% and 15.35% in the hatching rate compared to the control. The ratio of eupyrene sperm bundles to apyrene sperm bundles also decreased. PxAKT and PxCDK5 were identified as pivotal genes influencing male reproductive processes. We established a dose-response relationship for irradiation (0-200 Gy and 200-400 Gy) and derived the irradiation dose equivalent to RNA interference targeting PxAKT and PxCDK5. Combining RNA interference with low-dose irradiation achieved a sub-sterile effect on Plutella xylostella, surpassing either irradiation or RNA interference alone. This study enhances our understanding of the genes associated with the reproduction of Plutella xylostella and proposes a novel approach for pest management by combining irradiation and RNA interference.

Effects of radiation on the fitness, sterility and arbovirus susceptibility of a Wolbachia-free Aedes albopictus strain for use in the sterile insect technique

BACKGROUND

The sterile insect technique (SIT) is a green and species-specific insect pest control technique that suppresses target populations by releasing factory-reared, radiosterilized males into the wild. Once released, it is important to be able to distinguish the released males from the wild males for monitoring purposes. Several methods to mark the sterile males exist. However, most have limitations due to monetary, process efficiency, or insect quality. Aedes albopictus is naturally infected with Wolbachia at a high prevalence, therefore the elimination of Wolbachia can serve as a biomarker to distinguish factory-reared male mosquitoes from wild conspecifics.

RESULTS

In this study, a Wolbachia-free Ae. albopictus GT strain was developed and its fitness evaluated, which was found to be comparable to the wild GUA strain. In addition, GT male mosquitoes were irradiated at the adult stage and a dose of 20 Gy or more induced over 99% sterility. Moreover, a dose of 30 Gy (almost completely sterilizing male and female mosquitoes) had limited effects on the mating competitiveness of GT males and the vector competence of GT females, respectively. However, radiation reduced mosquito longevity, regardless of sex.

CONCLUSION

Our results indicate that the Ae. albopictus GT strain can be distinguished from wild mosquitoes based on Wolbachia status and shows similar fitness, radio-sensitivity and arbovirus susceptibility to the GUA strain, indicating that it is feasible to use the GT strain to suppress Ae. albopictus populations for SIT programmes. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

X-rays are as effective as gamma-rays for the sterilization of Glossina palpalis gambiensis Vanderplank, 1911 (Diptera: Glossinidae) for use in the sterile insect technique

An area-wide integrated pest management strategy with a sterile insect technique (SIT) component requires a radiation source for the sterilisation of male insects. Self-contained gamma irradiators, which were exclusively used in past SIT programmes, are now facing increasing constraints and challenges due to stringent regulations. As a potential alternative, new generation high output X-ray irradiators have been proposed. The feasibility of using X-ray irradiators was assessed by comparing the effects of both gamma- and X-ray irradiators on biological parameters of Glossina palpalis gambiensis (Vanderplank, 1911), that are important for SIT applications. The gamma irradiator Foss Model 812 and two X-ray irradiators, the Rad Source 2400 and the blood irradiator Raycell Mk2 were used. Glossina palpalis gambiensis males were exposed to radiation as pupae. A radiation dose of 110 Gy or above induced more than 97% sterility in females that mated with the irradiated males for all the irradiators. Adult emergence rate, flight propensity, survival and mating performance did not differ between gamma- and X-rays irradiators. These results suggest that irradiating pupae with a dose of 110 Gy is optimal for both gamma-and X-ray irradiators used in this study, to achieve a sterility of approximately 99%. Similar research on other tsetse species could gradually phase out the use of gamma-ray irradiators in favour of X-rays irradiators, especially for smaller SIT programmes.

Improved Quality Management of the Indian Meal Moth, Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae) for Enhanced Efficacy of the Sterile Insect Technique

The sterile insect technique (SIT) is considered an environmentally friendly, autocidal control tactic to manage insect pests. This work dealt with the improvement of quality management of the Indian meal moth Plodia interpunctella (Hübner) for enhanced efficacy of the SIT. The results indicated that egg hatching of irradiated mature eggs of P. interpunctella was higher than that of younger eggs, indicating that mature eggs were significantly more tolerant than younger eggs. Moreover, our data revealed that a dose of 500 Gy completely prevented pupal formation in irradiated young and mature larvae. Crosses between irradiated and non-irradiated adults resulted in considerable variations in fecundity. The mating competitiveness index (CI) value was higher for a ratio of 5:1:1 (sterile male, fertile male, and fertile female, respectively) as compared with the ratio 1:1:1 for the irradiated individuals of all life stages. Low temperature (5 °C) maintenance of irradiated pupae significantly affected adult emergence. Using cylinders to assess flight ability, we found that the flight performance of adults that were developed from cold treated irradiated pupae was influenced by cylinder diameter, cylinder height and the number of hours the insects were in the cylinders. The percentage of DNA damage of the reproductive organs of adults developed from cold treated pupae that were irradiated with 100 and 150 Gy varied significantly. The results of this study should be used to implement pilot-scale field tests aiming at a sterile- to-fertile male ratio of 5 to 1.

Quality Control and Mating Performance of Irradiated Glossina palpalis gambiensis Males

The biological quality of sterile male insects produced in a mass-rearing facility is a prerequisite for the success of the SIT, which is a component of area-wide integrated pest management (AW-IPM). Indeed, sterile male insects released in the field must have a good mating performance in order to compete with wild males, but they must also present the required level of sterility. In the present study, the biological quality of sterile male Glossina palpalis gambiensis produced in a mass-rearing insectary was assessed through quality control testing. The mating performance of irradiated males was assessed in walk-in field cages. Irradiation had no effect on adult emergence but significantly reduced the percentage of operational flies (from 89.58% to 79.87%) and male survival (from 5 to 4 days, on average). However, irradiation did not impact the sterile male insemination potential, with all females inseminated and more than 80% of the spermathecae completely filled. The rate of induced sterility in females was 89.67% due to a dose rate decrease of the radiation source. Moreover, sterile males were able to compete successfully with untreated fertile males for untreated females in walk-in field cages. This study confirmed that the flies were still competitive and stressed the importance of regularly checking the radiation source parameters.