The Insect Pest Control Laboratory of the Joint FAO/IAEA Programme: Ten Years (2010-2020) of Research and Development, Achievements and Challenges in Support of the Sterile Insect Technique

Investigating the Potential of X-Ray-Based Cancer Treatment Equipment for the Sterile Insect Technique in Aedes aegypti Control Programs

The sterile insect technique (SIT) stands as an eco-friendly approach for mosquito control, but it is impeded by the limited availability of γ-ray radiation source. This research sought to investigate a different radiation source-the Varian Clinac 23EX linear accelerator, which is frequently used for X-ray therapy in cancer treatment. Evaluation parameters including emergence rate, average survival time, induced sterility (IS), male mating competitiveness of irradiated males and fecundity (the number of eggs per female per batch), and the egg hatch rate of females mated with irradiated males were assessed to gauge the application potential of this cancer treatment equipment in the realm of the SIT. The results indicated that X-rays from radiation therapy equipment can effectively suppress the hatch rate of offspring mosquitoes without adversely affecting the emergence rate of irradiated males or the fecundity of females. In addition, at an X-ray dose of 60 Gy, the induced sterility in Ae. aegypti was comparable to the sterility induced by 40 Gy of γ-rays with both treatments resulting in 99.6% sterility. Interestingly, when a release ratio of 7:1 (irradiated males:unirradiated males) was used to competitively mate with females, the IS results resulted by 60 Gy X-rays and 40 Gy γ-rays were still at 70.3% and 73.7%, respectively. In conclusion, the results underscored the potential of the Varian Clinac 23EX linear accelerator as an X-ray source in SIT research.

Preparation for targeted sterile insect technique to control invasive Aedes aegypti (Diptera: Culicidae) in southern California: dose-dependent response, survivorship, and competitiveness

Aedes aegypti is fast spreading across California, with over 300 cities within 22 central and southern counties being infested since its introduction in 2013. Due to its cryptic breeding habitats, control efforts have not been successful so far. This calls for innovative tools such as sterile insect technique (SIT) to reinforce the existing integrated pest management (IPM). Here, we assessed fitness, survivorship, and dose response of X-ray irradiated male Ae. aegypti in California. Locally acquired Ae. aegypti eggs were hatched and reared in temperature-controlled laboratory setting at the West Valley Mosquito and Vector Control District in Ontario, California. Freshly emerged adult male mosquitoes were manually separated using motor-operated aspirators and treated with X-ray radiation at different dosage (42-60 Gy). Dose response of irradiated males was analyzed and induced sterility determined. Survivorship of males treated with different X-ray doses was compared. Fecundity of females that mated with irradiated males at different X-ray doses was generally comparable. Overall, induced sterility increased with higher X-ray doses. Nulliparous females that mated with male Ae. aegypti treated with 55-60 Gy laid eggs with over 99% sterility. Non-irradiated male mosquitoes had higher survivorship (mean = 0.78; P = 0.0331) than irradiated mosquitoes (mean range = 0.50-0.65). The competitiveness index of irradiated males decreased with increasing X-ray treatment doses, 1.14 at 55 Gy and 0.49 at 60 Gy, and this difference was significant (P < 0.01). Irradiated males showed high survivorship and competitiveness-key for the anticipated SIT application for the control of invasive Ae. aegypti in California.

Spatial and temporal characterization of Aedes albopictus oviposition activity in candidate urban settings for sterile insect technique testing in La Reunion Island

BACKGROUND

Understanding of mosquito spatiotemporal dynamics is central to characterize candidate field sites for the sterile insect technique (SIT) testing, and is critical to the effective implementation and evaluation of pilot sterile male release programs. Here, we present a detailed description of Aedes albopictus (Skuse) egg-laying activity over a 6-year period in urban areas identified as potential SIT testing sites on Reunion Island.

METHOD

Weekly entomological collections using ovitraps were carried out in residential and adjacent uninhabited habitats in two urban areas, Duparc and Bois Rouge, in the municipality of Sainte Marie, Reunion Island. Time-series data incorporating the frequency of positive ovitraps and the total number of eggs/ovitrap recorded each time at each locality during the study period from May 2013 to December 2018 were analyzed with multifaceted statistical approaches including descriptive statistics and spatiotemporal analyses incorporating the role of climatic factors on overall ovitrap productivity.

RESULTS

During the ovitrap survey, the proportion of egg-positive ovitraps differed among study sites (χ2 = 50.21, df = 2, P < 0.001), being relatively lower in Duparc (89.5%) than in Bois-Rouges (95.3%) and the adjacent buffer zone (91.2%). Within each neighborhood, Ae. albopictus egg abundance varied by month in a roughly seasonal pattern marked by a single peak occurring more regularly February each year, a decline at the onset of the austral winter in July, followed by a period of lower ovitrap productivity in August and September. Fluctuation in both positivity rate and eggs densities per ovitraps were related to annual and seasonal variations in local temperature and rainfall (P < 0.001 in all cases). The spatial analysis also captured substantial between- and within-habitats heterogeneity, whereby the overall ovitrap productivity was higher in residential areas than in the buffer zone.

CONCLUSIONS

Collectively, these results reveal that the distribution of Ae. albopictus oviposition activity is shaped by local habitat heterogeneity and seasonal climatic factors. Overall, this study provides baseline insights into the reproductive dynamics of Ae. albopictus, which would assist in planning locally tailored SIT interventions, while addressing concerns related to focal areas of high egg-laying intensity and potential immigration of females from natural areas.

Biotechnology-enhanced genetic controls of the global pest Drosophila suzukii

Spotted wing Drosophila (Drosophila suzukii Matsumura, or SWD), an insect pest of soft-skinned fruits native to East Asia, has rapidly spread worldwide in the past 15 years. Genetic controls such as sterile insect technique (SIT) have been considered for the environmentally friendly and cost-effective management of this pest. In this review, we provide the latest developments for the genetic control strategies of SWD, including sperm-marking strains, CRISPR-based sex-ratio distortion, neoclassical genetic sexing strains, transgenic sexing strains, a sex-sorting incompatible male system, precision-guided SIT, and gene drives based on synthetic Maternal effect dominant embryonic arrest (Medea) or homing CRISPR systems. These strategies could either enhance the efficacy of traditional SIT or serve as standalone methods for the sustainable control of SWD.

Simultaneous Silencing of Gut Nucleases and a Vital Target Gene by Adult dsRNA Feeding Enhances RNAi Efficiency and Mortality in Ceratitis capitata

Ceratitis capitata, known as the Mediterranean fruit fly (Medfly), is a major dipteran pest significantly impacting fruit and vegetable farming. Currently, its control heavily relies mainly on chemical insecticides, which pose health risks and have effects on pollinators. A more sustainable and species-specific alternative strategy may be based on double-stranded RNA (dsRNA) delivery through feeding to disrupt essential functions in pest insects, which is poorly reported in dipteran species. Previous reports in Orthoptera and Coleoptera species suggested that dsRNA degradation by specific nucleases in the intestinal lumen is among the major obstacles to feeding-mediated RNAi in insects. In our study, we experimented with three-day adult feeding using a combination of dsRNA molecules that target the expression of the ATPase vital gene and two intestinal dsRNA nucleases. These dsRNA molecules were recently tested separately in two Tephritidae species, showing limited effectiveness. In contrast, by simultaneously feeding dsRNA against the CcVha68-1, CcdsRNase1, and CcdsRNase2 genes, we observed 79% mortality over seven days, which was associated with a decrease in mRNA levels of the three targeted genes. As expected, we also observed a reduction in dsRNA degradation following RNAi against nucleases. This research illustrates the potential of utilizing molecules as pesticides to achieve mortality rates in Medfly adults by targeting crucial genes and intestinal nucleases. Furthermore, it underscores the importance of exploring RNAi-based approaches for pest management.

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.

Efficiency assessment of a novel automatic mosquito pupae sex separation system in support of area-wide male-based release strategies

This study provides a comparative analysis of two state-of-the-art automatic mosquito pupae sex sorters currently available: the ORINNO and the WOLBAKI Biotech pupae sex separation systems, which both exploit the sexual size dimorphism of pupae. In Aedes aegypti, the WOLBAKI sex sorter and the ORINNO with a sieve mesh size of 1.050 mm achieved sex separation with female contamination rates below 1%, low pupae mortality rates and high male flight capacity. However, in Ae. albopictus, there was more variability, with female contamination rates above the 1% threshold and pupae mortality reaching 27% when using the ORINNO sorter. On the other hand, the WOLBAKI sorter achieved a male pupae recovery of 47.99 ± 8.81% and 50.91 ± 11.77% in Ae. aegypti and Ae. albopictus, respectively, while the ORINNO sorter with a smaller sieve size achieved male pupae recoveries of 38.08 ± 9.69% and 40.16 ± 2.73% in Ae. aegypti and Ae. albopictus, respectively. This study provides valuable information for researchers and practitioners in the field, assisting in the selection of the most suitable system for mosquito control, management and research programs depending on their specific requirements.

Climate-influenced vector-borne diseases in Africa: a call to empower the next generation of African researchers for sustainable solutions

We look at the link between climate change and vector-borne diseases in low- and middle-income countries in Africa. The large endemicity and escalating threat of diseases such as malaria and arboviral diseases, intensified by climate change, disproportionately affects vulnerable communities globally. We highlight the urgency of prioritizing research and development, advocating for robust scientific inquiry to promote adaptation strategies, and the vital role that the next generation of African research leaders will play in addressing these challenges. Despite significant challenges such as funding shortages within countries, various pan-African-oriented funding bodies such as the African Academy of Sciences, the Africa Research Excellence Fund, the Wellcome Trust, the U.S. National Institutes of Health, and the Bill and Melinda Gates Foundation as well as initiatives such as the African Research Initiative for Scientific Excellence and the Pan-African Mosquito Control Association, have empowered (or are empowering) these researchers by supporting capacity building activities, including continental and global networking, skill development, mentoring, and African-led research. This article underscores the urgency of increased national investment in research, proposing the establishment of research government agencies to drive evidence-based interventions. Collaboration between governments and scientific communities, sustained by pan-African funding bodies, is crucial. Through these efforts, African nations are likely to enhance the resilience and adaptive capacity of their systems and communities by navigating these challenges effectively, fostering scientific excellence and implementing transformative solutions against climate-sensitive vector-borne diseases.

Mass irradiation of adult Aedes mosquitoes using a coolable 3D printed canister

In the last decade, the use of the sterile insect technique (SIT) to suppress mosquito vectors have rapidly expanded in many countries facing the complexities of scaling up production and procedures to sustain large-scale operational programs. While many solutions have been proposed to improve mass production, sex separation and field release procedures, relatively little attention has been devoted to effective mass sterilization of mosquitoes. Since irradiation of pupae en masse has proven difficult to standardise with several variables affecting dose response uniformity, the manipulation of adult mosquitoes appears to be the most promising method to achieve effective and reliable sterilization of large quantities of mosquitoes. A 3D-printed phase change material based coolable canister was developed which can compact, immobilize and hold around 100,000 adult mosquitoes during mass radio sterilization procedures. The mass irradiation and compaction treatments affected the survival and the flight ability of Aedes albopictus and Aedes aegypti adult males but the use of the proposed irradiation canister under chilled conditions (6.7-11.3 °C) significantly improved their quality and performance. The use of this cooled canister will facilitate adult mass irradiation procedures in self-contained irradiators in operational mosquito SIT programmes.

Radiation dose fractionation and its potential hormetic effects on male Glossina palpalis gambiensis (Diptera: Glossinidae): a comparative study of reproductive and flight quality parameters

One of the most critical factors for implementing the sterile insect technique for the management of tsetse is the production of large quantities of highly competitive sterile males in the field. Several factors may influence the biological quality of sterile males, but optimizing the irradiation protocols to limit unwanted somatic cell damage could improve male performance. This study evaluated the effect of fractionation of gamma radiation doses on the fertility and flight quality of male Glossina palpalis gambiensis. Induced sterility was assessed by mating irradiated males with virgin fertile females. Flight quality was assessed using a standard protocol. The male flies were irradiated as pupae on day 23-27 post larviposition with 110 Gy, either in a single dose or in fractionations of 10 + 100 Gy and 50 + 60 Gy separated by 1-, 2- and 3-day intervals or 55 + 55 Gy separated by 4-, 8-, and 24-hour intervals. All treatments induced more than 90% sterility in females mated with irradiated males, as compared with untreated males. No significant differences were found in emergence rate or flight propensity between fractionated and single radiation doses, nor between the types of fractionations. Overall, the 50(D0) + 60(D1) Gy dose showed slightly higher induced sterility, flight propensity, and survival of males under feeding regime. Dose fractionation resulted in only small improvements with respect to flight propensity and survival, and this should be traded off with the required increase in labor that dose fractionation entails, especially in larger control programs.

The Effect of X-ray Irradiation on the Fitness and Field Adaptability of the Codling Moth: An Orchard Study in Northeast China

The codling moth, Cydia pomonella (L.), is an invasive agricultural pest of pome fruits and walnuts in China that threatens the apple industry in the Loess Plateau and Bohai Bay; it has developed resistance to many insecticides. Sterile insect technique (SIT) combined with area-wide integrated pest management (AW-IPM) can reduce the risk of resistance to insecticides and effectively control some insect pest species. Our previous laboratory experiment found that irradiation with 366 Gy of X-ray caused the males of the codling moth to become sterile. However, the sterility and adaptability of males after being irradiated with 366 Gy X-ray in the field are still unclear. In this study, we investigated the effect of X-ray irradiation on the fitness of male adults that emerged from pupae irradiated with 366 Gy to explore their adaptability and mating competitiveness, and to examine the effect of releasing sterile male insects in orchards in northeast China on the fruit infestation rate of the Nanguo pear. The results showed that 366 Gy of X-ray irradiation significantly reduced the mating competitiveness of males and the hatching rate of the eggs laid by females pairing with sterile males. Meanwhile, the lifespan of the sterile male moths was significantly shorter than that of the normal ones in the field. A pilot test showed that the release twice of sterile male moths in the orchards had no significant effect on the fruit infestation rate. Our field experiments provide a scientific basis for the further optimization of the SIT technology program for controlling C. pomonella.

Female Sex Determination Factors in Ceratitis capitata: Molecular and Structural Basis of TRA and TRA2 Recognition

In the model system for genetics, Drosophila melanogaster, sexual differentiation and male courtship behavior are controlled by sex-specific splicing of doublesex (dsx) and fruitless (fru). In vitro and in vivo studies showed that female-specific Transformer (TRA) and the non-sex-specific Transformer 2 (TRA2) splicing factors interact, forming a complex promoting dsx and fru female-specific splicing. TRA/TRA2 complex binds to 13 nt long sequence repeats in their pre-mRNAs. In the Mediterranean fruitfly Ceratitis capitata (Medfly), a major agricultural pest, which shares with Drosophila a ~120 million years old ancestor, Cctra and Cctra2 genes seem to promote female-specific splicing of Ccdsx and Ccfru, which contain conserved TRA/TRA2 binding repeats. Unlike Drosophila tra, Cctra autoregulates its female-specific splicing through these putative regulatory repeats. Here, a yeast two-hybrid assay shows that CcTRA interacts with CcTRA2, despite its high amino acid divergence compared to Drosophila TRA. Interestingly, CcTRA2 interacts with itself, as also observed for Drosophila TRA2. We also generated a three-dimensional model of the complex formed by CcTRA and CcTRA2 using predictive approaches based on Artificial Intelligence. This structure also identified an evolutionary and highly conserved putative TRA2 recognition motif in the TRA sequence. The Y2H approach, combined with powerful predictive tools of three-dimensional protein structures, could use helpful also in this and other insect species to understand the potential links between different upstream proteins acting as primary sex-determining signals and the conserved TRA and TRA2 transducers.

How Rearing Systems for Various Species of Flies Benefit Humanity

Flies (Diptera) have played a prominent role in human history, and several fly species are reared at different scales and for different beneficial purposes worldwide. Here, we review the historical importance of fly rearing as a foundation for insect rearing science and technology and synthesize information on the uses and rearing diets of more than 50 fly species in the families Asilidae, Calliphoridae, Coelopidae, Drosophilidae, Ephydridae, Muscidae, Sarcophagidae, Stratiomyidae, Syrphidae, Tachinidae, Tephritidae, and Tipulidae. We report more than 10 uses and applications of reared flies to the well-being and progress of humanity. We focus on the fields of animal feed and human food products, pest control and pollination services, medical wound therapy treatments, criminal investigations, and on the development of several branches of biology using flies as model organisms. We highlight the relevance of laboratory-reared Drosophila melanogaster Meigen as a vehicle of great scientific discoveries that have shaped our understanding of many biological systems, including the genetic basis of heredity and of terrible diseases such as cancer. We point out key areas of fly-rearing research such as nutrition, physiology, anatomy/morphology, genetics, genetic pest management, cryopreservation, and ecology. We conclude that fly rearing is an activity with great benefits for human well-being and should be promoted for future advancement in diverse and innovative methods of improving existing and emerging problems to humanity.

From the Lab to the Field: Long-Distance Transport of Sterile Aedes Mosquitoes

Pilot programs of the sterile insect technique (SIT) against Aedes aegypti may rely on importing significant and consistent numbers of high-quality sterile males from a distant mass rearing factory. As such, long-distance mass transport of sterile males may contribute to meet this requirement if their survival and quality are not compromised. This study therefore aimed to develop and assess a novel method for long-distance shipments of sterile male mosquitoes from the laboratory to the field. Different types of mosquito compaction boxes in addition to a simulation of the transport of marked and unmarked sterile males were assessed in terms of survival rates/recovery rates, flight ability and morphological damage to the mosquitoes. The novel mass transport protocol allowed long-distance shipments of sterile male mosquitoes for up to four days with a nonsignificant impact on survival (>90% for 48 h of transport and between 50 and 70% for 96 h depending on the type of mosquito compaction box), flight ability, and damage. In addition, a one-day recovery period for transported mosquitoes post-transport increased the escaping ability of sterile males by more than 20%. This novel system for the long-distance mass transport of mosquitoes may therefore be used to ship sterile males worldwide for journeys of two to four days. This study demonstrated that the protocol can be used for the standard mass transport of marked or unmarked chilled Aedes mosquitoes required for the SIT or other related genetic control programs.

Radiation dose-fractionation in adult Aedes aegypti mosquitoes

Balancing process efficiency and adult sterile male biological quality is one of the challenges in the success of the sterile insect technique (SIT) against insect pest populations. For the SIT against mosquitoes, many stress factors need to be taken into consideration when producing sterile males that require high biological quality to remain competitive once released in the field. Pressures of mass rearing, sex sorting, irradiation treatments, packing, transport and release including handling procedures for each step, add to the overall stress budget of the sterile male post-release. Optimizing the irradiation step to achieve maximum sterility while keeping off-target somatic damage to a minimum can significantly improve male mating competitiveness. It is therefore worth examining various protocols that have been found to be effective in other insect species, such as dose fractionation. A fully sterilizing dose of 70 Gy was administered to Aedes aegypti males as one acute dose or fractionated into either two equal doses of 35 Gy, or one low dose of 10 Gy followed by a second dose of 60 Gy. The two doses were separated by either 1- or 2-day intervals. Longevity, flight ability, and mating competitiveness tests were performed to identify beneficial effects of the various treatments. Positive effects of fractionating dose were seen in terms of male longevity and mating competitiveness. Although applying split doses generally improved male quality parameters, the benefits may not outweigh the added labor in SIT programmes for the management of mosquito vectors.

Suitability of Raycell MK2 Blood X-ray Irradiator for the Use in the Sterile Insect Technique: Dose Response in Fruit Flies, Tsetse Flies and Mosquitoes

The sterile insect technique (SIT) is based on the inundatory field release of a target pest following their reproductive sterilization via exposure to radiation. Until recently, gamma irradiation from isotopic sources has been the most widely used in SIT programs. As isotopic sources are becoming increasingly expensive, especially for small programs, and regulations surrounding their procurement and shipment increasingly strict, irradiation capacity is one of the limiting factors in smaller or newly developing SIT projects. For this reason, the possibility of using X-ray irradiators has been evaluated in the recent decade. The availability of "off-the-shelf" blood X-ray irradiators that meet the technical requirements for insect irradiation can provide irradiation capacity for those SIT projects in which the acquisition of gamma ray irradiators is not feasible. Following the recent technical characterization of a Raycell MK2 X-ray blood irradiator, it was found in this study, that MK2 instruments were suitable for the sterilization of fruit flies, tsetse flies and mosquitoes, inducing comparable, even slightly higher, sterility levels compared to those achieved by gamma ray irradiation. This, together with its estimated processing efficiency, shows that MK2 irradiators are suitable for small- to mid-sized SIT programs.

Interactions between Glossina pallidipes salivary gland hypertrophy virus and tsetse endosymbionts in wild tsetse populations

BACKGROUND

Tsetse control is considered an effective and sustainable tactic for the control of cyclically transmitted trypanosomosis in the absence of effective vaccines and inexpensive, effective drugs. The sterile insect technique (SIT) is currently used to eliminate tsetse fly populations in an area-wide integrated pest management (AW-IPM) context in Senegal. For SIT, tsetse mass rearing is a major milestone that associated microbes can influence. Tsetse flies can be infected with microorganisms, including the primary and obligate Wigglesworthia glossinidia, the commensal Sodalis glossinidius, and Wolbachia pipientis. In addition, tsetse populations often carry a pathogenic DNA virus, the Glossina pallidipes salivary gland hypertrophy virus (GpSGHV) that hinders tsetse fertility and fecundity. Interactions between symbionts and pathogens might affect the performance of the insect host.

METHODS

In the present study, we assessed associations of GpSGHV and tsetse endosymbionts under field conditions to decipher the possible bidirectional interactions in different Glossina species. We determined the co-infection pattern of GpSGHV and Wolbachia in natural tsetse populations. We further analyzed the interaction of both Wolbachia and GpSGHV infections with Sodalis and Wigglesworthia density using qPCR.

RESULTS

The results indicated that the co-infection of GpSGHV and Wolbachia was most prevalent in Glossina austeni and Glossina morsitans morsitans, with an explicit significant negative correlation between GpSGHV and Wigglesworthia density. GpSGHV infection levels > 10^3.31 seem to be absent when Wolbachia infection is present at high density (> 10^7.36), suggesting a potential protective role of Wolbachia against GpSGHV.

CONCLUSION

The result indicates that Wolbachia infection might interact (with an undefined mechanism) antagonistically with SGHV infection protecting tsetse fly against GpSGHV, and the interactions between the tsetse host and its associated microbes are dynamic and likely species specific; significant differences may exist between laboratory and field conditions.

CRISPR/Cas9-Mediated Mutagenesis of Sex-Specific Doublesex Splicing Variants Leads to Sterility in Spodoptera frugiperda, a Global Invasive Pest

Spodoptera frugiperda (J. E. Smith), an emerging invasive pest worldwide, has posed a serious agricultural threat to the newly invaded areas. Although somatic sex differentiation is fundamentally conserved among insects, the sex determination cascade in S. frugiperda is largely unknown. In this study, we cloned and functionally characterized Doublesex (dsx), a "molecular switch" modulating sexual dimorphism in S. frugiperda using male- and female-specific isoforms. Given that Lepidoptera is recalcitrant to RNAi, CRISPR/Cas9-mediated mutagenesis was employed to construct S. frugiperda mutants. Specifically, we designed target sites on exons 2, 4, and 5 to eliminate the common, female-specific, and male-specific regions of S. frugiperda dsx (Sfdsx), respectively. As expected, abnormal development of both the external and internal genitalia was observed during the pupal and adult stages. Interestingly, knocking out sex-specific dsx variants in S. frugiperda led to significantly reduced fecundity and fertility in adults of corresponding sex. Our combined results not only confirm the conserved function of dsx in S. frugiperda sex differentiation but also provide empirical evidence for dsx as a potential target for the Sterile Insect Technique (SIT) to combat this globally invasive pest in a sustainable and environmentally friendly way.

Exploring Conditions for Handling Packing and Shipping Aedes aegypti Males to Support an SIT Field Project in Brazil

The sterile insect technique (SIT) application, as an alternative tool for conventional mosquito control methods, has recently gained prominence. Nevertheless, some SIT components require further development, such as protocols under large-scale conditions, focusing on packing and shipping mosquitoes, and considering transporting time. Immobilization of Aedes aegypti males was tested at temperatures 4, 7, 10, and 14 °C, and each temperature was assessed for 60, 90, and 120 min. The recovery after 24 h was also studied. Chilled and control-reared males had comparable survival rates for all conditions, although 4 °C for 120 min impacted male survival. The male escape rate was affected after 60 min of exposure at 4 °C; this difference was not significant, with 24 h of recovery. First, we defined the successful immobilization at 4 °C for 60 min, thus enabling the evaluation of two transportation intervals: 6 and 24 h, with the assessment of different compaction densities of 100 and 150 mosquitoes/cm³ at 10 °C to optimize the shipment. Compaction during simulated mosquito shipments reduced survival rates significantly after 6 and 24 h. In the mating propensity and insemination experiments, the sterile males managed to inseminate 40 to 66% for all treatments in laboratory conditions. The male insemination propensity was affected only by the highest compaction condition concerning the control. The analysis of the densities (100 and 150 males/cm³) showed that a higher density combined with an extended shipment period (24 h) negatively impacted the percentage of inseminated females. The results are very helpful in developing and improving the SIT packing and shipment protocols. Further studies are required to evaluate all combined parameters' synergetic effects that can combine irradiation to assess sexual competitiveness when sterile males are released into the field.

Standardization of the FAO/IAEA Flight Test for Quality Control of Sterile Mosquitoes

Successful implementation of the sterile insect technique (SIT) against Aedes aegypti and Aedes albopictus relies on maintaining a consistent release of high-quality sterile males. Affordable, rapid, practical quality control tools based on the male’s flight ability (ability to escape from a flight device) may contribute to meeting this requirement. Therefore, this study aims to standardize the use of the original FAO/IAEA rapid quality control flight test device (FTD) (version 1.0), while improving handling conditions and reducing the device’s overall cost by assessing factors that could impact the subsequent flight ability of Aedes mosquitoes. The new FTD (version 1.1) is easier to use. The most important factors affecting escape rates were found to be tube color (or “shade”), the combined use of a lure and fan, mosquito species, and mosquito age and density (25; 50; 75; 100 males). Other factors measured but found to be less important were the duration of the test (30, 60, 90, 120 min), fan speed (normal 3000 rpm vs. high 6000 rpm), and mosquito strain origin. In addition, a cheaper version of the FTD (version 2.0) that holds eight individual tubes instead of 40 was designed and successfully validated against the new FTD (version 1.1). It was sensitive enough to distinguish between the effects of cold stress and high irradiation dose. Therefore, the eight-tube FTD may be used to assess Aedes’ flight ability. This study demonstrated that the new designs (versions 1.1 and 2.0) of the FTD could be used for standard routine quality assessments of Aedes mosquitoes required for an SIT and other male release-based programs.

Impact of invasive insects on native insect communities

Several biophysical factors are leading to the loss of biodiversity, among them the dominance of exotic invasive species on native communities is important. Their dominance can lead to changes in the structure of insect communities, by competing and displacing native species to other crops or habitats. These changes can impact the herbivore's natural enemies in invaded areas by diverging them from suitable herbivores and altering their biological control process. The development of edible insects and derived products at an industrial scale can also have an impact on the local fauna by the risks of spillover and accidental release in nature. Several area-wide integrated pest management programs are also using the sterile insect technique to control insect pests and disease' vectors. This technique is becoming largely used; however, its application as 'non-intrusive to the environment' is controversial particularly when eradication is concerning species that are at the basis of food webs.

Bacterial Symbionts in Ceratitis capitata

Ceratitis capitata (Diptera: Tephritidae) is responsible for extensive damage in agriculture with important economic losses. Several strategies have been proposed to control this insect pest including insecticides and the Sterile Insect Technique. Traditional control methods should be implemented by innovative tools, among which those based on insect symbionts seem very promising. Our study aimed to investigate, through the 16S Miseq analysis, the microbial communities associated with selected organs in three different medfly populations to identify possible candidates to develop symbiont-based control approaches. Our results confirm that Klebsiella and Providencia are the dominant bacteria in guts, while a more diversified microbial community has been detected in reproductive organs. Concertedly, we revealed for the first time the presence of Chroococcidiopsis and Propionibacterium as stable components of the medfly’s microbiota. Additionally, in the reproductive organs, we detected Asaia, a bacterium already proposed as a tool in the Symbiotic Control of Vector-Borne Diseases. A strain of Asaia, genetically modified to produce a green fluorescent protein, was used to ascertain the ability of Asaia to colonize specific organs of C. capitata. Our study lays the foundation for the development of control methods for C. capitata based on the use of symbiont bacteria.

Effects of Chilling and Anoxia on the Irradiation Dose-Response in Adult Aedes Mosquitoes

The success of the sterile insect technique (SIT) relies on the achievement of high levels of sterility and mating success of the factory-reared sterile males and thus their biological quality, which can be enhanced by the reduction of stress factors encountered during rearing, handling, and irradiation procedures. The achievement of consistent sterility levels requires reliable and standard irradiation protocols. Additionally, mosquito adults require immobilization prior to, and during irradiation to increase processing efficiency and to avoid physical damage caused by movement in restricted space. Common methods for immobilization include chilling and anesthetics such as nitrogen. Here we assessed the effects of chilling and exposure to nitrogen on the irradiation dose-response of Aedes mosquitoes, and their downstream effects on some male quality parameters including longevity and flight ability. We found that chilling does not incur damage in the insects in terms of longevity and flight ability when chilling duration and temperature are carefully controlled, and a recovery phase is provided. Irradiation in nitrogen shows high radioprotective effects during irradiation, resulting in reduced induction of sterility. Overall, longevity of males can be improved by irradiating in anoxia, however the exposure to nitrogen itself comes with negative impacts on flight ability. The results reported here will assist in the standardization and optimization of irradiation protocols for the SIT to control mosquito populations of medical relevance.

Development of the Sterile Insect Technique to control the dengue vector Aedes aegypti (Linnaeus) in Sri Lanka

BACKGROUND

The Sterile Insect Technique (SIT) is presently being tested to control dengue in several countries. SIT aims to cause the decline of the target insect population through the release of a sufficient number of sterilized male insects. This induces sterility in the female population, as females that mate with sterilized males produce no offspring. Male insects are sterilized through the use of ionizing irradiation. This study aimed to evaluate variable parameters that may affect irradiation in mosquito pupae.

METHODS

An Ae. aegypti colony was maintained under standard laboratory conditions. Male and female Ae. aegypti pupae were separated using a Fay and Morlan glass sorter and exposed to different doses of gamma radiation (40, 50, 60, 70 and 80 Gy) using a Co60 source. The effects of radiation on survival, flight ability and the reproductive capacity of Ae. aegypti were evaluated under laboratory conditions. In addition, mating competitiveness was evaluated for irradiated male Ae. aegypti mosquitoes to be used for future SIT programmes in Sri Lanka.

RESULTS

Survival of irradiated pupae was reduced by irradiation in a dose-dependent manner but it was invariably greater than 90% in control, 40, 50, 60, 70 Gy in both male and female Ae. aegypti. Irradiation didn't show any significant adverse effects on flight ability of male and female mosquitoes, which consistently exceeded 90%. A similar number of eggs per female was observed between the non-irradiated groups and the irradiated groups for both irradiated males and females. Egg hatch rates were significantly lower when an irradiation dose above 50 Gy was used as compared to 40 Gy in both males and females. Irradiation at higher doses significantly reduced male and female survival when compared to the non-irradiated Ae. aegypti mosquitoes. Competitiveness index (C) scores of sterile and non-sterile males compared with non-irradiated male mosquitoes under laboratory and semi-field conditions were 0.56 and 0.51 respectively at 50 Gy.

SIGNIFICATION

Based on the results obtained from the current study, a 50 Gy dose was selected as the optimal radiation dose for the production of sterile Ae. aegypti males for future SIT-based dengue control programmes aiming at the suppression of Ae. aegypti populations in Sri Lanka.

Insecticide resistance in the Cydia pomonella (L): Global status, mechanisms, and research directions

The codling moth, Cydia pomonella (Lepidoptera: Tortricidae) is a major pest of pome fruit and walnuts worldwide. Although environmentally compatible integrated control strategies, such as mating disruption, attract-kill strategy, and sterile insect technique have been conducted for management of this notorious pest, effects to control of codling moth have mainly relied on insecticides. In consequence, different levels of insecticide resistance towards organophosphates, neonicotinoids, hydrazines, benzoylureas, pyrethroids, diamides, spinosyns, avermectins, JH mimics, carbamates, oxadiazines and C. pomonella granulovirus (CpGVs) have developed in codling moth in different countries and areas. Both metabolic and target-site mechanisms conferring resistance have been revealed in the codling moth. In this review, we summarize the current global status of insecticide resistance, the biochemical and molecular mechanisms involved, and the implications for resistance management.

Sterile Insect Technique (SIT) and Its Applications

Although most insect species have a beneficial role in the ecosystems, some of them represent major plant pests and disease vectors for livestock and humans. During the last six-seven decades, the sterile insect technique (SIT) has been used as part of area-wide integrated pest management strategies to suppress, contain, locally eradicate or prevent the (re)invasion of insect pest populations and disease vectors worldwide. This Special Issue on "Sterile insect technique (SIT) and its applications", which consists of 27 manuscripts (7 reviews and 20 original research articles), provides an update on the research and development efforts in this area. The manuscripts report on all the different components of the SIT package including mass-rearing, development of genetic sexing strains, irradiation, quality control as well as field trials.