Effect of irradiation on mating performance and mating ability in the West Indian sweetpotato weevil, Euscepes postfasciatus

The sterile insect technique (SIT) is widely used for suppressing or eradicating target pest insect populations. The effectiveness of SIT depends on the ability of released sterile males to mate with and inseminate wild females. Irradiation not only damages the reproductive cells but the somatic cells as well. The mating behavior of irradiated males may be altered over time due to the depressed metabolic activity brought about by sterilization. In this study, we evaluated the mating behavior (copulation behavior, mating performance, and ability of sperm transfer) of irradiated males in Euscepes postfasciatus (Fairmaire) (Coleoptera: Curculionidae) for 16 days after irradiation in the laboratory. The mating performance of males irradiated with a 150 Gy dose, as currently used in the SIT program in Okinawa prefecture for E. postfasciatus, decreased compared to that of control after day 7. As a result, we considered that irradiation had no major effect on male mating behavior for approximately 1 week after irradiation.     

Insect quality control: synchronized sex,mating system,and biological rhythm

The sterile insect technique (SIT) is a method of eradicating insects by releasing mass-reared sterilized males into fields to reduce the hatchability of eggs laid by wild females that have mated with the sterile males. SIT requires mass-production of the target insect, and maintenance of the quality of the mass-reared insects. The most important factor is successful mating between wild females and sterile males because SIT depends on their synchronized copulation. Therefore, understanding the mating systems and fertilization processes of target insects is prerequisite. Insect behavior often has circadian rhythms that are controlled by a biological clock. However, very few studies of relationships between sterile insect quality and circadian rhythm have been performed compared with the amount of research on the mating ability of target insects. The timing of male copulation attempts with receptivity of females is key to successful mating between released males and wild females. Therefore, we should focus on the mechanisms controlling the timing of mating in target insects. On the other hand, in biological control projects, precise timing of the release of natural enemies to attack pest species is required because behavior of pests and control agents are affected by their circadian rhythms. Involving both chronobiologists and applied entomologists might produce novel ideas for sterile insect quality control by synchronized sex between mass-reared and wild flies, and for biological control agent quality by matching timing in activity between predator activity and prey behavior. Control of the biological clocks in sterile insects or biological control agents is required for advanced quality control of rearing insects.     

Effect of irradiation on mating ability in the male sweetpotato weevil (Coleoptera: Curculionidae)

The sterile insect technique (SIT) is widely used for suppressing or eradicating target pest insect populations. The effectiveness of SIT depends on the ability of released sterile males to mate with and inseminate wild females. Irradiation is the effective manner to sterilize mass-reared insects. The negative impacts of this procedure are not limited to damage on reproductive cells. Gamma-radiation damages the epithelial tissue of midgut, which affects the alimentation in insects. Irradiated males alter their mating behavior over time because of the depression of metabolic activity by sterilization. In this study, we evaluated the male mating performance and sexually compatibility of irradiated male Cylas formicarius elegantulus (Summers) (Coleoptera: Curculionidae) with a 200-Gy dose, as currently used in the SIT program in Okinawa Prefecture, throughout 16 d after irradiation in the laboratory. The mating ability of irradiated males did not differ from that of control males for about a week. However, the mating ability of irradiated male drastically decreased thereafter. We consider that irradiated male C. formicarius elegantulus with a 200-Gy dose had no major effect on male mating behavior approximately for a week after irradiation.     

Evaluation of partial sterility in mating performance and reproduction of the West Indian sweetpotato weevil,Euscepes postfasciatus

The sterile insect technique (SIT) is based on population and behavioral ecology and is widely used to suppress or eradicate target pest insect populations. The effectiveness of SIT depends on the ability of the released sterile males to mate with and inseminate wild females. The use of gamma‐radiation to induce sterility is, however, associated with negative impacts not only on reproductive cells but also on somatic cells. Consequently, irradiation for sterilization diminishes mating performance over time. In this study, we evaluated the balance between the irradiation dose and both fertility and mating propensity in Euscepes postfasciatus (Fairmaire) (Coleoptera: Curculionidae) for 22 days following irradiation. The mating propensity of males irradiated with a 150‐Gy dose, as currently used to induce complete sterility of E. postfasciatus in the SIT program in Okinawa Prefecture, was equal to that of non‐irradiated weevils for up to 6 days, and the mating propensity of males irradiated with a dose of 125 Gy was equal to that of non‐irradiated weevils for twice this period (12 days). The fertilization ability of weevils irradiated with a dose of 125 Gy was reduced by 4.6% in males and 0.6% in females, compared to the potential fertilization ability. We also discuss the possibility of the application of partially sterilized insects in eradication programs.     

The Sterile Insect Technique for Controlling Populations of Aedes albopictus (Diptera: Culicidae) on Reunion Island: Mating Vigour of Sterilized Males

Reunion Island suffers from high densities of the chikungunya and dengue vector Aedes albopictus. The sterile insect technique (SIT) offers a promising strategy for mosquito-borne diseases prevention and control. For such a strategy to be effective, sterile males need to be competitive enough to fulfil their intended function by reducing wild mosquito populations in natura. We studied the effect of irradiation on sexual maturation and mating success of males, and compared the sexual competitiveness of sterile versus wild males in the presence of wild females in semi-field conditions. For all untreated or sterile males, sexual maturation was completed within 13 to 20 h post-emergence and some males were able to inseminate females when 15 h old. In the absence of competition, untreated and sterile males were able to inseminate the same number of virgin females during 48 h, in small laboratory cages: an average of 93% of females was inseminated no matter the treatment, the age of males, and the sex ratio. Daily mating success of single sterile males followed the same pattern as for untreated ones, although they inseminated significantly fewer females after the ninth day. The competitiveness index of sterile males in semi-field conditions was only 0.14 when they were released at 1-day old, but improved to 0.53 when the release occurred after a 5-day period in laboratory conditions. In SIT simulation experiments, a 5∶1 sterile to wild male ratio allowed a two-fold reduction of the wild population’s fertility. This suggests that sterile males could be sufficiently competitive to mate with wild females within the framework of an SIT component as part of an AW-IPM programme for suppressing a wild population of Ae. albopictus in Reunion Island. It will be of interest to minimise the pre-release period in controlled conditions to ensure a good competitiveness without increasing mass rearing costs.     

Prolongation of the effective copulation period by fractionated‐dose irradiation in the sweet potato weevil,Cylas formicarius

The sterile insect technique (SIT), based on the principles of population and behavioral ecology, is widely used to suppress or eradicate target pest insect populations. The effectiveness of SIT depends on the ability of released sterile males to mate with and inseminate wild females; however, the use of gamma radiation to induce sterility negatively affects both somatic cells as well as reproductive cells. Consequently, sterilization by irradiation drastically diminishes mating performance over time. It is well known that fractionated‐dose irradiation, in which a sterilizing dose is delivered via a series of smaller irradiations, reduces radiation damage. In the present study, we evaluated the effect of fractionated‐dose irradiation on fertility, longevity, and mating propensity in Cylas formicarius (Summers) (Coleoptera: Brentidae) for 16 days after irradiation. Fractionated‐dose irradiation with 200 Gy induced full sterility regardless of the number of radiation doses. Although the mating propensity of males sterilized by a single 200 Gy dose (the current standard of the Okinawa Prefecture SIT program) was equal to that of non‐irradiated weevils for the first 6 days, the mating propensity of males sterilized by a series of three doses was maintained for at least the first 12 days. These results demonstrated that fractionated‐dose irradiation can be highly advantageous in C. formicarius eradication programs.     

Remating in Ceratitis capitata sterile males: Implications in sterile insect technique programmes

Sterile insect technique (SIT) is used, among other biological control tools, as a sustainable measure for the management of Ceratitis capitata Wiedemann (Diptera: Tephritidae) in many agricultural regions where this pest can trigger severe economic impacts. The tendency of wild females to remate multiple times has been deeply studied; it has been a common point of controversy when evaluating SIT programmes. Nevertheless, the remating potential of the released sterile males remains unknown. Here, under laboratory conditions, the remating capability of mass-reared sterile males was determined. Wild-type virgin females were offered to sterile males (Vienna-8 strain), which had the opportunity to mate up to four consecutive times. The remating assays were carried out at 24 hr, 48 hr, 4 days and 7 days after the first mating. At the end of each tested time period, males were divided according to their mating response, mated or unmated, and subsequently reused for the next round of mating assays. The frequency of successful remating in each tested time period was obtained. Insemination was confirmed by determining the sperm transfer in mated female spermathecae by quantitative real-time PCR. Our results demonstrate that 73% of the mass-reared sterile males were able to remate 24 hr after the first mating, 55% of which remated again the day after. Close to 25% of the V8 sterile males tended to copulate in all of the four mating opportunities. The qPCR analysis of the spermathecae contents verified an effective transfer of V8 sperm to wild females with every mating; 99% of copulations resulted in sperm transfer. These findings shed light on the remating potential of V8 sterile males, an aspect until now underestimated in many SIT programmes.     

Assessment of effect of partial sterility on mating performance in sweetpotato weevil (Coleoptera: Curculionidae)

The sterile insect technique (SIT) is widely used to suppress or eradicate target pest insect populations. Although the effectiveness of SIT depends on the ability of released sterile males to mate with and inseminate wild females, the use of gamma radiation to induce sterility negatively impacts reproductive cells as well as somatic cells. Consequently, sterilization by irradiation drastically diminishes mating performance over time. In the current study, we evaluated the effect of irradiation dose intensity on fertility, mating propensity, and mating competitiveness in sweetpotato weevil, Cylas formicarius elegantulus (Summers) (Coleoptera: Curculionidae), for 16 d after irradiation. Although the mating propensity of males irradiated with 200 Gy, the dose currently used to induce complete sterility of C. f. elegantulus in the SIT program in Okinawa Prefecture, was equal to that of nonirradiated weevils for the first 6 d, the mating propensity of males irradiated with doses between of 75 and 150 Gy was maintained for the first 12 d. The potential fertilization ability of weevils was highly depressed compared with the control weevils, even in those treated with 75 Gy. Mating performance was severely compromised in weevils that were irradiated with a dose of 100 Gy or more. These results demonstrate that partial sterilization can be highly advantageous in eradication programs for the sweetpotato weevil. We discuss the advantages of the application of partial irradiation in insect eradication programs.     

Radiation effects on Drosophila suzukii (Diptera: Drosophilidae) reproductive behaviour

Female remating is a widespread behaviour, reported in several insect species. This behaviour can affect the efficiency of sterile insect technique (SIT); however, little is known about the postcopulatory behaviour of some pest species considered as candidates to be controlled by this technique, such as Drosophila suzukii (Matsumura, 1931) (Diptera: Drosophilidae). In this study, we investigated the effects of male and female sterilization on mating and remating behaviour of D. suzukii. First, we tested the occurrence of multiple mating in different combinations between sterile and fertile males and females. Then, we tested the effects of male and female sterility on female propensity to mate and remate. We found an overall low remating rate by D. suzukii females. Male sterility did not influence mating and remating likelihood; however, copula duration of sterile males was shorter compared to fertile males. On the other hand, sterile females were less likely to mate. Our findings encourage further research regarding the use of SIT to control D. suzukii.     

Suppressing effect of gamma-irradiated weevils on progeny production in the West Indian sweetpotato weevil Euscepes postfasciatus (Coleoptera: Curculionidae)

The West Indian sweet potato weevil Euscepes postfasciatus (Fairmaire) (Coleoptera: Curculionidae) is a major pest of sweet potato Ipomoea batatas (L.) in the tropical and subtropical regions. The sterile insect technique (SIT) could be used as one of the most effective methods for suppression or eradication of the weevil. The effectiveness of SIT depends on the ability of the released sterile males to mate with and inseminate wild females. However, the effect of sterile weevils on the fitness of E. postfasciatus has not been evaluated on natural density. Here, we investigated the effect of gamma-irradiated weevil density on the number of weevil progeny. When irradiated weevils were released in numbers equal to those of non-irradiated weevils, the number of progeny was reduced by half of that in the control treatment, and it remained at this state for 2?weeks. Our results show that irradiated weevils ensure adequate and efficient suppression of wild weevils. We conclude that the SIT programs will be employed as effective eradication method for E. postfasciatus.     

Multiple paternity in a natural population of a wild tobacco fly, Bactrocera cacuminata (Diptera: Tephritidae), assessed by microsatellite DNA markers

Mating frequency has important implications for patterns of sexual selection and sexual conflict and hence for issues such as speciation and the maintenance of genetic diversity. Knowledge of natural mating patterns can also lead to more effective control of pest tephritid species, in which suppression programmes, such as the sterile insect technique (SIT) are employed. Multiple mating by females may compromise the success of SIT. We investigated the level of polyandry and sperm utilization in a Brisbane field population of the tropical fruit fly, Bactrocera cacuminata (Hering), using seven polymorphic microsatellite loci. The offspring of 22 wild-caught gravid females were genotyped to determine the number of males siring each brood and paternity skew, using the programs gerud and scare. Our data showed that 22.7% of females produced offspring sired by at least two males. The mean number of mates per female was 1.72. Paternal contributions of double-sired broods were skewed with the most successful male having sired between 76.9% and 87.5% of the offspring. These results have implications for SIT, because the level of remating we have identified would indicate that wild females could mate with one or more resident fertile males.     

Laboratory adaptation reduces female mating resistance in the sweet potato weevil

Selection for genetic adaptation might occur whenever an animal colony is maintained in the laboratory. The laboratory adaptation of behavior such as foraging, dispersal ability, and mating competitiveness often causes difficulties in the maintenance of biological control agents and other beneficial organisms used in procedures such as the sterile insect technique (SIT). Sweet potato weevil, Cylas formicarius (Summers) (Coleoptera: Brentidae), is an important pest in sub‐tropical and tropical regions. An eradication program targeting C. formicarius using SIT was initiated in Japan with weevils being mass‐reared for 95 generations to obtain sufficient sterile males. The mass‐reared strain of C. formicarius exhibits weaker female resistance to male mating attempts compared with the wild strain. This could affect the success of SIT programs because mating persistence of mass‐reared males might be expected to decrease in response to weak female resistance. We show that high success of sperm transfer to mass‐reared females was due to weak female resistance to male mating attempts. However, the mating behavior of mass‐reared males did not change. In C. formicarius, the trait of male persistence to mate was not correlated with the female resistance traits. Our results suggest that mass‐rearing conditions do not have negative effects on the mating ability of the sterile males of this species, and thus that the current mass‐rearing procedures are suitable for production of sterile males for the weevil eradication program.     

Disparity in sexual behaviour between wild and mass‐reared Mexican fruit flies

The sterile insect technique (SIT) is currently used to control Mexican fruit fly Anastrepha ludens Loew (Diptera: Tephritidae). However, mass‐rearing can alter the quality of released males. If males that are mass‐reared have behaviours different from those of their wild counterparts, then this may diminish the effectiveness of SIT. Questions remain as to whether wild females may be able to detect the male condition before, during and/or after copulation with a mass‐reared male. In the present study, copula duration, female remating, female fecundity and fertility of both mass‐reared and wild A. ludens are evaluated. Marked differences are found between mass‐reared and wild females. Specifically, mating latency is longer and copula duration is shorter for wild females compared with mass‐reared females. Importantly, there are no significant differences in mating latency, copula duration or remating probability between wild females paired with either mass‐reared or wild males. All mass‐reared females remate, whereas only approximately half of the wild females remate after first mating with either a wild or mass‐reared male. Fecundity of wild females mated to either wild or mass‐reared males is approximately one‐third lower than that of mass‐reared females, confirming that mass‐reared females may have been selected for high fecundity and are adapted to laboratory conditions. Fertility of females that mate with a wild male for only 10 min is not significantly different from that achieved via a full‐length copulation. By contrast, females mating with mass‐reared males need copulation durations of at least 40 min to achieve fertility comparable with that achieved via a full‐length copulation. The findings of the present study have important implications for A. ludens controlled through SIT and broaden our understanding on the copulatory and post‐copulatory behaviours between wild females and mass‐reared males.     

Female presence enhances sexual performance of sterile Anastrepha ludens males of the Tapachula-7 GSS strain

The sterile insect technique (SIT), used for the control of many tephritid fly pests, is based on the rearing and release of large numbers of sexually competitive sterile insects into a wild population. In the interest of reducing expenses and increasing SIT effectiveness, genetic sexing strains (GSS) have been developed. These strains allow the production and release of only males. The objective of our study was to assess the effects of pre-release adult exposure to methoprene and to females on the mating propensity and mating competitiveness of GSS sterile males of Anastrepha ludens (Loew) (Diptera: Tephritidae). GSS sterile males were kept on a protein-sugar (protein-fed) or a protein-sugar-methoprene diet and were exposed to different proportions of females for the normal pre-release period of 5 days. Using laboratory and field-cage bioassays, we examined the influence of methoprene and female presence on the mating success of sterile males of 3–9 days old, in competition for wild females with untreated males and with wild males. Methoprene and female exposure had no significant effects on male mating success in the laboratory, whereas age had a positive relationship with the number of copulations observed. However, in field-cage bioassays, males exposed to females obtained a higher number of copulations than unexposed control males. Possible implications of these findings for programs that use GSS and especially for the campaign against Mexican fruit flies are discussed.     

Bacterially enriched diet improves sexual performance of sterile male Mediterranean fruit flies

The Mediterranean fruit fly, Ceratitis capitata (Wiedemann) (Diptera: Tephritidae), (medfly) is a polyphagous and cosmopolitan agricultural pest, targeted in many areas for control by the Sterile Insect Technique (SIT). Our objective in the present study was to test the hypothesis that a bacterially enriched diet provided to sterile males will improve their sexual performance in competitive settings that emulate natural conditions. Specifically we determined how feeding on diets enriched with Klebsiella oxytoca affected the ability of sterile males to compete for wild females against wild males, their ability to inhibit female receptivity, and their survival. We found that enriching the sterile male diet with K.oxytoca significantly improved mating competitiveness in the laboratory and in field cages. In addition, bacterially enriched sterile males inhibited female receptivity more efficiently than sugar fed males and survived longer duration of starvation. We conclude that inoculating mass reared sterile flies with bacteria prior to their release is a valid approach to improve the efficacy of SIT.     

Male lures and male mating competitiveness in a genetic sexing strain of the melon fly (Diptera: Tephritidae)

The Sterile Insect Technique (SIT) has been used to suppress or eradicate populations of the melon fly, Zeugodacus cucurbitae (Coquillett), a pest of various fruits and vegetables throughout Asia, Africa, and South Pacific Islands. The success of the SIT depends largely on the ability of released, sterile males to compete successfully with wild males for copulations with wild females. Unfortunately, both the sterilization process (via gamma irradiation) and artificial selection under mass-rearing conditions generally reduce the biological competence, including the mating competitiveness, of the released males. However, previous data on Z. cucurbitae show that irradiation has minimal adverse effects and prolonged mass-rearing may or may not result in decreased mating ability. The present study first confirms that neither irradiation nor mass-rearing had deleterious effects on male mating ability of males from a pupal-color, genetic sexing strain (termed T1). Then, the influence of male lures (cue-lure and its natural analogue raspberry ketone) on the mating success of sterile T1 males was measured as a potential pre-release procedure to improve the effectiveness of SIT. Feeding on cue-lure enhanced male mating success, but the effect was short-lived (1 d but not 3 d post-feeding). Feeding on raspberry ketone boosted mating success over a longer time, i.e., at least 5 d but not 10 d post-feeding. However, mixing raspberry ketone into the adult diet had no detectable effect on male mating performance. The implications of these results for SIT against Z. cucurbitae are discussed.     

Fractionated irradiation improves the mating performance of the West Indian sweet potato weevil Euscepes postfasciatus

• 1 The sterile insect technique (SIT) is widely used to suppress or eradicate target pest insect populations.
• 2 The effectiveness of SIT depends on the ability of released sterile males to mate with and inseminate wild females. The use of gamma radiation to induce sterility, however, negatively affects both somatic cells as well as reproductive cells. Consequently, mating performance of sterilized individuals decreases drastically over time. The mating propensity of sterilized Euscepes postfasciatus (Fairmaire) males irradiated with a single dose of 150 Gy (the current standard of the Okinawa Prefecture SIT programme) is equal to that of non‐irradiated weevils for the first 6 days.
• 3 Fractionated irradiation, in which a sterilizing dose is delivered over time in a series of smaller irradiations, reduces the damage of irradiation in insects. In the present study, we evaluated the effect of fractionated irradiation on male fertilization ability, longevity and mating propensity of E. postfasciatus for a period of 16 days after irradiation.
• 4 Although fractionated irradiation totalling 150 Gy was found to induce full sterility regardless of the number of individual doses, the mating propensity of male weevils sterilized by fractionated irradiation was maintained for the first 12 days. These results demonstrate that fractionated irradiation can be highly advantageous in programmes aimed at eradication of E. postfasciatus.

     

Genetic sexing through the use of Y-linked transgenes

Sterile insect technique (SIT)-based pest control programs rely on the mass release of sterile insects to reduce the wild target population. In many cases, it is desirable to release only males. Sterile females may cause damage, e.g., disease transmission by mosquitoes or crop damage via oviposition by the Mediterranean fruit fly (Medfly). Also, sterile females may decrease the effectiveness of released males by distracting them from seeking out wild females. To eliminate females from the release population, a suitable sexual dimorphism is required. For several pest species, genetic sexing strains have been constructed in which such a dimorphism has been induced by genetics. Classical strains were based on the translocation to the Y chromosome of a selectable marker, which is therefore expressed only in males. Recently, several prototype strains have been constructed using sex-specific expression of markers or conditional lethal genes from autosomal insertions of transgenes. Here, we describe a novel genetic sexing strategy based on the use of Y-linked transgenes expressing fluorescent proteins. We demonstrate the feasibility of this strategy in a major pest species, Ceratitis capitata (Wiedemann), and discuss the advantages and disadvantages relative to other genetic sexing methods and potential applicability to other species.     

Pupal X-ray irradiation influences protein expression in adults of the oriental fruit fly,Bactrocera dorsalis

The oriental fruit fly, Bactrocera dorsalis, is a pest of fruit in the Asia–Pacific region and also, due to quarantine restrictions, a threat to California fruit production. Area-wide suppression of B. dorsalis integrated several approaches including the sterile insect technique (SIT). SIT involves exposing juveniles to gamma radiation and releasing sterile males in substantial numbers, where they successfully compete for wild females. The resulting infertile eggs lead to reduction of the pest populations. Although these protocols are well documented, arising issues about the international transport and distribution of radioactive products is creating difficulties in use of radioactive sources for sterilizing radiation. This led to a shift toward use of X-ray irradiation, which also sterilizes male and female insects. However, use of X-ray technologies is in its infancy and there is virtually no information on the effects of irradiation, other than sterilization, at the physiological and molecular levels of fruit fly biology. We posed the hypothesis that sterilizing male oriental fruit flies via radiation treatment also influences protein expression in the flies. We found that exposing pupae to X-ray irradiation impacted expression of 26 proteins in adult females and 31 proteins in adult males. Seven proteins (glyceraldehyde-3-phosphate dehydrogenase, fructose-bisphosphate aldolase, larval cuticle protein 2, sarcoplasmic calcium-binding protein alpha-B and A chains, general odorant-binding protein 99b, polyubiquitin, and protein disulfide-isomerase) were impacted in both sexes. Some of the proteins act in central energy-generating and in pheromone-signal processing pathways; we infer that males sterilized by X-ray irradiation may be enfeebled in their ability to compete with wild males for females in nature.