A review of 16 years of quality control parameters at a mass‐rearing facility producing Queensland fruit fly,Bactrocera tryoni

From 1996 to 2012, the mass‐rearing facility at Camden (NSW, Australia) has been producing Queensland fruit flies, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae). During this time, the facility has regularly recorded fly quality parameters, creating a unique data set that provides an invaluable opportunity to evaluate the interrelationships among standard quality control (QC) parameters and test for redundant QC variables. Here, we conducted an exploratory data analysis to reveal relationships among the QC parameters. We found that pupal weight, adult lifespan, and longevity under nutritional stress (i.e., survival duration without food or water) had distinct monthly trends, suggesting that these QC parameters are sensitive to seasonal conditions. Furthermore, emergence percentage, flight ability, and adult lifespan were adversely affected by the dyeing/handling/irradiation process associated with sterile insect releases. Using a multivariate approach and controlling for monthly and yearly patterns, we showed that pupal weight and egg hatch are consistently negatively related and that percentage male and emergence rates are consistently negatively related. These results suggest that these correlation pairs measure similar quality information and hence one QC variable from each pair could be dropped. Flight ability was not strongly correlated with any of the QC variables, suggesting that this QC variable remains a useful QC metric. Finally, the longevity under nutritional stress QC appears to be fairly insensitive to QCs and we suggest that it should be replaced by the standard mortality under stress test.     

Fecundity,fertility and reproductive recovery of irradiated Queensland fruit fly Bactrocera tryoni

Pupae of the Queensland fruit fly or Q‐fly Bactrocera tryoni (Froggatt) are irradiated routinely to induce reproductive sterility in adults for use in sterile insect technique programmes. Previous studies suggest that adult sexual performance and survival under nutritional and crowding stress are compromised by the current target dose of radiation for sterilization (70–75 Gy), and that improved mating propensity and survival under stress by irradiated males may be achieved by reducing the target sterilization dose without reducing the level of induced sterility. This raises the question of the amount by which the irradiation dose can be reduced before residual fertility becomes unacceptable. The present study measures the levels of residual fertility in male and female irradiated Q‐flies at different irradiation doses (20, 30, 40, 50, 60 and 70 Gy), and investigates the possibility that fecundity and fertility increase between 10–15 and 30–35 days post emergence. Male flies require a higher dose than females to induce sterility, with no residual fertility found in females irradiated at doses of 50 Gy or above, and no residual fertility found in males irradiated at doses of 60 Gy or above. Irradiated females are more fecund at 30–35 days post emergence than at 10–15 days. However, fertility does not increase between 10 and 15 days post emergence and 30–35 days, even at doses below 50 Gy. The present study shows that there is scope to reduce the target sterilization dose for Q‐flies below that of the current dose range (70–75 Gy) at the same time as retaining an adequate safety margin above radiation doses at which residual fertility can be expected.     

Activity patterns of Queensland fruit flies (Bactrocera tryoni) are affected by both mass‐rearing and sterilization

Mass‐reared sterile tephritid flies released in sterile insect technique (SIT) programmes exhibit behaviour, physiology and longevity that often differ from their wild counterparts. In the present study, video recordings of flies in laboratory cages are used to determine whether the sequential processes of mass‐rearing and sterilization (using gamma radiation) that are integral to SIT affect general activity patterns of male and female Queensland fruit flies Bactrocera tryoni (Froggatt) (Diptera: Tephritidae) (‘Q‐flies'). Compared with wild flies, mass‐reared flies exhibit a marked reduction in overall activity, and further reduction is found after sterilization. In terms of the frequency of activities, both fertile and sterile mass‐reared Q‐flies fly less often and exhibit more bouts of inactivity and grooming than wild Q‐flies. In addition, in terms of the duration of activities, fertile and sterile mass‐reared Q‐flies spend less time flying and more time walking, grooming and being inactive than wild Q‐flies. Although fertile and sterile mass‐reared flies are similar in other regards, sterile mass‐reared flies spend more time being inactive than fertile mass‐reared flies. These findings raise new questions about how changes in behaviour and activity levels may influence the performance of mass‐reared sterile Q‐flies in the field, as well as the physiological and metabolic processes that are involved. The frequency and duration of inactivity could provide a simple but powerful and biologically relevant test for quality in mass‐rearing and SIT programs.     

Sexual development of wild and mass‐reared male Queensland fruit flies in response to natural food sources

Diet has a profound influence on the fitness of adult tephritid flies. Mass‐reared flies are provided yeast hydrolysate as a rich source of nutrition that supports rapid sexual development and mating success. In contrast, wild tephritid flies often live in environments where food may be hard to find, and these are the conditions that sexually immature mass‐reared sterile males encounter when released into the field during sterile insect technique campaigns. The effect of natural food sources (bat guano, bird droppings, citrus pollen, and wheat pollen) on the sexual development of adult mass‐reared fertile, mass‐reared sterile, and wild male Queensland fruit flies, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae), was determined by measuring ejaculatory apodeme size. Inclusion of yeast hydrolysate in the adult diet was associated with faster growth of the ejaculatory apodeme in comparison with all other diets. Effects of diet were far less pronounced in mass‐reared males, which may indicate reduced nutritional requirements, whereas the ejaculatory apodeme of wild males fed on natural sources of food or sucrose alone did not increase in size over the first 20 days of adult life.     

Effects of field cage colour and supplementary shade on environmental conditions and mating behaviour of Queensland fruit flies,Bactrocera tryoni

The mating performance field cage test is a required periodic quality‐control assessment for factory‐reared fruit flies used for the sterile insect technique. The FAO/IAEA/USDA guidelines for assessing fly quality state that if during tests a large proportion of flies call and mate on cage walls, away from host trees, then environmental conditions within the cage need to be adjusted and tests repeated. Here we test effects of cage design, specifically mesh colour (green, white) and addition of supplementary shade, on the mating behaviour of Queensland fruit fly (Q‐fly), Bactrocera tryoni Froggatt (Diptera: Tephritidae). Observations were made over a 4‐h period at dusk when these flies mate. Changes in environmental conditions in each cage over the dusk period varied with cage design. We recorded the highest proportion of matings taking place on trees as opposed to cage walls (>90%) in the unshaded white cage, the shaded white and un‐shaded green cages being intermediate (ca. 70%), and the shaded green cage had the least (ca. 40%). The effects of field cage colour and supplementary shade on mating behaviour are discussed. We recommend that Q‐fly field cage tests should be conducted in cages with a light coloured mesh, and that supplementary shading should only be applied if there is a need to adjust temperature and light within the cage.     

Oviposition responses of Queensland fruit fly (Bactrocera tryoni) to mineral oil deposits on tomato fruit

Behavioural responses of wild and laboratory‐culture females of Queensland fruit fly, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae), to mineral oil deposits on tomato fruit dipped in aqueous oil emulsions were assessed in a no‐choice test and three choice tests. The oils were two commercial products used to manage plant pests and diseases, Ampol D‐C‐Tron NR and SK EnSpray 99, one distillation fraction of the base oil of the former, and four distillation fractions of the base oil of the latter. The initial and final boiling points of the fractions were equivalent to those of n‐alkanes with chain lengths of C20–22 (Ampol), and C21–C23, C22–23.5, C22–24, and C22–24.5 (SK). For both fly types in the no‐choice test, numbers of punctures and eggs per fruit declined strongly as concentrations of the nC20–22 Ampol fraction in emulsions rose from 0.25 to 2% (vol/vol). Fly type affected the extent of responses but there was no significant interaction for fly type*oil concentration. Responses of laboratory‐culture females in the choice tests also declined as concentrations of SK and the four fractions of its base oil in emulsions rose from 0 to 0.25%. The SK nC22–24 and nC22–24.5 fractions had least impact. Responses of laboratory‐culture flies to 0.5% emulsions of the nC20–22 Ampol fraction and the nC21–23 SK fraction in choice tests were not significantly different. Likewise, responses of laboratory‐culture flies to 0.5% emulsions of the two commercial products were not significantly different. Emulsifier type did not affect numbers of punctures or eggs per fruit in choice responses of laboratory‐culture flies to 0.5% emulsions of the Ampol nC20–22 fraction or 0.5% emulsions of the SK nC21–23 fraction. If the equivalence of no‐choice and choice responses in the laboratory were to hold in the field, then unsprayed ‘sacrificial’ plants would not be necessary and oil emulsions could be used as cover sprays.     

Factors affecting the dispersal of large‐scale releases of the Queensland fruit fly,Bactrocera tryoni

This study investigated two factors potentially affecting the spatial distribution of Queensland fruit fly Bactrocera tryoni (Froggatt) after release from a single point. First, the effect of age at release was investigated using a single cohort of 205 000 males from a mass‐rearing strain used for sterile insect releases. Approximately half were released as immature males and the remainder as sexually mature males (1 week later). Males were collected over 3 weeks from a grid of 135 traps, each containing a pheromone/insecticide bait, positioned between 4 and 500 m from the release point. Variation in the distribution of fly density around the release point was assessed by regressing trapped fly counts against distance. Unexpectedly, no significant differences were found in the spatial distribution of the flies. Second, the effect of inbreeding on spatial distribution was investigated using replicated simultaneous releases of two strains of B. tryoni. One strain was the existing (inbred) mass‐rearing strain that has been selected for high productivity in a mass‐rearing facility. The second strain was deliberately outbred but also selected for high productivity. Almost 100 000 males of each strain were released over the two experiments. Regression of trappings against distance differed significantly between strains in only one of five releases, but in all cases the outbred strain had a greater dispersal distance. As our trapping grid was not regular but contained gaps of up to 100 m, a small preliminary experiment investigated whether flies move faster along tree rows or across open fields. At distances up to 100 m, we found no detectable difference in fly distributions. These results are primarily relevant to the large‐scale point releases carried out as part of an existing B. tryoni sterile insect programme and are discussed in that context.     

Evaluation of yeasts in gel larval diet for Queensland fruit fly,Bactrocera tryoni

Queensland fruit fly, Bactrocera tryoni (“Q‐fly”), is Australia’s most economically important insect pest of horticultural and commercial crops especially in the eastern regions. The sterile insect technique (SIT) has been adopted as an environmentally benign and sustainable approach for management of Q‐fly outbreaks. High‐performance larval diets are required to produce the millions of flies needed each week for SIT. Yeast products contribute amino acids (protein) to fruit fly larval diets, as well as carbohydrate, fat and micronutrients, but there can be substantial variation in the nutritional composition and suitability of yeast products for use in larval diets. Gel larval diets have recently been developed for large‐scale rearing of Q‐fly for SIT, and composition of these diets requires optimization for both performance and cost, including choice of yeast products. We assessed performance of Q‐flies reared on gel larval diets that contained debittered brewer’s yeast (Lallemand LBI2240), hydrolysed yeast (Lallemand FNILS65), inactivated brewer’s yeast (Lallemand LBI2250) and inactivated torula yeast (Lallemand 2160‐50), including blends. Q‐flies performed poorly when reared on diets containing only or mostly hydrolysed yeast in terms of pupal number, pupal weight and percentage of fliers. Performance was also poor on diets containing high proportions of torula yeast. Overall, debittered brewer’s yeast is recommended as the best option for Q‐fly gel larval diet, as it is cheap, readily available, and produces flies with good performance in quality control assays. Inactivated brewer’s yeast produced flies of comparable quality with only a modest increase in cost and would also serve as an effective alternative.     

Dispersal of mass‐reared sterile,laboratory‐domesticated and wild male Queensland fruit flies

Queensland fruit flies, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae) (‘Q‐flies’) were released as sexually immature adults from a point within an orchard. Marked male Q‐flies were recaptured in the trap furthest from the release point (1087 m) by 2 weeks after release, although 98.25 ± 1.04% of recaptured males were trapped <500 m from the release point. Comparison of gamma‐irradiated (sterile), laboratory‐adapted and wild male Q‐flies indicated that dispersal distance was not significantly affected by fly type. There was no significant correlation between temperature and mean dispersal distance, but total recaptures were significantly negatively correlated with increasing daily maximum, minimum and average temperature.     

Effects of irradiation dose rate on quality and sterility of Queensland fruit flies, Bactrocera tryoni (Froggatt)

Queensland fruit fly (Bactrocera tryoni; Q‐fly) pupae are routinely irradiated to induce reproductive sterility in adults released in a sterile insect technique programme. Although there have been some studies of how total dose influences fly quality, dose rate has not been considered. In the present study, pupae were irradiated at a target dose range of 70–75 Gy at dose rates of approximately 5, 7, 26, 57 and 80 Gy/min and were then subjected to routine IAEA/FAO/USDA quality control tests including emergence, flight ability, mortality under stress and sterility induction. No significant effects of dose rate were found on emergence or flight ability. Sterility induction was also found to be independent of dose rate, a result conforming to a ‘one‐hit’ ionizing event hypothesis. Flies irradiated at higher dose rates suffered increased mortality under stress. This appears to stem from an increased tendency to over‐shoot the target dose when irradiating at high dose rates. We recommend that, to reduce potential error in total target dose, the lowest practical dose rate be used when irradiating Q‐fly pupae for use in the sterile insect technique.     

Feeding on yeast hydrolysate enhances attraction to cue‐lure in Queensland fruit flies,Bactrocera tryoni

Feeding on yeast hydrolysate (a source of nitrogen) has a strong influence on the physiology and behaviour of the Queensland fruit fly (Q‐fly), Bactrocera tryoni (Froggatt) (Diptera: Tephritidae), affecting longevity, sexual maturation, oogenesis, and mating performance. In this study, we demonstrate that access to yeast hydrolysate also influences the development of attraction to cue‐lure in Q‐flies. We provided virgin Q‐flies various periods of access to yeast hydrolysate (continuous, 48 h, 24 h, or deprived). Attraction of males to cue‐lure was increased and occurred at an earlier age when they were fed yeast hydrolysate. Males given continuous access were strongly attracted to cue‐lure at a younger age (8 days after emergence), but by 12 days after emergence attraction of males given access to yeast hydrolysate for 48 h did not differ from males given continuous access. Attraction by males deprived or given just 24 h access to yeast hydrolysate was always significantly lower than those of males with continuous access. Male attraction to cue‐lure was highest in the early morning. While cue‐lure is most often thought of as a male attractant, virgin female Q‐flies were caught in cue‐lure traps at dusk at ages when they are known to be sexually mature. We suggest that cue‐lure or similar natural chemicals play a role in the Q‐fly mating system. γ‐Irradiation used to induce sterility had no significant effect on attraction to cue‐lure by Q‐flies.     

Mate choice by wild and mass‐reared females of the Mediterranean fruit fly

The sterile insect technique (SIT) is used to control Mediterranean fruit fly, Ceratitis capitata (Wiedemann), but its effectiveness is limited by low sexual competitiveness of mass‐reared males. This study investigated whether wild and mass‐reared [from a temperature sensitive lethal (tsl) genetic sexing strain] females display similar mate preferences and thus exert similar selective forces on the evolution of male courtship behaviour. Wild females preferred wild males over tsl males, whereas tsl females mated indiscriminately. The probability that mounting resulted in copulation was related to the duration of pre‐mount courtship for wild females, and wild males performed longer courtships than tsl males. Copulation occurred independently of courtship duration in tsl females. Counter to the aim of the SIT, female choice by tsl females appears to promote the evolution of male behaviour disfavoured by wild females.     

Behavioural responses of female Queensland fruit fly, Bactrocera tryoni, to mineral oil deposits

Behavioural responses of Queensland fruit fly, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae), females to fruit dipped in water and fruit dipped in 0.5% (vol/vol) aqueous emulsions of a mineral oil were determined and analysed. The mineral oil was an nC20–22 distillation fraction of the base oil used to produce an nC23 horticultural mineral oil. Females caged with oil‐treated fruit had significantly longer prelanding intervals than females caged with water‐dipped fruit. The latter was attacked immediately or shortly after being caged with flies whereas some oil‐dipped fruit was not attacked within 180 min. The percentage of landings that led to oviposition on water‐ and oil‐treated fruit were 58 and 13%, respectively, and the percentages ovipositing after probing were 74 and 25%, respectively. Likewise, average times spent probing were 7 vs. 31 s whereas average times spent ovipositing were 321 vs. 223 s. Females spent less than half as much time on oil‐treated fruit than on water‐treated fruit. Transition probabilities of rejection, when applied to the behaviour sequence indicated that oil‐treated fruits are about nine times less likely to be infested with B. tryoni.     

Wild bacterial probiotics fed to larvae of mass‐reared Queensland fruit fly [Bactrocera tryoni (Froggatt)] do not impact long‐term survival,mate selection,or locomotor activity

Queensland fruit fly [Bactrocera tryoni (Froggatt), Diptera, Tephritidae] is the most devastating insect pest impacting Australian horticulture. The Sterile Insect Technique (SIT) is an important component of tephritid pest management programs. However, mass‐rearing and irradiation (to render insects sterile) may reduce the fitness and performance of the insect, including the ability of sterile males to successfully compete for wild females. Manipulation of the gut microbiome, including the supplementation with bacterial probiotics shows promise for enhancing the quality of mass‐reared sterile flies, however there are fewer published studies targeting the larval stage. In this study, we supplemented the larval stage of mass‐reared B. tryoni with bacterial probiotics. We tested several individual bacteria that had been previously isolated and characterized from the gut of wild B. tryoni larvae including Asaia sp., Enterobacter sp., Lactobacillus sp., Leuconostoc sp. We also tested a consortium of all four of these bacterial isolates. The fitness parameters tested included adult survival in field cages, laboratory mate selection of bacteria supplemented males by bacteria nonsupplemented females, and laboratory locomotor activity of adult flies. None of the bacterial probiotic treatments in the current study was significantly different to the control for field survival, mate selection or locomotor activity of adult B. tryoni, which agree with some of the other studies regarding bacterial probiotics fed to the larval stage of tephritids. Future work is needed to determine if feeding the same, and/or other probiotics to adults, as opposed to larvae can positively impact survival, mating performance, mating competitiveness and locomotor activity of B. tryoni. The bacterial group(s) and function of bacterial species that increase fitness and competitiveness is also of interest to tephritid mass‐rearing programs.     

Mechanism and effectiveness of safflower oil against female Queensland fruit fly Bactrocera tryoni

Queensland fruit fly, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae), infests many horticultural fruit crops in the eastern part of Australia. Farmers usually apply synthetic insecticides to control this pest. Little is known on the use of plant products especially vegetable oils for fruit fly control although they are considered to be safer than synthetic insecticides. In this study, safflower oil was investigated for its mechanism and effectiveness against female B. tryoni. In a laboratory test, safflower oil treatments (2.5 and 5.0 ml l^?1) reduced the number of fly punctures on treated artificial fruits, no matter whether pre‐punctures were present or absent. Safflower oil treatments also reduced the number of fly landings and eggs laid, but only when the treated artificial fruits were without pre‐punctures. These results confirmed that safflower oil is active against female B. tryoni mainly by preventing this fruit fly from making oviposition punctures, not by discouraging them from depositing eggs or by repelling them. The slippery nature of safflower oil is considered to be responsible for a reduction in the susceptibility of artificial fruit to fruit fly punctures. Further investigation using fruit‐bearing tomato plants (a no‐choice test) in a glasshouse situation revealed that safflower oil application at concentrations of 10 and 15 ml l^?1 reduced the number of oviposition punctures but failed to reduce the number of eggs laid. To increase efficacy of safflower oil under field conditions, multiple tools may be needed to reduce fruit fly populations and oviposition behaviour, such as the addition of trap‐crops, provision of artificial oviposition sites, or mixing the insecticides with the oil.     

Methoprene modulates the effect of diet on male melon fly,Bactrocera cucurbitae,performance at mating aggregations

The effect of access to dietary protein (P) (hydrolyzed yeast) and/or treatment with a juvenile hormone analogue, methoprene (M), (in addition to sugar and water) on male aggregation (lekking) behaviour and mating success was studied in a laboratory strain of the melon fly, Bactrocera cucurbitae (Coquillett) (Diptera: Tephritidae). Six‐day‐old males were treated with (1) protein and methoprene (M+P+), (2) only protein (M?P+), or (3) only methoprene (M+P?), and compared with 14‐day‐old sexually mature untreated males (M?P?). The lekking behaviour of the four groups of males when competing for virgin sexually mature females (14 –16 days old) was observed in field cages. The following parameters were measured at male aggregations: lek initiation, lek participation, males calling, male–male interaction, female acceptance index, and mating success. For all these parameters, the M+P+ males significantly outperformed the other males. Moreover, for all parameters, there was a similar trend with M+P+ > M?P+ > M?P? > M+P?. More M+P+ males called and initiated and participated in lek activities than all other types of male, which resulted in higher mating success. They had also fewer unsuccessful copulation attempts than their counterparts. Whereas treatment with methoprene alone had a negative effect in young males with only access to sugar, access to dietary protein alone significantly improved young male sexual performance; moreover, the provision of methoprene together with protein had a synergistic effect, improving further male performance at leks. The results are of great relevance for enhancing the application of the sterile insect technique (SIT) against this pest species. The fact that access to dietary protein and treatment of sterile males with methoprene improves mating success means that SIT cost‐effectiveness is increased, as more released males survive to sexual maturity.     

Mass-rearing and sterilisation alter mating behaviour of male Queensland fruit fly, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae)

Abstract  The effects of domestication and irradiation on the mating behaviour of males of Bactrocera tryoni (Froggatt) (Diptera: Tephritidae) were investigated by caging wild, laboratory-domesticated and sterile (laboratory-domesticated, gamma-irradiated) males with wild females. Mating behaviour of mass-reared males was different from that of wild males, although behaviour of wild and sterile males was similar. Mass-reared males engaged in mounting of other males much more frequently than wild and sterile males, and began calling significantly earlier before darkness. Unnatural selection pressures imposed in mass-rearing conditions may explain these changes in mass-reared male behaviour. Male calling did not appear to be associated with female choice of mating partners, although this does not exclude the possibility that calling is a cue used by females to discriminate among mating partners. Despite differences in behaviour, frequency of successful copulations and mating success were similar among wild, mass-reared and sterile males.     

Pre-release feeding on yeast hydrolysate enhances sexual competitiveness of sterile male Queensland fruit flies in field cages

Recent laboratory studies of mass‐reared flies in small cages have found that periods of just 24‐ or 48‐h access to yeast hydrolysate can substantially enhance mating performance of mass‐reared male Queensland fruit flies, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae) (‘Q‐flies’). Using field cage tests that provide a better approximation of nature, we here investigated whether access to yeast hydrolysate for 48 h after adult emergence improves the ability of male and female mass‐reared, sterile Q‐flies to compete sexually with wild‐type flies that had been provided continuous access to yeast hydrolysate. Mating probability of sterile males was significantly increased by 48‐h access to yeast hydrolysate; sterile males provided 48‐h access to yeast hydrolysate had mating probability similar to that of wild males provided continuous access to yeast hydrolysate, whereas sterile males deprived of access to yeast hydrolysate had much lower mating probability. Unlike males, access to yeast hydrolysate for 48 h did not increase mating probability of sterile female Q‐flies. We instead found that wild females provided continuous access to yeast hydrolysate had higher mating probability than sterile females that did or did not have 48‐h access to yeast hydrolysate. This result raises the possibility that a bisexual Q‐fly strain might operate essentially as a male‐only release when the flies are given access to yeast hydrolysate during a 48‐h pre‐release holding period. Sterile males given access to yeast hydrolysate for 48 h mated significantly earlier in the evening than wild males and, as in other recent studies, this tendency was associated with an increased tendency to mate on the trees rather than the cage walls. There was no evidence of sexual isolation in this study, as wild and sterile mass‐reared flies showed no evidence of preferential mating with their own kind. Further studies are now needed to assess the potential for pre‐release access to yeast hydrolysate to improve sexual performance and longevity of sterile, mass‐reared, Q‐flies in the field.     

Sexual competitiveness of a transgenic sexing strain of the Mediterranean fruit fly, Ceratitis capitata

The sterile insect technique (SIT), when used for the control of the Mediterranean fruit fly (medfly), Ceratitis capitata (Wiedemann) (Diptera: Tephritidae), generally relies on the release of sterile flies of only the male sex. Male selection is achieved through the use of a genetic sexing strain (GSS) in which females are killed by heat treatment in the generation prior to release. Transgenic sexing strains (TSS) have been developed that perform the same function of female-lethality, this time by withholding tetracycline (or related compounds) from the larval diet. The use of TSS may allow for certain problems associated with conventional GSS, such as strain instability and reduced productivity in mass-rearing, to be avoided. The performance, and principally the sexual competitiveness, of released male flies is important for the success of an SIT control programme. This study describes field cage experiments in which the competitiveness of males from a TSS (OX3376B) was compared with that of a conventional GSS (VIENNA-8) and two wild-type strains (TOLIMAN and ARG). When competing for female mates with wild-type males, OX3376B male performance was acceptable. When OX3376B males competed directly for mates with VIENNA-8 males, VIENNA-8 slightly outperformed the TSS males. Parallel tests, in which wild-type males competed with either OX3376B or VIENNA-8 males, showed that males from both sexing strains were highly competitive with wild-type males. These results suggest that OX3376B in particular, and TSS in general, show sufficiently good mating competitiveness to merit further research into their suitability for eventual use in SIT programmes.