Lifespan and patterns of accumulation and mobilization of nutrients in the sugar-fed phorid fly, Pseudacteon tricuspis

Abstract. The effect of sugar feeding on the survival of adult phorid fly Pseudacteon tricuspis is investigated. Flies fed 25% sucrose in aqueous solution continuously throughout their lifespan have greater longevity (mean ± SE longevity: female = 7.9 ± 0.8 days, male = 8.9 ± 0.9 days) than completely starved (provided no water and no sugar solution) flies, sugar-starved (provided water only) flies, or flies fed sugar solution only on their first day of adult life. Completely starved flies rarely lived beyond one day. Provision of water increases longevity by 2 days, and one full day of sugar feeding further increases longevity by an additional 1–2 days. Flies fed 50% sucrose have similar survivorship as those fed 25% sucrose. The temporal patterns of nutrient accumulation and utilization are also compared in P. tricuspis fed different diets: sugar-starved, sucrose-fed on the first day of adult life only, and sucrose-fed continuously. Adult P. tricuspis emerge with no gut sugars, and only minimal amounts of body sugars and glycogen. Although the levels of body sugars and glycogen decline gradually in sugar-starved flies, a single day of sugar feeding results in the accumulation of maximum amounts of gut sugars, body sugars and glycogen. High levels of these nutrients are maintained in female and male phorid flies fed on sucrose continuously over the observation period, whereas nutrient levels decline in flies fed only on the first day of life, beginning 1 day postfeeding. Female and male P. tricuspis emerge with an estimated 12.3 ± 2.3 and 7.2 ± 1 g of lipid reserves per fly, respectively. These teneral amounts represent the highest lipid levels detected in adult flies, irrespective of their diet, and are maintained over the life times of sucrose-fed female and male flies, but declined steadily in sugar-starved females. These data suggest that adult P. tricuspis are capable of converting dietary sucrose to body sugars and glycogen, but not lipids.     

Influence of floral resources on sugar feeding and nutrient dynamics of a parasitoid in the field

Abstract.  1. To investigate how floral resources impact feeding and nutrient dynamics of parasitoids, the ichneumonid Diadegma insulare (Cresson) was monitored in cabbage fields with and without buckwheat Fagopyrum esculentum Moench borders. Biochemical analyses of field-collected wasps determined whether sugar feeding occurred, and whether it enhanced nutrient levels and increased parasitism of diamondback larvae Plutella xylostella (L.) from 2000 to 2003. The effects of experimental spatial scale on sugar feeding dynamics were also evaluated in 2001.
2. The majority of D. insulare foraging on cabbage had fed upon sugar, but floral borders did not consistently increase the proportion of sugar-fed wasps or their levels of glycogen and lipid over all 4 years. Floral borders increased sugar storage by females in 2001. More females fed on sugar when experimental plots were separated by at least 800 m compared with 67 m.
3. Feeding on buckwheat nectar vs. honeydew produced by the soybean aphid Aphis glycines Matsumura in surrounding fields was distinguished using the ratio of fructose to total sugars, which was higher in nectar- than in honeydew-fed parasitoids. Floral borders increased the probability that females collected from adjacent crops were nectar-fed in 2002, demonstrating that floral resources were utilised by foraging parasitoids. Diadegma insulare that fed upon any sugar source had higher levels of sugar and glycogen than unfed wasps, and feeding on honeydew appeared to enhance nutrient storage.
4. The prevalence of sugar or nectar feeding by female D. insulare was not correlated with parasitism rates over the 4 years from all plots, but nectar feeding was positively correlated with parasitism rates within floral plots.     

Sugar feeding and fluid destination control in the phlebotomine sandfly Lutzomyia longipalpis (Diptera: Psychodidae)

Sandfly feeding behaviour and destination of coloured sugar meals in the gut of Lutzomyia longipalpis were investigated with particular attention to the role of the crop. Sandflies were able to ingest sugar from liquid drops, microcapillaries, a slice of pear and even sugar powder. In most cases the flies adopted a 'sugar feeding mode' with raised palps. As the fruit dried, flies of both sexes fed by piercing the tissue with the proboscis. All sugar-fed flies had a full crop plus a small amount of sugary fluid in the thoracic mid-gut, i.e. past the stomodaeal valve. Dissections of flies interrupted during feeding showed that the very first trace of sugar passed through the stomodaeal valve, but that the rest of the meal was diverted into the crop. This suggests that closure of the stomodaeal valve is initiated only after a small volume of sugar solution has passed through it.     

Effects of supplementary feeding on growth and reproduction of three carnivorous plant species in a subarctic environment

1 The effect of supplementary feeding on growth and reproduction of three carnivorous plants species was investigated over a 6‐year period. Pinguicula alpina , P. villosa and P. vulgaris populations growing at two altitudes in a subarctic environment were fed with fruit flies ( Drosophila melanogaster ).
2 Fed plants increased in size relative to control plants during the first years. Subsequently a stable size difference between feeding levels was established. The weight of the over‐wintering part was higher in fed plants than in control plants.
3 The flowering frequency (i.e. proportion of plants in a population carrying flowers) was also higher in the fed plants. The proportion of flowering plants increased in the feeding treatment compared to control plants during more‐or‐less the whole experimental feeding period (5–6 years). Seed production also increased slowly in response to feeding.
4 No feeding effect on nitrogen and phosphorus concentrations in seeds and over‐wintering parts was found.
5 In most characteristics the high‐altitude populations were less responsive than populations growing at low altitude. In the high‐altitude population reproduction failed repeatedly, suggesting that seed output may be more dependent on abiotic factors than on resource availability in cold environments.
6 Pinguicula villosa and P. vulgaris used the new resources the same summer for increased leaf rosette growth and current reproduction, showing these species to be income breeders, while the third species ( P. alpina ) allocated a larger proportion to storage and future reproduction, characteristic of a capital breeder.     

Insecticide,sugar, and diet effects on feeding and mortality in Rhagoletis indifferens (Dipt., Tephritidae)

The effects of spinosad bait and various insecticides, the presence of sugar in insecticides, and diet on feeding responses and mortality in western cherry fruit fly, Rhagoletis indifferens Curran (Dipt., Tephritidae), were determined. Numbers of feeding events on insecticides with sugar were greater than on insecticides alone, but there was only a small effect of diet on feeding responses to insecticides with sugar. Feeding durations on imidacloprid, thiamethoxam and acetamiprid with sugar were shorter than on sugar water and spinosad bait, as the neonicotinoids paralysed flies quickly. Flies that fed on sugar only (nitrogen‐starved) suffered higher mortalities when exposed to spinosad, thiamethoxam and azinphos‐methyl than to imidacloprid, acetamiprid and indoxacarb, and mortality in between these two groups of treatments when exposed to spinosad bait. Mortalities were greater when sugar was added to insecticides, and were higher in nitrogen‐starved than fully‐fed (yeast extract + sugar fed) flies. Flies that fed once on thiamethoxam were killed more quickly than those that fed once on spinosad bait and spinosad. Results suggest that thiamethoxam is comparable to spinosad in its effects on mortality, and that using it with sugar in bait may also have similar results as using spinosad bait or spinosad. One benefit of using thiamethoxam with sugar may be that it kills flies more quickly, before they can oviposit, than spinosad bait, although whether a fly will feed on it may depend on how much sugar or nitrogenous food it has eaten.     

Comparing the effects of five naturally occurring monosaccharide and oligosaccharide sugars on longevity and carbohydrate nutrient levels of a parasitic phorid fly, Pseudacteon tricuspis

Abstract.  The longevity and nutrient levels of Pseudacteon tricuspis provided with 1  m solutions of five naturally occurring sugars, fructose, glucose, sucrose, trehalose and melezitose, are compared. All but melezitose, result in significant increases in the longevity of P. tricuspis in comparison with sugar-starved flies (flies provided with water only). Sugar-starved female and male P. tricuspis have an average longevity of 3.3 and 4.1 days, respectively. Provision of free water in addition to sugar solution is necessary for optimum longevity by female and male flies. Longevity is increased by 2.4–2.6-fold by the two monosaccharides, fructose and glucose, and by 2.6–2.8-fold by the disaccharides, sucrose and trehalose. Phorid flies provided with the trisaccharide sugar, melezitose, had a marginal increase in lifespan (approximately 1 day), but this is not significantly different from the longevity of sugar-starved flies. Significantly greater levels of total sugars are detected in P. tricuspis fed the disaccharide sugars (sucrose, trehalose) or the monosaccharide sugars (fructose, glucose), compared with flies provided with melezitose (trisaccharide), or to sugar-starved flies. Fructose is not detected in sugar-starved flies, or in flies fed glucose or trehalose. However, high levels of fructose are detected in flies fed sucrose or fructose, whereas levels of fructose in melezitose-fed flies are intermediate. In general, significantly greater glycogen levels are detected in P. tricuspis fed sucrose, glucose, trehalose or fructose, compared with melezitose-fed or sugar-starved flies. Levels of total sugars and glycogen in sugar-fed flies are positively correlated with wing length, possibly indicating a higher accumulation of storage sugars by larger flies. These results are discussed in relation to the nutritional ecology of the phorid fly.     

Feeding behaviour,midgut distension and ovarian development in Phlebotomus papatasi (Diptera: Psychodidae)

Phlebotomus papatasi females were fed through membranes or from cotton wool soaked in blood, water, sucrose or sodium chloride solutions. In membrane-fed flies, all diets were routed to the midgut and not to the crop. Following the meals that went to the midgut, females showed ovarian development at least 3 times greater than in sucrose-fed, autogenous control flies. Neither small quantities of water arriving in the midgut following drinking from soaked cotton wool, nor piercing of a membrane without feeding, stimulated ovarian development. Flies exhibited different feeding behaviour namely, blood feeding, sugar feeding, and water drinking. The blood-feeding behaviour was typical of flies ingesting any of the experimental diets through membranes, or blood or saline from cotton wool. The other two types of behaviour were observed in flies which fed from soaked cotton wool. The type of behaviour was characterized by the depth of penetration of the mouthparts into the substrate, the deployment of the palps and the degree of contact between the palps and the surface. It is suggested that the stimuli which control the routing of meals to the crop or to the midgut are derived from these types of behaviour.     

Consumption of sugars by Anastrepha suspensa (Diptera: Tephritidae)

We were interested in determining the feeding response of the Caribbean fruit fly, Anastrepha suspensa Loew (Diptera: Tephritidae), to various sugar concentrations to develop an improved bait for adults. We compared the consumption of 0.01-1.00 M concentrations of glucose, fructose, raffinose, and sucrose in no-choice tests for 24-h- and 6-d-old male and female flies. Sucrose was the most consumed sugar or within the most consumed group of sugars at 0.02-0.20 M concentrations. There were no differences in consumption among sugars at 0.01, 0.40, and 1.00 M. Consumption generally increased with increasing sugar concentration except that sucrose consumption peaked at 0.20 M. Twenty-four-hour females consumed less fructose than other sugars; 24-h males consumed more sucrose than fructose or raffinose, with an intermediate response to glucose. Females in the 6-d group consumed more sucrose than the other three sugars, whereas 6-d males exhibited no difference in consumption among sugars. In choice tests, flies consumed more sugar solution than water, but the difference between 0.20 M fructose and water was not significant for 24-h males or 24-h females. In choice tests between 0.20 M fructose and 0.20 M sucrose, both 24-h and 6-d females showed a preference for fructose. Males of both age classes showed no preference. These results indicate that the responses of flies to different sugars can vary by sugar, gender, and age.     

A survival and reproduction trade-off is resolved in accordance with resource availability by virgin female mosquitoes

The first 2-4 days after an Anopheles gambiae female mosquito emerges are critical to her survival and reproductive success. Yet, the order of behavioural events (mating, sugar feeding, blood feeding) during this time has received little attention. We discovered that among female cohorts sampled from emergence, sugar feeding had a higher probability than blood feeding of occurring first, and mating rarely occurred before a meal was taken. The night after emergence, 48% of females fed on sugar in mesocosms, and 25% fed on human blood; in the absence of sugar, 49% of females fed on human blood. After 5 days, 39% of the sugar-supplied females had blood fed and mated, and were fructose negative, whereas only 8% of the sugar-denied females had both blood fed and mated by this time. The model that best explained the transitions suggests that females made use of two distinct behavioural pathways, the most common one being to sugar-feed, then mate, and then seek blood. Other females sought blood first, then mated, and forwent a sugar meal. Lipid levels were higher in females with access to sugar than in females without access to sugar, particularly for those in later gonotrophic stages, while glycogen levels in the sugar-supplied group were higher throughout. In single-night experiments with females having had access to sucrose since emergence, those given a blood meal 1 day before spending a night with males had higher insemination rates than those not receiving the blood meal. These results indicate that the trade-off between survival and immediate reproduction is resolved by young adult females in accordance with availability of resources and gonotrophic state.     

Effect of semolina-jaggery diet on survival and development of Drosophila melanogaster

Drosophila melanogaster is an ideal model organism for developmental studies. This study tests the potential of semolina-jaggery (SJ) diet as a new formulation for bulk rearing of flies. Semolina and jaggery are organic products obtained from wheat endosperm and cane sugar, respectively. Semolina is a rich source of carbohydrates and protein. Jaggery has a high content of dietary sugars. Moreover, preparation of semolina jaggery diet is cost-effective and easy. Thus, the current study aimed to compare survival and developmental parameters of flies fed the SJ diet to flies fed the standard cornmeal-sugar-yeast (CSY) diet. SJ diet enhanced survival of flies without affecting fecundity; male flies showed increased resistance to starvation. A higher number of flies emerged at F₂ and F₃ generation when fed the SJ diet than when fed the control CSY diet. SJ diet did not increase fly body weight and lipid percentage. Therefore, SJ diet can be used for bulk rearing of healthy flies at par with the standard cornmeal-sugar-yeast diet.     

Effect of semolina-jaggery diet on survival and development of Drosophila melanogaster

Drosophila melanogaster is an ideal model organism for developmental studies. This study tests the potential of semolina-jaggery (SJ) diet as a new formulation for bulk rearing of flies. Semolina and jaggery are organic products obtained from wheat endosperm and cane sugar, respectively. Semolina is a rich source of carbohydrates and protein. Jaggery has a high content of dietary sugars. Moreover, preparation of semolina jaggery diet is cost-effective and easy. Thus, the current study aimed to compare survival and developmental parameters of flies fed the SJ diet to flies fed the standard cornmeal-sugar-yeast (CSY) diet. SJ diet enhanced survival of flies without affecting fecundity; male flies showed increased resistance to starvation. A higher number of flies emerged at F₂ and F₃ generation when fed the SJ diet than when fed the control CSY diet. SJ diet did not increase fly body weight and lipid percentage. Therefore, SJ diet can be used for bulk rearing of healthy flies at par with the standard cornmeal-sugar-yeast diet.     

Effect of bacteria on nutritional status and reproductive success of the Mediterranean fruit fly Ceratitis capitata

Abstract.  Mediterranean fruit flies ( Ceratitis capitata Wiedemann, Diptera: Tephritidae) harbor a community of diazotrophic bacteria in their digestive system. The present study aims to test the hypothesis that bacteria contribute to fly fitness by enhancing copulatory success and egg production in males and females, respectively. After eclosion, flies were fed a full diet containing peptides, sugar and minerals, or a sugar diet, lacking peptides. Subgroups from each diet were fed a mixture of the antibiotics ciprofloxacin and piperacillin. The presence of bacteria, food consumption, weight gain, lipid and protein levels, oviposition in females and copulatory success of males were quantified in the four groups. The antibiotic treatment effectively cleared the gut of bacteria. The relative amounts of food consumed (with or without antibiotics) are similar in all groups. The antibiotics do not inhibit feeding, and their ingestion does not affect dry weight or the amount of protein stored, yet females feeding on the full diet without antibiotics have increased lipid levels. Females fed the full diet produce significantly more eggs than females on the sugar diet, but the presence of bacteria does not affect numbers of eggs produced. However, in the absence of bacteria, the oviposition rate of nutritionally stressed females is significantly accelerated. The presence of bacteria in sugar fed males does not provide them with a mating advantage. Conversely, in males fed a full diet, the presence of bacteria is associated significantly with a shorter latency to mate. It is concluded that, because the bacterial community is present at all stages of the fly's life cycle, at least some species are effectively transmitted from parents to offspring, and removal of bacteria affects measurable physiological and behavioural parameters related to fitness, the association between bacteria and the medfly is mutualistic.     

Blood meal as a regulator of triacylglycerol synthesis in the haematophagous stable fly,Stomoxys calcitrans

Summary Accumulation of triacylglycerol and glycogen reserves following meals of blood and/or sugar-water was investigated in the haematophagous stable fly,Stomoxys calcitrans. In the starved fly, triacylglycerol reserves appear to be utilized predominantly for energy requirements. When the starved flies were fed one meal of 1.0 M sucrose-water, they accumulated considerable amounts of glycogen but there was no increase in the triacylglycerol content. Starved flies fed one meal of blood accumulated large quantities of triacylglycerol but no glycogen; in those flies fed sugar-water and blood, glycogen and triacylglycerol accumulated. This study shows that the stable fly preferentially utilizes blood for triacylglycerol synthesis and sugar for glycogen synthesis. It is suggested that one or more factor(s) in the blood meal influences accumulation of triacylglycerol in this insect, possibly through an hormone from the corpora cardiaca-corpora allata complex.     

Effects of disodium octaborate tetrahydrate on survival, behavior, and egg viability of adult muscoid flies (Diptera: Muscidae).

Disodium octaborate tetrahydrate (Na2B8O13(.)4H2O) was mixed with sugar and fed to adult Musca domestica L. and Fannia canicularis (L.) to determine concentration-mortality relationships. LC50s (48-h exposure) were 5.7% for M. domestica and 1.0% for F. canicularis. Rates of 1 and 2% were used to test effects on M. domestica mortality and egg hatch over an 8-d period. Reduced egg hatch was evident after 1 d of feeding on the treated mixtures and was greatest (less than 10% egg hatch) after flies fed only on treated mixtures for 2 d. A partial rebound in egg hatch occurred after 3-4 d of feeding on treated diet. Sperm motility in females fed treated sugar was apparently normal. Fertile egg placed on treated poultry manure did not hatch, indicating embryonic death, which also may have been involved in the low hatch of eggs observed from treated flies. When flies were exposed to treated sugar for 2 d then returned to untreated diet, delayed mortality effects and reduced egg hatch persisted for at least 3 d. Behavioral assays (feeding) with M. domestica demonstrated that flies rejected borate-sugar mixtures in favor of sugar alone when the concentration of borate was greater than 2%. Given a choice of treated and untreated poultry manure for oviposition, flies also rejected the treated manure. The potential of borates in adult bait formulations or applied to developmental substrates for fly control is discussed.     

Size‐dependent insect flight energetics at different sugar supplies

Under most circumstances, large body size confers a higher fitness and is positively selected, whereas selection against large size is empirically poorly documented. Physiologically, according to the ¾ power law, larger animals have lower relative but higher absolute energy demands, such that large body size may become disadvantageous, particularly under fast locomotion in food‐limited environments. After a period of initial feeding on different sugar concentrations, we investigated size‐dependent energy content (reserves) at baseline and of females unflown (i.e. resting) or flown for 18 h in two (replicate) insect species: the yellow dung fly Scathophaga stercoraria and the yellow fever mosquito Aedes aegypti. Tethered adults of various sizes were tested in a flight mill. In both species, teneral glycogen, sugar, and lipid content increased with sugar availability, and isometrically or even hyper‐allometrically (slope > 1) with body size. Activity treatment also revealed the expected consumption effects. Both species increased their flight distance with sugar supply, although only larger mosquitoes flew longer. Crucially, larger females of both species disproportionately exhausted more glycogen and sugars (but not lipid) during flight. The mosquitoes appeared to adjust their flight more finely to their size‐dependent energy reserves at all sugar availabilities, whereas, in the dung flies, size‐dependent energy demands were detectable only with a low but not with an overly high sugar supply. Although we found a greater absolute and relative locomotory energy demand for the larger flies, which is in agreement with interspecific patterns in insects, this was (more than) compensated by their greater baseline energy reserves, resulting in the greater net flying capacity of larger individuals. Consequently, we found no evidence for energetic mechanisms limiting the performance of large flying insects under food limitation. The differences between the two species presumably relate to mosquitoes inherently being long distance flyers and dung flies being short distance flyers. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ●● , ●●–●●.     

Ingestion of sugar diets correlates with spike activity in labellar chemosensilla of the flesh-fly, Neobellieria (= Sarcophaga ) bullata

Abstract .Female 2-day-old Neobellieria (= Sarcophaga ) bullata (Parker) (Diptera: Sarcophagidae) were exposed to different concentrations of sucrose, glucose and fructose in a single-choice potometer, and the volume ingested in the first hour was measured. Nerve spike activity in response to the same sugars was recorded from medium labellar taste hairs of similar flies by tip-recording. Two classes of chemosensory cells responded to sucrose, glucose and fructose. Cell 1 showed an increasing spike activity with sugar concentration, whereas cell 2 did not; cell 1 was identified as the 'sugar cell'.
For both spike activity in cell 1 and feeding, sucrose was the most stimulatory sugar. The dose–response curves for glucose and fructose crossed over at about 200 m m . At higher concentrations, glucose was more stimulatory for both cell 1 and for feeding, and at lower concentrations, fructose. The pattern of spike activity supports a separate location on the sensory cells of receptors for pyranose and fructose forms of sugar. The strong correlation between volume ingested and spike activity indicates that sugar feeding is controlled by sensory input from the 'sugar' cells of labellar chemosensilla. Moreover, the results suggest that the flies do not distinguish between these sugars except by apparent 'sweetness'.     

Further comment on the role of sugars for the successful infection of blood-sucking Diptera by protozoa that are the causative agents of human diseases

Possible reasons of a successful development of Plasmodium gallinaceum in Aedes aegypti, which were not given sugar feeding, are analysed. Such reasons are assumed to be high contents of sugars (up to 200 mg/% in the blood of chickens-donors) and retention of sugar in the crop of mosquitoes, which were fed from a tampon, in control.     

Post-alighting responses of Mexican fruit flies (Dipt., Tephritidae) to different insecticides in paint on attractive spheres

Abstract: Two new, comparatively safe insecticides (spinosad and imidacloprid) were compared with dimethoate (each at 1.5% active ingredient) for behavioural and mortality effects on Mexican fruit flies, Anastrepha ludens . Insecticide was mixed with sugar (as a feeding stimulant) and yellow latex paint (as an extending agent) applied to the surface of fruit-mimicking biodegradable 7 cm spheres made of sugar, flour and glycerin. Flies feeding on spinosad-treated spheres did not differ from flies feeding on untreated spheres in post-feeding intra-tree flight capability, amount of oviposition or mortality. Flies that fed on imidacloprid- or dimethoate-treated spheres for as little as 30 s experienced both high reduction in oviposition and high mortality compared with flies that fed on untreated spheres, and the flies from imidacloprid-treated spheres also showed a much reduced intra-tree flight capability. If baited with attractive odour, biodegradable yellow spheres treated with a surface coating of imidacloprid in latex paint and sugar could have potential for suppressing Mexican fruit flies on host trees.     

Age‐related activity patterns are moderated by diet in Queensland fruit flies Bactrocera tryoni

Life‐history parameters and the fitness of tephritid flies are closely linked to diet. Studies of locomotor behaviour can provide insights to these links, although little is known about how locomotor behaviour is influenced by diet. In the present study, video recordings of Queensland fruit flies Bactrocera tryoni Froggatt (Diptera: Tephritidae) (‘Q‐flies’) that are maintained individually in cages are used to determine how diet affects the activity patterns (flight, walking, grooming, inactivity) of males and females at ages ranging from 4 to 30 days. The frequency and total duration of activities over 10‐min trials are affected by diet, age and sex. Supplementation of diet with hydrolysed yeast results in a higher frequency and duration of flight in flies of all ages and both sexes. The effect of diet on other activities varies with age. Q‐flies fed sugar only increase walking frequency steadily from 4 to 30 days post‐eclosion, whereas flies fed sugar + yeast have higher walking frequencies at 4 and 10 days than flies fed sugar only, although they then exhibit a sharp decline at 30 days post‐eclosion. The frequency and duration of inactivity remain consistent in flies fed sugar + yeast, whereas flies fed sugar only exhibit a marked increase in inactivity from 4 to 30 days post‐eclosion. Compared with older flies, 4 day‐old Q‐flies fed sugar only spend considerably more time grooming. The potential of activity monitoring as a quality control test for flies that are mass‐reared for use in sterile insect technique programmes is discussed.     

Kinetics of Colonization of Adult Queensland Fruit Flies (Bactrocera tryoni) by Dinitrogen-Fixing Alimentary Tract Bacteria

The average total population of bacteria remained constant in the alimentary tracts of adult laboratory-raised Queensland fruit flies (Bactrocera tryoni) although the insects had ingested large numbers of live bacteria as part of their diet. The mean number of bacteria (about 13 million) present in the gut of the insects from 12 to 55 days after emergence was not significantly modified when, at 5 days after emergence, the flies were fed antibiotic-resistant bacteria belonging to two species commonly isolated from the gut of field-collected B. tryoni. Flies were fed one marked dinitrogen-fixing strain each of either Klebsiella oxytoca or Enterobacter cloacae, and the gastrointestinal tracts of fed flies were shown to be colonized within 7 days by antibiotic-resistant isolates of K. oxytoca but not E. cloacae. The composition of the microbial population also appeared to be stable in that the distribution and frequency of bacterial taxa among individual flies exhibited similar patterns whether or not the flies had been bacteria fed. Isolates of either E. cloacae or K. oxytoca, constituting 70% of the total numbers, were usually dominant, with oxidative species including pseudomonads forming the balance of the population. Antibiotic-resistant bacteria could be spread from one cage of flies to the adjacent surfaces of a second cage within a few days and had reached a control group several meters distant by 3 weeks. Restriction of marked bacteria to the population of one in five flies sampled from the control group over the next 30 days suggested that the bacterial population in the gut of the insect was susceptible to alteration in the first week after emergence but that thereafter it entered a steady state and was less likely to be perturbed by the introduction of newly encountered strains. All populations sampled, including controls, included at least one isolate of the dinitrogen-fixing family Enterobacteriaceae; many were distinct from the marked strains fed to the flies. Nitrogenase activity detected by the acetylene reduction assay was associated with flies fed dinitrogen-fixing bacteria as well as with control groups given either no supplement or free access to a yeast hydrolysate preparation. Nitrogen fixed from the atmosphere may supplement the nutrition of the alimentary tract microbial population of B. tryoni. Transmission electron microscopy showed that the principal site of bacterial colonization in the abdominal alimentary tract was the lumen of the midgut inside the peritrophic membrane. No intracellular symbionts were seen in the gut tissues nor were bacteria found attached to the cuticular folds of the hindgut. The ultrastructure of the gut resembled that of other fly genera except that the intercellular spaces between rectal epithelial cells were more extensive, suggesting a role for unspecialized epithelium in water and solute uptake in B. tryoni.