Dung Beetle Onthophagus Taurus Research Articles

Offensive and defensive sperm competition roles in the dung beetle Onthophagus taurus (Coleoptera: Scarabaeidae)

Sperm competition is predicted to generate opposing selection pressures on males. On one hand, selection should favour ‘defensive’ adaptations that protect a male’s ejaculate from displacement, while, on the other hand, selection should favour ‘offensive’ adaptations that overcome paternity assurance mechanisms of rivals. Here, we use the sterile male technique to assess sperm precedence when a male dung beetle Onthophagus taurus mates in both a defensive (first male) and an offensive (second male) role. Significant variation in a male’s sperm precedence (both P1 and P2) was detected, and an individual’s defensive (P1) and offensive (P2) abilities were positively correlated. Thus, it appears that sexual selection simultaneously selects for ‘defensive’ and ‘offensive’ adaptations in O. taurus. We discuss a variety of male traits in O. taurus that potentially contribute to a male’s ability to be successful when mating in an ‘offensive’ and a ‘defensive’ role.

The evolution of male genitalia: patterns of genetic variation and covariation in the genital sclerites of the dung beetle Onthophagus taurus

Three main hypotheses, have been invoked to explain divergent genital evolution, the lock and key, pleiotropy, and sexual selection hypotheses, each of which make different predictions about how genital traits are inherited. Here we used a half-sib breeding design to examine the patterns of genetic variation and covariation between male genital sclerites, and their covariance with general body morphology in the dung beetle Onthophagus taurus. We found CV(A)'s and CV(P)'s were similar for both genital and general morphological traits and that CV(R)'s were large for both trait types. We found that male genital sclerites were negatively genetically correlated with general morphological traits. Variation in male genital morphology has direct implications for a male's fertilization success and the resulting sexual selection acting on male genitalia is predicted to maintain high levels of additive genetic variance. Contrary to this prediction, we found that individual genital sclerites all had low levels of additive genetic variance and large maternal and environmental sources of variation. Our data suggest that the genital sclerites in O. taurus are not inherited independently but as a genetically integrated unit. More importantly, the way the different sclerites function to influence male fertilization success reflects this genetic integration. Even though levels of V(A) in individual genital sclerites may be low, there may still be sufficient V(A) in multivariate trait space for selection to generate evolutionary change in the overall morphology of male genitalia.

Relative influence of male and female genital morphology on paternity in the dung beetle Onthophagus taurus

Studies of fertilization success have demonstrated that male effects are often a strong and important source of variation in P2 (the proportion of offspring that are fertilized by the second male to mate). More recently there has been emphasis on female processes that occur during and after copulation that might bias the outcome of male-male interactions. Here we used the sterile male technique to evaluate whether female genital morphology influences the repeatability of P2 when the same pair of male dung beetles, Onthophagus taurus, copulated with a series of full-sib females or unrelated females that were all unrelated to the male pair. Repeatability estimates of measures of female genital morphology showed that full-sib females varied less in their genital morphology than did unrelated females. Therefore, if female genital traits are an important source of variation in male fertilization success, P2 should be more repeatable across full-sib than unrelated females. Contrary to this prediction, we show that the repeatability of P2 did not differ between female groups. Moreover, specific dimensions of the female genitalia (sclerotized vagina and bursa) did not contribute significantly to variance in P2. In contrast, male effects had consistent and repeatable influences on paternity across females. These were partly explained by variation in the morphology of male genital sclerites.

Optimal maternal investment in the dung beetle Onthophagus taurus ?

Optimal investment theory is based on the assumption that the proximate constraint acting on parental investment is resource based. A trade-off between per offspring investment and total investment seems intuitive. Consequently, a parent’s investment strategy is expected to represent a trade-off between the benefits of investment for current offspring and the costs to future reproduction for parents. In this study, we provide clear evidence that the costs and benefits of maternal provisioning in the dung beetle Onthophagus taurus influence the amount of provisions provided by the mother. Horse dung is typically of a higher nutritional value than cow dung and females were shown to provide 20% less dung to offspring when provisioning with horse dung. By reducing their investment per offspring and exhibiting a clear preference to provision offspring with horse dung, females were able to produce significantly more offspring. Females provisioning with horse dung received greater fitness returns per unit of investment and experienced lower provisioning costs, in terms of the minimum amount of dung required to produce a surviving offspring, than females provisioning with cow dung. Females provisioning in soil of low moisture content were found to have higher tunneling costs than those provisioning in soil of high moisture content, while the fitness returns per unit of investment did not differ. We adopted a marginal value theorem (MVT) approach to calculate the theoretical optimal level of investment for each dung type and for each soil moisture. Predicted levels of provisioning were lower for horse dung than for cow dung and for moist soil than for dry soil. Therefore, the results of this study are in qualitative agreement with MVT predictions and provide empirical support for the proposal that females can adaptively adjust their level of investment in response to resource and/or habitat quality. However, the theoretically predicted optimal investment yielded a poor quantitative fit with our observed levels of investment, with females providing over twice the investment predicted by the MVT approach. We suggest that this difference may reflect either our inability in directly quantifying all the necessary costs and benefits of investment in O. taurus and/or the applicability of the underlying assumptions of MVT.

The behavioral ecology of threshold evolution in a polyphenic beetle

Facultative expression of alternative male morphologies is thought to allow individual males to select the phenotype with the highest fitness gain given their competitive status relative to other males with which they compete for females. Choice of, or switching between, morphs commonly relies on developmental threshold responses. Evolutionary changes in developmental threshold responses are thought to provide an important avenue for phenotypic diversification and the evolution of morphological and behavioral novelties. However, the extent to which alternative male phenotypes and their underlying threshold responses actually evolve in natural populations is unclear. Likewise, the ecological factors that shape the evolution of threshold responses in natural populations are unexplored for most organisms, as are the consequences of such modifications for patterns of morphological diversity. I examined the ecological basis of rapid threshold evolution in exotic populations of the horn-polyphenic dung beetle Onthophagus taurus. Male O. taurus vary continuously in body size as a function of larval feeding conditions. Only males that exceed a critical threshold body size develop a pair of long horns on their heads, whereas males below this threshold remain hornless. Populations in two exotic ranges of this species, the eastern United States and western Australia, have diverged in the mean threshold body size, which has resulted in the evolution of highly divergent and novel horn length–body size allometries in these populations. Populations in a third and previously unstudied exotic range of O. taurus in eastern Australia exhibit threshold body sizes roughly intermediate between the eastern U.S. and western Australian populations. I tested three hypothesis to explain how differences in ecological and demographic factors can drive allometric divergences between populations, using data derived from comparative, standardized sampling of a large number of populations in each exotic range. Results suggest that differences in the intensity of both intra- and interspecific competition have contributed to the evolution of divergent thresholds in these populations. My results do not support the hypothesis that shifts in threshold body sizes to larger body sizes are a consequence of increases in the mean body size of competing males. I discuss my results in the context of Onthophagus mating systems and the evolutionary implications of threshold evolution.

Males influence maternal effects that promote sexual selection: a quantitative genetic experiment with dung beetles Onthophagus taurus.

Recently, doubt has been cast on studies supporting good genes sexual selection by the suggestion that observed genetic benefits for offspring may be confounded by differential maternal allocation. In traditional analyses, observed genetic sire effects on offspring phenotype may result from females allocating more resources to the offspring of attractive males. However, maternal effects such as differential allocation may represent a mechanism promoting genetic sire effects, rather than an alternative to them. Here we report results from an experiment on the horned dung beetle Onthophagus taurus, in which we directly compare genetic sire effects with maternal effects that are dependent on sire phenotype. We found strong evidence that mothers provide more resources to offspring when mated with large-horned males. There were significant heritabilities for both horn length and body size, but when differential maternal effects were controlled, the observed estimates of genetic variance were greatly reduced. Our experiment provides evidence that differential maternal effects may amplify genetic effects on offspring traits that are closely related to fitness. Thus, our results may partly explain the relatively high coefficients of additive genetic variation observed in fitness-related traits and provide empirical support for the theoretical argument that maternal effects can play an important role in evolution.

Genital morphology and fertilization success in the dung beetle Onthophagus taurus: an example of sexually selected male genitalia.

In animals with internal fertilization and promiscuous mating, male genitalia show rapid and divergent evolution. Three hypotheses have been suggested to explain the evolutionary processes responsible for genital evolution: the lock-and-key hypothesis, the pleiotropy hypothesis and the sexual-selection hypothesis. Here, we determine whether variation in male genital morphology influences fertilization success in the dung beetle Onthophagus taurus, as predicted by the sexual-selection hypothesis. Variation in four out of five genital sclerites of the endophallus influenced a male's fertilization success, supporting the general hypothesis that male genitalia can evolve under sexual selection. Furthermore, different genital sclerites were found to enhance first versus second male paternity, indicating that different sclerites serve offensive and defensive roles. Genital-trait variability was comparable to that in other species but was less variable than a non-genital sexually selected trait (head horns). We suggest that directional selection for genital elaboration may be countered by natural selection, which should favour genitalia of a size and shape necessary for efficient coupling and sperm transfer.

Evolution of ejaculates: patterns of phenotypic and genotypic variation and condition dependence in sperm competition traits.

Sperm competition is widely recognized as a potent force in evolution, influencing male behavior, morphology, and physiology. Recent game theory analyses have examined how sperm competition can influence the evolution of ejaculate expenditure by males and the morphology of sperm contained within ejaculates. Theoretical analyses rest on the assumption that there is sufficient genetic variance in traits important in sperm competition to allow evolving populations to move to the evolutionarily stable equilibrium. Moreover, patterns of genotypic variation can provide valuable insight into the nature of selection currently acting on traits. However, our knowledge of genetic variance underlying traits important in sperm competition is limited. Here we examine patterns of phenotypic and genotypic variation in four sperm competition traits in the dung beetle Onthophagus taurus. Testis weight, ejaculate volume, and copula duration were found to have high coefficients of additive genetic variation (CV(A)S), which is characteristic of fitness traits and traits subject to sexual selection. Heritabilities were high, and there was some evidence for Y-linked inheritance in testis weight. In contrast, sperm length had a low CV(A), which is characteristic of traits subject to stabilizing selection. Nevertheless, there was little residual variance so that the heritability of sperm length exceeded 1.0. Such a pattern is consistent with Y-linked inheritance in sperm length. Interestingly, we found that testis weight and sperm length were genetically correlated with heritable male condition. This finding holds important implications for potential indirect benefits associated with the evolution of polyandry.

Behavioural dynamics of biparental care in the dung beetle Onthophagus taurus

In the dimorphic dung beetle Onthophagus taurus major males provide assistance during offspring provisioning. We examined the behavioural dynamics of biparental care to quantify directly how males and females allocate time to parental and nonparental behaviours and to determine whether parents adjust their level of investment relative to their partner's contribution. Females allocated more of their time budget to parental behaviours than males. The proportion of time females allocated to parental behaviours increased after oviposition while that of a male decreased. Male paternity assurance behaviours were negatively associated with male and female parental behaviours. Theoretical models predict that the investment provided by the members of a cooperative pair should be negatively correlated and that any shortfall of one parent should be partially compensated for by the other. In the absence of a male, unassisted females allocated more time to parental care, and performed more parental behaviours. However, compensation was incomplete as unassisted females performed fewer parental behaviours than pairs, resulting in significantly lighter brood masses (the egg and its associated dung supply). Males performed more parental behaviours when paired with small females, and small females more than large females. Contrary to prediction, the investments provided by males and females in a cooperative pair were positively correlated. Males coordinated their parental behaviours with the females rather than acting independently. Since parental behaviours were directly related to the weight of brood masses, the observed parental interactions will have important fitness consequences in this species. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.

EVOLUTION OF EJACULATES: PATTERNS OF PHENOTYPIC AND GENOTYPIC VARIATION AND CONDITION DEPENDENCE IN SPERM COMPETITION TRAITS

Sperm competition is widely recognized as a potent force in evolution, influencing male behavior, morphology, and physiology. Recent game theory analyses have examined how sperm competition can influence the evolution of ejaculate expenditure by males and the morphology of sperm contained within ejaculates. Theoretical analyses rest on the assumption that there is sufficient genetic variance in traits important in sperm competition to allow evolving populations to move to the evolutionarily stable equilibrium. Moreover, patterns of genotypic variation can provide valuable insight into the nature of selection currently acting on traits. However, our knowledge of genetic variance underlying traits important in sperm competition is limited. Here we examine patterns of phenotypic and genotypic variation in four sperm competition traits in the dung beetle Onthophagus taurus. Testis weight, ejaculate volume, and copula duration were found to have high coefficients of additive genetic variation (CVAs), which is characteristic of fitness traits and traits subject to sexual selection. Heritabilities were high, and there was some evidence for Y-linked inheritance in testis weight. In contrast, sperm length had a low CVA, which is characteristic of traits subject to stabilizing selection. Nevertheless, there was little residual variance so that the heritability of sperm length exceeded 1.0. Such a pattern is consistent with Y-linked inheritance in sperm length. Interestingly, we found that testis weight and sperm length were genetically correlated with heritable male condition. This finding holds important implications for potential indirect benefits associated with the evolution of polyandry.

A comparison of the development and survival of the dung beetle, Onthophagus taurus (Schreb.) when fed on the faeces of cattle treated with pour-on formulations of eprinomectin or moxidectin

Faeces voided by 1-year old cattle at 3–70 days after treatment with a pour-on formulation of moxidectin had no detectable effects on development or survival of the common dung beetle Onthophagus taurus. In contrast, faeces voided by cattle treated with a pour-on formulation of eprinomectin were associated with high juvenile mortality during the first 1–2 weeks after treatment. Increased mortality also occurred among newly emerged beetles fed on faeces collected 3 days after eprinomectin treatment and there was evidence of suppressed brood production among those that survived. This effect was still apparent even after insects fed for a further 10 days on the faeces of untreated cattle. A model simulating the effects of drug residues on dung beetle populations suggests that in the absence of immigration a single treatment of eprinomectin is capable of reducing beetle activity in the next generation by 25–35%. Effects are likely to be greatest when treatment coincides with emergence of a new generation of beetles.

Food availability controls the onset of metamorphosis in the dung beetle Onthophagus taurus (Coleoptera: Scarabaeidae)

SummaryIn nature, larvae of the dung beetle Onthophagus taurus (Schreber 1759) are confronted with significant variation in the availability of food without the option of locating new resources. Here we explore how variation in feeding conditions during the final larval instar affects larval growth and the timing of pupation. We found that larvae respond to food deprivation with a reduction in the length of the instar and premature pupation, leading to the early eclosion of a small adult. To achieve pupation, larvae required access to food for at least the first 5 days of the final instar (= 30% of mean third‐instar duration in control individuals), and had to exceed a weight of 0.08 g (= 58% of mean peak weight in control individuals). Larvae that were allowed to feed longer exhibited higher pupation success, but increased larval weight at the time of food deprivation did not result in increased pupation success except for larvae weighing > 0.14 g. Larvae responded to food deprivation by initiating and undergoing the same sequence of developmental events, requiring the same amount of time, as ad libitum‐fed larvae once those had reached their natural peak weight. Our results reveal a striking degree of flexibility in the dynamics and timing of larval development in O. taurus. They also suggest that premature exhaustion of a larva's food supply can serve as a cue for the initiation of metamorphosis. Premature metamorphosis in response to food deprivation has been documented in amphibians, but this is, to the best of our knowledge, the first time such a behaviour has been documented for a holometabolous insect. We discuss our findings in the context of the natural history and behavioural ecology of onthophagine beetles.

Towards a resolution of the lek paradox.

Genetic benefits in the shape of 'good genes' have been invoked to explain costly female choice in the absence of direct fitness benefits. Little genetic variance in fitness traits is expected, however, because directional selection tends to drive beneficial alleles to fixation. There seems to be little potential, therefore, for female choice to result in genetic benefits, giving rise to the 'lek paradox'. Nevertheless, evidence shows that genetic variance persists despite directional selection and genetic benefits of female choice are frequently reported. A theoretical solution to the lek paradox has been proposed on the basis of two assumptions: that traits are condition-dependent, and that condition shows high genetic variance. The observed genetic variability in sexual traits will be accounted for, because a proportion of the genetic variance in condition will be captured and expressed in the trait. Here we report results from experiments showing that male courtship rate in the dung beetle Onthophagus taurus is a condition-dependent trait that is preferred by females. More importantly, male condition has high genetic variance and is genetically correlated with courtship rate. Our results thereby represent a significant step towards a resolution of the lek paradox.

Allometric relationships and character evolution inOnthophagus taurus(Coleoptera: Scarabaeidae)

Allometric relationships in primary sexual traits (male and female genitalia), secondary sexual traits (male horns and female carinae), and non-sex-related traits (external body traits, epipharynx traits) were studied in the dung beetle Onthophagus taurus. Model II regressions of log-transformed data were used to quantify relationships, with pronotum width as regressor and indicator of overall body size. Slopes (allometric values) for the different trait categories were significantly different, with secondary sexual traits showing the highest values (higher than 1.0), followed by external body traits (slightly lower than 1.00) and epipharynx traits (around 0.2). Primary sexual traits and body size were mostly uncorrelated and genital sizes were virtually constant. Allometries of secondary sexual traits were quite different in the two sexes: the relationship between male horn length and pronotum width was approximately sigmoidal, while that between female carina length and pronotum width was linear. External body traits had significantly higher allometric values in females than in males. Our results suggest that traits in the different categories are under different kinds of selection. Genital allometries can be explained on the basis of sexual selection by cryptic female choice or by the lock-and-key hypothesis. Among secondary sexual characters, male horn morphology seems to be mostly "environmentally" determined and sexual selection would affect only a component of the developmental mechanism of horn expression. External body characters are likely under natural selection, even though a few traits could be sex-related. Finally, in both sexes, internal epipharynx traits seem to be subject to the same selective pressure, probably because males and females use the same feeding niche. The constancy of genital sizes in O. taurus suggests that in developmental processes, more is invested in primary sexual traits (to produce genitalia of the proper size) than in secondary sexual or body traits. Males receiving low quantities of food may incur costs associated with a small horn or small body size, but do not incur costs associated with small genitalia. Females probably share the same developmental pattern.

Allometric relationships and character evolution in <i>Onthophagus taurus</i> (Coleoptera: Scarabaeidae)

Allometric relationships in primary sexual traits (male and female genitalia), secondary sexual traits (male horns and female carinae), and non-sex-related traits (external body traits, epipharynx traits) were studied in the dung beetle Onthophagus taurus. Model II regressions of log-transformed data were used to quantify relationships, with pronotum width as regressor and indicator of overall body size. Slopes (allometric values) for the different trait categories were significantly different, with secondary sexual traits showing the highest values (higher than 1.0), followed by external body traits (slightly lower than 1.00) and epipharynx traits (around 0.2). Primary sexual traits and body size were mostly uncorrelated and genital sizes were virtually constant. Allometries of secondary sexual traits were quite different in the two sexes: the relationship between male horn length and pronotum width was approximately sigmoidal, while that between female carina length and pronotum width was linear. Extern...

Dispersal of dung containing ivermectin in the field by Onthophagus taurus (Coleoptera: Scarabaeidae)

AbstractFifty-five species of exotic dung beetles have been introduced into Australia and in many areas, several species are now established and often abundant. Controversy exists in the dung beetle literature concerning the use of ivermectin as an anthelmintic treatment for livestock and its subsequent lethal and sub-lethal effects on adult dung beetles. A study was conducted in the field exposing replicate dung pats containing residues of ivermectin excreted 0–15 days post-injection. At the time of the study, the introduced species of dung beetle Onthophagus taurus (Schreber) was present in high numbers (>1000 beetles per pat). After 24 h exposure, dung pats collected from cattle treated 7 and 10 days earlier with ivermectin were dispersed significantly less than untreated dung. A similar pattern was evident in the numbers of dung beetles present in each pat. This study is discussed in the context of anthelmintic usage in cattle and their potential ecotoxic effects against dung beetles.

Proximate determination of male horn dimorphism in the beetle Onthophagus taurus (Coleoptera: Scarabaeidae)

Abstract The existence of discrete phenotypic variation within one sex poses interesting questions regarding how such intrasexual polymorphisms are produced and modified during the course of evolution. Approaching these kinds of questions requires insights into the genetic architecture underlying a polymorphism and an understanding of the proximate mechanisms determining phenotype expression. Here we explore the genetic underpinnings and proximate factors influencing the expression of beetle horns – a dramatic sexually selected trait exhibiting intramale dimorphism in many species. Two relatively discrete male morphs are present in natural populations of the dung beetle Onthophagus taurus (Scarabaeidae, Onthophagini). Males exceeding a critical body size develop a pair of long, curved horns on their heads, while those smaller than this critical body size remain essentially hornless. We present results from laboratory breeding experiments designed to assess the relative importance of inherited and environmental factors as determinants of male morphology. Using father–son regressions, our findings demonstrate that horn length and body size of male progeny are not predicted from paternal morphology. Instead, natural variation in an environmental factor, the amount of food available to larvae, determined both the body sizes exhibited by males as adults and the presence or absence of horns. The nonlinear scaling relationship between the body size and horn length of males bred in the laboratory did not differ from the pattern of variation present in natural populations, suggesting that nutritional conditions account for variation in male morphology in natural populations as well. We discuss our results by extending ideas proposed to explain the evolution of conditional expression of alternative phenotypes in physically heterogeneous environments toward incorporating facultative expression of secondary sexual traits. We use this synthesis to begin characterizing the potential origin and subsequent evolution of facultative horn expression in onthophagine beetles.

Sperm numbers, their storage and usage in the flyDryomyza anilis

In the fly Dryomyza anilis females have two kinds of sperm storage organs: one bursa copulatrix and three spermathecae (two spermathecae with a common duct form the doublet, and the third is a singlet spermathecal unit). At the beginning of a mating the male deposits his sperm in the bursa copulatrix. After sperm transfer the male taps the female's abdomen with his claspers. This behaviour has been shown to increase the male's fertilization success. After mating, the female discharges large quantities of sperm before oviposition. To find out where the sperm remaining in the female are stored, I counted the number of sperm in the droplet and in the female's sperm storage organs after different types of mating. I carried out three mating experiments. In experiment 1, virgin females were mated with one male and the matings were interrupted either immediately after sperm transfer or after several tapping sequences. The results show that during male tapping more sperm moved into the singlet spermatheca. In addition, the total number of sperm correlated with sperm numbers in all sperm storage organs, and male size was positively related to the number of sperm remaining in the bursa. In experiment 2, females mated with several males. The number of sperm increased with increasing number of matings only in the doublet spermatheca. No increase in the number of sperm in the singlet spermatheca during consecutive matings suggests that sperm were replaced or did not reach this sperm storage organ. In experiment 3, virgin females were mated with a single male and half of them were allowed to lay eggs. The experiment showed that during egglaying, females primarily used sperm from their singlet spermatheca. The results from the three experiments suggest that sperm stored in the singlet spermatheca is central for male fertilization success and male tapping is related to sperm storage in the singlet spermatheca. The different female's sperm storage organs in D. anilis may have separate functions during sperm storage as well as during sperm usage.

Testing the functional significance of tail streamers

Studies of the evolution of elaborate ornaments have concentrated on their role in increasing attractiveness to mates. The classic examples of such sexually selected structures are the elongated tails of some bird species. Elongated tails can be divided into three categories: graduated tails, pin tails and streamers. There seems to be little debate about whether graduated and pin tails are ornaments; i.e. costly signals used in mate choice. However, in the case of streamers there is considerable discussion about their function. It has been suggested that tail streamers could be (i) entirely naturally selected, (ii) entirely sexually selected, (iii) partly naturally and partly sexually selected. The prime example of a species with tail streamers is the swallow (Hirundo rustica) in which both sexes have tail streamers. In this paper we discuss the aerodynamic consequences of different types of manipulation of the streamer and/or outer tail feather. We make qualitative predictions about the aerodynamic performance of swallows with manipulated tail streamers; these predictions differ depending on whether streamers have a naturally or sexually selected function. We demonstrate that these hypotheses can only be separated if tail streamers are shortened and changes in aerodynamic performance measured during turning flight.

Population status of the bush fly, Musca vetustissima (Diptera: Muscidae), and native dung beetles (Coleoptera: Scarabaeinae) in south-eastern Australia in relation to establishment of exotic dung beetles

AbstractWe compare the pest status and age structure of bush fly,Musca vetustissima Walker, populations, and the abundance of native dung beetles at Uriarra, ACT, for five fly seasons before (1976–81) and three seasons after (1990–93) the exotic dung beetles Euoniticellus fulvus (Goeze) and Onthophagus taurus (Schreber) became established in the area during the late 1980s. Fly populations in all post-introduction seasons of 1990–93 were much older than in the pre-introduction seasons, indicating reduced levels of local fly breeding and higher proportions of immigrants. During the pre-introduction period, the fly annoyance index exceeded the ‘minimum discomfort’ threshold of 10.0 in parts of all seasons. The mean index was 11.3, and this did not differ significantly between seasons. The total dry weight of dung beetles at the time of the first influx of bush flies each year was consistently below 3 g/dung pad which was apparently too low to depress bush fly breeding below their replacement level of 3%. The mean annoyance index of 3.1 for the 1990–91 season was significantly lower than for the pre-introduction period and the total dry weight of beetles during November-December exceeded 3 g/dung pad on all sampling occasions. In the 1991–92 and 1992–93 seasons fly annoyance indices reverted to pre-introduction levels (seasonal means of 13.7 and 7.8 respectively) and total dry weight of beetles was again below 3 g/dung pad when the bush flies first appeared. Survival of immature bush flies in local dung pads remained below the replacement level of 3% during 1990–91, but exceeded 3% on two occasions during 1991–92. Apart from 1976–77, when mean catches of native dung beetles (O. australis and O. granulatus) were exceptionally high, mean catches of native dung beetles during pre- and post-introduction years were very similar, but the mean dung beetle biomass increased following the establishment of exotic species.