Spatial relationships between mate acquisition probability and aggregation size in a gregarious coreid bug, (Colpula lativentris): A case of the ideal free distribution under perceptual constraints

Spatial relationships of mate acquisition probability for individuals of both sexes of a gregariously-mating coreid bug, Colpula lativentris, were studied in relation to aggregation size. Operational sex ratio was always strongly male biased. Mate acquisition probability of females was rather constant and independent of aggregation size, as predicted by an ideal free distribution. Moreover laboratory experiments showed that both multiple mating and rearing density little affected female fecundity, suggesting ideal free distribution of females in terms of reproductive success. On the other hand, mate acquisition probability of males was higher in larger aggregations, where more receptive females were available. This male discrepancy from an ideal free distribution was similar to the patterns predicted by an ideal free distribution under perceptual constraints (Abrahams, 1986), but not by that under unequal competitive ability.     

Mating aggregation of a stink bug,Megacopta punctissimum (Montandon) (Heteroptera: Plataspidae)

Summary Adults of a stink bug,Megacopta punctissimum, form aggregations on stems of the bush-clover,Lespedeza crytobotria, in spring. They do not oviposit there, but feed and mate within aggregations. Oviposition is made on other leguminous plants such as the kudzu-vine,Pueraria lobata. Mean size of aggregations (groups of two or more individuals which sit on a stem at distance less than 3 cm from their nearest neighbors) consisted of 4.4 individuals. The sex ratios within aggregations were similar to overall sex ratio of the population including solitary individuals (0.58). More than 50% of females found in aggregations were copulating. Percentage of femalesin copula in larger aggregations was higher than that in smaller aggregations, and this difference was considered to be caused by the higher chance of sexual enconters in larger aggregations. It was suggested that the aggregation of this species on the bush-clover is not a ‘harem’ (a male monopolizes a group of females reported in some other stink bugs) but is mating aggregation to raise the chance of mating. This work was partially supported by Grant-in-Aid (No. 56480039) from the Ministry of Education, Science and Culture.     

AGGREGATION FORMATION AND ASSORTATIVE MATING IN TWO MELOID BEETLES

Males and females of the meloid beetles Lytta magister and Tegrodera aloga form large aggregations in Sonoran Desert habitat. Males and females of L. magister fly to prominent ridgetop landmarks, where they feed on flowering shrubs, mate (probably just once in any one aggregation), and disperse, with the group forming and disbanding in a few days. Males and females of T. aloga form very large, mobile bands that march across the desert flatland feeding and mating; females probably leave after a single copulation, with the result that the group becomes increasingly male-biased in its sex ratio. The aggregations persist for a minimum of 2–3 weeks. Great variation in body size characterizes both species. Positive assortative mating occurs in L. magister but not in T. aloga, possibly because males and females of the two species incur different costs and benefits for selective mating. It is probable that body size in Lytta correlates both with female fecundity and with male spermatophore size or amount of cantharidin contributed during copulation. This should favor males that mate with large females while also favoring females that mate with large males. If large individuals pair off, this will leave smaller individuals to settle for one another, leading to positive assortative mating. The same argument may not apply to T. aloga, perhaps because its copulations are shorter and its aggregations longer-lived, and therefore any one mating does not eliminate chances to copulate again within the band.     

Social system of an inshore stock of the red hind grouper,Epinephelus guttatus (Pisces: Serranidae)

Synopsis According to sex allocation theory, the decision by a female in protogynous fish species to change sex or not should be influenced by, among other things, the mating sex ratio during spawning periods and/or by factors that vary directly with the spawning sex ratio, such as relative rates of behavioral interaction with males and females outside of spawning periods. In groupers that only spawn during a few weeks of the year in large aggregations, individuals must assess the relative value of changing sex or not entirely within the aggregation unless the social system during the remainder of the year provides a behavioral equivalent of the mating sex ratio. Fifty-five individuals of the red hind,Epinephelus guttatus, were tagged and repeatedly located during a 152-day period within a 100 × 100 m grid on a shallow forereef off southwestern Puerto Rico. The home ranges of 22 tagged individuals sighted 10 or more times were 112–5636 m² in area. Individual home ranges overlapped with the home ranges of 1–18 other individuals. Home ranges of small fish were not clustered within the borders of the home range of larger fish, i.e. fish did not form spatially defined social units. At the end of the study, 31 tagged individuals remained on the grid together with five newly sighted fish. All 36 individuals proved on histological examination to be females similar in size to females in the spawning aggregation of the following year. The sex ratio of this all-female inshore stock differed significantly from the sex ratio of that spawning aggregation. Hence, information predicting the reproductive value of a sex change is not available to females in the inshore stock during nonspawning months.     

DIPLOID MALES IN A PRIMITIVELY EUSOCIAL BEE,LASIOGLOSSUM (DIALICTUS) ZEPHYRUM (HYMENOPTERA: HALICTIDAE)

Hymenoptera are characterized by a haplo-diploid mechanism of sex determination. Females are diploid and males are haploid. However, in many species diploid males may occur if individuals are homozygous at a sex determining locus. Diploid males were found in three out of four populations (nest aggregations) of the primitively eusocial, halictine bee Lasioglossum zephyrum for which samples of males were examined electrophoretically. The frequency of diploid males was greater in a small, geographically isolated population (the “Robinson” nest aggregation) than in a large population that had nearby neighboring populations (the “Salmon Creek A” nest aggregation). In addition, the proportion of polymorphic loci was lower in the Robinson nest aggregation suggesting that a bottleneck event or loss of alleles due to small population size occurred in the Robinson population that involved a loss in the number of alleles at the sex determining locus.     

Female choice for male gregariousness in a Stink bug,Megacopta punctissimum (Montandon) (Heteroptera,Plataspidae)

Adults of a stink bug,Megacopta punctissimum, form mating aggregations on their host plants, based on the gregarious habit of males. A female was released on a stem on which there was an aggregation of 2 mating pairs and 1 bachelor male. Next, the bachelor male was released on a stem on which no bugs were present, and the same female was released there. Sequences of courtship behavior were compared between them. This experiement was repeated for 41 pairs of males and females, and about half (20 cases) of these experiments were made in the reverse order. Males were the active sex in courtship, whilst females either accepted the courtship, or escaped from courting males. Females accepted courtship with a higher probability when males courted in aggregations (73%) compared to solitary conditions (22%). This was because the escape behavior of females from the males was reduced if females detected the presence of other bugs near the males. It was concluded that female choice is a selective force for gregariousness in males.     

Precise control of sex allocation in pseudo-arrhenotokous phytoseiid mites

Can sex allocation be controlled by haplo-diploid organisms where both males and females arise from fertilised eggs but males become effectively haploid by paternal genome loss (pseudo-arrhenotoky)? If so, how does the control of mean and variance of sex allocation compare with haplo-diploids where males arise from unfertilised eggs (arrhenotoky)? These questions are addressed by experiments with two species of pseudo-arrhenotokous plant-inhabiting predatory mites: Typhlodromus occidentalis Nesbitt and Phytoseiulus persimilis Athias-Henriot (Acari: Phytoseiidae). It is shown that females conditionally adjust offspring sex ratio in response to the presence of conspecifics or their cues. The sex ratios are precise in that their variance is less than expected from a binomial distribution. Because eggs are produced one-by-one at regular intervals, it is not possible to designate separate clutches, thus rendering conventional clutch-based estimates of precision inadequate. To remedy this a range of time scales was investigated and this showed an increase in precision with time scale (and hence “clutch” size). Markovian analysis of son-daughter sequences showed that this increase arises only if the predator “memorizes” the mean sex ratio of all eggs laid before. Control of mean and variance of sex allocation is selectively advantageous when local mating groups vary in size and are usually small, as is the case for the phytoseiid mites under study. Predictions of the optimal sex ratio from local mate competition models were in agreement for T. occidentalis. However, P. persimilis, exhibited a stronger female bias than predicted. We suggest that this may be due to selection levels operating at a larger spatial scale than the local mating group. Control of sex allocation seems as good as in arrhenotokous arthropods, suggesting that — in this respect — pseudo-arrhenotoky is not at a disadvantage compared to arrhenotoky.     

Male-biased sex ratios,female promiscuity,and copulatory mate guarding in an aggregating tropical bug,Dysdercus bimaculatus

The ecological and social bases of the mating system of the seed-feeding bug, Dysdercus bimaculatus(Hemiptera: Pyrrhocoridae), were studied in the lab and in aggregations at the host tree, Sterculia apetala(Malvales: Malvaceae), in Panama. On theoretical grounds, two factors are predicted to be of importance in determining the evolution of male mating tactics in Ms species: the operational sex ratio and the probability that undefended females will mate with other males, subjecting the gametes of deserters to sperm competition. Results of a study of a related species suggested that sperm displacement is probably substantial. Adult sex ratios at numerous sites were significantly male biased, and females whose mates were removed remated before oviposition (i. e., sperm utilization). These results predict that a mate defense tactic is likely to be superior to a nondefense tactic. The biological significance of the parameters is supported by observations that captive pairs often remained in copulafor several days, until just before oviposition. However, substantial variation in copulation duration was also observed, and possible causes of this variation are considered. Causes of male biased adult sex ratios were investigated by monitoring demographic changes within a single aggregation over 2 months. Both female juvenile and adult mortality rates were greater than male. In addition, dissections of reproductive adults showed that the flight muscles of females, but not males, had histolyzed, so that female reproduction is physiologically limited to a single site. Greater rates of immigration among both mature and young males suggests that an excess of males may also be found in the populations of bugs that subsequently colonize other host plants, so that female scarcity is typical of aggregations in all stages of development. The evolution of sex-limtied flight muscle histolysis may be explained by greater patchiness of females than males as mating resources, plus a lower energetic benefit/cost ratio of histolysis for males.     

Formation and maintenance of mating aggregations in a stink bug,Megacopta punctissimum (Montandon) (Heteroptera: Plataspidae)

Adults of a stink bug,Megacopta punctissimum, formed mating aggregations on their host plants: a few pairs in copula and a few bachelor males (males not in copula) stayed in the aggregation and the bachelor males waited for arriving females to mate with. The processes of formation and maintenance of the aggregations were observed using the individual marking technique. Aggregations initiated by 2 males or by 1 male and 1 female were usually joined by 1 or more individuals and lasted for several days, but aggregations initiated by 2 females broke up within 1 h. Aggregations were not maintained by the same members. The residence time (time from joining an aggregation to leaving the aggregation without copulating) of males was longer than that of females. There was a negative correlation between the residence time of males and the number of bachelor males in an aggregation when bugs joined it, while the residence time of females was positively correlated with the number of bachelor males in the aggregation. When bugs copulated after joining an aggregation, the postcopulatory residence time (time from completing copulation to leaving the aggregation) was longer in males than in females. Thus, males had a stronger tendency to initiate and maintain aggregations than females.     

Attraction of the sexes inFormica lugubris Zett (Hymenoptera: Formicidae)

Summary Sexuals ofFormica lugubris fly to mating places, where females attract males by using a sex pheromone. Females collected on the nest surface before departing on a mating flight are much less attractive than those collected on the mating place after the mating flight, suggesting that the mating flight triggers the release of the sex pheromone. Olfactory cues are essential for males to locate females while they patrol. Males probably use visual cues to locate females once they have alighted nearby them. Males are also attracted by aggregations of other males on the ground, probably because one or several females are likely to be close to male aggregations.     

The mating system of the true fruit fly Bactrocera tryoni and its sister species,Bactrocera neohumeralis

The frugivorous “true” fruit fly, Bactrocera tryoni (Queensland fruit fly), is presumed to have a nonresourced‐based lek mating system. This is largely untested, and contrary data exists to suggest Bactrocera tryoni may have a resource‐based mating system focused on fruiting host plants. We tested the mating system of Bactrocera tryoni, and its close sibling Bactrocera neohumeralis, in large field cages using laboratory reared flies. We used observational experiments that allowed us to determine if: (i) mating pairs were aggregated or nonaggregated; (ii) mating system was resource or nonresource based; (iii) flies utilized possible landmarks (tall trees over short) as mate‐rendezvous sites; and (iv) males called females from male‐dominated leks. We recorded nearly 250 Bactrocera tryoni mating pairs across all experiments, revealing that: (i) mating pairs were aggregated; (ii) mating nearly always occurred in tall trees over short; (iii) mating was nonresource based; and (iv) that males and females arrived at the mate‐rendezvous site together with no evidence that males preceded females. Bactrocera neohumeralis copulations were much more infrequent (only 30 mating pairs in total), but for those pairs there was a similar preference for tall trees and no evidence of a resource‐based mating system. Some aspects of Bactrocera tryoni mating behavior align with theoretical expectations of a lekking system, but others do not. Until evidence for unequivocal female choice can be provided (as predicted under a true lek), the mating system of Bactrocera tryoni is best described as a nonresource based, aggregation system for which we also have evidence that land‐marking may be involved.     

Simultaneous polygyny of the gobiid fish Asterropteryx semipunctata in relation to mate availability

Nesting males of Asterropteryx semipunctata conducted spawning behavior with 2–6 females simultaneously. We carried out field observations on a rocky reef in Kagoshima, Japan, to examine the hypotheses that large males will show multi-female spawning behavior because of their mating advantage, and that simultaneous multi-female spawning will occur when the operational sex ratio (OSR; the ratio of receptive males to receptive females) becomes female-biased. Contrary to our prediction, neither the total number of multi-female spawnings during a spawning season nor mean number of spawning females at a time were correlated with nesting male sizes. This indicates that larger males often did not conduct multi-female spawnings. As predicted, the incidence of multi-female spawning followed the change in the OSR over time—as the OSR in the study area became biased toward females, the incidence of multi-female spawnings gradually increased. Our results suggest that mate availability affects mating patterns in A. semipunctata.     

Do assortative mating and immigrant inviability help maintain population genetic structuring of an herbivore on a crop and a wild relative?

Population genetic structuring is common among herbivorous insects and frequently is associated with divergent host plants, such as crops and their wild relatives. Previous studies showed population genetic structuring in corn leafhopper Dulbulus maidis in Mexico, such that the species consists of two sympatric, host plant-associated populations: an abundant and widespread "pestiferous” population on maize (Zea mays mays), and a small and localized "wild" population on perennial teosinte (Zea diploperennis). a maize wild relative with a limited distribution. This study addressed whether assortative mating and immigrant inviability mediate genetic structuring of corn leafliopper by comparing the mating and reproductive successes of pestiferous and wild females that colonize their nonassociated host plants against the successes of females colonizing their associated host plants. Assortative mating was assessed by comparing mating frequencies and premating and mating times among females of each population on each host plant: immigrant inviability was assessed by comparing, across two generations, the fecundity, survival, development time, sex ratio, and population growth rate among leafhopper populations and host plants. Our results showed that on maize, and compared to resident, pestiferous females, wild females were more likely to mate, and greater proportions of their offspring survived to adult stage and were daughters;consequently, the per-generation population growth rate on maize was greater for immigrant, wild leafhoppers compared to resident, pestiferous leafhoppers. Our results suggested that wild leafhoppers emigrating to maize have a fitness advantage over resident, pestiferous leafhoppers, while immigrant pestiferous and resident wild leafhoppers on teosinte have similar fitnesses.     

The potential for sexual selection in males: Effect of sex ratio and spatiotemporal distribution of receptive females

Summary An animal mating system characterized by male-male competition and active searching for sexually receptive females was modelled to study how varying sex ratio and spatiotemporal distribution of receptive females can affect the variance in male mating success (i. e. potential for sexual selection) in males. The temporal distribution of female receptivity periods appeared to be the variable that had the most pronounced effect on the potential for sexual selection in males. The potential for sexual selection increased monotonically as the degree of asynchrony of female reproduction increased. Female spatial distribution and sex ratio were important only when female reproduction was asynchronous. Then, the potential for sexual selection in males was at its peak when females were overdispersed in space and the sex ratio was female biased. Some of the results derived from the model analysis contrast with predictions from previous studies. The deviating results are most likely caused by different assumptions about modes of mate acquisition in males.     

Mating Aggregations and Mating Success in the Flower Thrips, Frankliniella schultzei (Thysanoptera: Thripidae), and a Possible Role for Pheromones

Aggregations of Frankliniella schultzei males were observed on the corollas of Hibiscus rosasinensis and Gossypium hirsutum flowers in southeast Queensland. Aggregations were seen only on the upper surfaces of corollas but may have occurred on other flower parts, which were hidden from view. Conspecific females entered aggregations and a small proportion of them mated [18% (n = 163), H. rosasinensis; 30% (n = 181), G. hirsutum]. Most females (87 and 72%, respectively) that did not mate in aggregations walked to other flower parts. Behavior was difficult to observe on these parts, but mating was sometimes observed there. The number of females that landed within aggregations on the upper surfaces of both H. rosasinensis and G. hirsutum corollas was highly correlated with the number of males (r = 0.88, r = 0.93, respectively; P < 0.001). Significantly more mating pairs were observed in high-density aggregations (mean ± SE, 1.10 ± 0.22 and 4.44 ± 0.48, respectively) than in low-density aggregations (0.37 ± 0.11 and 1.67 ± 0.29, respectively) (P < 0.05) on flowers of both species. More F. schultzei females were attracted to sticky traps baited with live conspecific males set among flowering Ipomoea indica (mean ± SE, 8.83 ± 0.32) and G. hirsutum (10.90 ± 0.79) plants than to control traps (0.10 ± 0.05 and 0.70 ± 0.25, respectively) (P < 0.05), presumably in response to male-produced pheromones. Significantly more females were attracted to traps with high male densities than to traps with low densities. We found no statistical evidence that aggregation size influenced mating success (proportion males that mated). Mating success, however, should be evaluated with respect to mating on all flower parts and not just the upper surfaces of corollas. The results of this study constitute the first behavioral evidence for an attractant sex pheromone in thrips.     

Eusocial bee male aggregations: spatially and temporally separated but genetically homogenous

Male insects mostly aggregate near sites where sexually reproductive virgin females are found and where mating occur. This reproductive strategy is quite common in Hymenoptera and appears to decrease the chances of inbreeding. In Hymenoptera, inbred mating frequently result in sterile diploid males. Production of diploid males may reach high proportions in small bee populations, and it usually lead to population extinction within a small number of generations. Aggregation of males during a short period of time allow the mixing of local genes. In this paper, we analyzed male aggregations of Tetragonisca angustula (Latreille) (Hymenoptera: Apidae), a eusocial stingless bee, using microsatellite molecular markers. We used population genetic statistics for haplodiploid organisms to address genetic structuring among male aggregations. Our findings indicate that, in general, male aggregations of T. angustula occurring over short time scales and in close proximity are genetically homogeneous as if a single aggregation. We conclude that T. angustula males randomly disperse within a population looking for mating chances rather than settle on a distinct male aggregation. This behavior seems to contribute to the reduction of inbreeding in this species.     

Mating system of Bracon hebetor (Hymenoptera: Braconidae)

Abstract.

• 1 We report on the mating system of a field population of the parasitic wasp, Bracon hebetor, on a corn pile infested by the Indian meal moth, Plodia interpunctella. We demonstrate that the mating system is based upon male scramble competition polygyny with male aggregations on high places on the corn.
• 2 The sex ratio among adults was greater than 80% males on the surface of the corn, whereas below the surface the sex ratio was less than 45%. Males actively courted females on the surface, but there were no aggressive interactions among males during courtship or mating.
• 3 Approximately 20% of the females found on the surface of the corn had no sperm in their spermathecae, regardless of age, but the numbers of unmated females decreased later during the day.
• 4 In laboratory studies we showed that females from this population oviposit a female biassed sex ratio, and that only 14% of females were mated before dispersing from their place of emergence.
• 5 Thus sib-mating is unlikely in this gregarious parasitoid. This outcrossing mating system probably arose because of severe inbreeding depression that B.hebetor suffers via a sex locus: diploids that are heterozygous at the sex locus develop into females, but homozygous diploids are male and are generally inviable. The female biassed sex ratio may have evolved in B. hebetor in response to males being the more expensive sex, females dispersing more frequently from the population than males, or a fraction of females remaining unmated in the population.

     

Host instar, body size and fitness in the koinobiotic parasitoid Aphidius nigripes

In aphidiine parasitoids, resources for growth and adult body size increase with host instar used by ovipositing females, but the fitness consequences of body size on fitness are poorly documented. We compared the fitness of male and female A. nigripesadults that varied in size as a consequence of developing in different instars of their host Macrosiphum euphorbiae. When reproductive fitness was measured without considering time, female wasps from small and large hosts performed similarly, contributing 125–175 foundresses plus 100–180 sons to the next generation. However, when expressed as the innate capacity for increase (r _m), female fitness correlated with host-induced variation of wasp size, indicating that micropopulations initiated by large wasps would increase faster. In a wind-tunnel, a sex pheromone plume from large female wasps induced more males to fly upwind when released at a distance of 50 cm downwind than small females, indicating that large females were sexually more attractive. With respect to male body size effects on fitness, large individuals performed similar to small ones, whether fitness was measured by lifetime mating frequency, fertile inseminations, or proportion of daughters among progeny born from their mates. When young naive males of unequal size were directly competing for mating with a virgin female, small and large males had equal mating success, and large individuals were no more successful than small ones at displacing a competitor already positioned on a receptive female. In a wind-tunnel test where males were scored on their ability to reach a female pheromone source, small and large males were equally affected by wind speed but reached the source located 50 cm downwind in equal proportions, suggesting similar capacity for finding mates by flying upwind. Our results indicate that despite host resources not being fixed at the time of attack for the koinobiont A. nigripes, fitness consequences of resource limitation by the mother may be perceived to be greater for daughters than sons, which would explain male-biased sex ratio in early-instar hosts.     

Intranidal mating,emergence, and sex ratio in a communal beeAndrena jacobi Perkins 1921 (Hymenoptera: Andrenidae)

The extent of preemergence intranidal mating, schedules of emergence, and sex ratio at emergence were documented forAndrena jacobi, a communal, univoltine bee, by collecting and dissecting adults as they emerged from their fossorial nests in 1994. Over 70% of females had mated intranidally with nestmate males, thus potentially incestuously, before emerging. Preemergence intranidal mating did not vary during a day or between nests within a day, though it was less frequent at the start and end of the period of emergence. It was independent of a female's size.A. jacobi was protandrous, though some males emerged after all females. The sex ratio at emergence was remarkably female biased, possibly a consequence of local mate competition. Intranidal mating may represent a characteristic trait of communal bees where a high density of receptive females are predictably aggregated within a nest.     

Does time until mating affect progeny sex ratio? A manipulative experiment with the parasitoid wasp Aphelinus asychis

In haplodiploid organisms, unmated or sperm depleted females are “constrained” to produce only male progeny. If such constrained females reproduce, the population sex ratio will shift toward males and unconstrained females will be selected to produce more females. Assuming that a female's own time spent constrained is an index of the population-wide level of constrained oviposition, and that constrained and unconstrained females reproduce at the same rate, the proportion of sons that females produce when unconstrained should decrease with increasing time spent constrained. Alternatively, if females cannot measure time spent constrained or if time spent constrained is not an index to the level of constrained oviposition in the population, the proportion of sons among progeny produced when unconstrained should not depend upon time spent constrained and should be female biased to an extent depending upon the average time spent constrained over evolutionary time. To test these predictions, we manipulated the amount of time spent virgin in the parasitoid wasp Aphelinus asychis Walker (Hymenoptera: Aphelinidae) and measured the number of males and females among progeny produced before and after mating. First, we found no interaction between age and age at mating in their effect on fecundity, which suggests that mating does not change fecundity. Second, we found that females mated at 8 days and 15 days produced equal sex ratios after mating but these were slightly more female biased than the sex ratios of females mated at 1 day. This observed “step response” suggests that females may perceive time from emergence to mating as a discrete rather than a continuous variable (i.e., short versus long), or that females do not perceive time per se but assess their age class (i.e., young versus old) at the time of mating.