Contest and scramble competitions inCallosobruchus maculatus (Coleoptera: Bruchidae) II. Larval competition and interference mechanisms

Competition between contest and scramble strategists was examined using two strains of Callosobruchus maculatus, the contest strain (iQ) and the scramble strain (tQ). The direct larval interference experiment within each strain showed that the contest strain performed strong direct interference throughout its larval stage against the conspecific opponent(s). The scramble strain also performed its interference, but only during the 3rd and the 4th larval stages. The inter-strain larval competition experiments inside a large and small mung bean (Vigna radiata) showed that the contest strain was, in general, superior to the scramble strain in competition, but the competition results were density- and frequency-dependent. In the large beans, the proportion of adult emergence of the contest strain increased with the increase of its own initial density but decreased as the initial density of the scramble strain increased. The shape of the proportion of adult emergence became nonlinear in the small beans. The cause of the complexity of inter-strain competition results was discussed in the light of the difference in larval interference ability between the two strains.     

Competitive exclusion between contest and scramble strategists in <Emphasis Type="Italic">Callosobruchus</Emphasis> seed–beetle modeling

Interspecific competition between an intermediate contest strain of Callosobruchus maculatus and a scramble strain of C. chinensis was investigated on two types of resource beans: the azuki (Vigna anguralis, small seed) and the black-eye bean (Vigna unguiculata, large seed). We conducted both single-generation competition experiments and multiple-generation experiments and then analyzed the system with a discrete version of the Lotka–Volterra competition model. The estimated competition coefficients showed the competitive dominance of the C. maculatus strain over the C. chinensis strain in both types of bean. Zero-growth isocline analysis based on the predicted model showed that an overall dominance of the contest species, C. maculatus, in azuki beans, whereas the outcome of competition was dependent on the initial population size of each species in the black-eye beans. In the multiple-generation experiments, C. maculatus was the successful competitor irrespective of the ratio of the two beans, whereas C. chinensis overcame C. maculatus in one of five replicates with the black-eye beans system. When we incorporated the cost of scramble competition, or density-dependent reduction in body mass into the predicted model, the model alteration decreased the survival region of C. chinensis in phase space on the black-eye bean. Thus, the competitive equations, which do not consider the density-dependent body size reduction, tend to overestimate the advantages of scramble strategists. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Larval competition within seeds: From the behaviour process to the ecological outcome in the seed beetle Callosobruchus maculatus

Abstract Environmental conditions experienced by organisms during development can have profound impacts on adult fitness and behaviour. Internally feeding larvae unable to leave the seed selected by their mother face limitations of resource suitability and competition. The host seed may guide the larval behaviour within the seed leading to differential intensity of competition and determining its process and outcome, which varies in strains of the legume seed beetle Callosobruchus maculatus (Coleoptera: Bruchidae). However, the intensity, process and outcome of larval competition in different hosts have yet to be simultaneously considered, the objective of the present study. Here we assessed the intensity, process and outcome of intrastrain larval competition as related to host type, and how they are interrelated. Larval competition was faced with two distinct strategies – scramble and contest competition depending on the insect strain and host seed species. The intensity of competition did not show any straight link with the process and outcome of competition. Only a single strain showed a contest competition process with likely interference between larvae, while the four other strains studied showed the process of scramble competition. The process of scramble competition, however, led to variable outcomes in mung beans based on larval competition curves. Such differences were not apparent on cowpea seeds and either the plateau or the peak expected on the larval fitness curves were not reached preventing the distinction of the competition outcome, a likely consequence of the egg laying behaviour of these strains limiting the maximum number of eggs laid per seed. Seed host species rather than seed size are the likely cause of the differences observed from the initial expectation. The strain showing the process of contest competition increased larval fitness with density of larvae emerged per seed regardless of the host species, an unexpected outcome based on theoretical models. In this case the egg laying behaviour of the adult female is probably the main fitness determinant of its progeny.     

Larval competition causes the difference in male ejaculate expenditure in Callosobruchus maculatus

The seed beetle Callosobruchus maculatus larvae exhibit two types of resource competition: scramble, in which a resource is shared, and contest, in which the resource is monopolized. This difference in larval behavior results in different adult densities. Under contest competition, adult density remains constant regardless of larval density, but under scramble competition, adult density increases with larval density. This in turn affects mating frequency during adulthood, and thus, the intensity of sexual selection operating on males. In this study, we examined the relationship between larval competition types and male reproductive investment in mating. We assessed the male ejaculate expenditure per mating across geographic strains of C. maculatus. The male investment (ejaculate expenditure) increased with the degree of scramble competition and decreased with the degree of contest competition. We therefore suggest that males experience different selective pressures depending on the type of larval competition: scramble type males are selected for increased reproductive investment.     

Contest and scramble competitions in two bruchid species,Callosobruchus analis andC. phaseoli (Coleoptera: Bruchidae)

Larval competition between contest and scramble strategists was investigated using the two bruchid species, C. analis (contest species) and C. phaseoli (scramble species) with two different sized mung beans (large and small beans). In both sized beans, the adult emergences of each species dependen on total density of the initial larval densities of the two species and the ratio of the two densities. The emergence of one species was suppressed by the existence of the other species when the initial larval density per bean of the former species was less than that of the latter one. There were many cases in which both C. analis and C. phaseoli emerged from one bean in large beans, but such cases were quite rare in small beans. C. analis performed interference behavior only at late larval stages, whereas C. phaseoli was superior in exploitative competition all through their larval stages. These, combined with the niche segregation inside a bean, are throught to be the major factors of observed density- and frequency-dependent competition results. Based on the above experimental results, long-term competition results between the contest and scramble species were predicted.     

Wall-making behavior as a proximate mechanism to generate variation in larval competition in Callosobruchus maculatus (Coleoptera: Bruchidae)

Variation from contest to scramble in larval competition types was observed among laboratory lines derived from a geographic strain of Callosobruchus maculatus. In contest competition, only one adult can emerge from a small bean because the successful larva monopolizes resources. In scramble competition, however, multiple adults can emerge from the bean because larvae share resources. To explain the variation in competition types, we used six lines of the geographic strain to test the hypothesis that the larval competition type is determined by the larval behavior of building walls, which prevent larvae from interfering with each other, allowing multiple adults to emerge from a single bean. We also investigated the proportions of wall-making in contest-scramble hybrid lines to test whether the formation of a wall structure was genetically determined. Results support our hypothesis that wall-making behavior determines the type of larval competition within a geographic strain, and that the behavior is genetically determined. Scramble-type lines exhibited higher frequencies of wall-making than contest-type lines when two larvae of the same line infested a bean. Larval competition type and the tendency towards wall formation in contest-scramble hybrid lines ranged intermediate of parental lines. We concluded that the variation in larval competition type is determined by the variation in larval wall-making behavior among laboratory lines derived from the geographic strain. We will discuss the evolution of scramble-type larvae in C. maculatus based on our results.     

Polymorphism of competition type and its genetics in Callosobruchus maculatus (Coleoptera: Bruchidae)

Callosobruchus maculatus has both contest and scramble competition strategies. The currently existing theoretical models using game theory suggest that the contest strategy should be selected for. However, most geographic strains of C. maculatus show scramble competition. We experimentally crossed the representative strains of contest and scramble. We expressed the degree of contest competition by a continuous value named the C-value, which ranges from zero (pure scramble) to unity (pure contest). The competition types expressed in the C-value were genetically additive. Their larval developmental rates were negatively correlated with C-values. Multiple-generation experiments of the mixed strains confirmed that there were no overwhelming advantages of contest over scramble type. Most of the mixed strains remained in the intermediate states. We discuss the results in terms of the resource size necessary for developmental success and developmental speed. Received: April 19, 2001 / Accepted: November 2, 2001     

Avoidance of reproductive interference causes resource partitioning in bean beetle females

Reproductive interference is any interspecific sexual interaction that adversely affects female fitness through indiscriminate reproductive activities. It can be a driving force of resource partitioning in conjunction with resource competition. We previously showed that the bean beetle Callosobruchus maculatus is superior in larval resource competition, but vulnerable in reproductive interference, compared with its congener C. chinensis. We hypothesized that these two species might use two resources differently if one of the resources modified the intensity of reproductive interference or resource competition. We observed that C. maculatus females often enter gaps between beans to avoid mating attempts of heterospecific males, and hypothesized that removing bean gaps would strengthen reproductive interference. Therefore, we provided normal beans with gaps and split beans without gaps to females of the two species housed with conspecific or heterospecific males or no males and compared the number of eggs on each bean type among treatments. Callosobruchus maculatus females housed with heterospecific males were more likely to oviposit on normal beans than C. chinensis females. As a result, more C. chinensis adults hatched from split beans and more C. maculatus hatched from normal beans when females and males of both species were housed together. Thus, oviposition resource partitioning resulted from the combination of female avoidance of reproductive interference and resource competition.     

Contest‐type competition between age classes in scramble‐type Callosobruchus maculatus (Coleoptera: Bruchidae)

We examined the effect of age differences on competition type in individuals of a scramble‐type strain of Callosobruchus maculatus (F.). When oviposition of two individuals on a bean was manipulated to introduce time intervals using two lines with different adult body colors, the frequency of two‐adult emergence decreased with the introduction of sequential oviposition. This result indicates that an age difference between two individuals induces contest competition. The frequency of adult emergence in older individuals decreased, whereas in younger individuals it increased with the introduction of sequential oviposition. Using a dissecting microscope, we observed that bodies of older individuals that died in the bean during the 4‐day oviposition interval were crushed at the pupal stage under the pupal chambers of younger individuals. These results show that an age difference between two larvae in a bean causes contest competition due to one‐sided interference by a younger individual during pupation of an older individual. Based on these experimental results, we discuss the ecological cause of contest competition and the population‐level consequences of identified interactions in scramble‐type C. maculatus.     

Experimental evolution of competing bean beetle species reveals long‐term reversals of short‐term evolution,but no consistent character displacement

Interspecific competition for shared resources should select for evolutionary divergence in resource use between competing species, termed character displacement. Many purported examples of character displacement exist, but few completely rule out alternative explanations. We reared genetically diverse populations of two species of bean beetles, Callosobruchus maculatus and Callosobruchus chinensis, in allopatry and sympatry on a mixture of adzuki beans and lentils, and assayed oviposition preference and other phenotypic traits after four, eight, and twelve generations of (co)evolution. C. maculatus specializes on adzuki beans; the generalist C. chinensis uses both beans. C. chinensis growing in allopatry emerged equally from both bean species. In sympatry, the two species competing strongly and coexisted via strong realized resource partitioning, with C. chinensis emerging almost exclusively from lentils and C. maculatus emerging almost exclusively from adzuki beans. However, oviposition preferences, larval survival traits, and larval development rates in both beetle species did not vary consistently between allopatric versus sympatric treatments. Rather, traits evolved in treatment‐independent fashion, with several traits exhibiting reversals in their evolutionary trajectories. For example, C. chinensis initially evolved a slower egg‐to‐adult development rate on adzuki beans in both allopatry and sympatry, then subsequently evolved back toward the faster ancestral development rate. Lack of character displacement is consistent with a previous similar experiment in bean beetles and may reflect lack of evolutionary trade‐offs in resource use. However, evolutionary reversals were unexpected and remain unexplained. Together with other empirical and theoretical work, our results illustrate the stringency of the conditions for character displacement.     

Female Oviposition Decisions and Their Impact on Progeny Life-History Traits

An important factor affecting the life-history of an organism is parental investment in reproduction: reproductive decisions are almost invariably costly. Therefore, reproductive decisions should be beneficial in terms of increased offspring number or fitness. For example, egg laying decisions in many insects can influence resource availability of the offspring through changes in the larval density, and resource availability will have effects on many life-history traits. Here we studied whether female reproductive decisions affect offspring fitness in Callosobruchus maculatus seed beetles. Females laid more eggs on black-eye beans than on mung beans. However, when the difference in the surface area of the beans was accounted for, the number of eggs was not higher in black-eye beans. This together with the poisson distribution of eggs on each of the bean types suggests that females tend to lay their eggs randomly. We found that development time was longer, larval mortality lower and adult survival higher in black-eye beans. We also found interactions between bean type and larval density on size of the offspring such that in mung beans the emergence mass and pronotum width decreased with increasing larval density, but in black-eye beans larval density did not affect the size measures. We conclude that when there is a risk that larval denisty will become high within a bean and there is variable resources available, there exist clear benefits that females might obtain by choosing black-eye beans as a resource for their offspring. However, in contrast to many earlier studies, our results suggest that females may not be making any active oviposition decisions. Therefore, to unequivocally determine whether females do capitalise the potential benefits by active decision making, some further experimentation is required.     

Contest and scramble competitions in two bruchid species,Callosobruchus analis andC. Phaseoli (Coleoptera: Bruchidae)

We performed multiple-generation competition experiments between Callosobruchus analis and C. phaseoli with different bean sizes. In each system, we supplied 5 g of mung beans (Vigna radiata) every 10 days. We examined three types of bean conditions: 5 g of large beans, 5 g of small beans, and a mixture of 2.5 g of large and small beans. In small bean condition, C. analis dominated C. phaseoli in all three replicates and C. phaseoli was extinct by the 260th day. On the contrary, C. phaseoli overcame C. analis within 250 days in large beans in all three replicates. In mixed beans condition the two bruchid species coexisted more than 500 days in two out of the three replicates. Even in the exceptional case, both species coexisted for 460 days. These results were examined in the light of the predictions from short-term larval competition experiments and a game theoretical model by Smith and Lessells (1985). The density and frequency dependent results during larval competition inside a bean was concluded to be a main factor to produce the above long-term competition results.     

Competition strategies and correlated selection on responses to polyandry in the seed beetle Callosobruchus maculatus

Abstract Polyandry reflected in multiple mating with different mates is regarded as favoured by natural selection in males but not necessarily in females, where conflicting effects on fitness components can occur. The present study aims to provide empirical evidence to predict which fitness components may be affected in this sexual conflict using a species that demonstrates potential between‐population variation in their resolution: the cowpea weevil Callosobruchus maculatus. Two strains showing contrasting competition outcomes (scramble × contest) and contrasting life‐history strategies based on trade‐offs between longevity and fecundity are crossed for subsequent selection based on larval‐competition strategy, expecting the production of a correlated response to multiple (polyandrous) mating. Such a response is expected because the scramble strain shows high fecundity (and lower longevity) and would benefit from multiple mating, in contrast with the contest strain, which shows high juvenile mortality. The scramble‐selected lines would evolve a response of increased fecundity and reduced longevity under multiple and potentially polyandrous mating but the contest‐selected lines would not respond to multiple (polyandrous) mating. Instead, both scramble‐ and contest‐selected lines show increased fecundity and reduced longevity with multiple (polyandrous) matings, which did not affect egg weight. Indirect benefits of multiple (polyandrous) mating appear to be relevant for lines showing contest competition among juveniles.     

Coevolution of competing Callosobruchus species does not stabilize coexistence

Interspecific resource competition is expected to select for divergence in resource use, weakening interspecific relative to intraspecific competition, thus promoting stable coexistence. More broadly, because interspecific competition reduces fitness, any mechanism of interspecific competition should generate selection favoring traits that weaken interspecific competition. However, species also can adapt to competition by increasing their competitive ability, potentially destabilizing coexistence. We reared two species of bean beetles, the specialist Callosobruchus maculatus and the generalist C. chinensis, in allopatry and sympatry on a mixture of adzuki beans and lentils, and assayed mutual invasibility after four, eight, and twelve generations of evolution. Contrary to the expectation that coevolution of competitors will weaken interspecific competition, the rate of mutual invasibility did not differ between sympatry and allopatry. Rather, the invasion rate of C. chinensis, but not C. maculatus, increased with duration of evolution, as C. chinensis adapted to lentils without experiencing reduced adaptation to adzuki beans, and regardless of the presence or absence of C. maculatus. Our results highlight that evolutionary responses to interspecific competition promote stable coexistence only under specific conditions that can be difficult to produce in practice.     

Studies on the interspecies competition between the azuki bean weevil and the southern cowpea weevil IV. Competition between strains

This study analyzes results of interspecies competition between the azuki bean weevil, Callosobruchus chinensis and the southern cowpea weevil, C. maculatus using the four geographical strains of each species.

1. Fifteen combinations of one strain from each species were set up to compete. The strains of C. chinensis, rather than of C. maculatus, characteristically determine the eventual outcomes of competition. Some strains of C. chinensis (jC and kC) invariably won every encounter with C. maculatus. Another strain (nC) always lost its encounters, while one other strain (iC) won some encounters (:cQ and :cQ) and lost others (:tQ and :bQ). For any combination the competition results were deterministic and no reversal in outcome was observed among the replicates.
2. The weakness of strain nC in competition is explained by its characteristics in single species populations, different greatly from other strains of C. chinensis, although it had the similar primary (individual) characteristics as other strains of C. chinensis. Strain iC had the shortest adult longevity in the four strains of C. chinensis although it behaved similar to strain jC at the single species population level. Especially under the food supply schedule of the present experiment the shortness of adult longevity might be the important factor determining the outcome.
3. The relationship of the eight strains based on the individual and single species population characteristics, and that based on the competition results were very similar. Even when the competition results could not be explained by any one or two of these characteristics alone, the outcome would be largely decided by the overall biotic profile made by several characteristics.

     

Studies on the interspecies competition between the azuki bean weevil,Callosobruchus chinensis,and the southern cowpea weevil,C. maculatus V. The role of adult behavior in competition

Summary One strain (iC) ofC. chinensis and two strains (aQ and tQ) ofC. maculatus were used to study the temporal pattern in adult distribution in oviposition site. The density of iC in the oviposition site decreased faster than the density in the population cage, but the pattern was not affected by the presence of the other species. The density in ovipositon site of aQ and tQ controls was far lower than that of iC. The density changes in the controls of both strains were rather parallel to the pattern in the population cage. The density of aQ and tQ in the oviposition site in mixed cultures was affected by the presence of the other species. The density of aQ was less than half that in the controls. The pattern of density change was almost the same as in the controls. The density of tQ in the mixed cultures, although lowered initially by the presence of iC, increased after the density of iC decreased. Coupled with its longer female longevity, this characteristic of tQ behavior at the oviposition site apparently contributed to its winning the competition with iC. The adult distribution pattern observed in the present experiments provides an explanation for the peculiar two-fold increase in mean individual weight ofC. chinensis in mixed cultures compared with controls, whileC. maculatus adults were a little lighter in mixed cultures than the controls. Contribution from the Entomological Laboratory, Kyoto University, No. 432.     

Damped oscillation of population density at equilibrium

It has been theoretically assumed that the population density at the equilibrium oscillates with damping from generation to generation. In the adult population of the southern cowpea weevil, Callosobruchus maculatus, it was exemplified. But, it was not so clear in the adult population of the azuki bean weevil, C. chinensis as seen in that of C. maculatus. This difference seems to be due to the scramble type of competition that occurs in larval stage in C. maculatus, instead of in the egg stage as C. chinensis. Comparing with the oscillation from generation to generation obtained in the present experiment to that of Lucilia population found byNicholson , the oscillation in Lucilia population is composed of the cycle in a generation and the descending phase of each cycle of it is not regulated density-dependently. The present result seems to be more appropriate for the demonstration of the theory of self-adjustment of population.     

The dynamics of laboratory populations ofCallosobruchus chinensis andC. Maculatus (Coleoptera: Bruchidae) in patchy environments

Experiments are described showing the long-term dynamics of two species of bruchid beetles (Callosobruchus chinensis and C. maculatus) in arenas in which the resource of 50 black-eyed beans is divided between 5, 10 or 50 ‘patches’. Both species of adult beetles exhibit clumped distributions between patches. Within a patch there is a tendency for a density dependent reduction in (1) eggs laid per female, (2) the proportion of eggs hatching per bean (C. chinensis only) and (3) larval survival which is strongly overcompensating (particularly in C. maculatus). A discrete generation model is used as a framework to draw these results together and show how the different factors affecting natality and mortality can influence the population dynamics. Finally, the importance of the resource renewal interval in influencing the period of the population cycles is discussed.     

Frequency‐dependent and density‐dependent larval competition between life‐history strains of a fly,Lucilia cuprina

1. Competition was created between the larvae of two life‐history strains of the blowfly Lucilia cuprina (Wiedemann) that have different requirements for larval resource acquisition. Adult females of one strain had the ability to mature eggs in the absence of adult feeding (autogeny) whereas the other strain lacked this ability. Autogeny shifts the burden of resource acquisition from adults to larvae, potentially leading to greater competition at this earlier life history stage. 2. A replacement series was used to determine the per‐capita competitive effect between strains relative to the intra‐strain effect, and density‐ and frequency‐dependent variation in this per‐capita effect was then evaluated. Evidence was found of competitive superiority of autogenous larvae when they occurred at a low frequency and low density, but their competitive ability was lost or reversed at higher frequencies and densities. 3. A dynamic competitive environment created by frequency and density dependence can account for the maintenance of genetic diversity for major life‐history traits. Such competition may explain why autogeny is rare in field populations of L. cuprina even although underlying genetic variation for the trait seems to be present.