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.     

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.     

Contest and scramble competitions inCallosobruchus maculatus (Coleoptera: Bruchidae)

Larval competition curves and resource sharing patterns of 5 strains of Callosobruchus maculatus (iQ, yQ, aaQ, wQ, and tQ) were examined. Offspring emergences as a function of the initial larval density were recorded to construct competition curves. Elytron length of emerged adults was used as the indicator of resource sharing patterns among competing larvae inside a bean. In the large beans, strain iQ showed a saturated competition curve and tQ strain showed a humped curve. Competition curves of the other 3 strains (yQ, aaQ, and wQ) were between those two extremes. In the small beans, strains iQ and tQ also showed a saturated and a humped competition curves, respectively, whereas the competition curves of the 3 intermediate scramble strains could not be distinguished from that of the iQ strain. Thus, the classification based on competition curves was sensitive to the resource condition (bean size). In both the large and the small beans, the elytron lengths of iQ strain remained constant irrespective of initial larval density. On the contrary, the elytron lengths of the 4 other strains decreased monotonically with higher initial larval density. Thus, the judgment based on the resource sharing pattern was shown to be robust. Only iQ strain should be designated as a contest type, and the remaining strains as scaramble types. Contest and scramble types in C. maculatus were also compared with those observed in C. analis and C. phaseoli using competition curves, resource sharing patterns, and other physiological characters.     

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‐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.     

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     

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.     

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.     

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.     

Intraspecific competition and density dependence in an Ephestia kuehniella–Venturia canescens laboratory system

A model host-parasitoid system of Ephestia kuehniella and Venturia canescens was used to examine the influence of host and parasitoid density on host and parasitoid life-history parameters via a two-way factorial experimental design (5 initial host densities×3 parasitoid densities). In the absence of parasitoids, E. kuehniella experienced scramble-type competition with reduced growth, diminished adult size and a subsequent fecundity trade-off for mortality. The mortality that did occur was confined to the late larval and pupal stages. In the presence of parasitoids attacking the late larval stage, competition changed from scramble for food to contest for enemy-free space, with hosts escaping parasitism being small with low fecundity and reduced egg size, and with parasitoid adult size inversely dependent on host density. Total insect emergence (host+parasitoid), a measure of the influence of host resource competition on survivorship, exhibited a threshold effect as a function of initial host density; the threshold value was increased to a higher initial host density in the presence of parasitoids. Models of host self-limitation were fitted to the data, with the generalized Beverton-Holt model that incorporates a threshold effect providing the best fit, and the Ricker model with no threshold providing a very poor fit to the data.     

Both Contest and Scramble Competition Affect the Growth Performance of Brown Trout, Salmo Trutta, Parr of Wild and of Sea-ranched Origins

The effect of contest and scramble competition on the growth performance of wild and sea-ranched juvenile (0+) brown trout, Salmo trutta, originating from the River Dalälven, Sweden was scrutinised. In a mirror image stimulation (MIS) experiment, and in a 35000 1 stream-water aquarium the trout was studied for three weeks (20 individuals in each of four replicates). Activity in MIS was correlated with swimming activity in the stream-water aquarium. The MIS results could not be used for predicting any social behaviour patterns or the growth performance of a fish. No behavioural differences between the two strains were noted. However, the sea-ranched strain grew faster than the wild one, both in regard to the RNA/DNA ratio and the weight-specific growth rate. Because the strains had the same genetic background and prior to the experiments were raised under similar hatchery condition, the results of this study suggest that the sea-ranching process selects for faster juvenile growth in brown trout. The ultimate mechanisms underlying the faster growth by the domesticated strain probably involves both contest and scramble competition.     

Inter-strain competition and dispersal in aphids: evidence from a greenhouse study

1. Crops are often colonised by aphids having different life-history traits, and the aphids and their offspring compete for the shared resource. The intraspecific competition and dispersal characteristics of two strains (A and B) of the cotton aphid Aphis gossypii on cucumber were examined in a greenhouse study. Strain A normally feeds on cucumber, whereas strain B originated from melon but develops on cucumber. Both strains reproduce exclusively via parthenogenesis and can be discriminated using molecular markers. 2. The strains were released at the same time on different plants in a greenhouse. Total density and their dispersion within and between plants were recorded through time, and the frequency of each strain was assessed using DNA fingerprinting. The Lotka–Volterra competition model was used to test for inter-strain competition and to estimate its intensity. 3. Strain A had the highest rate of increase and carrying capacity, and became the most frequent strain, nearly displacing strain B. Strain B was the most common only on the plants it infested first. The competition was unbalanced as strain B was affected strongly by strain A, but strain A was only affected minimally by strain B. This asymmetrical relationship for strain B was not due only to competition for the shared resource. 4. Such competition may in part explain the reduction of genetic polymorphism observed by others in the field where colonising strains coming from different hosts compete and some are eliminated. Polymorphism in A. gossypii populations in nature is thought to be maintained by heterogeneous patches of hosts differentially favourable for different clones.     

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.     

Egg dispersion is more important than competition type for herbivores attacked by a parasitoid

Herbivore fitness can be altered by a combination of interacting organisms, such as its food plant, conspecifics, and predators/parasitoids. Here, we tested relative effects of plant species, herbivore intraspecific competition type, and spatial distribution of the herbivore among plant units on herbivore survival and whether parasitoids modified these effects. We used an endophagous bruchine seed predator Callosobruchus maculatus for the herbivore, and a braconid wasp Heterospilus prosopidis for the parasitoid. The survival rate of C. maculatus was measured for each of 16 combinations of two plants (bean species, Vigna unguiculata and V. radiata), two competition types of C. maculatus larvae (contest and scramble), two spatial distributions of hosts [sparse (1 C. maculatus larva per seed over 20 seeds) and dense (2 C. maculatus larvae per seed over ten seeds)], and with/without a parasitoid pair. In the absence of the parasitoid, C. maculatus survival rate was lower with V. radiata and in the contest type. With the parasitoid, the proportion parasitized hosts was independent of total host density. Neither the proportion of parasitized hosts nor host survival rate was affected by plant species or host strain, but they were affected by host spatial distribution. When host distribution was dense, a higher proportion of hosts were parasitized, and C. maculatus survival rate was lower. Here we discuss parasitoid potential as a selective agent for the sparse within-pod distribution of its hosts in the field.     

Examination of the influences of density pressure on the pattern of adult emergence with reference to the azuki bean weevil,Callosobruchus chinensis

Summary The effect of competition pressure on the sequential pattern of adult emergence was studied by using the experimental population of the azuki bean weevil. The density and pattern of emergence curves of the parental adult induced the changes in the shape of emergence curve of the progeny as well as the number of progeny individuals. In order to explain these changes, a simple mathematical model which has two independent variables, the population density and the capacity of environment, was introduced. The model generated basically similar patterns of emergene curves as observed in the present experiments. This means that the model displayed well the competition pressure which differentially acted upon early-born and late-born individuals in the populations being at different densities and which were differentially received by individual progeny according to the sequential distributions of parents. This different severity of competition pressure resulted in the difference of time when the pressure compelled influences on the sequential distribution of adult emergence. Therefore, early developed individuals may have good chance to survive, but the competition pressure works as a factor modifying their fundamental superiority, especially in the scramble type of competition. Intense crowding of the azuki bean weevil gave rise to the scramble type of competition in the relation between the numbers of adults emerged in two successive generations. When parental emergence concentrated in a short term and at high density, the competition becomes intense, resulting in the inferiority of early developed individuals, in the decrease in number at the next generation and in the increase of duration for emergence.     

Effects of larval movement behavior and density on emergence success and adult body size in a commensal midge

Changes in larval density and movement behavior of a commensal midge, Nanocladius (Plecopteracoluthus) sp. #4, were monitored for 26 weeks in recirculating laboratory streams. Adults were captured at emergence, sexed, and weighed to assess the effect of larval density and movement behavior on emergence success and adult size. The density of midges on hosts declined with time and coincided with a springtime increase in larval movement frequency. Midges residing on hosts with high spring densities emerged significantly less than midges on hosts with low densities. Resident midge density on hosts did not influence the likelihood of successful colonization by commensals and colonizers showed no preference for initial attachment site on hosts. However, colonizing midges emerged significantly less than resident midges. Similarly, successful emergers changed tube positions significantly less often as larvae relative to non-emergers. There was no difference in adult body weight of resident midges and colonists/movers, but adult males which emerged from commensal-laden (high density) hosts were significantly smaller than males from low density hosts. These data indicate larval density and movement behavior may have strong fitness consequences for commensal midges.     

Bottom-up derivation of population models for competition involving multiple resources

This paper provides first-principles derivations of population models for competition involving multiple resources with different competition types, based on resource partitioning between individuals. The following two cases are investigated. The first is the case in which the resource competed for and its competition type change depending on life stages from scramble to contest competition, or from contest to scramble competition. The second is the case in which individuals compete for two resources simultaneously with scramble and contest types, respectively. In both cases, population models are derived analytically, and in particular, the Hassell model is derived in the second case. The nature of reproduction curves and the stability properties of three population models derived are compared with each other. These models provide three representative models for competition involving both scramble and contest types.     

Larval competition in serially transferred populations of Drosophila melanogaster

Summary Experiments showed that larval competition for food is not always the result of decreases in the amount of food available per larva as population density increases. The feeding period during which the larvae try to attain the minimum survival weight may be restricted when food quantity is not limiting. The scramble type competition involves both exploitation and interference components, in degrees which vary with population density.     

Life-history variation in a seed beetle: adult egg-laying vs. larval competitive ability

Summary Populations of the seed beetle Callosobruchus maculatus differ genetically in several traits that mediate intraspecific competition. This study examined competitive interactions between larvae from two strains that differed in their propensity to oviposit on occupied hosts. In a strain (S) where females avoided laying >1 egg/seed, larvae were highly competitive; if two larvae entered a small host simultaneously, only one adult emerged. In a strain (I) whose females were sloppier in their egg-laying decisions, more than half of the seeds bearing two larvae yielded two adults. If seeds contained one larva from each strain, only one adult emerged per seed, and 70% of these adults belonged to the more competitive S strain. A larva's probability of emergence could be increased if it entered the seed before its competitor. A two-day headstart was needed merely for I larvae to compete equally with S larvae. Competition also affected development time and adult weight, but its effect was highly strain- and sex-specific. Adult life-history traits also differed substantially between strains. Compared to I beetles, S beetles exhibited decreased longevity, lower fecundity, a truncated period of oviposition, and larger egg and body size. Fecundity was linearly related to body size in the I strain, but was largely independent of size in the S strain. When faced with a shortage of hosts, S females (whose progeny are highly competitive) withheld eggs and died without depositing 40% of their lifetime supply. In contrast, the fecundity of I females was independent of seed availability. Comparisons with previous studies suggest that both highly uniform egg-laying and strong interference among larvae may be a consequence of chronic association with a relatively small host. Results from the larval competition experiments were used as parameter estimates for a simple, game-theoretic model that postulates interference vs. exploitative strategies. Fitness comparisons suggest that a strategy employing interference competition cannot be invaded by a more exploitative form of competition in a small host.