Factors affecting egg maturation in the bean weevil Callosobruchus maculatus

Abstract. This paper concerns the effects of mate and seed availability on the rate of egg maturation in the bean weevil Callosobruchus maculatus (F.). Egg maturation starts before emergence from the seed and, provided that both oviposition sites and mates are available, eggs are laid at a rate determined by the number of oviposition sites, and mature at a similar rate. If seeds or mates are absent then a small number of eggs are laid, but oocytes continue to mature until the oviduct-capacity is approached. The number of eggs that a female can store is dependent on her body weight and does not correlate with the number of ovarioles. If, after a period in which oocyte development has been halted, conditions for egg-laying become suitable, then egg maturation can be re-started, but only after the oviducts have been emptied of eggs. The rate of egg maturation is then similar to that for females of the same age which have been maturing eggs since emergence.     

Effects of search experience in a resource-heterogeneous environment on the oviposition decisions of the seed beetle, Callosobruchus maculatus (F.)

Abstract.  1. This study investigates how female seed beetles, Callosobruchus maculatus , distribute their eggs on various-sized seeds when the size of seed was varied during the egg-laying period.
2. Beetles were allowed to lay eggs on one of three arrays of 64 adzuki beans ( Vigna angularis ). Each array contained four size classes of seed, ranging from small (5.0–5.5 mm diameter) to large (6.5–7.0 mm), but differed in how they were distributed within the environment. In the most heterogeneous condition (the 64-patch design), the four sizes were interspersed, while in the least heterogeneous condition (the four-patch design) they were grouped into four separate blocks. Thus, a beetle exploring the 64-patch design would frequently encounter all four seed sizes, whereas a beetle exploring the four-patch design would only rarely encounter a change in bean size.
3. Beetles experiencing greater seed size heterogeneity were more likely to lay eggs on larger seeds, whereas those in the blocked condition were more likely to oviposit on small seeds. Beetle responses to seed size heterogeneity suggest that the degree of preference for large seeds depends on a female's recent experience.
4. Female beetles exhibited size discrimination throughout their egg-laying process; however, there was a trade-off between seed size and egg discrimination (i.e. avoiding those seeds already containing developing eggs) in response to the change in fitness gained from either laying on larger seeds or lower egg-load seeds during the egg-laying process.
5. Our model provides the first evidence that evolving seed size discrimination ability is adaptive for the seed beetle with egg-discrimination ability.     

The fecundity and oviposition behavior of a South Indian strain of Callosobruchus maculatus

The fecundity of the South India strain of Callosobruchus maculatus (Fab.) (Bruchidae: Coleoptera) is 73 eggs for females developing in 31 days and increases to 94 eggs for females emerging after 41 days. Fed females lay 45–55 more eggs which is the egg-equivalent of the reserves unfed females had to sacrifice for activity. Competition as a larva did not reduce a female's fecundity. Oviposition is inhibited when females are given 25 or fewer beans for oviposition and it is reversed if females find fresh beans. Oviposition is generally released by beans carrying a below average number of eggs, thus, the eggs are almost uniformly distributed over the beans. Eggs are preferentially placed on the largest egg-free beans as expected if oviposition is released when a beetle goes from a smaller to a larger bean. The cues resulating host preferences, cowpea > mung pigeonpea chickpea, are secondary to the cues producing hyperdispersion and the selection of larger beans. We present a model for the oviposition behavior that accounts for the observed responses to species, size, and egg load of oviposition sites.

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Résumé La fécondité de femelles isolées avec un mâle, sans aliment ni eau, et laissées sans perturbation avec 50 graines au moins sert de fécondité standard. Elle augmente avec le temps de développement de l'uf à l'adulte de 73 ufs (32 j) à 94 ufs (44 j). La fécondité potentielle, obtenue avec des femelles alimentées sur sucre est de 125 ufs. Des femelles maintenues avec moins de 25 graines réduisent leur fécondité.Plus de la moitié des femelles distribuent leurs ufs uniformément jusqu'à une densité de 1,5 uf. L'uniformité peut provenir de l'induction de réactions de ponte par les graines portant moins d'ufs que l'effectif moyen. L'hyperdistribution au dessus des densités de 3 ufs/graine révèle l'aptitude à distinguer les graines avec 3 ufs des graines avec 4 ufs. Il y a moins d'écarts par rapport à l'uniformité (erreurs) avec des graines à surface rugueuse. Des femelles avec 10 graines font moins d'erreurs que des femelles avec 50 ou 100 graines.Les ufs sont ajoutés aux plus grosses graines sans ufs; ainsi, plus il y a de graines qui portent des ufs, plus le poids des graines avec ufs diminue. Un tel schéma est prévisible si la ponte est induite par des graines plus grosses que les graines rencontrées précédemment. Les femelles présentent un ordre de préférence clair quand elles sont en rapport avec leurs hôtes classiques: Vigna unguiculata > V. radiata > Cajanus cajan > Cicer arietinum Les caractères spécifiques de l'hôte, plutôt que la taille ou la texture de la surface servent à la distinction entre les graines d'un même hôte.Ces résultats expérimentaux peuvent être utilisés pour la modélisation du comportement de ponte d'une femelle.

     

Copulating with multiple mates enhances female fecundity but not egg-to-adult survival in the bruchid beetle Callosobruchus maculatus

Postcopulatory sexual selection theory has come a long way since the evolutionary implications of sperm competition were first spelled out by Parker (1970). However, one of the most enduring questions remains: why do females copulate with multiple males? Here we show that females copulating with multiple males lay more eggs than those copulating repeatedly with the same male. We also show egg‐to‐adult survival to be more variable when females copulate multiply with different males and less variable when they copulate multiply with the same male. This supports the notion that egg‐to‐adult survival may depend on the genetic compatibility of males and females. However, pre‐adult survival was highest when females copulated repeatedly with the same male rather than with different males. Thus, it would appear that polyandry in this species does not function to reduce the risk of embryo failure resulting from fertilization by genetically incompatible sperm.     

Genetic architecture of population differences in oviposition behaviour of the seed beetle Callosobruchus maculatus

Few studies have examined the genetic architecture of population differences in behaviour and its implications for population differentiation and adaptation. Even fewer have examined whether differences in genetic architecture depend on the environment in which organisms are reared or tested. We examined the genetic basis of differences in oviposition preference and egg dispersion between Asian (SI) and African (BF) populations of the seed beetle, Callosobruchus maculatus. We reared and tested females on each of two host legumes (cowpea and mung bean). The two populations differed in mean oviposition preference (BF females preferred cowpea seeds more strongly than did SI females) and egg dispersion (SI females distributed eggs more uniformly among seeds than did BF females). Observations of hybrid and backcross individuals indicated that only the population difference in oviposition preference could be explained by complete additivity, whereas substantial dominance and epistasis contributed to the differences in egg dispersion. Both rearing host and test host affected the relative magnitude of population differences in egg dispersion and the composite genetic effects. Our results thus demonstrate that the relative influence of epistasis and dominance on the behaviour of hybrids depends on the behaviour measured and that different aspects of insect oviposition are under different genetic control. In addition, the observed effect of rearing host and oviposition host on the relative importance of dominance and epistasis indicates that the genetic basis of population differences depends on the environment in which genes are expressed.     

The effects of copulation duration in the bruchid beetle Callosobruchus maculatus

Control over copulation duration is a potentially importantgenerator of sexual conflict that has received little empiricalattention. The copulatory behavior of the bruchid beetle Callosobruchusmaculatus may reflect a sexual conflict over copulation duration.Males have spines on their intromittent organs that puncturethe female reproductive tract, and females kick their matesduring copulation. If females are prevented from kicking, copulationslast longer and the injuries females sustain are more severe.Males supposedly use the spines as anchors to prolong copulationduration, and females kick to terminate copulations. We manipulatedcopulation duration experimentally and quantified its effectson male and female fitness components to test whether or notthere is a conflict over copulation duration in C. maculatus.Females did not suffer from long copulations but instead experiencedincreased lifetime fecundity. Ejaculate size increased withcopulation duration, and females apparently derive materialbenefits from the ejaculates. Males that mated first and hadlong copulations were relatively unsuccessful when competingwith sperm from other males. However, there was a trend forfemale remating propensity to decrease with long copulationdurations, and first males may therefore also benefit from longcopulations. The copulation duration of the second male to matedid not have a significant effect on sperm precedence. We concludethat even though it seems likely that the male spines have evolvedto act as an anchor during copulation, there seems to be littleconflict over copulation duration per se in C. maculatus.     

The effect of male mating history on paternal investment, fecundity and female remating in the seed beetle Callosobruchus maculatus

1. In many organisms, males provide nutrients to females via ejaculates that can influence female fecundity, longevity and mating behaviour. The effect of male mating history on male ejaculate size, female fecundity, female longevity and female remating behaviour in the seed beetle Callosobruchus maculatus was determined.
2. The quantity of ejaculate passed to females declined dramatically with successive matings. Despite the decline, a male's ability to fertilize a female fully did not appear to decline substantially until his fourth mating.
3. When females multiply mated with males of a particular mated status, the pattern of egg production was cyclic, with egg production increasing after mating. Females multiply mated to virgins had higher fecundity than females mated to non-virgins, and females mated to twice-mated males had disproportionately increased egg production late in their life.
4. Females that mated to multiple virgins, and consequently laid more eggs, experienced greater mortality than females mated only once or mated to non-virgins, suggesting that egg production is costly, and rather than ameliorating these costs, male ejaculates may increase them by allowing or stimulating females to lay more eggs.
5. Females mating with non-virgin males remated more readily than did females mated to virgins. Females given food supplements were less likely to remate than females that were nutritionally stressed, suggesting that females remate in part to obtain additional nutrients.     

Expression of a life-history trade-off in a seed beetle depends on environmental context

Within populations of the seed beetle Callosobruchus maculatus (F.) (Coleoptera: Bruchidae), female longevity is positively correlated with fecundity, both phenotypically and genetically. Yet manipulations of egg-laying rate (through deprivation of seeds or mates) consistently suggest a trade-off between these traits. In this study, females were exposed to four levels of seed availability and the relationship between fecundity and longevity was examined both across and within environments. The expected trade-off was observed across environments, as females with few or no seeds laid fewer eggs and lived longer than females with many seeds. Within environments, however, the relationship was inconsistent; individual longevity and fecundity were positively correlated when seeds were abundant, uncorrelated when seeds were few and negatively correlated when seeds were absent. Body mass at adult emergence was positively correlated with fecundity, but its effect was stronger when seeds were present than when they were absent. After the effects of mass were statistically controlled, longevity and fecundity remained positively correlated among females with many seeds and negatively correlated among those without seeds. Thus, even a single technique (phenotypic correlation) can produce divergent conclusions about the ‘cost of reproduction’ in insects. The reversal in the correlation appears to depend on variation in reproductive allocation; when seeds are scarce or absent, differences in reproductive effort among individuals may be large enough to overwhelm differences in resource acquisition.     

Functional anatomy of the egg pore in Callosobruchus maculatus: a trade-off between gas-exchange and protective functions?

Abstract. The chorion of the egg in Callosobruchus maculatus (Coleoptera: Bruchidae) provides protection but also has to allow exchange of respiratory gases. A single opening in the chorion (the egg pore) allows diffusion of gases, and young larvae die if the egg pore is blocked. Comparison of respiration rates using a Gilson respirometer showed an increase in oxygen uptake from the first to the seventh day after the egg was laid; the increase was from 6.8 to 11.2 pl/egg/day in a Brazil strain and from 8.6 to 23.5 pl/egg/day in a Yemen strain. The Yemen strain had double the metabolic rate of Brazil-strain insects in the larvae, which are exposed to seed toxins, but not in the non-feeding adults. It is suggested that the higher metabolic rate in the Yemen larvae is related to their ability to develop in seeds that are toxic to the Brazil strain. The egg pore also differed in size and shape between the two strains. The egg pore was cylindrical in the Brazil strain, but shorter and funnel-shaped with an increased external diameter in the Yemen strain. Calculation showed that the different shape and size of the Yemen egg pore would allow a 2–3-fold increase in gas conductance compared with the Brazil strain, and this increase was consistent with the enhanced oxygen requirement of the Yemen larvae, which was a little over twice that of the Brazil larvae at 7 days. The anatomy of egg pores in Cmaculatus thus seems to represent a trade-off between two functions, allowing adequate gas exchange while maintaining the protective function of the chorion.     

Response of Callosobruchus maculatus (F.) to varying temperature and relative humidity under laboratory conditions

The effect of temperature and relative humidity were determined on the development of Callosobruchus maculatus (F.) on stored bean (Vigna unguiculata) seeds exposed to five temperatures (20, 25, 30, 35 and 40 °C) and six relative humidity levels (40, 50, 60, 70, 80 and 90%). Oviposition and total adult progeny responded in a curvilinear pattern to temperature and relative humidity while developmental period presented a linear response. Egg laying (117.33 ± 3.21) and adult emergence (35.00 ± 1.70) were least at 20 °C and 90% R.H. but those showed the optimal values at 30 °C and 70% R.H. The implication of these findings is that the growth attributes of C. maculatus relate with the weather variables studied in a curvilinear manner and thus quadratic equations generated could be used for the prediction of optimum temperature and relative humidity in a given area for the management of C. maculatus in stored beans.     

Biological activity of tree marigold, Tithonia diversifolia, on cowpea seed bruchid, Callosobruchus maculatus (Coleoptera: Bruchidae)

The powder and ethanol extract of Tithonia diversifolia leaves were tested for their efficacy at five different concentrations (0.0%, 0.5%, 1.0%, 1.5% and 2.0%) on bruchid mortality, oviposition and adult emergence of cowpea seed beetle Callosobruchus maculatus at ambient tropical storage conditions of temperature and relative humidity. Results showed that the leaf extract had a higher bioactivity on oviposition, adult emergence and mortality of C. maculatus. The mean number of eggs laid on seeds treated with extract was reduced from 20.7 in the solvent-treated to 4.7 at 2%Tithonia extract concentration while adult emergence reduced from a mean of 92.2 in the solvent-treated seeds to 72.2 at 2.0% extract treatment. Mortality was 100% at higher concentrations of 3%, 4% and 5% within 24 h of extract application but at lower concentrations mortality was 73.3% and 93.3% at 1% and 2%, respectively after 24 h. Fourty-eight hours after application, 100% mortality of adult C. maculatus was obtained at all concentrations. The mean number of eggs laid was reduced from 41.3 in the untreated to 17.3 at 2.0% powder concentration while adult emergence dropped from 98.5% in the control to 74.2% at the highest powder concentration. The powder was effective at higher concentrations and longer exposure time. At 3%, 4% and 5%, 63–75% mortalities occurred within 24 h of application while at 48 h, mortality was 76–98% at 3–5% powder application rates. The results of this study revealed T. diversifolia as a potential candidate for bioinsecticide preparations because of antiovipositional, ovicidal and knockdown properties of its products, which have some volatile components.     

Relationships between larval density, adult size and egg production in the cowpea seed beetle, Callosobruchus maculatus

ABSTRACT.

• 1 The eggs of the cowpea seed bettle Callosobruchus maculatus (Fab.) are attached to cowpeas. On hatching, the larvae penetrate the testa and remain in the seed until development is complete and adult eclosion has occurred. The adults do not need to feed and were not allowed to do so.
• 2 Strains from Brazil and Nigeria can produce more than twelve adults from a seed bearing numerous eggs, whereas a strain from the Yemen Republic rarely produced more than three.
• 3 In all three strains the mean weight of the adults produced from a single cowpea declined with increasing initial larval density in the seed.
• 4 Egg production by females is positively correlated with their weight at the time of mating, shortly after emergence.
• 5 Lifetime egg production by females of the Brazilian and Nigerian strains was lower if they came from cowpeas with higher initial larval densities. No such relationship could be demonstrated in the Yemen strain.
• 6 The fecundity of one generation of these beetles, at least in some geographical strains, is significantly affected by the number of larvae entering the hosts in which the adults of that generation have developed.

     

Evolution of clutch size in insects. II. A test of static optimality models using the beetle Callosobruchus maculatus (Coleoptera: Bruchidae)

Wilson and Lessells (1993) analysed the effect of constraint assumptions on the predictions of static optimality models for insect clutch size. They concluded that the models could be reliably distinguished between (and hence the main constraints identified) only after precise quantitative predictions had been examined. The present paper describes a series of laboratory experiments, using the bruchid beetle Callosobruchus maculatus, that allow these quantitative predictions to be made and tested. Experiments in which female encounter rate with hosts was altered gave qualitative support for 3 out of 6 basic (single oviposition) models, but the quantitative fit of them all was poor. However, when the (a priori) condition was included in these models that several other females would oviposit on the same hosts (the multiple oviposition models), the time limiting multiple oviposition model alone produced quantitative predictions that were supported by observations. In other words, the results suggest that the main constraints on bruchid oviposition behaviour are the amount of time available for laying eggs and the number of other females ovipositing. However, additional qualitative predictions indicate that the number of eggs available to the female may also constrain clutch size evolutionarily. The usefulness of static optimality models for examining clutch size decisions in insects is discussed in the context of these results.     

Effect of seed moisture content and d‐limonene on oviposition decisions of the seed beetle Callosobruchus maculatus

Callosobruchus maculatus (Fabricius) (Coleoptera: Bruchidae) is a cosmopolitan pest of stored pulses and legumes. Under legume storage conditions neither adults nor larvae drink, with larvae obtaining their hydration from metabolic water. However, lack of water limits female fecundity and longevity, thus females might be expected to preferentially oviposit on seeds with high moisture content so as to minimize water stress on their offspring. This was tested by offering females a choice of seeds that differed in relation to their level of water content. After controlling for differences in seed size, females were found to preferentially oviposit on the driest seeds. Offspring size and larval survival were equivalent on seeds that differed in moisture content, which in combination with the preference to lay on the driest seeds would indicate that the egg laying decisions of C. maculatus females do not function to minimize larval water stress. Analysis of the chemical volatiles released by seeds that differed in their moisture content revealed a greater level of d ‐limonene in the environment surrounding high moisture seeds. d ‐limonene has known anti‐herbivory properties, thus we suggest the observed female preference for dry seeds may be related to the observed difference in the level of d ‐limonene in the immediate vicinity of beans that differ with respect to their moisture content.     

Consequences of inbreeding for the cowpea seed beetle, Callosobruchus maculatus (F.)(Coleoptera: Bruchidae)

Inbreeding is said to reduce vigour and fitness. It may also determine how a population responds to selection. Local populations of Callosobruchus maculatus , the cowpea seed beetle, are established annually from small numbers of founders and the species has been distributed to many parts of the world where isolated populations may have been founded by very small numbers of individuals. After more than 20 generations of inbreeding, inbred lines have been shown to diverge from a common ancestral stock in similar directions with respect of some variables such as developmental speed, but haphazardly in respect of other parameters such as male weight. The respective roles of drift and of selection as effective evolutionary forces in inbred lines are discussed in the light of these results. It is argued that some intraspecific differences in C. maculatus may be explained as a product of periodic inbreeding, but that the process does not impair the ability to adapt to local conditions so contributing to the status of the species as a pest of international importance.     

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.