Adaptive plasticity of egg size in response to competition in the cowpea weevil,Callosobruchus maculatus (Coleoptera: Bruchidae)

Life history theory predicts that larger propagules should be produced when the offspring are expected to experience intense competition. This study tested whether female cowpea weevils responded to high larval or adult density by producing larger eggs. In a splitbrood design I measured the effect of density experienced by females at their larval stage (1 vs. 4–6 larvae/cowpea) on the size of eggs produced just after emergence. The females were then kept either at low adult density (1 female+1 male per vial), or at high adult density (10 females+10 males) for 2 days, and tested for the effect of this adult density treatment on the size of eggs laid subsequently. I measured egg length and width, as well as the diameter of the entrance tunnel made by the larva, which can be regarded as a crude measure of larval size. Females that experienced high adult density subsequently laid slightly wider eggs than those kept at low density. This difference, albeit small (about 1–4% after correction for female weight and the effect of family, depending on the statistical model used), was statistically significant and robust to alterations of the statistical model. It may be a remnant of a larger plastic response of egg size to competition that has become eroded during many generations in high-density laboratory cultures. There was no difference in egg length or the diameter of the entrance tunnel. Eggs laid just after emergence by females reared at high larval density also tended to be wider than those produced by females that had no competitors. This effect was only marginally significant, however, and sensitive to the statistical model. Both egg length and width and the diameter of the entrance tunnel increased with female weight and decreased with female age. The tunnel diameter was positively correlated with both egg length and width, but the effect of width was larger.     

Oviposition deterrents of Callosobruchus maculatus (Coleoptera: Bruchidae)

Abstract Many bruchids distribute their eggs uniformly among the host seeds available to them and tend to avoid adding eggs to seeds bearing any eggs if pristine seeds remain. Several authors have postulated that the adult cowpea seed beetles, Callosobruchus maculatus (F.) (Coleoptera: Bruchidae), produce pheromones inhibiting oviposition by females exposed to them. It is shown that both sexes produce compounds which can be washed from glass surfaces in a variety of organic solvents which do not deter oviposition as such, but seeds coated with such washings are avoided by ovipositing females when they are selecting seeds for egg-laying and uncoated seeds are available. Some components, at least, of the products persist for at least 30 days, longer than the normal development time of insects in the seeds. The compounds affect the distribution of eggs among the available seeds. At least three different biotypes of the species produce, recognize, and respond in the same way to each other's products. It is emphasized that the eggs themselves may not be the site of the product produced by females but that the physical presence of eggs cannot be overlooked in any discussion of the possible role of pheromones in oviposition and egg distribution by bruchids.     

The Effect of Single, Double, and Triple Matings on the Lifetime Fecundity of Callosobruchus analis and Callosobruchus maculatus (Coleoptera: Bruchidae)

This study examined the effect of single, double, and triple matings on female lifetime fecundity in two closely related species of beetle, Callosobruchus analis and C. maculatus. Multiple mating resulted in elevated lifetime fecundities in both species, although the pattern of elevation differed between the two species. When oviposition resource was plentiful the elevation was apparent after a second but not a third copulation for C. maculatus and after a second and after a third for C. analis. By altering the availability of oviposition sites to C. maculatus females, we were able to alter the pattern of fecundity elevation between matings. When oviposition sites were limited, fecundity increased after a third mating but not a second mating. We suggest that the pattern of fecundity elevation associated with multiple mating in these species is primarily the result of oviposition stimulants delivered by males, rather than a nutritional donation brought about by the metabolism of ejaculates. We also suggest that the intra- and interspecific differences in the relationship between mating frequency and fecundity are the result of differences in egg maturation rates.     

Effect of host quality of Callosobruchus maculatus (Coleoptera: Bruchidae) on performance of the egg parasitoid Uscana lariophaga (Hymenoptera: Trichogrammatidae)

Development and reproductive success of the solitary egg parasitoid Uscana lariophaga Steffan were examined after development in eggs of the bruchid storage pest Callosobruchus maculatus Fabricius reared at either low or high densities on cowpea seeds and laid at day 1 and 4 of maternal life. Both bruchid larval competition and maternal age negatively affected egg size, but the latter more than the former. Uscana lariophaga reared in small hosts developed slower, were smaller and produced fewer eggs compared to parasitoids reared in large hosts. Fecundity of the parasitoid was heavily influenced by host egg size. This was reflected in the values for the intrinsic rate of increase of U. lariophaga, which differed for wasps that developed in host eggs laid by bruchid females of different age. Wasps allocated marginally more female offspring to larger hosts.     

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     

The function of multiple mating in oviposition and egg maturation in the seed beetle Callosobruchus maculatus

Abstract.  To comprehend the significance of multiple mating in female Callosobruchus maculatus , the material-benefits hypothesis is tested, as well as the potential function and mechanism of additional mating for females. The results show that longevity and lifetime fecundity are significantly higher in doubly-mated females, whereas females subject to an interrupted second copulation (which transfers no ejaculate) show resemblance with singly-mated females in lifetime fecundity and daily fecundity, supporting the material-benefits hypothesis. Female lifetime fecundity increases with ejaculate size in doubly-, but not singly-mated females. Doubly-mated females exhibit an immediate increase in fecundity on the day of remating. Moreover, dissection of ovaries after remating shows that the number of unlaid eggs is lower in doubly-mated females in comparison with singly-mated females, whereas the total number of unlaid and laid eggs shows no significant difference. This suggests that the second mating increases the oviposition rate, but not the egg maturation rate in female C. maculatus . Thus, ejaculate serves as a stimulus for oviposition, at least on the remating day. However, because doubly-mated females lay more eggs in total than singly-mated females, doubly-mated females appear to mature more eggs than the singly-mated counterparts. This is attributed to the 'indirect-driven' hypothesis, which states that the reduction in the number of mature eggs in the oviducts brought about by oviposition stimulates the maturation of oocytes in ovarioles to replenish the number of mature eggs.     

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.     

Multiple mating reduces male survivorship but not ejaculate size in the polygamous insect Stenomacra marginella (Heteroptera: Largidae)

In polygamous species, successful males should be able to inseminate multiple females, to defeat sperm from previous males, to avoid sperm displacement by other males, and to induce females to use his sperm during fertilization. Since resources are limited, adaptations to perform any of these functions may conflict with each other (and with other life-history traits) and trade-offs are expected to evolve. We studied if males of the polygamous true bug Stenomacra marginella face a trade-off between multiple mating and survivorship, by comparing the survivorship of virgin and multiply mated males. We also looked for physiological costs of ejaculate production by examining ejaculate production in consecutive matings in multiple mated males. Multiply mated males were able to produce ejaculates of similar size in up to six consecutive copulations but they had decreased survivorship in comparison with virgin males. There was no difference in survivorship between males mated three and six consecutive times, suggesting that the negative relation between survivorship and number of copulations is not linear. The decrease in survivorship seems to be a cost of mating and ejaculate production. This cost could favor the evolution of prudence in the allocation of resources to ejaculate production (e.g., cryptic male choice).     

Effects of maternal age on reproductive traits and fitness components of the offspring in the bruchid beetle, Callosobruchus chinensis (Coleoptera: Bruchidae)

In many insect species, the size and number of eggs decrease with maternal age. Thus, both the size and number of eggs must be considered to know the exact cost of reproduction with maternal age. The resource depletion hypothesis was examined in the bruchid beetle Callosobruchus chinensis. The hypothesis explains why the egg size decreases with maternal age based on the decline of the female's reproductive capacity. A decrease was found in reproductive effort (= egg size × the number of eggs) and the fitness component of offspring with maternal age. The effects of the female's nutritional status on the relationship between maternal age and the reproductive effort of females with and without food and water were also examined. The results indicate that the decrease in size and number of eggs with maternal age can be explained by the resource depletion hypothesis in C. chinensis.     

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.     

Location and ultrastructure of sex pheromone glands in female Callosobruchus maculatus (Fabricius) (Coleoptera : Bruchidae)

The ultrastructure of the female sex pheromone glands in Callosobruchus maculatus (Coleoptera : Bruchidae) were localized using a masking technique, combined with eiectro-antennography and by a comparison of the glandular cells of sexually active (flightless) females and non-sexually active (flight-form) females. Each unicellular gland is an invagination of the integumental membrane capped by a single secretory cell. These glands are situated on the fine intersegmental membrane, which joins the pygidium to the ovipositor. The secretory cells of the glands of active females are characterized by well-developed microvilli, with many elongated mitochondria among the latter. The high metabolic activity of these cells is revealed by the presence of heterogeneous secretion vesicles, some of which contain abundant crystallized material. Deep basal invaginations indicate the uptake of substances from the haemolymph. The receptor canal is a network of fine cuticular filaments which have the same structure regardless of the female's sexual status. Cells from the glands of non-sexually active females are underdeveloped and show no invaginations of the basal membrane and very few microvilli. The localization of these glands was made possible by the use of SEM, TEM and EAG as well as by masking the suspected zones and by comparing females in different physiological states: flightless females, which were sexually active and producing pheromones; and flight-form females, non-sexually active and producing no sex pheromones. Only by adopting such a stringent method was it possible to confirm the function of the glands whose ultrastructure was studied.     

Talisia esculenta lectin and larval development of Callosobruchus maculatus and Zabrotes subfasciatus (Coleoptera: Bruchidae)

Bruchid larvae cause major losses in grain legume crops throughout the world. Some bruchid species, such as the cowpea weevil and the Mexican bean weevil, are pests that damage stored seeds. Plant lectins have been implicated as antibiosis factors against insects, particularly the cowpea weevil, Callosobruchus maculatus. Talisia esculenta lectin (TEL) was tested for anti-insect activity against C. maculatus and Zabrotes subfasciatus larvae. TEL produced ca. 90% mortality to these bruchids when incorporated in an artificial diet at a level of 2% (w/w). The LD(50) and ED(50) for TEL was ca. 1% (w/w) for both insects. TEL was not digested by midgut preparations of C. maculatus and Z. subfasciatus. The transformation of the genes coding for this lectin could be useful in the development of insect resistance in important agricultural crops.     

Costs of mating and egg production in female Callosobruchus chinensis

Costs of reproduction include the costs of mating and egg production. Specific techniques such as irradiation or genetic mutation have been used to divide the expense into costs of mating and egg production in previous studies. We tried to divide the costs in the adzuki bean beetle, Callosobruchus chinensis (Coleoptera: Bruchidae), which needs some kinds of bean as an oviposition substrate. Mated females that were not allowed to lay eggs had a shorter life span than virgin females, but they had a longer life span than mated females that were allowed to lay eggs. The results showed two independent significant costs, mating and egg production, on the life span in C. chinensis. Costs of mating, however, include the costs of sexual harassment by males and copulation itself, and we need further studies to divide the costs. The present method for dividing the cost of reproduction into costs of mating and egg production can be applied to a broad taxonomic range of insect species, and thus it will be a useful model system for inter-specific comparisons of costs of mating and egg production.     

昆虫病原真菌与硅藻土混用对四纹豆象的毒力效果(英文)

在室内条件下检测了白僵菌Beauveria bassiana (Balsamo) Vuillemin 菌株IRAN-187C 和金龟子绿僵菌Metarhizium anisopliae (Metschnikow) Sorokin菌株A-396分别与硅藻土（DE）制剂InsectoSec混用对四纹豆象Callosobruchus maculatus成虫的毒力效果。结果表明：硅藻土与致死剂量和亚致死剂量的真菌分离物混用导致了四纹豆象成虫的高死亡率,且这些处理均缩短了真菌的致死中时(LT₅₀)。LC₅₀ 的白僵菌B. bassiana菌株IRAN-187C和 LC₅₀ 的DE混用对四纹豆象的LT₅₀ (122.2 h)最低,在95%置信限上与LC50 的金龟子绿僵菌M. anisopliae菌株A-396 与LC₅₀的DE混用得到的LT₅₀ (128.2 h) 相当,说明两种真菌分离物与DE混用对四纹豆象成虫都具有较好的毒力效果。用四纹豆象成虫检测3个温度（22, 27和30℃）和2个相对湿度（40%和55%）组合对DE活性的影响,结果显示30℃和40%相对湿度条件下得到的LT₅₀ (110.11 h)最低,说明高温低湿条件下DE活性较高。     

在玻璃表面上短时接触硅藻土和多杀菌素对四纹豆象的致死效应和亚致死效应（英文）

四纹豆象Callosobruchus maculatus (F.)是伊朗豇豆种子上的主要贮藏害虫。控制这一害虫时, 用生物杀虫剂比用常规杀虫剂更为合适。本研究评价了室内条件下在玻璃表面上硅藻土和多杀菌素对四纹豆象成虫的致死效应和亚致死效应。结果表明： 硅藻土处理24 h和48 h后, 对四纹豆象成虫的LC50 值分别为 1.47和0.2 g/m²; 多杀菌素处理24 h和48 h后, 对四纹豆象成虫的LC50 值分别为102.9 和68.8 mg ai/L, 说明两种化合物都对四纹豆象成虫具有较高的急性毒性。通过检测生物学参数, 研究了LC₂₀浓度的硅藻土和多杀菌素对四纹豆象的亚致死效应。LC₂₀浓度的硅藻土和多杀菌素使四纹豆象成虫的繁殖力分别比对照降低了71.5%和17.2%, 卵孵化率降低了57.5%和27.8%, 成虫寿命缩短了74.7%和17.1%。接触LC₂₀浓度的硅藻土和多杀菌素使这一害虫的蛹期分别比对照延长了4.8%和2.3%。亚致死效应研究表明, 硅藻土和多杀菌素对四纹豆象的生命参数均产生了负面影响。总之, 致死效应和亚致死效应综合显示, 硅藻土在防治四纹豆象上具有较大的潜力。     

Rhythmicity of mating and oviposition inCallosobruchus subinnotatus (Pic) (Coleoptera: Bruchidae)

Mating and oviposition behaviors were studied inCallosobruchus subinnotatus. Copulation was most frequent during the late scotophase, 2–3 h before onset of photophase. The females were less willing to mate during photophase, which increased the time to initiate mating while decreasing the duration of mating. Females exhibited increased movement prior to mating, resting immediately after mating, and remained stationary for 6 h when oviposition commenced. Multiple mating by both males and females affected the number of eggs laid, duration of mating, and uncoupling time at the end of mating. Females that mated two or three times laid more eggs than females that mated once or more than three times. Females that remainedin copula for less than 18 min showed greater readiness to remate than those that remainedin copula longer. There was a gradual decrease in the number of eggs females could lay with an increase in the number of previous matings by males.     

Bionomics and polymorphism in Callosobruchus subinnotatus (Coleoptera: Bruchidae)

Callosobruchus subinnotatus (Pic) is the major insect pest of stored bambara groundnuts, Vigna subterranea (L.) Verdcourt, in sub-Saharan West Africa, but little is currently known about its biology or how it may be controlled. A series of laboratory studies was performed to investigate the bionomics of and differences between two apparently different morphs of adult of each sex of this species, here termed 'active' and 'normal'. Major differences in their morphology, physiology and behaviour were identified and are described in detail for the first time. They provide clear evidence of the existence of an adult polymorphism among populations of this species, which is comparable in certain respects to that previously described for C. maculatus (Fabricius) and C. chinensis Linnaeus. Adults can be separated into the correct morph based on characteristic differences in elytral and pygidial colour and pattern. 'Normal' adults are characterized by having high fecundity, short adult life and are relatively sedentary while 'active' adults exhibit reproductive diapause (suspension of reproductive activity), are long lived, and show (at least in females) increased dispersal tendencies. These characteristics suggest adaptation of the 'active' and 'normal' morphs respectively to the different environments of field and seed stores, and the significance of the polymorphism in the life history of C. subinnotatus is discussed in this context. The design of any effective control regime for this bruchid needs to take account of and could potentially be based upon the existence of polymorphism in C. subinnotatus.     

Identification and validation of stage-specific reference genes for gene expression analysis in Callosobruchus maculatus (Coleoptera: Bruchidae)

In light of a number of recent studies highlighting the increasing research interest in bruchids, it is crucial to validate suitable reference genes that could be used in quantitative gene expression studies. Callosobruchus maculatus is a serious pest of stored grains and field legumes in which reference genes have not been assessed and validated to date. The present study aimed to identify and validate reference genes in different developmental stages of C. maculatus shortlisted from commonly used reference genes such as VATPase, TRIP12, TBP, TF11D, ACTIN, GST, ANNEXIN, PTCD3, RPL32, and β -Tub in various insects. Dedicated algorithms like GeNorm, NormFinder, and BestKeeper were used to analyze the stability of these candidate genes, which revealed GST for third instar, ANNEXIN and PTCD3 for the fourth instar, TF11D and VATPase for male pupa, RPL32 and β-tub for female pupa, β-tub and TBP for adult male and VATPase and GST for adult females as suitable reference genes for expression studies in C. maculatus. The final comprehensive ranking using RefFinder identified GST and TBP as the best reference genes for all the developmental stages of C. maculatus. To the best of our knowledge, this is the first report which evaluates and validates stable reference genes in C. maculatus. The information of stage-specific gene expression, generated in this study will be useful for future molecular, physiological, and biochemical studies on C. maculatus and other closely related bruchids.     

Mechanisms of the insecticidal action of TEL (Talisia esculenta lectin) against Callosobruchus maculatus (Coleoptera: Bruchidae)

Plant lectins have insecticidal activity that is probably mediated through their ability to bind carbohydrates. To examine the influence of sugars on the insecticidal activity of a lectin from Talisia esculenta seeds (TEL), the lectin was mixed with mannose, glucose, or mannose plus glucose. Mannose abolished the insecticidal activity. Affinity chromatography showed that TEL bound to midgut proteins of the insect Callosobruchus maculatus. Immunoblotting showed that TEL recognized some proteins, probably glycoproteins, present in the midgut membrane of this insect. The principal proteases responsible for digestive proteolysis in fourth instar larvae of C. maculatus were purified by chromatography on activated thiol-Sepharose. These purified proteases were unable to digest TEL after a 15-h incubation. These results suggest that the insecticidal activity of TEL involves a specific carbohydrate-lectin interaction with glycoconjugates on the surface of digestive tract epithelial cells, as well as binding to assimilatory glycoproteins present in midgut extracts and resistance to enzymatic digestion by cysteine proteinases.