Cylas Formicarius Research Articles

Inbreeding avoidance or tolerance? Comparison of mating behavior between mass-reared and wild strains of the sweet potato weevil

Inadvertent selection is an important genetic process that frequently occurs during laboratory culture. The mass-reared strain of the sweet potato weevil Cylas formicarius exhibits stronger inbreeding depression than the wild strain does. When inbreeding depression occurs in a population, mating with a close relative is often considered maladaptive; however, in some contexts, the inclusive fitness benefits of inbreeding may outweigh the costs, favoring individuals that tolerate a low level of inbreeding depression. Theory predicts that mass-reared strain weevils will avoid inbreeding while wild strain weevils will tolerate inbreeding. To examine this prediction, we compared the effect of relatedness on the mating and insemination successes in mass-reared and wild strains of C. formicarius. While close relative pairs of the wild strain copulated less frequently than non-kin pairs, almost all mass-reared strain pairs copulated irrespective of relatedness. The results showed that the strain with weak inbreeding depression (wild strain) avoided inbreeding, whereas the strain with strong inbreeding depression (mass-reared strain) tolerated inbreeding. The contradiction between the theoretical prediction and our results is discussed from the perspective of laboratory adaptation, mating systems, and life history of C. formicarius.

Age‐dependent investment in death‐feigning behaviour in the sweetpotato weevil Cylas formicarius

Because life‐history theory predicts that risky behaviours such as mating should increase as life expectancy decreases, predatory avoidance is expected to decrease with age. However, this prediction has not been examined. In the present study, the effect of age on death‐feigning behaviour, a form of predatory avoidance behaviour in the sweetpotato weevil Cylas formicarius (Summers) (Coleoptera: Brentidae), is investigated by performing a longitudinal study. Because the effects of mating history and age usually cannot be distinguished, mating history is controlled. The results show that only female weevils decrease the investment in death‐feigning behaviour with age, whereas male weevils do not show any age‐related change. In addition, death‐feigning behaviour of mated females is longer than that of virgin females, possibly because additional mating partners would be not needed by mated females.

Effect of temperature on the development of the West Indian sweet potato weevil, Euscepes postfasciatus (Fairmaire) (Coleoptera: Curculionidae) on an artificial diet

The relationship between temperature and the development of the West Indian sweet potato weevil, Euscepes postfasciatus, on an artificial larval diet containing powdered sweet potato root, was examined at different fixed temperatures from 22 to 31°C. The developmental periods for egg, larvae, and pupae stages shortened in correlation with increased temperature. The thermal constant was 769.2 degree-days and the developmental zero for female and male was 11.1 and 11.7°C, respectively. Although we can rear this weevil at temperatures ranging from 22 to 31°C, rearing temperatures should be kept between 25 and 28°C because the developmental stages were too long at 22°C and the larval period was delayed at 31°C. The basis for these developmental data will be a useful key factor in designing a plan to eradicate the weevil by using a mass-rearing system and SIT.

Ants as biological control agents in agricultural cropping systems

AbstractAnts positively impact agricultural systems by rapidly consuming large numbers of pest insects, disturbing pests during feeding and oviposition, and increasing soil quality and nutrients. The ability of ants to control pest species has been recognized since the year 300 A.D. and farmers continue to conserve and promote ant populations in agricultural systems worldwide. Naturally occurring ant species in milpas, mango, citrus, coconut, cashews, and cotton control many pest insects. Through judicious insecticide application and changes in management practices such as tillage, and other manipulations of vegetation and crop structure, beneficial ant populations are conserved in a variety of agroecosystems. The first recorded example of biological control was the manipulation of ants throughout citrus orchards in Asia. Augmentation continues in citrus, and methods of ant introduction have been developed in Malaysian and Indonesian cocoa plantations, as well as to control sweet potato and banana weevils in Cuba. Ant species have been formally incorporated into other integrated pest management programs for cashew in Australia, cocoa in Papua New Guinea, and mango in Australia and Vietnam. With efforts to reduce chemical pesticide input in agricultural systems, research evaluating the ability of generalist ant species to control pest insects must continue.

採卵用人工飼料に混合する植物粉末の種類と飼料調製法がイモゾウムシの産卵に及ぼす影響

採卵用人工飼料に混合する植物粉末の種類と飼料調製法がイモゾウムシの産卵に及ぼす影響

成虫用人工飼料がイモゾウムシの蔵卵数と交尾能力に与える影響

The West Indian sweetpotato weevil Euscepes postfasciatus (Fairmaire) (Coleoptera: Curculionidae) is a major pest of the sweet potato Ipomoea batatas (L.) Lam (Solanales: Convolvulaceae) in some countries. To improve mass rearing systems involved in an eradication program employing the sterile insect technique (SIT), we examined the effect of an artificial adult diet on the reproductive performance of both male and female E. postfasciatus. The artificial diet had a negative effect on female fecundity but no effect on male mating ability. Our results suggest that the artificial diet should be improved to prevent adverse effects on female fecundity.

イモゾウムシを人工飼料から簡便に回収する方法

The West Indian sweetpotato weevil Euscepes postfasciatus (Fairmaire) (Coleoptera: Curculionidae) is a major pest of the sweet potato Ipomoea batatas (L.) Lam (Solanales: Convolvulaceae) in some countries. In order to improve the mass-rearing systems involved in an eradication program employing the sterile insect technique (SIT), we improved the efficiency of the method used for collecting adult weevils from artificial larval diets; this was done by using the combination of a sieve and Tullgren apparatus. Using this combination, we could collect 95.4% of weevils from artificial diets. Furthermore, the time spent in collection with this method was approximately one-fifth of that spent with the existing hand-sorting method. These results showed that this new collection method will improve the efficiency of mass rearing of West Indian sweetpotato weevils.

Assessment of Effect of Partial Sterility on Mating Performance in Sweetpotato Weevil (Coleoptera: Curculionidae)

The sterile insect technique (SIT) is widely used to suppress or eradicate target pest insect populations. Although the effectiveness of SIT depends on the ability of released sterile males to mate with and inseminate wild females, the use of gamma radiation to induce sterility negatively impacts reproductive cells as well as somatic cells. Consequently, sterilization by irradiation drastically diminishes mating performance over time. In the current study, we evaluated the effect of irradiation dose intensity on fertility, mating propensity, and mating competitiveness in sweetpotato weevil, Cylas formicarius elegantulus (Summers) (Coleoptera: Curculionidae), for 16 d after irradiation. Although the mating propensity of males irradiated with 200 Gy, the dose currently used to induce complete sterility of C. f. elegantulus in the SIT program in Okinawa Prefecture, was equal to that of nonirradiated weevils for the first 6 d, the mating propensity of males irradiated with doses between of 75 and 150 Gy was maintained for the first 12 d. The potential fertilization ability of weevils was highly depressed compared with the control weevils, even in those treated with 75 Gy. Mating performance was severely compromised in weevils that were irradiated with a dose of 100 Gy or more. These results demonstrate that partial sterilization can be highly advantageous in eradication programs for the sweetpotato weevil. We discuss the advantages of the application of partial irradiation in insect eradication programs.

Biological Role of Nardonella Endosymbiont in Its Weevil Host

Weevils constitute the most species-rich animal group with over 60,000 described species, many of which possess specialized symbiotic organs and harbor bacterial endosymbionts. Among the diverse microbial associates of weevils, Nardonella spp. represent the most ancient and widespread endosymbiont lineage, having co-speciated with the host weevils for over 125 million years. Thus far, however, no empirical work on the role of Nardonella for weevil biology has been reported. Here we investigated the biological role of the Nardonella endosymbiont for the West Indian sweet potato weevil, Euscepes postfasciatus. This insect is an experimentally tractable pest insect that can easily be reared on a natural diet of sweet potato root as well as on an agar-based artificial diet. By larval feeding on an antibiotic-containing artificial diet, Nardonella infection was effectively eliminated from the treated insects. The antibiotic-treated insects exhibited significantly lighter body weight and lower growth rate than the control insects. Then, the antibiotic-treated insects and the control insects were respectively allowed to mate and oviposit on fresh sweet potatoes without the antibiotic. The offspring of the antibiotic-treated insects, which were all Nardonella-negative, exhibited significantly lighter body weight, smaller body size, lower growth rate and paler body color in comparison with the offspring of the control insects, which were all Nardonella-positive. In conclusion, the Nardonella endosymbiont is involved in normal growth and development of the host weevil. The biological role of the endosymbiont probably underlies the long-lasting host-symbiont co-speciation in the evolutionary course of weevils.

Pre-exposure to sex pheromone did not affect mating behavior in the sweetpotato weevil Cylas formicarius

In sterile insect technique (SIT) programs, the released sterile males are important for their effectiveness. The use of sex pheromones to collect males is economical; however, pre-exposure to sex pheromones may affect male mating behavior, and would thus reduce the effectiveness of the SIT programs. Males exposed to sex pheromone may become attractive to other males due to pheromone adsorption on their body impregnation, prevent mating and reduce the dispersal of sterile males. We examined the effect of pre-exposure to sex pheromone on the mating behavior of male sweetpotato weevil Cylas formicarius. Pre-exposure to high-dose sex pheromone for 24 h did not affect the male mating behavior of C. formicarius, and the exposed males did not attract other males. These results suggest that male C. formicarius that were collected from a mass-rearing facility using sex pheromone can be effectively used in SIT programs.

The neogregarine protozoan Farinocystis sp. reduces longevity and fecundity in the West Indian sweet potato weevil, Euscepes postfasciatus (Fairmaire)

The number of West Indian sweet potato weevils, Euscepes postfasciatus, being mass-reared in a facility for use in sterile insect technique (SIT) eradication programs has undergone a drastic reduction. A neogregarine protozoan pathogen Farinocystis sp. (an undescribed species) was detected in vivo in the mass-reared E. postfasciatus. We investigated the effects of this disease on the longevity and fecundity of host weevils and the incubation time of the disease in the host body under mass-rearing conditions. Our results demonstrated that infection by this Farinocystis sp. decreased both longevity and fecundity in E. postfasciatus. In particular, the pathogen severely limited the production of progeny by infected females compared to healthy females. Therefore, we consider this protozoan infection to be the major cause of the decreased E. postfasciatus production in the mass-rearing facility.

The effect of mass‐rearing on death‐feigning behaviour in the sweet potato weevil (Coleoptera: Brentidae)

AbstractThe aim of our study was to examine the effect of mass‐rearing, in which there is no exposure to predators, on antipredator traits of insects for improving sterile insect technique programs. The duration of death‐feigning (antipredator behaviour) in sweet potato weevil Cylas formicarius (Summers) (Coleoptera: Brentidae) after mass‐rearing for 71 generations were compared with those in wild strain. There was no significant difference in the duration of death‐feigning between wild and mass‐reared strains. This indicated that the death‐feigning behaviour of mass‐reared strain was maintained for 71 generations even in the absence of predators. We discuss the reasons why death‐feigning behaviour is maintained after mass‐rearing.

Toxicity of Seven <I>Bacillus thuringiensis</I> Cry Proteins Against <I>Cylas puncticollis</I> and <I>Cylas brunneus</I> (Coleoptera: Brentidae) Using a Novel Artificial Diet

"Sweetpotato weevils" Cylas puncticollis (Boheman) and Cylas brunneus F. (Coleoptera: Brentidae) are the most important biological threat to sweetpotato, Ipomoea batatas L. (Lam), productivity in sub-Saharan Africa. Sweetpotato weevil control is difficult due to their cryptic feeding behavior. Expression of Cylas-active Bacillus thuringiensis (Bt) Cry proteins in sweetpotato could provide an effective control strategy. Unfortunately, Bt Cry proteins with relatively high toxicity against Cylas spp. have not been identified, partly because no published methodology for screening Bt Cry proteins against Cylas spp. in artificial diet exists. Therefore, the initial aim of this study was to develop an artificial diet for conducting bioassays with Cylas spp. and then to determine Bt Cry protein efficacy against C. puncticollis and C. brunneus by using this artificial diet. Five diets varying in their composition were evaluated. The highest survival rates for sweetpotato weevil larvae were observed for diet E that contained the highest amount of sweetpotato powder and supported weevil development from first instar to adulthood, similar to sweetpotato storage roots. Seven coleopteran-active Bt Cry proteins were incorporated into diet E and toxicity data were generated against neonate C. puncticollis and second-instar C. brunneus. All Bt Cry proteins tested had toxicity greater than the untreated control. Cry7Aa1, ET33/34, and Cry3Ca1 had LC50 values below 1 microg/g diet against both species. This study demonstrates the feasibility of using an artificial diet bioassay for screening Bt Cry proteins against sweetpotato weevil larvae and identifies candidate Bt Cry proteins for use in transforming sweetpotato varieties potentially conferring field resistance against these pests.

Bioactivity of Amphitoxin, the Major Constituent of the Jamaican Sponge Amphimedon compressa

Fractionation of the MeOH/CH2Cl2 extract of the sponge Amphimedon compressa afforded the secondary metabolite amphitoxin (1), the structure of which was elucidated by interpretation of 1H- and 13C-NMR data. The crude extract and the fractions containing the metabolite 1 were assessed for ichthyotoxic and insecticidal activity towards Xiphophorus variatus (moon fish) and Cylas formicarius elegantulus (sweet potato weevil), respectively. In addition, the ability of 1 to cause mortality (toxicity and lethal effect) in the rodent Rattus norvegicus (Norway rat) was examined. Moderate insecticidal activity was observed, while the toxicity towards the moon fish was evidenced by the high mortality rates for all the fractions tested. In contrast, the rodent was not affected by the metabolite.

Effect of Mass Rearing on Life History Traits and Inbreeding Depression in the Sweetpotato Weevil (Coleoptera: Brentidae)

Inadvertent selection is an important genetic process that frequently occurs during laboratory culture and maintenance of biological control agents and other beneficial organisms used in procedures such as the sterile insect technique (SIT). We investigated effects of mass rearing and inbreeding depression on life history traits (number of progeny emerging from host plants, body weight, developmental period, and starvation tolerance) in the sweetpotato weevil, Cylas formicarius elegantulus (Summers) (Coleoptera: Brentidae). The effect of inbreeding was measured by comparing the results obtained from the full-sib crosses with those obtained from nonkin crosses in both wild and mass-reared strains. The mass-reared strain had more progeny than the wild strain. The developmental period of mass-reared strain was shorter than that of the wild strain. Other traits did not differ significantly between strains. We detected inbreeding depression effects on numbers of progeny, and the effects were more pronounced in the mass-reared strain. Hence, laboratory adaptation to mass rearing can produce changes in important biological attributes of sweetpotato weevils.

Chemical Composition and Antimicrobial Activity of the Essential Oils from Cleome Spinosa

Five different essential oil extractions of the aerial parts of Cleome spinosa Jacq. were examined. The oils obtained by hydrodistillation of the whole aerial parts, aerial parts without flowers (fruit, leaves and stem), flowers, fruits and leaves have been examined by GC-FID and GC-MS. The chemical profiles of the oils reveal the dominance of oxygenated sesqui- and diterpenes, with the exception of the fruit oil, which contained a high content of fatty acids. The most abundant compounds from the whole aerial parts were (Z)-phytol (31.3%), integerrimine (5.5%) and incensole (4.0%). The major compounds from the aerial portion without flowers were caryophyllene oxide (10.5%), (-)-spathulenol (7.5%) and Z-phytol (6.9%). In the flower oil, the main components were 7-alpha-hydroxy manool (23.8%), incensole (9.2%) and sclareol (8.7%). The chief constituents in the fruit oil were tetradecanoic acid (40.6%), (Z)-phytol (6.58%) and sclareol (4.5%). In the leaf oil, (Z)-phytol (19.5%), 7-alpha-hydroxy manool (6.8%) and caryophyllene oxide (4.36%) were the predominant compounds. Antimicrobial activity of the oil obtained from the whole aerial part was evaluated against nine microbial strains using a filter paper disc-diffusion method. The volatile oil showed moderate action against seven of the eight bacteria strains used, with significant inhibitory activity against Streptococcus pyogenes Group A when compared with the standard antibiotics, ampicillin and gentamicin. The fungus, Candida albicans was less sensitive to the essential oil. The oils showed moderate insecticidal activity against Cylas formicarius elegantalus, but possessed no antioxidant activity as indicated by the DPPH method. This represents the first report on the chemical composition of the essential oils from C. spinosa found in Jamaica and the in vitro antioxidant, insecticidal and antimicrobial potential of the oil from the aerial parts.

Evaluation of partial sterility in mating performance and reproduction of the West Indian sweetpotato weevil, Euscepes postfasciatus

AbstractThe sterile insect technique (SIT) is based on population and behavioral ecology and is widely used to suppress or eradicate target pest insect populations. The effectiveness of SIT depends on the ability of the released sterile males to mate with and inseminate wild females. The use of gamma‐radiation to induce sterility is, however, associated with negative impacts not only on reproductive cells but also on somatic cells. Consequently, irradiation for sterilization diminishes mating performance over time. In this study, we evaluated the balance between the irradiation dose and both fertility and mating propensity in Euscepes postfasciatus (Fairmaire) (Coleoptera: Curculionidae) for 22 days following irradiation. The mating propensity of males irradiated with a 150‐Gy dose, as currently used to induce complete sterility of E. postfasciatus in the SIT program in Okinawa Prefecture, was equal to that of non‐irradiated weevils for up to 6 days, and the mating propensity of males irradiated with a dose of 125 Gy was equal to that of non‐irradiated weevils for twice this period (12 days). The fertilization ability of weevils irradiated with a dose of 125 Gy was reduced by 4.6% in males and 0.6% in females, compared to the potential fertilization ability. We also discuss the possibility of the application of partially sterilized insects in eradication programs.

Intensive resistance by females before copulation induces insemination failure in the West Indian sweet potato weevil Euscepes postfasciatus

AbstractPersistent mating attempts by males (sexual harassment) are frequently observed among animals. For females, resisting persistent males can be costly because vigorous resistance increases both energy expenditure and the possibility of injury. Although one tactic for coping with male harassment is to cease resistance and mate with the persistent partner, the females of several species are able to prevent the fertilization of their egg(s) despite copulation. In this study, we used three different sex ratios to investigate whether a male's mating persistence affects his mating success in the West Indian sweet potato weevil Euscepes postfasciatus, in which males mount females both before and after copulation. Consistent with our predictions, we found that female weevils resist and manipulate sperm transfer either before or during copulation according to their preferences. Female weevils were able to reject the sperm of persistent males despite having copulated with them. However, neither copulation and/or post‐copulatory mounting affected insemination success. We speculate that the intensive resistance shown by females before copulation may induce mechanical sterility in E. postfasciatus.

Population dynamics of the West Indian sweetpotato weevil Euscepes postfasciatus (Fairmaire): a simulation analysis

AbstractThe West Indian sweetpotato weevil Euscepes postfasciatus (Fairmaire) is a major pest of the sweet potato Ipomoea batatas (L.) Lam. and this weevil is a target of an eradication program using the Sterile Insect Technique in Okinawa Prefecture, Japan. Understanding the population ecology is essential in the planning of an eradication program; hence, a host‐plant infestation survey and light trap survey have been conducted to monitor the population dynamics of the weevil on Kume Island (Okinawa Prefecture), which is the target area of the trial weevil eradication project. Seasonal tendencies of weevil density were found in these field surveys, but the tendency found in the host‐plant infestation survey was not seen every year, and the effectiveness of the light trap is somewhat suspect. To confirm the reliability of the tendency observed in these field surveys, the present study attempted to explain the tendency by a seasonal temperature change using a temperature‐based model of weevil population dynamics. The seasonal changes of weevil density differed according to host plants and host‐plant fields. The seasonal changes of weevil density inside the host plant Ipomoea indica and outside the host plants in I. indica fields were consistent with those predicted by the model. However, those inside the host plant Ipomoea pes‐caprae in the host‐plant infestation survey were contrary to the predicted ones, and those observed outside host plants in I. pes‐caprae fields by the light trap survey were not in good agreement with the predicted ones. It was concluded that the seasonal change of the weevil density observed in I. indica and I. indica fields can be explained by a seasonal temperature change, but factors other than seasonal temperature change are needed to explain those in I. pes‐caprae and I. pes‐caprae fields.

Effect of body size and sex ratio on male alternative mating tactics of the West Indian sweetpotato weevil, Euscepes postfasciatus

AbstractMale body size is considered to be one of the major determinants of mating success among many insect species. Because the effectiveness of the sterile insect technique (SIT) depends on the ability of released sterile males to mate with and inseminate wild females, it is indispensable to understand the effect of male body size on the mating behavior of both sexes for the progress of the SIT program. We investigated how male body size and the presence of other rival males affect the guarding and copulatory durations of the West Indian sweetpotato weevil, Euscepes postfasciatus (Fairmaire) (Coleoptera: Curculionidae). In this species, males guard females before and after copulation. By observing the mating behavior under two sex‐ratio conditions (male‐to‐female ratios of 1:1 and 2:1), we found that small males hastened to court females when rival males were present, but the females rejected these small males as mates. Therefore, we consider that female weevils adopt a counter‐adaptation for mate preference in response to this male mating strategy. Body size did not affect the durations of copulation and post‐copulatory guarding. Although we found a conditional mating strategy for body size in E. postfasciatus, it is unlikely to have a large influence on the weevil‐eradication program using SIT.