West Indian Sweet Potato Weevil Research Articles

Spatio-temporal distribution of adults and eggs of the West Indian sweetpotato weevil Euscepes postfasciatus (Coleoptera: Curculionidae) on sweet potato stems

The West Indian sweetpotato weevil, Euscepes postfasciatus, a serious pest of sweet potatoes, is being eradicated by sterile insect technique (SIT) in the south-western islands of Japan. Information on the diurnal movement of the target pests on host plants and where mating and egg-laying behavior occurs on the host is important for the application of SIT, which eradicates the target pest through mating of released sterile males and wild females. However, little such information is available on this species. In this study, male and female adults were released on host plants to examine the diurnal distribution on seedlings according to sex, as well as the sites where mounting behavior and egg laying occurs. The results showed that females left the host plant more frequently at night, whereas males were more likely to remain on the host plant at night. Both males and females stayed on the nodes of the host plant during the daytime. Mounting behavior also tended to occur more often at nodes. Furthermore, compared to unmated females, mated females stayed at the vertical top of the seedlings. However, it was found that eggs were often laid close to the roots rather than at the top of the vertical stems, even when the seedlings were placed upside down. The results of previous studies and this study will be discussed from the perspective of the application of SIT against E. postfasciatus.

The female Euscepes postfasciatus refractory period is induced by the male but length is determined by the female

Females of many animals mate multiple times during their lives (i.e., polyandry). The period between matings (mating interval) is called the refractory period (RP). In the West Indian sweet potato weevil (Euscepes postfasciatus), males use the ejaculate to induce the RP in females to prevent re-mating. By measuring the RP, a large variation of 1–49 days was observed. This variation may be due to the males (ejaculate quantity and quality) and females (ejaculate sensitivity/degradation ability and body size) and their interactions, but the exact mechanisms are currently unclear. Here, we investigated a tendency towards a particular female RP duration and the associated traits of males and females to test the following three factors responsible for variation in the length of the RP: male manipulation of ejaculate volume, individual differences in male ejaculation substances, and ejaculate sensitivity/degradation ability in females. We prepared virgin males and females to create mating pairs. The following day, another mate was introduced to the females, and the first RP was measured. The same procedure was used for measuring the second RP. The males were also provided with another female (second female), mated, and then the RP of the second female was measured. In addition, the relationship between the length of the RP and female fitness was investigated. The results showed that there was a significant positive correlation between the first and second RP in the focal females, while no significant correlation was observed between the RP of the first and second females induced by the same male. It was also found that the length of the RP did not affect female fitness. This indicated that the males did not adaptively manipulate ejaculation volume depending on the quality of the females, and variance in the length of the RP may be explained by variation in the female physiological ability against ejaculate.

Effects of Larval Diet on the Male Reproductive Traits in the West Indian Sweet Potato Weevils Euscepes postfasciatus (Coleoptera: Curculionidae).

Simple SummaryIn insects, it is known that the diet during the larval stage affects traits in the adult stage. However, it is still unclear how it affects reproductive traits such as ejaculation. The ejaculate contains many proteins and therefore requires much nutrition, so the larval diet strongly influences it. Males of the West Indian sweet potato weevil Euscepes postfasciatus use accessory gland substances to inhibit remating by females. Crossing experiments were conducted using lines reared on artificial diets or sweet potato tubers during the larval stage, and the refractory period was examined. The results showed that the larval stage diet had a significant effect on the refractory period of females. We also found one protein of approximately 15 kDa in size expressed only in the treatments reared on sweet potatoes. To our knowledge, this is the first study to show that larval diet qualitatively influences male ejaculate and female refractory period.Larval diet significantly affects adult traits, although less is known about how they affect reproductive traits. Males of West Indian sweet potato weevil Euscepes postfasciatus deliver a remating inhibitor along with sperm to their mates during mating, leading to a refractory period (the period before females mate again). Crossing experiments were conducted using lines reared on artificial diets, including sweet potato powder (AD) or sweet potato tubers (SP) during the larval stage, and the refractory period was examined. We also examined whether the larval diet qualitatively or quantitatively altered male ejaculate. The results showed that the refractory period was significantly longer in the SP treatment than in the AD treatment for males and females. There was no significant difference in ejaculate volume. However, the number of sperm in the testes-seminal vesicles complex was significantly higher in the SP treatment. Additionally, SDS-PAGE revealed that the ejaculate was qualitatively different depending on the larval diet, and one protein of approximately 15 kDa in size was expressed only in the SP treatments. Revealing how larval diet affects reproductive traits in adult males will help shed light on the diverse evolution of insect mating systems and reproductive behavior.

Female Marking via Rubidium-Labeled Ejaculates in the West Indian Sweetpotato Weevil (Coleoptera: Curculionidae)

The West Indian sweetpotato weevil, Euscepes postfasciatus (Fairmaire), is one of the pests of the sweet potato, Ipomoea batatas, in tropical and subtropical countries. Although an eradication program using the sterile insect technique (SIT) for this weevil is now underway in Japan, the lack of potent attractants such as sex pheromones for this weevil species prevents effective monitoring and hampers pest control operations. New methods to monitor the performance of sterile males using trapped wild females are needed. In this study, we tested whether the ejaculate of the labeled males that were fed an artificial diet labeled with the trace element, rubidium (Rb), is detectable in the fertilized females. We fed an artificial diet treated with Rb (1.00, 0.50, 0.25, and 0.00%) to adult male weevils for 14 d after emerging and quantified the rubidium content in inseminated females. We also examined the side effects of the artificial diet on mating performance and longevity. The advantages of monitoring the spatial level of sterility using data from females labeled with ejaculate of Rb-fed males in an SIT eradication program are discussed.

Effects of Storage Periods of an Artificial Larval Diet on the Yield and Quality of Mass-Reared West Indian Sweet Potato Weevil (Coleoptera: Curculionidae).

Artificial diets have been employed for the mass-rearing of numerous insects because of their ease of use and standardized quality. An ability to store artificial diets under nonrefrigerated conditions over the long term could improve the efficacy of mass-rearing systems considerably. However, it remains largely unknown how long artificial diets can be stored at such temperatures without any adverse effects on the insects reared. In this study, we investigated yield, body size, and reproductive potential of West Indian sweet potato weevil, Euscepes postfasciatus (Fairmaire), which is a major sweet potato pest, under management using the sterile-insect technique in Japan and reared using artificial diets with different storage periods (14, 28, and 42 d) at nonrefrigerated temperatures (25 ± 1°C), and compared them with those of the control (0 d). Notably, E. postfasciatus yield and reproductive potential increased significantly with an increase in storage period (28 and 42 d). Conversely, male body size decreased significantly following feeding with artificial diet stored for 42 d, when compared with the control, while there were no significant differences in female body size between the control and all the treatments. We discuss the potential causes of such varying effects between yield and body size and conclude that E. postfasciatus artificial diet can be stored for at least 28 d without any adverse effects on weevil yield and weevil quality. To the best of our knowledge, this is the first report revealing the positive effects of long-term storage of the artificial diet on mass-reared insects.

Male body size does not affect the refractory period of females in the West Indian sweet potato weevil Euscepes postfasciatus (Fairmaire) (Coleoptera: Curculionidae) and the seed bug Togo hemipterus (Scott) (Heteroptera: Lygaeidae)

Multiple mating by females increases both the risk and intensity of sperm competition for males. The ability to temporarily or permanently inhibit female receptivity to further mating represents a highly adaptive strategy to reduce the risk of sperm competition. Males of the West Indian sweet potato weevil Euscepes postfasciatus (Fairmaire) (Coleoptera: Curculionidae) and the seed bug Togo hemipterus (Scott) (Heteroptera: Lygaeidae) use accessory gland substances to inhibit remating in females. It is known that there is variance in the length of the female refractory period (i.e., the effectiveness of male ejaculate substances that inhibit female remating) within a species. However, it is unclear what types of male traits affect the length of the female refractory period. The mechanism of the female refractory period is not sufficiently revealed. We focused on male body size, which is often a target of sexual selection, and investigated the relationship between male body size and both the refractory period of females and mating duration. Contrary to our thought, male body size did not significantly influence the refractory period of females or mating duration in either species. Smaller male body size was not particularly favored by post-copulatory sexual selection in either species.

Effects of high CO2 on mating success, mating duration, and sperm transfer in the West Indian sweet potato weevil, Euscepes postfasciatus

AbstractMost terrestrial animals live in natural atmospheric conditions, but some are also adapted to low oxygen (hypoxic) or high‐carbon dioxide (hypercapnic) conditions, such as in social insect nests, soil, caves, wood, and decaying material. Although it is possible that individuals adapt their behaviour to the environmental condition of their habitats, mating behaviour under ecologically possible ranges of CO2 has not been well studied. We compared walking activity, duration of mating behaviour, and sperm transfer ability in the West Indian sweet potato weevil, Euscepes postfasciatus Fairmaire (Coleoptera: Curculionidae), in high CO2 (10 000 p.p.m.) vs. normal atmospheric conditions (laboratory air, ca. 800 p.p.m.). We found that high CO2 enhanced walking activity and mounting frequency. Under such circumstances, we predicted that enhanced activities under a high‐CO2 environment would increase the risk of sperm competition, which induces an extended copulation period and an increase in sperm transfer. However, weevils shortened the mating period and did not alter their sperm transfer ability under high CO2. These findings are, as far as we are aware, the first report of the effect of ecologically relevant high CO2 on insect mating behaviour under mass‐rearing conditions. The effect of ambient conditions on mating behaviour and sperm transfer is discussed in relation to the intensity of female refusal behaviour directed against males.

When a male perceives a female: the effect of waxy components on the body surface on decision-making in the invasive pest weevil.

Insects use various semiochemicals for sexual communication and mate recognition; these can therefore be used to govern the behaviours of harmful pest species, and several candidate chemicals have been explored for this purpose. For the West Indian sweet potato weevil, Euscepes postfasciatus, which is one of the most serious pests of sweet potato, no effective capture techniques, such as sex pheromone lures, exist. Toward exploring promising procedures for monitoring these weevils, we assessed the effect of secretions on the body surface on the recognition of congeners and on courtship behaviour in the weevils. Our study clearly demonstrated that weevils responded to extracts from the body surface, and the behaviour adopted by the weevils varied significantly depending on the condition of the extracts. Furthermore, we found a significantly prolonged retention time for males on glass beads covered with extracts of females based on survival analysis. These findings are, as far as we are aware, the first to show the effect of lipid components of the body surface on decision-making in these economically important pest weevils.

Effects of X-ray irradiation on male sperm transfer ability and fertility in the sweetpotato weevils Euscepes postfasciatus (Coleoptera: Curculionidae) and Cylas formicarius (Coleoptera: Brentidae)

Gamma radiation from isotopic sources has been used in sterile insect technique (SIT) programs worldwide, but it might be difficult to continue using these sources in future SIT programs because of social issues. Therefore, an alternative sterilization source to gamma rays, such as X-rays, needs to be developed. The physical properties of radiation are different between gamma rays and X-rays: for example, X-rays have a shorter penetration depth than gamma rays. Therefore, X-rays may not fully confer male sterility, depending on the target pest insects. The present study investigated whether the West-Indian sweetpotato weevil Euscepes postfasciatus (Fairmaire) and the sweetpotato weevil Cylas formicarius (Fabricius) are sterilized by X-rays generated in a low-energy X-ray irradiator, without deterioration of male mating ability, at the doses currently used in the eradication programs for E. postfasciatus (150 Gy) and C. formicarius (200 Gy) using gamma rays at Okinawa, Japan. The results demonstrated that it is possible to use X-rays in future SIT programs for E. postfasciatus and C. formicarius, because X-ray irradiated males were almost completely sterilized without deterioration of their mating ability.

Appropriate number of inoculated eggs for mass-rearing the West Indian sweet potato weevil, Euscepes postfasciatus (Coleoptera: Curculionidae)

It is necessary to establish an economical mass-rearing system for the West Indian sweet potato weevil Euscepes postfasciatus (Fairmaire) so that large numbers can be created for use in the sterile insect technique (SIT) to control wild infestations. Both the quality of offspring and the number of adult weevils produced are important. As an adult female with a dry weight of less than 1.4 mg has an extremely low rate of production of offspring, we investigated the effect of an inoculated dose of egg suspension on both yield rate and body size, in order to determine the appropriate dose to attain the maximum yield of productive females. The number of emerged weevils increased as the inoculated dose of egg suspension increased. In brief, both yield rate and female body size significantly decreased as the inoculated dose of egg suspension increased, perhaps as a result of density effects. We conclude that the appropriate dose of inoculated egg suspension to attain the maximum yield from productive females of E. postfasciatus in the current mass-rearing system is 3 mL (containing approximately 450 eggs) per rearing container.

Limited mobility of target pests crucially lowers controllability when sterile insect releases are spatiotemporally biased

The sterile insect technique (SIT) is a genetic pest control method wherein mass-reared sterile insects are periodically released into the wild, thereby impeding the successful reproduction of fertile pests. In Okinawa Prefecture, Japan, the SIT has been implemented to eradicate the West Indian sweet potato weevil Euscepes postfasciatus (Fairmaire), which is a flightless agricultural pest of sweet potatoes. It is known that E. postfasciatus is much less mobile than other insects to which the SIT has been applied. However, previous theoretical studies have rarely examined effects of low mobility of target pests and variation in the spatiotemporal evenness of sterile insect releases. To theoretically examine the effects of spatiotemporal evenness on the regional eradication of less mobile pests, we constructed a simple two-patch population model comprised of a pest and sterile insect moving between two habitats, and numerically simulated different release strategies (varying the number of released sterile insects and release intervals). We found that spatially biased releases allowed the pest to spatially escape from the sterile insect, and thus intensively lowered its controllability. However, we showed that the temporally counterbalancing spatially biased releases by swapping the number of released insects in the two habitats at every release (called temporal balancing) could greatly mitigate this negative effect and promote the controllability. We also showed that the negative effect of spatiotemporally biased releases was a result of the limited mobility of the target insect. Although directed dispersal of the insects in response to habitats of differing quality could lower the controllability in the more productive habitat, the temporal balancing could promote and eventually maximize the controllability as released insects increased.

Symptoms shown by late-stage larvae of the West Indian sweet potato weevil Euscepes postfasciatus (Coleoptera: Curculionidae) infected with Farinocystis sp. (Neogregarinida: Lipotrophidae)

Pathogen outbreaks often represent a major issue when mass rearing target insects for biological control, even when applying sterile insect techniques (SIT). Here, we aimed to improve existing screening techniques used to detect a neogregarine Farinocystis sp. that infects the West Indian sweet potato weevil Euscepes postfasciatus Fairmaire (Coleoptera: Curculionidae) mass reared under the SIT program. Our results revealed that infected late-instar larvae exhibited swelling symptoms, and that their body weight and head width significantly increased compared to uninfected larvae. The head width to body weight ratio differed between infected and uninfected larvae; yet, head width was highly correlated with body weight in both cases. Visual diagnosis based on the appearance of the late-instar larvae showed concordance of almost 90 % with the current diagnostic methods of optical microscopy and PCR. Thus, the swelling symptoms observed in late-instar larvae were directly noticeable. Therefore, visual diagnosis, based on larval appearance, represents a feasible, low-cost technique for the early detection of Farinocystis sp. infection of the West Indian sweet potato weevil.

長期累代飼育がイモゾウムシの交尾能力に及ぼす影響

A lot of sterile males need to be released continuously in the target area in the sterile insect technique (SIT). Therefore, mass-rearing of the target pest insects is indispensable for SIT programs. The West Indian sweet potato weevil, Euscepes postfasciatus, has been maintained for over 44 generations (～8 years) for its eradication program in the Okinawa Prefectural Plant Protection Center (Naha, Okinawa, Japan). The selective pressures under mass-rearing conditions are likely to be very different from those of wild ones. The specific selective pressures over many generations under mass-rearing conditions would adversely affect the reproductive capacity of mass-reared insects. Therefore, evaluation of the mating performance of mass-reared males compared with that of wild ones is indispensable in SIT eradication programs. Although we compared mating performance with respect to four reproductive characteristics (duration of mating behavior, mating success, mating competitiveness ability and number of transferred sperm in female spermatheca during copulation) between mass-reared and wild males in E. postfasciatus, there was no evidence that the mass-reared males were inferior to the wild males in terms of mating performance. Thus, we considered that the mass-rearing procedure for at least 44 generations did not adversely influence the male mating activity in E. postfasciatus.

鹿児島県指宿市に侵入したイモゾウムシの定着に関する空間疫学ならびにリスク要因分析

鹿児島県指宿市に侵入したイモゾウムシの定着に関する空間疫学ならびにリスク要因分析

奄美群島におけるアリモドキゾウムシおよびイモゾウムシの分布（2010-2011年）

The geographical distribution of the sweetpotato weevil, Cylas formicarius and the West Indian sweetpotato weevil, Euscepes postfasciatus were investigated on sweetpotato, Ipomoea batatas and wild host plants, I. india and I. pes-caprae on the Amami Islands except Kikai Island during 2010 to 2011. The number of C. formicarius male adults was determined using simple sticky traps with synthetic sex pheromone at 235 sites during November 2010 to February 2011. The occurrence of the two weevils were investigated at 109 sweetpotato fields and 97 creeping colonies of wild host plants, I. india and I. pes-caprae, sites during November 2010 to February 2011. Both weevil species were found in all islands of the Amami Islands. This was consistent with the result of 1997-1998, and the distribution of both weevils species have not changed on the Amami Islands. The occurrence of E. postfasciatus was on the increase from 1997. On Okinoerabu Island, the number of male adults of C. formicarius captured using pheromone traps were fewer than the other islands.

Suppressing effect of gamma-irradiated weevils on progeny production in the West Indian sweetpotato weevil Euscepes postfasciatus (Coleoptera: Curculionidae)

The West Indian sweet potato weevil Euscepes postfasciatus (Fairmaire) (Coleoptera: Curculionidae) is a major pest of sweet potato Ipomoea batatas (L.) in the tropical and subtropical regions. The sterile insect technique (SIT) could be used as one of the most effective methods for suppression or eradication of the weevil. The effectiveness of SIT depends on the ability of the released sterile males to mate with and inseminate wild females. However, the effect of sterile weevils on the fitness of E. postfasciatus has not been evaluated on natural density. Here, we investigated the effect of gamma-irradiated weevil density on the number of weevil progeny. When irradiated weevils were released in numbers equal to those of non-irradiated weevils, the number of progeny was reduced by half of that in the control treatment, and it remained at this state for 2 weeks. Our results show that irradiated weevils ensure adequate and efficient suppression of wild weevils. We conclude that the SIT programs will be employed as effective eradication method for E. postfasciatus.

Effects of intra‐ and inter‐specific competition on fitness of sweetpotato weevil and West Indian sweetpotato weevil

AbstractThe sweetpotato weevil Cylas formicarius (Fabricius) (Coleoptera: Brentidae) and West Indian sweetpotato weevil Euscepes postfasciatus (Fairmaire) (Coleoptera: Curculionidae) are major pests of sweet potato Ipomoea batatas (L.) Lam., in tropical and subtropical regions. Effects of intra‐ and inter‐specific competition on the number of progeny (fecundity), body weight and developmental time of C. formicarius and E. postfasciatus were examined in single‐ and mixed‐species treatments under laboratory conditions. Cylas formicarius tended to outcompete E. postfasciatus, whereas E. postfasciatus rather than C. formicarius tolerated higher conspecific densities. We discuss the implications of the results for pest management and resource partitioning of pestiferous weevils.

Ultraviolet light-emitting diode (UV LED) trap the West Indian sweet potato weevil, Euscepes postfasciatus (Coleoptera: Curculionidae)

The West Indian sweet potato weevil Euscepes postfasciatus (Fairmaire) is a troublesome pest insect of sweet potato that originally came from the Caribbean, but is now expanding its distribution into the Pacific Islands. Although sterile insect techniques have been used against this pest in a demonstration experiment on Kume Island [Ohno et al. (2006) Kontyu to Shizen 41:25–30], effective methods of monitoring E. postfasciatus are scarce. It is necessary to detect the weevils at an early stage of invasion in uninvaded areas, and an attractant trap can be used to achieve this. Thus, we developed an ultraviolet (UV) light-emitting diode trap, invented a method for diffusing the light to attract more insects, and investigated the attractiveness of the light trap to E. postfasciatus under laboratory conditions. Our results indicate that diffused UV light has a higher potential to attract E. postfasciatus than direct UV light. Furthermore, sweet potato is an effective bait to use to capture the weevils attracted by UV light. Thus, E. postfasciatus can be trapped using diffused UV light and sweet potato bait.

Effect of low temperature between fractionated-dose irradiation doses on mating of the West Indian sweetpotato weevil, Euscepes postfasciatus (Coleoptera: Curculionidae)

The effectiveness of the sterile insect technique (SIT) depends on the ability of released sterile males to mate with and inseminate wild females, but the use of gamma radiation to induce sterility negatively affects both somatic and reproductive cells of the sterilized insects. Recently, the effectiveness of fractionated-dose irradiation (FI), in which a sterilizing dose is delivered over time in a series of small irradiations, has been demonstrated in the West Indian sweetpotato weevil Euscepes postfasciatus (Fairmaire). FI improves male mating propensity compared with acute irradiation; however, this renewed technique takes a long time (72 h in the present circumstances) compared with the traditional technique (approximately 20 min) using single-dose irradiation (150 Gy) to fully sterilize this weevil. The extra time required by FI might negatively affect the quality of released sterile males, because weevils expend limited resources, such as metabolic energy or sperm, in mating freely in this period. We evaluated whether the temporal storage of weevils at low temperature (5°C and 15°C) improves the quality of sterile males compared with the normal condition (25°C). Temporal storage at low temperature in FI improves male mating propensity. For example, the sexually active phase of males exposed to 15°C was 18 days at least after irradiation. This period was longer than that of normal males (14 days). Meanwhile, this manner delayed male reproductive development and temporarily reduced mating competitiveness ability. If considering the long active phase of sterile males exposed to 15°C, these disadvantage would be cancelled out. We discuss the advantage of FI with temporal storage at low temperature in the eradication program using SIT for E. postfasciatus.

The Effect of Inbreeding on Mating Behaviour of West Indian Sweet Potato Weevil Euscepes postfasciatus

When there is an inbreeding depression, mating with a kin individual is generally considered maladaptive behaviour. However, in some conditions, the inclusive fitness benefits from inbreeding may outweigh the costs of inbreeding depression, and thus, inbreeding tolerance is often adaptive. Inbreeding depression and the effect of relatedness on mating behaviour in the West Indian sweet potato weevil Euscepes postfasciatus were examined. No significant inbreeding depression was detected as indicated by body weight and number of progeny emerging from sweet potato roots. Male mating performance (i.e. number of mating occurrences per night) was adversely affected by inbreeding depression, but the effect was low (fitness loss was 6.3%). Although there were no significant differences in latency to mounting, pre-copulatory guarding, copulation and post-copulatory guarding duration between full-sib and non-kin pairs, the copulation rate of full-sib pairs was significantly higher than that of non-kin pairs. These results support the theoretical prediction that when inbreeding depression is weak, copulation with close relative individuals is favoured.