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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.

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.

Quarantine pests in plants can be a serious agricultural problem; many eradication programs using area-wide control measures have been implemented worldwide to combat this threat. Surveillance measures using sex pheromone (in general, male-attractant) traps are also widely implemented for rapid control and eradication of invasive pests. If initial pest colonization can be determined based on temporal count data of trapped insects (i.e., males), and countermeasures are applied only during colonization, costs incurred by these countermeasures would be dramatically reduced, especially in areas with frequent invasions. In this study, we developed a system to detect initial pest colonization, and to narrow down colonized regions using estimated temporal count data of the sweet potato weevil, Cylas formicarius Fabricius (Coleoptera: Curculionidae), in Tsuken Island, Okinawa, Japan. We verified the system by comparing our estimates to actual colonization data obtained via regular host plant surveys. Results indicated that our system was able to successfully detect pest colonization and estimate colonized regions. In this study, we discuss the conditions (i.e., pest biology, environment, etc.) that are optimal for application of our system.

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.

The sterile insect technique (SIT) involves periodically releasing artificially sterilized insects to inhibit normal mating between wild insect pests, ultimately resulting in the eradication of wild pest populations. It has often been discussed whether releasing either one sex, mainly males, of sterile insects (i.e., a unisexual release) can enhance the pest-control effect of the SIT more than releasing both sexes (i.e., a bisexual release). We constructed a mathematical model to examine the contribution of sterile males and females to the pest-control effect and the synergy between them. We consider that males seek out and court females in accord with their own female searching ability and preference, and that females subsequently choose one male from among males courting them in accordance with their own preference. Using this model, we compared the pest-control effect of bisexual and unisexual release, focusing on the difference in mating systems of the targeted insects. We showed that for swarm-type mating systems (with few courtship chances with higher encounter rates), bisexual release was the most effective, irrespective of the relative female searching ability between wild and sterile males. In this case, sterile females indirectly reduce wild females mating with either male by absorbing courtship from both wild and sterile males. By contrast, bisexual release is the most effective for scramble-type mating systems (more courtship chances with lower encounter rates) only when the female searching ability of sterile males is lower than that of wild males. In this case, sterile females absorb courtship from males with higher searching abilities. Therefore, the net impact of sterile females depends on the difference in sexual performance between wild and sterile males. Because the sexual performance of sterile insects is often degraded during the process of sterilization, we suggest that bisexual release can be a compatible measure to efficiently suppress wild pest populations.

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.

Cultivation management to improve quality or yield of crops causes periodic disturbances in agricultural fields and increases mortality of arthropods. Thus, it may interfere with biological control of pests. An herbivorous pest thrips ( Frankliniella intonsa (Trybom)) is a significant pest for strawberry ( Fragaria x ananassa ) in a greenhouse and a polyphagous predatory thrips (Haplothrips brevitubus (Karny)) is its natural enemy. Because strawberry flowers bloom sequentially, thinning of juvenile fruits and harvesting of mature fruits are also sequentially implemented. Such management practices work as periodic disturbances for both the pest and natural enemy. We constructed a spatially explicit metapopulation model to examine effects of periodic disturbances on efficiency of biological control. The natural enemy was more susceptible to periodic disturbances than the pest, because the former lost diets whereas predation pressure on the latter was weakened. However, the high mobility and alternative diet of the natural enemy could compensate for the negative effect of periodic disturbances. If economic value of individual harvested fruits was diminished by a burden of excess fruits, total yield could increase with the intensity of fruit thinning. The more efficient release strategy of the natural enemy was either the clumped or dispersed depending on the mobility and initial number of the natural enemy and the intensity of periodic disturbances. Thus, for consistently practicing cultivation management and biological control, it is important to apply appropriate intensity of fruit thinning and to release a sufficient number of natural enemies in proper arrangement depending on the intensity of periodic disturbances.

One of the most important questions in biological control is whether multiple natural enemies can provide greater suppression of agricultural pests than a single best enemy. Intraguild predation (IGP) among natural enemies has often been invoked to explain failure of biological control by multiple enemies, and classical theoretical studies on IGP have supported this view. However, empirical studies are inconclusive and have yielded both positive and negative results. We extend classical models by considering anti-predator behavior of pests and diet switching of omnivorous natural enemies, and examine their effects on pest control. We assume that the pest can adaptively allocate effort toward the specific defense against each predator, and that the omnivorous natural enemy can consume disproportionately more of the relatively abundant prey (switching predation) by type III functional responses to prey items. The model predicts that adaptive defense augments pests but favors introduction of multiple natural enemies for controlling pests if IGP is weak. In contrast, switching predation does not make pest control by multiple natural enemies advantageous as in classical studies, in the absence of adaptive defense. However, switching predation reduces the necessity of defense by the pest against the omnivore and offsets the effect of adaptive defense. Thus, it makes the introduction of multiple natural enemies advantageous for pest control when the pest employs adaptive defense even if IGP is strong. These results suggest that types and combinations of behavior of prey and predators may greatly affect qualitative outcomes of biological control by multiple natural enemies.

Evolution and maintenance of mutualism have been one of the major questions in evolutionary ecology, because it is often susceptible of invasion of non-mutualistic strategy. Some previous studies using dual-lattice model suggest that spatial structures of habitat can prevent non-mutualism from prevailing over mutualism, while the detail of the dynamics is not fully revealed. Here we explore population dynamics of the two strategies (mutualism and non-mutualism) in two species engaged in Prisoner's Dilemma game on a dual-lattice space, especially focusing on whether mutualists and non-mutualists can coexist in long-term dynamics. The habitat consists of two layers, each of which a population of species inhabits, and interspecific interaction is restricted between two corresponding sites of the layers. Each individual of the both species is either a mutualist or a non-mutualist and only the former pay cost c for benefit of the partner b. The payoff of the game affects the individuals' fecundity, while the mortality is constant. Reproduction is restricted to neighboring vacant sites of the focal individuals. Our computer simulations of the model show that even if b/c ratio remains constant, mutualists become dominant in both species over wider ranges of basic reproduction rate (reproduction rate without interspecific interaction) as b and c increase. If basic reproduction rates are asymmetric between the species or basic reproduction rates were sufficiently large, mutualists and non-mutualists can coexist in one or both species, while their population sizes often fluctuate. Transition of the final state between mutualism and non-mutualism happens rather discontinuously, then total population sizes change drastically at the transition. Moreover, we also find paradoxical cases of unilateral exploitation, i.e. one species consists of mutualists and other species non-mutualists. Additional simulations reveal that accidental extinction of the non-mutualists of one species can result in extinction of mutualist of the other species.