Insecticide‐mediated behavioral avoidance by bean bug,Riptortus pedestris (Heteroptera: Alydidae)

Bean bug, Riptortus pedestris (Fabricius) (Heteroptera: Alydidae), is an economically important pest of soybean and tree fruit. Behavioral responses of R. pedestris to olfactory cues from insecticide‐treated foliage were assessed in a Y‐tube olfactometer. Ten insecticides were applied to kidney bean plants at the recommended dosage, and then bug avoidance was assessed at 0, 3, 5, 7 and 13 days after treatment (DAT). Each bug was allowed 5 min to choose one of the olfactometer's arms. Even though the degree of avoidance was variable to the insecticides tested, bean bugs significantly avoided the insecticide‐treated leaf. The pyrethroid bifenthrin was the most avoided of the insecticides tested. In all treatments, avoidance decreased as DAT increased. Also, the movement speed was higher to the untreated leaf than to the insecticide‐treated leaf. The degree of avoidance was positively correlated to the vapor pressure of the chemical; this finding implies the interaction of the treated chemicals and plant volatiles may be used as cues for bean bug selection of the host‐plant.     

Pars intercerebralis promotes oviposition in the bean bug,Riptortus pedestris (Heteroptera: Alydidae)

Effects of the pars intercerebralis (PI) on the rate of oviposition were examined in adult females of Riptortus pedestris (F.) under long-day conditions that promote reproduction. When the neurosecretory cells in the PI were surgically removed from the females, they laid significantly fewer eggs than control females. When the PI from reproductive females was transplanted to PI-removed females, the number of eggs was significantly greater than that in PI-removed females without transplantation. Therefore, we suggest that the PI neurons promote fecundity or oviposition in R. pedestris.     

Diurnal rhythm of male–male combat behavior in the bean bug Riptortus pedestris (Heteroptera: Alydidae)

In many insects, mating is affected by the day–night cycle, i.e., diurnal rhythm. Although there are many reports that mating and other reproductive behaviors are controlled by daily rhythms in various taxonomic insect species, little attention has been paid to the effect of daily rhythms on male fighting behavior. Here, we investigate whether the frequency and escalation of male–male aggressive interaction exhibit diurnal rhythms under a long‐day condition in the bean bug Riptortus pedestris. Despite the fact that male aggressive behaviors were most often observed in the middle of the later half of light periods, no interaction was found between escalation of fighting and the time period. The results, at least, suggest that male aggressive behaviors are influenced by diurnal rhythms like other reproductive behaviors in R. pedestris.     

Locomotion ability variation among instars of the bean bug, Riptortus pedestris (Fabricius) (Heteroptera: Alydidae) nymphs

The nymphal locomotion ability (walking distance) of the stenophagous bean bug Riptortus pedestris (Fabricius) was studied in each instar. We measured the walking distance using two systems. The walking distance in photophase was measured for 6 h using a tracking system with a charge coupled device (CCD) camera and computer software. The daily activity of nymphs was measured by an actograph system counting the number of infrared beam intercepts. The actograph data were converted to distance using a linear regression against the data of the tracking system. The longevity of nymphs without food was also studied to estimate the potential walking distance. Using both the tracking and actograph systems, it was determined that first instars walked less than the other instars (only 10.7 m within 6 h). The second to fifth instars could move 20-25 m within 6 h, and this distance did not differ among instar. This indicates that first instars seldom move after hatching in the field. The walking distance for 24 h varied and was greatest for the third instars (80.8 m). The potential longevity of nymphs was found to increase with instar age. Potential locomotion ability (walking distance for 24 hxpotential longevity) was high in the third to fifth instars (approximately 340 m). The potential locomotion ability for the second instars was relatively low compared with the elder instars (approximately 180 m). From these results, nymphs of R. pedestris seem to adapt by identifying feeding site locations after hatching and elder instars may be able to find a novel feeding site after the degradation of previous habitat.     

Changes in cuticular hydrocarbons in different developmental stages of the bean bug,Riptortus pedestris (Hemiptera: Alydidae)

The function of “cuticular hydrocarbons” (CHCs) in insects is to protect the body as well as chemical communication and recognition between species. Gas chromatography and mass spectrometry were used to compare the composition of CHCs in eggs, all nymph instars, and male and female adults of the bean bug, Riptortus pedestris (Hemiptera: Alydidae). CHC carbon chain numbers of each developmental stage ranged from 14 to 19 in eggs to the fourth nymph and were 24 in the last nymph. CHCs' carbon chain numbers in male and female adults increased from 16 to 23 and 16 to 27 until 6 days of age, respectively. Major components consisted of n-nonacosane in the larval stages, which changed to n-hentriacontane, n-heptacosane and 13,17-;15,19-demethyltritriacontane in adults. The major component from eggs was not identified. The major component from newly emerged adults was n-heptacosane. CHCs in different developmental stages of the bean bug consisted of n-alkanes with saturated hydrocarbons (27–63%), followed by mono or di-methyl alkanes. However, the CHCs of newly emerged female and male adults consisted mostly of methyl-alkanes. The major constituents of CHCs in the developmental stages of R. pedestris were proportionally different but showed only slight differences in composition.     

Purification and characterization of biliverdin-associated cyanoprotein from eggs and hemolymph of the bean bug,Riptortus clavatus (Heteroptera: Alydidae)

Biliverdin binding protein, cyanoprotein (CP), was purified from egg extracts of the bean bug, Riptortus clavatus. The preparation was shown to be homogeneous by DEAE-chromatography and polyacrylamide gel electrophoresis (PAGE). The native molecular weight (MW) of egg CP (CP_egg) was estimated to be 320,000 by PAGE and 335,000 by gel filtration. The apoprotein consisted of identical subunits with MW of 76,000. CP_egg had a high content of aromatic amino acids (17% mol ratio) with absorbance peaks at 370 and 620 nm which are characteristic of biliverdins. Native CP_egg was associated non-covalently with 16 molecules of biliverdin. CP_egg contained about 13.0% neutral sugar (mainly mannose) associated covalently with the apoprotein, no lipid was detected. In the hemolymph of the bugs, four CP bands (CP1, 2, 3 and 4) were detected by native PAGE. These bands were immunologically related and showed properties similar to CP_egg in biochemical analysis. Only CP1 was identical to CP_egg. CPs specific to the hemolymph, CP2, 3 and 4, were different from CP_egg in mobility by PAGE, pI and peptide mapping profiles.     

Receptivity of female remating and sperm number in the sperm storage organ in the bean bug,Riptortus clavatus (Heteroptera: Alydidae)

This study examines the relationship between the number of sperm in the seminal receptacle (spermatheca) and the receptivity of female remating in the bean bugRiptortus clavatus Thunberg. On the 21 st day after the first mating when receptivity to remating was > 70%, females receptive to remating had significantly fewer sperm ( < 40 on average) in the spermathecae than females reluctant to do (about 150 on average). However, averages of the number of eggs laid by receptive and reluctant females within 21 days were almost same. The proportion of fertilized eggs for receptive females at 15–21 days after copulation was significantly lower than that for reluctant females. Spermatozoa transferred from a male to a female’s spermatheca were detected 5 min after copulation and then increased continuously to about 500 with the first hour. When copulation durations were manipulated artificially, the shorter the copulation period (=females had less sperm in their spermathecae), the higher the remating rate became. Females may perceive the number of sperm in their seminal receptacles and then determine whether they copulate or not. These results support the hypothesis that females mate multiply in order to replenish inadequate sperm supplies to fertilize all eggs produced.     

Positive genetic correlation between brain size and sexual traits in male guppies artificially selected for brain size

Brain size is an energetically costly trait to develop and maintain. Investments into other costly aspects of an organism's biology may therefore place important constraints on brain size evolution. Sexual traits are often costly and could therefore be traded off against neural investment. However, brain size may itself be under sexual selection through mate choice on cognitive ability. Here, we use guppy (Poecilia reticulata) lines selected for large and small brain size relative to body size to investigate the relationship between brain size, a large suite of male primary and secondary sexual traits, and body condition index. We found no evidence for trade‐offs between brain size and sexual traits. Instead, larger‐brained males had higher expression of several primary and precopulatory sexual traits – they had longer genitalia, were more colourful and developed longer tails than smaller‐brained males. Larger‐brained males were also in better body condition when housed in single‐sex groups. There was no difference in post‐copulatory sexual traits between males from the large‐ and small‐brained lines. Our data do not support the hypothesis that investment into sexual traits is an important limiting factor to brain size evolution, but instead suggest that brain size and several sexual traits are positively genetically correlated.     

Immature performance linked with exaggeration of a sexually selected trait in an armed beetle

Exaggerated traits can be costly and are often trade-off against other characters, such as life-history traits. Thus, the evolution of an exaggerated trait is predicted to affect male life-history strategies. However, there has been very little experimental evidence of the impact of the evolution of sexually selected traits on life-history traits. This study investigated whether increased investment in exaggerated traits can generate evolutionary changes in the life-history strategy for armed males. Male flour beetles, Gnatocerus cornutus, have enlarged mandibles that are used in male-male competition, but females lack this character exaggeration completely. We subjected these weapons to 11 generations of bidirectional selection and found a correlated response in pupal survival but not in larval survival or adult longevity in the male. That is, selecting for male mandibles negatively impacted survival during the production of mandibles. There is no correlated response in the life-history traits of the female.     

Pheromonal cross-attraction in true bugs (Heteroptera): attraction of Piezodorus hybneri (Pentatomidae) to its pheromone versus the pheromone of Riptortus pedestris (Alydidae)

We investigated the attractiveness of a synthetic form of the pheromone of the soybean stink bug, Piezodorus hybneri (Gmelin), under field conditions, and compared it with that of (E)-2-hexenyl (E)-2-hexenoate, a pheromone component of a competitor, Riptortus pedestris (Fabricius). Many adult stink bugs were attracted to traps baited with 100 mg of the synthetic pheromone (1: 1: one mixture of β-sesquiphellandrene, (R)-15-hexadecanolide, and methyl (Z)-8-hexadecenoate), but few were attracted to 1 or 10 mg. More than twice as many females as males were attracted to this male-produced pheromone. None of the individual pheromone components (30 mg) attracted conspecifics. In summer (June-July), when field P. hybneri were not in diapause, (E)-2-hexenyl (E)-2-hexenoate was more attractive to P. hybneri than the synthetic pheromone. The sex ratio of the adults attracted to the synthetic pheromone was highly female-biased, yet almost equal numbers of both sexes were attracted to (E)-2-hexenyl (E)-2-hexenoate. Most females attracted to both attractants were mated and had mature ovaries. However, adults attracted to (E)-2-hexenyl (E)-2-hexenoate were likely to have less food in their stomach than those attracted to the synthetic pheromone. In late autumn (October-November), when the bugs were in reproductive diapause, both attractants attracted many sexually immature female and male adults that had well-developed fat body. The synthetic pheromone also attracted a large number of conspecific nymphs. These results suggest that P. hybneri pheromone and R. pedestris pheromone component, respectively, have different functions for P. hybneri. The male-produced pheromone system of P. hybneri seems to be sex-related but to have other roles.     

Condition-dependent traits and the capture of genetic variance in male advertisement song

The occurrence of additive genetic variance (VA) for male sexual traits remains a major problem in evolutionary biology. Directional selection normally imposed by female choice is expected to reduce VA greatly, yet recent surveys indicate that a substantial amount remains in many species. We addressed this problem, also known as the 'lek paradox', in Achroia grisella (Lepidoptera: Pyralidae), an acoustic moth in which males advertise to females with a pulsed ultrasonic song. Using a standard half-sib/full-sib breeding design, we generated F1 progeny from whom we determined VA and genetic covariance (COVA) among seven traits: three song characters, an overall index of song attractiveness, nightly singing period, adult lifespan, and body mass at adult eclosion. Because A. grisella neither feed nor drink as adults, the last trait, eclosion body mass, is considered a measure of 'condition'. We found significant levels of VA and narrow-sense heritabilities (h2) for all seven traits and significant genetic correlations (= COVAi,j / radical (VA i x VA j)) between most pairs of traits (i, j). Male attractiveness was positively correlated with body mass (condition), adult lifespan, and nightly singing period, which we interpret as an energy constraint preventing males in poor condition from singing attractively, from singing many hours per night, and from surviving an extended lifespan. The positive genetic correlation (r = 0.79) between condition and attractiveness, combined with significant levels of VA for both traits, indicates that much of the variation in male song can be explained by VA for condition. Finally, we discuss the morphological and physiological links between condition and song attractiveness, and the ultimate factors that may maintain VA for condition.     

Crowd control: sex ratio affects sexually selected cuticular hydrocarbons in male Drosophila serrata

Although it is advantageous for males to express costly sexually selected signals when females are present, they may also benefit from suppressing these signals to avoid costly interactions with rival males. Cuticular chemical profiles frequently function as insect sexual signals; however, few studies have asked whether males alter these signals in response to their social environment. In Drosophila serrata, an Australian fly, there is sexual selection for a multivariate combination of male cuticular hydrocarbons (CHCs). Here, we show that the ratio of females to males that an adult male experiences has a strong effect on his CHC expression, with female‐biased adult sex ratios eliciting greater expression of CHC profiles associated with higher male mating success. Classical models predict that male reproductive investment should be highest when there is a small but nonzero number of rivals, but we found that males expressed the most attractive combination of CHCs when there were no rivals. We found that male CHCs were highly sensitive to adult sex ratio, with males expressing higher values of CHC profiles associated with greater mating success as the ratio of females to males increased. Moreover, sex ratio has a stronger effect on male CHC expression than adult density. Finally, we explore whether sex ratio affects the variance among a group of males in their CHC expression, as might be expected if individuals respond differently to a given social environment, but find little effect. Our results reveal that subtle differences in social environment can induce plasticity in male chemical signal expression.     

Circadian clock regulates photoperiodic responses governed by distinct output pathways in the bean bug,Riptortus pedestris

Effects of RNA interference (RNAi) targeted against circadian clock genes on two distinct types of photoperiodic responses – ovarian development and lipid accumulation – were investigated in a bean bug Riptortus pedestris, to explore which physiological process in the photoperiodic response involved the circadian clock. Ovarian development and lipid accumulation are known to be regulated by distinct output pathways. Control insects showed clear photoperiodic responses; i.e. induction of ovarian development and suppression of lipid accumulation under long-day conditions, whereas opposite characteristics under short-day conditions. We found that RNAi directed against period, a negative element of the circadian clock, produced a long-day effect for both the ovarian development and lipid accumulation, while RNAi directed against Clock, a positive element of the circadian clock, produced a short-day effect for both, irrespective of photoperiod. These results indicate that the circadian clock comprised of these genes regulates a process governing both distinct photoperiodic responses.     

Flight behavior of the bean bug,Riptortus clavatus (Thunberg) (Hemiptera: Alydidae), relative to adult age,sex and season

Studies related to the flight behavior of the bean bug, Riptortus clavatus, an insect pest of soybean, provide information, which can aid the development of management tactics. R. clavatus flight activities were determined using the tethered flight technique in the laboratory and a field study. We compared the flight parameters (flight distance, duration, speed and frequency) of laboratory-reared mated or unmated females and males of different adult age groups, and field-collected females and males from different seasons in a year. Mating effect was only significant for flight frequency, which was higher for unmated adults. Only the flight frequency was different between females and males. Among age groups, 25 d old individuals were shown to have higher flight parameters while the 35 or 45 d old groups showed the lower flight. Among the season, flight activities decreased as the season progressed. The flight distance of R. clavatus in a day was estimated to be 1.6–5.1 km with an average speed of 0.8 m/s from the laboratory data. However, from the field study, it was shown that bean bugs flew a 45–54 m distance with a flight speed of 3.0–3.6 m/s for the first single flight. The second flight was much shorter and slower. We discuss the possible difference of flight parameters between the laboratory and field studies with features of flight mill, variable field conditions and host plant finding behaviors. Further study on flight behavior may provide a better understanding of R. clavatus biology which may guide proper management.     

Genetic variance of sexually selected traits in waxmoths: maintenance by genotype x environment interaction

When traits experience directional selection, such as that imposed by sexual selection, their genetic variance is expected to diminish. Nonetheless, theory and findings from sexual selection predict and demonstrate that male traits favored by female choice retain substantial amounts of additive genetic variance. We explored this dilemma through an ecological genetic approach and focused on the potential contributions of genotype x environment interaction (GEI) to maintenance of additive genetic variance for male signal characters in the lesser waxmoth, Achroia grisella (Lepidoptera: Pyralidae). We artificially selected genetic variants for two male signal characters, signal rate (SR) and peak amplitude (PA), that influence female attraction and then examined the phenotypic plasticity of these variants (high- and low-SR and high- and low-PA lines) under a range of environmental conditions expected in natural populations. Our split-family breeding experiments indicated that two signal characters, SR and PA, and several developmental characters in both high- and low-SR and high- and low-PA lines displayed considerable phenotypic plasticity among the environments tested. Moreover, strong GEIs leading to crossover between high- and low-SR lines were found for SR and developmental period. Therefore, neither high- nor low-SR genetic variants would achieve maximum attractiveness and fitness in every environment, and those variants producing unattractive signals with low SRs under normal conditions may remain in populations provided that gene flow across environments or generation overlap are sufficiently high. We speculate that the phenotypic plasticity for SR and developmental period is adaptive in A. grisella populations experiencing a range of temperature and density conditions. Females mating with attractive (high-SR) males may be assured of obtaining good genes because these males sire offspring that develop more rapidly and a crossover for developmental period may parallel that for SR. Such parallel crossovers may be expected wherever good-genes sexual selection mechanisms operate.     

Phenotypic and genetic variation in male genitalia in the seedbug, Lygaeus equestris (Heteroptera)

Male genitalia evolve through sexual selection and, in insects, tend to show negative static allometry, low phenotypic variation, and are usually relatively small. Much less is known about the genetic variation and heritability of male genitalia. Additionally, in instances where the intromittent organ is greatly elongated, it is unclear whether typical patterns of genital scaling and variation also apply. In the present study, we investigated the allometry, variation, and heritability of male genital length in the seedbug, Lygaeus equestris , a species with a greatly elongated intromittent organ (i.e. almost as long as male body size). We found that genital length was negatively allometric, in spite of its great length, and was no more variable than nongenital traits. Additionally, genital length was significantly heritable and had considerable evolvability.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 98 , 400–405.     

Selection of male traits during and after copulation in the seedbug Lygaeus simulans (Heteroptera, Lygaeidae)

In order to evaluate selection of male morphological traits during copulation, a laboratory experiment was performed with the promiscuous seedbug Lygaeus simulans. Three male traits suspected as putative targets of selection were measured: weight, fluctuating asymmetry of a measure on the forewings, and length of a conspicuous genital structure, the processus gonopori. As fitness measures we considered total fecundity (number of fertilized eggs), insemination and fertilization success (established if a female laid fertilized eggs after copulation), and the interval between copulation and oviposition. Eighty-four males were allowed a single copulation with one virgin female each. Out of 67 copulations, 27 (40.2%) resulted in fertilized eggs and the oviposition latency ranged from 6 to 26 days. Regressions of male traits on the fitness measures showed significant phenotypical selection of two male traits: (1) males of average weight are more likely to achieve fertilization and (2) the oviposition latency was shorter for males with lower asymmetry. The copulation-oviposition interval may be especially important for male fertilization success because Lygaeus males perform copulatory mate guarding and the last male copulating with a female fertilizes most of the eggs. No selection of the genitalic trait was detected.     

Interspecific larval competition between two egg parasitoids in refrigerated host eggs of Riptortus pedestris (Hemiptera: Alydidae)

In a previous study, we found that soybean fields could be supplemented with refrigerated eggs of Riptortus pedestris (Fabricius) (Hemiptera: Alydidae) to enhance parasitism. As a part of a study to evaluate the effect of host egg refrigeration on parasitism, host acceptance behavior and interspecific larval competition between Gryon japonicum (Ashmead) (Hymenoptera: Scelionidae) and Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae) were studied in multiparasitized unrefrigerated and refrigerated eggs. O. nezarae showed complete host acceptance behavior when offered refrigerated host eggs that were preparasitized by G. japonicum. Adult emergence rate of O. nezarae was 43 and 74% when the interval between the first and second oviposition was 0 and 4 days, respectively, and was not different between refrigerated and unrefrigerated eggs. Refrigeration did not change host acceptance behavior of G. japonicum, but adult emergence declined from 80% in unrefrigerated eggs to 37% in refrigerated eggs that were pre-parasitized by O. nezarae on the same day. No negative effects of refrigeration on sex ratio, adult longevity, and adult size of the both parasitoids were found. Generally host egg refrigeration did not negatively affect host acceptance behavior of the both parasitoids on preparasitized eggs or larval competition between the two parasitoids in multiparasitized host eggs with exceptions in the development time and emergence rate of G. japonicum. Therefore, host egg refrigeration may not interrupt interactions between the parasitoid populations in the field.     

Genetic diversity of Riptortus pedestris (Hemiptera: Alydidae) populations in the Republic of Korea based on COI sequence

Riptortus pedestris (Fabricius) has caused severe agricultural damage in Asian regions. To clarify genetic relationships among different populations of R. pedestris in Korea, 294 COI sequences were generated from 294 individual samples collected in 42 local regions. In total, 36 haplotypes were detected from the 294 COI sequences, and the genetic distances among the 42 local populations ranged from 0.00 % to 1.50 %. In the AMOVA results, a variability of more than 98 % was observed within populations, and the median joining (MJ) networks revealed that R. pedestris has expanded by crossing the nine geographical groups. Most of the samples shared one haplotype, H2; however, some samples obtained from the same regions displayed slight genetic differences. These results indicate that R. pedestris have undergone a series of genetic variations.