Effects of Host Diet on Romanomermis culicivorax,a Mermithid Parasite of Mosquitoes

When larval mosquitoes (Aedes aegypti) infected with the mermithid nematode Romanomermis culicivorax were fed on a diet low in quantity or protein content or both, the number of postparasites which emerged from the hosts decreased and host mortality increased marginally. Parasitic development was prolonged and became asynchronous in nutritionally deprived hosts. Nematodes emerged from insects infected by more than one nematode before the remaining juveniles comprising such infections had completed parasitic development; this resulted in substantial reductions in postparasite nttmbers. Host development was retarded by low protein and/or reduced diets. Postparasites emerging from second and third instars were reduced in size and in the amount of stored nutriment compared to those recovered from hosts fed on a high protein diet ad libitum. A greater proportion of the rnermithids developed into males in hosts fed on reduced diets but not in hosts fed on low protein diets.     

Penetration and Development of the Mermithid Nematode Reesimermis nielseni in Eighteen Species of Mosquitoes

The susceptibility of 18 species of mosquitoes to the infective stage of the mermithid nematode Reesimermis nielseni was compared to that of Culex pipiens quinquefasciatus. Thirteen species were more susceptible, with three Anopheles species and Culiseta inornata the most susceptible. Aedes triseriatus, Culex territans, and Psorophora ferox were highly resistant. Resistance to R. nielseni appeared to be behavioral, physical, or physiological, and some host species exhibited one or more types of defense mechanisms. No noticeable differences were apparent in the degree of susceptibility of a native-and a laboratory strain of C. p. quinquqfasciatus to R. nielseni.     

Effect of the Entomogenous Nematode Nemplectana carpocapsae on the Tachinid Parasite Compsilura concinnata (Diptera: Tachinidae)

The entomogenous nematode Neoaplectana carpocapsae and its associated bacterium, Xenorhabdus nematophilus, could not infect the pupal stage of the tachinid Compsilura concinnata through the puparium. N. carpocapsae had an adverse effect on 1-, 2- and 3-day-old C. concinnata larvae within the armyworm host in petri dish tests. All 1-day-old larvae treated with nematodes died in their hosts, whereas 61% and 69% of 2- and 3-day-old larvae treated with nematodes, respectively, died. However, the survivors developed to adults. Nine to thirty-seven percent of adult tachinids which emerged from nematode-treated soil (50 nematodes/cm²) were infected with N. carpocapsae. The nematode adversely affects C. concinnata directly by the frank infection of the tachinid and indirectly by causing the premature death of the host which results in tachinid death.     

A Comparative Study of Sex Ratio and Clutch Size in Gregarious Ichneumonoid Wasps

Some convincing support for sex ratio theory comes from the cross-species relationship between sex ratio and brood size in gregarious bethylid wasps (Hymenoptera: Bethylidae), in which the proportion males declines as brood size increases as predicted under local mate competition. It is unknown how widely such relationships hold within parasitoid wasps as a whole. We assemble a dataset on sex ratio and brood size for gregarious Braconidae and Ichneumonidae. Their sex ratios deviate substantially from those of bethylids; sex ratios differ widely across species; and they are not significantly related to brood size across species. Several factors explain the heterogeneity in sex ratios including across-species differences in mating system, sex determining mechanism, and sexual asymmetries in larval competition and polyembryony leading to single-sexed broods.     

Laboratory Culture and Life History of Heleidomermis magnapapula in Its Host, Culicoides variipennis (Diptera: Ceratopogonidae)

The mermithid parasite Heleidomermis magnapapula was maintained in larvae of the midge Culicoides variipennis for 20 months in enamel pans containing nutrient-rich water and polyester pads as a substrate. Inseminated female mermithids were introduced to the pad surface when the host was in the late second or early third-instar. Host larvae were harvested from the pans 9 days after exposure and held in tap water for nematode emergence. Preparasite yield was positively correlated with female nematode size and averaged 1,267 preparasites/female. Male and female nematodes emerged an average of 12.2 and 13.4 days after host exposure, respectively. Supplemental host food (Panagrellus) during the final days of parasitism did not alter time of emergence. Parasites emerging singly were 64% females, whereas superparasitized hosts yielded males (up to nine/host). Nematode carryover into the adult midge normally occurred at a level of 0.5-2.5%. Parasite load (nematodes/ parasitized individual) in midge adults was lower than that of larvae from the same cohort, and adult midges were more likely to harbor female parasites. Exposure of fourth-instar host larvae resulted in higher levels of adult parasitism (up to 17%).     

Superparasitism and sex ratio in the solitary parasitoidEpidinocarsis lopezi

Highly variable sex ratios are found in the solitary parasitoidEpidinocarsis lopezi, both in the field and in a mass-rearing situation. Superparasitism is one of a number of factors which can influence sex ratios in parasitoid Hymenoptera. We show that superparasitism inE. lopezi is common in the field. Sex allocation decisions when parasitizing unparasitized hosts are not different from those with parasitized hosts; neither does differential mortality occur between the sexes in superparasitized hosts. Therefore superparasitism does not contribute to the variable sex ratio ofE. lopezi. Both the occurrence of superparasitism and the sex produced when ovipositing are shown to be functional forE. lopezi.     

Temperature Effects on the Parasitic Phase of Romanomermis culicivorax in Culex pipiens

The developmental period for the parasitic stage of Romanomermis culicivorax in Culex pipiens was determined at constant and fluctuating temperatures. The median developmental times at 15, 18, 20, 27, and 32 C were 27.6, 17.2, 11.5, 7.1, and 5.8 days, respectively. The optimum temperature range for development of the parasitic stage in C. pipiens was 20 to 32 C. The threshold for development was calculated as 10.4 C, and the heat units required for development were 122.2 ± 3.6 day-degrees. Development at fluctuating temperatures conformed to that predicted by the constant temperature data.     

Effects of Selected Insecticides and Nematicides on the In Vitro Development of the Entomogenous Nematode Neoaplectana carpocapsae

The effects of organophosphates (mevinphos, phenamiphos, trichlorfon), carbamates (carbofuran, methomyl, oxamyl), a formamidine (chlordimeform), a synthetic pyrethroid (fenvalerate), a chlorinated hydrocarbon (methoxychlor). and an insect growth regulator (diflubenzuron) on in vitro development and reproduction of Neoaplectana carflocapsae were tested by incorporating each chemical into a nematode rearing medium. Organophosphates and carbamates adversely affected development and reproduction at concentrations ≥ 0.1 mg/ml. Phenamiphos was the most toxic, with no nematode reproduction at 0.01 mg/ml. Inoculated infective juveniles developed to adults with some of the organophosphates and carbamates, but limited or no reproduction occurred. Chlordimeform inhibited development at 1.0 mg/ml, while diflubenzuron, fenvalerate, and methoxychlor did not significantly (P > 0.05) reduced reproduction at 1.0 mg/ml. The organophosphate and carbamate nematicides in use for control of plant-parasitic nematodes may be toxic to N. carpocapsae in the soil.     

Hexamermis cathetospiculae n. sp. (Mermithidae: Nematoda), a Parasite of the Rice Stemborer,Tryporyza incertulas (Wlk.) (Pyralidae: Lepidoptera) in Malaysia

Hexamermis cathetospiculae n. sp. (Mermithidae: Nematoda), a parasite of the rice stemborer, Tryporyza incertulas (Wlk.), is described. This mermithid was observed to control the stemborer in Malaysia. It is distinguished from other members of the genus by the shape of the vagina, the nearly straight spicules, and the form of the amphids.     

Effect of Aedes aegypti (Diptera:Culicidae) Age on Sex Ratios in Romanomermis culicivorax (Nematoda:Mermithidae)

First, second, third, early and late fourth-instar larvae, and pupae of Aedes aegypti were infected with Romanomerrais culicivorax and reared at 20, 25, and 30 C. An increase in the ratio of male to female nematodes was observed with increase in host age at the time of infection at each temperature. The number of pupal and late fourth-instar infections was low, but R. culicivorax continued to develop in adult A. aegypti. Since male nematodes were recovered from both male and female hosts infected as late fourth instars or pupae, the sex of the host did not influence the sex of the nematode.     

Variable Female Mating Positions and Offspring Sex Ratio in the Spider Pityohyphantes phrygianus (Araneae: Linyphiidae)

Chromosomal sex determination and male heterogamety have been thought to seriously impede direct sex ratio control. However, in Pityohyphantes phrygianus, a solitary sheetweb spider with a skewed sex ratio, earlier experimental studies suggested that there are options for female control of offspring sex ratio, if females change their position during the normal mating sequence. Here we show that under natural conditions there is considerable between-female variation in positions, especially after termination of mating. Computer simulations of the orientation of female inner genitalia suggest that sperm are placed in different storage sites depending on the positions adopted. This means that a specific position after mating might potentially influence offspring sex ratio. The variance in offspring sex ratio among females in earlier experiments was binomially distributed, which leads us to conclude that females control the mean sex ratio but do not exercise direct control of the sex of individual offspring.     

Temperature Effects on Survival and Development of Heleidomermis magnapapula in the Laboratory

The mermithid Heleidomermis magnapapula Poinar and Mullens, a parasite of the biting midge Culicoides variipennis (Coquillett), was exposed to constant temperatures in the laboratory. Survival of the free-living stages and development times of eggs and the parasitic phase were inversely related to temperature. Average preparasite longevity was 70, 46, 42, and 22 hours at 15.6, 21.1, 26.7, and 32.2 C, respectively. Females survived significantly longer than males. Longevity in days (females/males) at different temperatures was 17.3/11.0 at 4.4 C, 9.0/8.2 at 15.6 C, 5.9/5,1 at 21.1 C, 5.2/4.7 at 26.7 C, and 4.4/3.6 at 32.2 C. Embryogenesis required 44 ± 2 degree days above a thermal minimum of 10.1 C, while parasitic development in host larvae required 214 ± 10 degree days above a thermal minimum of 8.9 C. Parasite responses to temperature were very closely related to temperature-dependent host development patterns.     

Host exploitation and contest behavior in a generalist parasitoid partially reflect quality of distinct host species

Information use determines parasitoid adaptive behavior in general, and host specialization or fitness in specific. Information regarding host suitability could affect sex allocation behavior, host exploitation, or aggressiveness in dyadic contests. In this paper, we relate aggressiveness of the pupal parasitoid Pachycrepoideus vindemmiae (Hymenoptera: Pteromalidae) with sex allocation and host exploitation when presented with different host species. More specifically, we presented parasitoids with puparia of five different Dipteran species: Drosophila melanogaster (Drosophilidae), Musca domestica (Muscidae), Anastrepha obliqua, Anastrepha fraterculus, and Ceratitis capitata (Tephritidae). Puparia of above species greatly varied in size and volume, were parasitized to differing extent and had varying fitness implications for P. vindemmiae. Using a composite measure of selected fitness currencies (i.e., parasitism level, offspring size, longevity and sex ratio), we typified D. melanogaster and A. obliqua as ‘low quality’ hosts for P. vindemmiae while puparia of C. capitata and A. fraterculus were considered of ‘high quality’. In contest dyads, female aggressiveness and host exploitation behavior differed between host species. Wasps exhibited highest frequencies of antennal striking and rival pursuit, and high degrees of puparium mounting, antennating and probing on C. capitata. Antennal striking frequency however was equally high on ‘low quality’ hosts such as D. melanogaster and A. obliqua. This work shows that a generalist parasitoid such as P. vindemmiae assesses host quality when confronted with hosts of differing species, size or nutritional suitability and employs such to define sex allocation, host exploitation, and contest behavior. However, contest and exploitation behavior only partially indicate host quality and broader parasitoid fitness implications. This work has further implications for parasitoid mass rearing and use of P. vindemmiae for biological control of Dipteran pests.     

Host selection and sex allocation in Leptomastidea abnormis, a parasitoid of the citrus mealybug Planococcus citri

Leptomastidea abnormis (Girault) and Leptomastix dactylopii (Howard) [Hymenoptera: Encyrtidae] are the two parasitoid species frequently used for biological control of the citrus mealybug Planococcus citri (Risso) [Homoptera: Pseudococcidae]. As part of a programme to determine the best control practice, host size selection and sex allocation were studied in L. abnormis, and compared with those of L. dactylopii. L. abnormis showed a preference for the second size class, but also attacked third and fourth size class and adult mealybugs, while in previous studies L. dactylopii was found to prefer the third and fourth size class and adults, and never attacked second size class mealybugs. Female L. abnormis reared from parasitized mealybugs had a higher mean body mass than males, as is predicted by Charnov's theory on sex allocation. However, sex ratios of emerging L. abnormis did not differ between mealybug size classes. These results imply possibile coexistence of the two parasitoids. As field evidence suggests a more complex interaction between the two species, studying direct competition would be a useful next step.     

Variation in Sex Ratio and Evolutionary Rate of Hemiclonal Rana esculenta Populations

In many plant and animal taxa mutation rates are higher in males than in females. As a result, the evolutionary speed of genes depends on how much time they spend in either sex. Usually, this time differs between genes located on sex chromosomes but not between those on autosomes. Here we present an unusual system with a partially sex-linked inheritance of autosomes: the hemiclonal frog Rana esculenta (E) which is originally a hybrid between the sexual species R. lessonae (L) and R. ridibunda (R). Rana esculenta excludes the L genome prior to meiosis, produces eggs or sperm containing an unrecombined R genome and restores hybridity by mating with R. lessonae (‘hybridogenesis’). Matings between L males and E females result in offspring with an even sex ratio, whereas the reverse combination produces only daughters. The extent of the resulting female bias and the proportion that R alleles have spent in either sex depend on the relative survival (b) and the relative reproductive contribution (a) of E males vs. E females. In this paper, we analyze mathematically how different combinations of a and b influence the sex ratio in R. esculenta populations and, combined with the male/female mutation rate ratio (α), the evolutionary rate of the clonally transmitted R genome. We find that this rate is higher than in an asexual population and lower than in a sexual one. Hence, clonal diversity through new mutations is more easily achievable than in purely asexual species. In contrast, the occurrence and accumulation of deleterious mutations is lower than in a comparable sexual species. We conclude that these intermediate mutation rates improve the ecological and evolutionary potential of hemiclonal organisms, and we draw attention to the implications for the use of microsatellites. Co-ordinating editor: L. Hurst     

Genetics and Genomics of an Unusual Selfish Sex Ratio Distortion in an Insect

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Pairing Patterns in Japanese Beetles (Popillia japonica Newman): Effects of Sex Ratio and Time of Day

Size-related patterns between unpaired and paired individuals and between males and females of a given pair give clues about both a species' sexual behavior and the environmental factors affecting its sexual behavior. We studied the mating patterns of Japanese beetles (Popillia japonica) in east–central Illinois. The frequency of male–female pairs varied significantly among days and within a day, with pairs being significantly more common in the morning and the evening. The sex ratio on the food plants was significantly male biased, but although the sex ratio fluctuated among days and among time periods, the variation in the frequency of mating pairs was not explained by variation in the sex ratio. We found no assortative pairing with respect to size, but sizes of paired and unpaired individuals did differ. Paired females were larger than unpaired females at all time periods. In contrast, paired males were larger at 0700 and smaller at 1000, and little difference existed at other times of the day. The size of males and females, sex ratio, and pairing frequency also differed among days. Much of this variation in size and pairing frequency was related to a seasonal effect: later in the summer, beetles of both sexes were smaller and pairs were less common. Interestingly, pairs were also less frequent on days with higher average temperatures. This between-day variation in pairing, in combination with the within-day pairing differences, suggests that the temperature may alter the cost, and hence likelihood, of pairing in this species.