Why does onlyAthalia Japonica enter summer diapause among three sympatricAthalia sawflies feeding on crucifers?

Summary This report assesses the primary factor for the evolution of summer diapause of the three species of sawfly,Athalia japonica, A. rosae andA. infumata that feed on cruciferous plants and coexist in the same area.A. japonica has two discrete spring and autumn generations, butA. rosae andA. infumata 5–6 generations. OnlyA. japonica enters summer diapause in response to the long daylengths in spring. Although these three sawflies usually feed on the same cultivated crucifers, they differ markedly in the utilization of wild crucifers. They oviposit only on young leaves.A. japonica mainly usesCardamine plants which sprout in spring and autumn.A. rosae andA. infumata primarily use hosts with new leaves all the year round, i.e. cultivated crucifers andRorippa indica, respectively. The thermal threshold for development is lower inA. japonica than in the other two species. The low heat tolerance ofA. japonica is adapted only to cool shady habitats whereCardamine grows. Presumably, summer diapause ofA. japonica is adaptation to the deterioration of the primary host plants rather than unfavorable climatic conditions. This interpretation is supported by the movement patterns of the threeAthalia sawflies, alternative means to escape from deteriorated habitat conditions.     

Movement patterns of threeAthalia sawflies in relation to the spatio-temporal distributions of their habitats

Summary I compared the differences in the movement intensity of three species of sympatricAthalia sawflies,A. japonica, A. rosae andA. infumata feeding on cruciferous plants. Mark-release-recapture census was conducted to estimate movement distance, sex ratio and age composition of adult sawflies. In addition, the sex ratio of newlyemerged adults was examined by rearing field-collected larvae until adult emergence. Age composition and longevity of adults were estimated experimentally. The movement intensity was evaluated mostly with the indirect information thus obtained. Females moved more actively than males in all three sawflies.A. japonica females of all age classes moved actively in spring and autumn, but in summer they disappeared. Also,A. rosae females of all age classes moved actively in spring and autumn. In summer, in contrast withA. japonica, A. rosae females moved most actively among the three species in all seasons.A. infumata females, in particular the young females, moved most actively among the three species, exceptA rosae in summer. The movement patterns of the three sawflies were deduced in relation to the spatio-temporal distributions of their habitats. In spring and autumn, when host plants were abundant,A. japonica andA. rosae females were dispersed among the host patches within the census are. In summer, however, when host plants were scarce,A. japonica entered diapause, whereasA. rosae migrated to neighboring areas. On the other hand,A. infumata, in particular young famale, innately dispersed to seek for temporary host plants throughout the census seasons.     

Warning coloration in sawflyAthalia rosae larva and concealing coloration in butterflyPieris rapae larva feeding on similar plants evolved through individual selection

Summary Green larvae of the butterflyPieris rapae and black larvae of the sawflyAthalia rosae feed on green leaves of the same cruciferous plants. To demonstrate thatP. rapae has concealing coloration and thatA. rosae has warning coloration, the larvae of the two species were supplied to naive chicksGallus gallus on white, green or black backgrounds.P. rapae larvae were palatable and their green body color acted as a concealing coloration. On the other hand,A. rosae larvae were unpalatable and their black body color acted as a warning coloration. There is a general consensus that warning coloration is an altruistic character which needs victims, and thus can evolve through kin selection or green beard selection. However, blackA. rosae larvae were seldom injured by chicks' attack, in particular, on the green background. Therefore, the warning coloration ofA. rosae larvae can be a selfish character and hence can evolve through individual selection as well as concealing coloration ofP. rapae.     

Comparative studies on the seasonal occurrence, nymphal development and food menu in two giant water bugs,Diplonychus japonicus vuillefroy andDiplonychus major esaki (Hemiptera: Belostomatidae)

Summary We compared the seasonal changes in population density and nymphal development at different water temperatures and under different food conditions between two giant water bugs,Diplonychus japonicus andD. major, in Okayama, Japan. D. japonicus produced 1–2 generations a year, whileD. major was strictly inivoltine. The developmental velocity was higher inD. japonicus than inD. major. The thermal constant ofD. japonicus was less than that ofD. major. These results suggest thatD. japonicus is adapted, to higher water temperature thanD. major. In the field,D. japonicus preyed predominately on Lymnaeidae and Physidae snails, whileD. major preyed on aquatic insects such as dragonfly nymphs.     

The effects of wild cruciferous host plants on morphology, reproductive performance and flight activity in the diamondback moth,Plutella xylostella (Lepidoptera: Yponomeutidae)

The wild crucifers,Rorippa indica andLepidium virginicum, are known to serve as host plants for the diamondback moth (DBM),Plutella xylostella, but they are less suitable than the preferred cultivated cruciferous plant, cabbage, in terms of adult body size and fecundity. The life history traits and flight activity of DBM adults grown on various host plants were investigated. The adults thus reared on each host plant were divided into three size groups (small, medium and large). In general, female adults grown on the wild crucifers were less fecund and lived longer than those reared on cabbage. Flight activity was higher in adults grown on wild crucifers than in those reared on cabbage. Male adults flew longer than females. Fecundity, longevity, flight activity and morphometrical characters of adults were positively correlated with pupal weight in individuals reared on the same host plant. A negative relationship was found between fecundity and flight activity in females of the same size group, but a positive one was observed in females reared on the same host plant.     

Dynamics of buried seed population and seedling cohorts of two dominant weeds in a hill agroecosystem of the humid subtropics of India

Dynamics of the buried seeds and plant population of two dominant weeds, viz.,Emilia sonchifolia (Linn.) DC. andRichardsonia pilosa HBK were studied in the crop fields of Meghalaya, north-east India during radish and maize cropping and intervening fallow periods. The total buried seed population ofR. pilosa was always larger than that ofE. sonchifolia, but the germinable fraction was invariably greater in the latter. A major portion (39–41%) of the viable (germinable+dormant) seed population in both weeds was confined to the surface soil layer (0–5 cm). The viable seed population ofE. sonchifolia peaked during April, while that ofR. pilosa showed two peaks (during August and December). The survival pattern and half-lives of seedling cohorts showed, some differences in the two weed species, but both being summer annuals, their populations behaved in a similar manner by showing higher seedling recruitment (K) and survivorship (p) rates in the summer crop (maize) than in the winter crop (radish). However, the density of plants that could attain adulthood was significantly higher inE. sonchifolia thanR. pilosa which might have resulted in greater seed input of the former to the soil leading to its greater abundance in the crop fields. Supported by the University Grants Commission, New Delhi (Grants No. F. 3-37/87 SR II)     

The evolutionary significance of wing dimorphism in carabid beetles (Coleoptera: Carabidae)

A review of data on the background of wing dimorphism in carabid beetles (Coleoptera: Carabidae) and especially of the closely relatedCalathus cinctus andC. melanocephalus is given. In bothCalathus species wing dimorphism is inherited in a simple Mendelian fashion with the brachypterous condition dominant, but inC. melanocephalus the expression of the long winged genotype is under environmental control as well. The development of long winged phenotypes in the latter species is favoured by relatively favourable environmental conditions, such as high temperatures and a high food-supply. The higher fecundity of the larger and heavier long winged females of both species may compensate for losses of long winged phenotypes by flight activities. The evolutionary significance of both types of inheritance is discussed in relation to dispersal. The ‘fixed type’ as found inC. cinctus is considered an opportunistic short term ‘between sites strategy’, whereas the ‘dynamic type’ ofC. melanocephalus represents a flexible long term ‘within sites strategy’.     

Seasonal host alternation by the andromeda lace bug,Stephanitis takeyai (Heteroptera: Tingidae) between its two main host-plant species

I studied the seasonal occurrence of the andromeda lace bug,Stephanitis takeyai, on its two main host-plant species. In a secondary forest in Kyoto, this bug altered its hosts seasonally, i.e., from an evergreen shrub,Pieris japonica, in winter to a deciduous shrub,Lyonia elliptica, in summer. In contrast, in Nara park where fewL. elliptica were available, the bug exploited onlyP. japonica. Thus, seasonal host alternation by this bug is not obligate. A comparison of adult longevity and fecundity on the two host-plant species demonstrated the higher quality ofL. elliptica as a food resource. Corresponding to this difference in host quality, there was a dramatic difference in the seasonal population growth in the two study sites. In Nara, the population size at the beginning of the 2nd generation was almost the same as in the overwintered generation, whereas in Kyoto the population size in the 2nd generation was approximately one hundred times as large as in the overwintered generation. Thus seasonal host alternation is adaptive for the bug. In a previous study, I reported that overwintering as eggs in living leaves of their hosts is likely to be common among all the related species of this bug. Thus, this trait can be considered to be a phylogenetic constraint to the group. I speculate that host alternation by this bug has been derived because it is more adaptive from autoecy on an evergreen plant, similar to the pattern currently found in Nara, and that this bug can not only exploit deciduous host due to a phylogenetic constraint.     

Comparison of dispersal ability between the green leafhoppers,Nephotettix virescens andN. cincticeps (hemiptera: Cicadellidae) by tethered flight

Summary Dispersal propensity of green leafhoppers was compared between a tropical species,Nephotettix virescens, and a temperate species,N. cincticeps. The flight ability was measured with tethered flight technique under laboratory conditions (25°C, 16L-8D). The pre-flight period was shorter and the flight duration was longer inN. virescens than inN. cincticeps in both sexes. No significant correlations were found between the flight activity and morphometric characters for either of the two species. The results suggested that dispersal propensity ofN. virescens is higher than that ofN. cincticeps.     

Inbreeding effects on competition inTribolium

When replicate cultures ofT. confusum andT. castaneum are husbanded together under identical treatment conditions, sometimesT. confusum eliminatesT. castaneum, and other times,T. castaneum wins (i.e., competitive indeterminacy occurs). While several plausible explanations were advanced, the results of Mertz et al. (1976) implicated demographic stochasticity and not classical genetic founder effect as the predominant factor influencing the identity of the winning species. They also observed, however, that the size of the founding population had an influence on the competitive strength ofT. castaneum. The present study shows that the decline in competitive strength that accompanied decreasing founder size inT. castaneum can be amply explained by simple inbreeding depression. The eggs of inbred adults showed an approximate 15% reduction in hatchability when compared to outbred adults. No evidence was found that the decrease in competitive strength was due either to prior history differences or reduced genetic heterogeneity of the founding adults.     

Preference and performance linkage in aPhyllocolpa sawfly on the willow,Salix miyabeana, on Hokkaido

A leaf-folding sawfly in the genusPhyllocolpa (Hymenoptera: Tenthredinidae) attackingSalix miyabeana (Salicaceae) was studied near Sapporo, Hokkaido, along the Ishikari River in 1993. Host plant individuals were young trees 4–7 years old which were growing rapidly, producing some long shoots with large leaves. On a gradient of shoot length classes from 0–5 cm long to over 80 cm long, shoots were much more abundant in the shorter shoot length classes. However, attacks by ovipositing females increased as shoot length increase from 0 attacks on the shortest shoots to 5.17 attacks per shoot on the longest shoots. The frequency of attack per leaf increased from 0 to 0.13 over the same range of shoot lengths. This pattern of attack resulted in a high frequency of larval establishment in feeding sites, between 0.96 and 1.00, in all attacked shoot length categories. However, probability of survival to a late instar larva increased with shoot length and corresponded to the attack pattern, indicating a preference-performance linkage between female ovipositional decisions and larval survival. The patterns found for thisPhyllocolpa species are similar for galling sawflies in North America and Europe, especially in the genusEuura, members of which make stem, bud and leaf midrib galls. Extending the pattern to aPhyllocolpa species broadens identification of pattern and ultimately the generality of the emerging theory on populations of galling sawflies.     

Spatio-temporal distribution patterns of two tachinid flies,Epicampocera succincta andCompsilura concinnata parasitizingPieris

Summary Larvae of genusPieris in the northern part of Kyoto City are parasitized by two tachinid flies:Epicampocera succincta, a specialist on genusPieris, andCompsilura concinnata, a generalist with very wide host-range. We surveyed the parasitism rates ofPieris by both flies for two years at six study areas. In these study areas, there lived three host species in the genusPieris: P. rapae, P. melete, andP. napi, but neither tachinid parasitizedP. napi to any significant extent. In the mountainous district,P. rapae andP. melete coexisted and their populations were relatively continuous, while in the lowland, onlyP. rapae larvae were abundant in spring and autumn, but even they disappeared in summer. Parasitisms byE. succincta occurred mainly in mountainous district and never in the lowland.C. concinnata parasitizedPieris in all the areas, but its parasitisms occurred mainly in autumn. We analyzed the factors affecting the spatial and temporal patterns of parasitism rates and presumed that the temporal discontinuity of host population restricted the distribution of the specialist parasitoid.     

Deme formation in gall-making aphidsAdelges japonicus (Homoptera: Adelgidae)

The deme formation hypothesis in herbivorous insects states that herbivores differentiate genetically into small demes, each specialized in the idiosyncratic traits of individual plants. We examined this hypothesis in gall-forming aphids,Adelges japonicus, by reciprocal transfers of aphids among three susceptible clones ofPicea jezoensis standing in proximity. Performance of aphids transferred to their natal clone was compared with that of aphids transferred to other novel clones. ANCOVA showed no significant donor (source of aphids) by receptor (recipient of aphids) interactions for both total aphid performance on the host tree over a life cycle and its three subdivisions(q ₁,q ₂ andq ₃). In addition, the total performance on the natal clone was lowest among those in the three clones, which was explicitly against the prediction from the local adaptation. In contrast, there were significant receptor effects onq ₁ (fecundity of gallicolae multiplied by survivorship of fundatrices while feeding on needles) andq ₃ (fecundity of fundatrices multiplied by survivorship of gallicolae before emergence from the galls), suggesting genetic differences in resistance among clones. The two-fold difference inq ₃ among clones was due to different gall size, which was highly correlated with the number of gallicolae that emerged from the gall. The two-fold difference inq ₁ seemed to be caused by different survivorship of larval fundatrices while feeding on needles.     

The role of parasitoids in evolution of habitat and larval food plant preference by three Pieris butterflies

This article attempts to explain that parasitoids provide the evolutionary pressure responsible for relationships between habitat use and larval food plant use in herbivorous insects. Three species of butterflies of the genus Pieris, P. rapae, P. melete, and P. napi use different sets of cruciferous plants. They prefer different habitats composed of similar sets of cruciferous plants. In our study, P. rapae used temporary habitats with ephemeral plants, P. melete used permanent habitat with persistent plants, although they also used temporary habitats, and P. napi used only permanent habitat. The choice experiment in the field cages indicated that each of the three butterfly species avoided oviposition on plants usually unused in its own habitat, but accepted the unused plants which grew outside its own habitat. Their habitat use and plant use were not explained by intrinsic plant quality examined in terms of larval performance. Pieris larvae collected from persistent plants or more long lasting habitats were more heavily parasitized by two specialist parasitoids, the braconid wasp Cotesia glomerata and the tachinid fly Epicampocera succincta. The results suggest that Pieris habitat and larval food plant use patterns can be explained by two principles. The evolution of habitat preference may have been driven by various factors including escape from parasitism. Once habitat preference has evolved, selection favors the evolution of larval food plant preferences by discriminating against unsuitable plants, including those which are associated with high parasitism pressures. Received: December 3, 1998 / Accepted: January 20, 1999     

Comparative population studies of threePieris butterflies,P. rapae, P. melete andP. napi, living in the same area

Summary Utilization of patchy habitats by adult populations of threePieris butterflies,P. rapae, P. melete andP. napi was studied throughout the flight season in an area of their coexistence, about 3×1.5 km, in a farm village in the mountains in Inabu, Aichi Prefecture. Field study was by the mark-recapture method. Results were analyzed by dispersal distances and recapture duration decay curves for adults of different age-classes estimated on the basis of physical condition of their wings, together with supplementary information of daliy egg-laying rate of females, obtained in field cages. Sexually immature, mated femals ofP. rapae after teneral stage showed a migratory flight. On the other hand, reproductive females and all males ofP. rapae were strongly resident within suitable habitats, and reproductive females begun to lay eggs abundantly at sunny places of newly suitable areas within a short period.P. melete seemed to disperse gradually from emerged stites and females of this species continued to lay some constant numbers of eggs for more than ten days over a wider area.P. napi appeared more likeP. melete thanP. rapae. The habitats of the three species can be characterized as follows:P. rapae, temporary, continued for pre-reproductive females but localized for reproductive females and all males, and unstable;P. melete, permanent, widespread, and stable;P. napi, permanent, localized, and stable. The numbers of generations ofP. rapae, P. melete andP. napi were estimated to be about six, three and three, respectively. Seasonal fluctuations in the number of adults were influenced by the stability of their habitats, i. e., the population size fluctuated sharply inP. rapae, but it was much more stable inP. melete andP. napi. In view of these results, it can be said thatP. rapae fits the general characteristics of a r-strategist whereasP. melete andP. napi are more K-strategic thanP. rapae.     

Estimation of prey consumption of a mantid,Paratenodera angustipennis (S.) in a natural habitat

Summary Using a relationship between prey consumption and growth rate, field prey consumption of adults ofParatenodera angustipennis (S.) in a paddy field was estimated. Since a great number of grasshoppers (Oxya japonica) lived in the research area and the mantids had frequently eatenO. japonica, we presumed that they consumed onlyO. japonica. As a result, it was estimated that average prey consumptions for 6 days were 406 mg for the females and 23 mg for the males. From the result of this estimation and the feeding rhythm of the mantids, it is concluded that each female captured one grasshopper on average every a few days. By comparing this actual feeding level ofP. angustipennis with those of other predacious arthropods, characteristics ofP. angustipennis as a predator were discussed.     

Coexistence and weak amensalism of congeneric gall-forming aphids on the Japanese elm

Closely related species of gall-forming aphids are often associated with a single host species. SixTetraneura species coexist on the Japanese elm,Ulmus davidiana, in Sapporo, northern Japan. This paper describes the probabilities of coexistence on macro- and microgeographic scales (i.e., on host trees and host leaves) and examines whether coexistence with conspecific or heterospecific galls on leaves or shoots has any effect on the fecundity of each aphid species using multiple regression. A Monte Carlo simulation showed that the frequency distribution of the numbers of species on individual host trees differed significantly from that expected from the null model. There were significantly positive or negative associations between species.Tetraneura radicicola andsorini always coexisted with other species on trees they infested. Multiple regression revealed that the coexistence of conspecific or heterospecific galls on individual leaves had no influence on the fecundity ofradicicola andsorini, but had a negative influence on that of sp. O. On average,radicicola andsorini produced a smaller number of offspring in galls than sp. O, and obviously consumed only a small part of resources available on the galled leaves. Evidence available suggests that although amensalism does arise between sp. O and other species, its influence in not so strong as to exclude sp. O competitively from theTetraneura community.     

Accumulation of phyllodulcin in sweet-leaf plants ofHydrangea serrata and its neutrality in the defence against a specialist leafmining herbivore

Among wild plants ofHydrangea serrata (Hydrangeaceae) in Japan, there are sweet plants whose leave contain a kind of isocoumarin, phyllodulcin, which happens to be 350 times as sweet as sucrose to the human tongue. In a primary beech forest in Ashu, Kyoto, the spatial distribution of sweet plants and temporal and the spatial distribution of phyllodulcin within and among plants were investigated using a high performance liquid chromatograph. The distribution of sweet plants was confined within a valley and was parapatric with non-sweet plants. A plant's characteristic phyllodulcin accumulation did not change, even when transplanted into the different habitats. The phyllodulcin content of the sweet plants varied greatly among plants, and the population mean peaked in July when the plants flowered. Within a plant, phyllodulcin content was elevated by partial defoliation. We examined the possible effect of phyllodulcin on herbivory by a specialist leafmining herbivore,Antispila hydrangifoliella (Lepidoptera: Heliozelidae). We transplanted sweet and nonsweet plants reciprocally between their original habitats, excluded attacks by parasitoids, and compared performance of the leafminer. Leafminer colonization and larval survivorship on transplanted andin situ plants was not significantly different between sites. The fact that accumulation of phyllodulcin did not augment a defensive function, at least against herbivory by the leafminer, and the sporadic distribution of phyllodulcin-accumulating plants, suggest that the genotypes synthesizing phyllodulcin emerged independently at separate localities by mutation, and that the genotypes are almost adaptively neutral in defence against the specialist herbivore.     

Niche modification and stability of competitive systems. II. Persistence of interspecific competitive systems with parasitoid wasps

Summary Effects of niche shift in ecological time scale on the population dynamics of competing species were studied in the experimental populations of two parasitoid wasp species,Anisopteromalus calandrae andHeterospilus prosopidis (both are solitary parasites), on a host, the azuki bean weevil,Callosobruchus chinensis. Four resource conditions were set up with combination of kind of bean (azuki or black eye), and host distribution (uniform or clumped). In each resource condition, four developmental stages of hosts were provided as a resource spectrum for parasitoid wasps. Population dynamics of the two wasp populations were investigated in each resource condition in Multi-Generation Competitive Systems (MGCS), in which fresh hosts of four developmental stages were periodically introduced and were parasitized competitively by the two wasp species. Competitive coexistence of both wasps occurred in the azuki-clumped condition, where the peaks of the resource utilization curves separated in the two species; pupae inA. calandrae and the early or late fourth instar inH. prosopidis, A. calandrae was eliminated in the azuki-uniform condition andH. prosopidis went extinct in two black eye conditions irrespective of host distributions. The degrees of overlap of the resource utilization patterns of the two wasp species during population dynamics were not significantly different among resource conditions irrespective of the results of coexistence or extinction. Even in the azuki-clumped condition, however, extinction ofA. calandrae was observed when resource partitioning could not be realized with only the late fourth instar larvae available to wasps. Further analytical experiments showed that parasitizing ability ofA. calandrae increased with host density per bean with azuki beans, butA. calandrae could express higher parasitizing ability with black eye beans thanH. prosopidis irrespective of host density per bean. The flexibility in parasitizing ability byA. calandrae for various host stages under different resource conditions was thought to be the major factor in determining the competitive coexistence or the extinction of either species under different resource conditions. The present experiments also suggested that different second-best host stages between competitors could be a major contributing factor to competitive coexistence.