Natural enemies for the cabbage webworm, Hellula undalis (Fabr.) (Lepidoptera: Pyralidae) in Malaysia
A. Sivapragasam and T. H. Chua
Strategic, Environment and Natural Resources Center, MARDI, G.P.O. Box 12301, 50774, Kuala Lumpur, Malaysia
Department of Zoology, Universiti Malaya, 50603 Kuala Lumpur, Malaysia.[email: chua@zoology.um.edu.my]
Abstract. Four species of larval parasitoids were reared from larvae of the cabbage webworm (CWW), Hellula undalis (Fabr.) (Pyralidae: Glaphyriinae) collected from various cruciferous plants and a Capparidaceous weed, Cleome rutidosperma (DC). On cabbage, only two species were recorded, viz, Bassus sp. (Braconidae) and Trathala flavoorbitalis (Cam.) (Ichneumonidae). The other parasitoids were braconids, Chelonus sp. and Phanerotoma sp. Egg or pupal parasitoids were not recorded. The parasitoids were not an important mortality factor of CWW on cabbage because they were usually present at the end of the crop season and their numbers were generally low. On the other hand, predator-exclusion experiments indicated that predators were important in determining the density of CWW on cabbage and the within-generation survival in the field. The major predator was the fire-ant, Solenopsis geminata (F.), which foraged on the prepupae and pupae.
Key words: Hellula undalis, crucifers, Cleome rutidosperma, natural enemies, Malaysia
Possible influences of habitat characteristics on the evolution of semelparity and cannibalism in the hump earwig Anechura harmandi
Katsuyuki Kohno
Laboratory of Entomology, Faculty of Agriculture, Kyoto University, Kyoto 606-01, Japan
Present Address: Kurume Branch, National Research Institute of Vegetables, Ornamental Plants and Tea, Mii-machi 1823, Kurume, Fukuoka 839 Japan [E-mail: kohno@nivot-krm.affrc.go.jp]
Abstract. The possible influences of life history and habitat characteristics on the evolution of semelparity and cannibalism in the hump earwig Anechura harmandi were studied. This species is univoltine and overwinters as an adult. Females laid single egg-batches during winter in nests under stones at a riverside in a valley. They took care of the eggs which hatched in early spring and the offspring ate their mother before dispersing. The valley was sometimes flooded in summer. Nymphs emerged as adults and dispersed to elsewhere before the rainy season arrived. They returned to the riverside after the rainy season. The flooding and/or summer heat seemed to be the selective force for the evolution of dispersal behavior and semelparity in this species. The cannibalism of the female parent by her offspring seemed to have readily evolved after the evolution of semelparity. The unfavorable environmental conditions seemed to have a large effect on the evolution of semelparity and cannibalism in this species.
Key words: earwig, Anechura harmandi, semelparity, cannibalism against female parent, life history, habitat characteristics
Deme Formation in Gall-Making Aphids Adelges japonicus (Homoptera: Adelgidae)
Kenichi Ozaki and Naoei Itahana
Hokkaido Research Center, Forestry and Forest Products Research Institute, Hitsujigaoka 7, Sapporo 062, Japan [E-mail: ozaki@ffpri-hkd.affrc.go.jp]
Hokkaido Breeding Office, National Forest Tree Breeding Center, Ebetsu 069, Japan
Abstract. 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 of Picea 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 (q1, q2 and q3). 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 on q1 (fecundity of gallicolae multiplied by survivorship of fundatrices while feeding on needles) and q3 (fecundity of fundatrices multiplied by survivorship of gallicolae before emergence from the galls), suggesting genetic differences in resistance among clones. The two-fold difference in q3 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 in q1 seemed to be caused by different survivorship of larval fundatrices while feeding on needles.
Key words: fine-scale adaptation, genetic differentiation, plant resistance, insect-plant interaction, gall-forming aphid, Picea jezoensis
An Island Biogeographical Approach to the Analysis of Butterfly Community Patterns in Newly Designed Parks
Masahiko Kitahara1) and Koichi Fujii
Graduate School of Environmental Sciences, The University of Tsukuba, Tsukuba, Ibaraki 305, Japan and Institute of Biological Sciences,The University of Tsukuba, Tsukuba, Ibaraki 305, Japan
1) Present address: Laboratory of Animal Ecology, Yamanashi Institute of Environmental Sciences, Ken-marubi, Fujiyoshida, Yamanashi 403, Japan. [E-mail: mkita@yies.pref.yamanashi.jp]
Abstract. We analyzed the butterfly communities in the newly designed city parks (area C), (IR(Jnewly opened habitat islands(IS(J, of Tsukuba City, central Japan. The area constituted a natural ecological experiment on the mainland for clarifying the pattern and process of faunal immigration. We compared butterfly communities in area C with those in two other areas in the light of the theory of island biogeography and the concept of generalist/specialist. Our results showed the following: (1) Fewer species were found in area C than in other areas, due largely to the absence of many specialist types, restricted and habitat specialists, and/or low density species in the area. Generalist types, widespread and habitat generalists, and/or high density species predominated in area C. (2) The difference in the species numbers among the three sections within area C could be explained by the habitat structure in and around the respective sections. (3) The densities of many species were low in area C, probably due to its man-modified habitat structure. In particular, several species occurred at extremely low densities in area C, but at high densities in other areas. (4) The internal structure of the habitat island butterfly community in area C was almost perfectly consistent with that of (IR(Jquasi-equilibrium(IS(J communities that appear during the colonization of an island. Our results demonstrate that the synergetic application of the generalist/specialist concept and the island biogeography theory is effective for the understanding of the patterns and structures of habitat island communities.
Key words. island biogeography theory, island patterns, generalist/ specialist, habitat island, newly designed city parks, butterfly communities.
Population stability in relation to resource availability in an introduced population of an herbivorous lady beetle
Takayuki OHGUSHI1) and Hiroichi SAWADA2)
Laboratory of Entomology, Faculty of Agriculture, Kyoto University,
Kyoto 606, Japan
1) Present address: Institute of Low Temperature Science, Hokkaido University, Sapporo 060, Japan [E-mail: ohgushi@orange.lowtem.hokudai.ac.jp]
2) Present address: School of Environmental Science, the University of Shiga Prefecture, Hikone, Shiga 522, Japan
Abstract. Mechanisms responsible for population stability in relation to resource availability were studied in an introduced herbivorous lady beetle, Epilachna niponica. The introduced population was relatively constant over a seven-year study period. Egg density was related to the variation in host-plant abundance in different years, and was highly stabilized during the period from reproductive adult to egg stage. Two density-dependent processes were identified in the reproductive season: (1) density-dependent reduction in fecundity and (2) density-dependent increase in female mortality and/or emigration, all of which operated early in the season. As a result, temporal variability in cumulative egg density was greatly reduced by mid-May, by which time approximately 40 % of total eggs were laid. A field cage experiment demonstrated that egg-laying of individual females was largely limited by resource availability even at low levels of leaf herbivory. Since movement activity of ovipositing females increased in a density-dependent manner, inter-plant movement is more likely to cause density-dependent reduction in fecundity and female loss, due to enhanced energy expenditure. The introduced population was less stable than the source population, probably because of decreased inter-plant movement of females and the unlikelihood of egg resorption, both of which contribute significantly to the temporal stability of E. niponica population densities.
Key words: Epilachna niponica, density-dependence, inter-plant movement, introduced population, oviposition limitation, population stability.
Simulation of Generation Times of the Rusty Grain Beetle, Cryptolestes ferrugineus, in Farm stored Grain in the Canadian Prairies, 1952-1990
S. M. Woods, N. D. G. White1) and R. N. Sinha
Agriculture and Agri-Food Canada, Cereal Research Centre, 195 Dafoe Road, Winnipeg, MB,
Canada R3T 2M9.
1) E-mail: NWHITE@EM.AGR.CA
Abstract. The number of potential annual generations of the rusty grain beetle, Cryptolestes ferrugineus, was simulated in wheat stored in granaries for all crop districts in the prairieprovinces of Canada each year from 1952 to 1990 using a population dynamic model driven by ecological variables. Granary size was assumed to be 6 m in diameter. Historical data for temperatures at harvest and times when storage began were used in the simulation model. A second model, which predicted the rate of temperature change at the centre of a 6-m-diameter bulk of wheat, determined environmental parameters for the population dynamic model. (Grain moisture content was assumed constant at 14.5% wet mass basis.) The combined model shows that the initial storage temperature is the most important factor responsible for predicting the number of generations and levels of infestation of C. ferrugineus. This findingwas largely validated by historical grain storage and infestation data. For various years initialgrain temperature ranged from 17.7 to 37.4¡C and harvest dates were between 1 August and20 October. The number of generations annually in simulations based on field conditionsranged from 0.35 to 6.77 with a mean of 3.29. Three or more generations result in a severeinfestation and every year at least three simulated generations were completed in some cropdistricts. In one year, at least three generations were completed in every crop district. Harvest temperature and date permit prediction of crop districts that will potentially have the largest populations of C. ferrugineus so that early monitoring of wheat for infestations can be targeted to areas most at risk.
Key words: Cryptolestes ferrugineus, generations, granaries, population model, stored wheat, western Canada.
Extinction of Populations Due to Inbreeding Depression with Demographic Disturbances
Yoshinari TANAKA
Laboratory of Theoretical Ecology, Institute of Environmental Science and Technology, Yokohama National University, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240, Japan
[E-mail: tanaka@kan.ynu.ac.jp]
Abstract. The process of population extinction due to inbreeding depression with constant demographic disturbances every generation is analysed using a population genetic and demographic model. The demographic disturbances introduced into the model represent loss of population size that is induced by any kind of human activities, e.g. through hunting and destruction of habitats. The genetic heterozygosity among recessive deleterious genes and the population size are assumed to be in equilibrium before the demographic disturbances start. The effects of deleterious mutations are represented by decreases in the growth rate and carrying capacity of a population. Numerical simulations indicate rapid extinction due to synergistic interaction between inbreeding depression and declining population size for realistic ranges of per-locus mutation rate, equilibrium population size, intrinsic rate of population growth, and strength of demographic disturbances. Large populations at equilibrium are more liable to extinction when disturbed due to inbreeding depression than small populations. This is a consequence of the fact that large populations maintain more recessive deleterious mutations than small populations. The rapid extinction predicted in the present study indicates the importance of the demographic history of a population in relation to extinction due to inbreeding depression.
Key Words: Extinction, inbreeding depression, deleterious mutation, genetic load, conservation biology
Competition and Evolutionary Stability of Plants in a Spatially Structured Habitat.
Yasuto Takenaka1), Hiroyuki Matsuda1) and Yoh Iwasa2)
Department of Biology, Faculty of Science, Kyushu University, Fukuoka 812-81, Japan
1) Present address: Ocean Research Institute, University of Tokyo, Minamidai 1-15-1, Nakano-ku, Tokyo 164, Japan
2) To whom all the correspondence should be addressed.
E-mail: yiwasscb@mbox.nc.kyushu-u.ac.jp
Abstract. We modelled the population dynamics of two types of plants with limited dispersal living in a lattice structured habitat. Each site of the square lattice model was either occupied by an individual or vacant. Each individual reproduced to its neighbors. We derived a criterion for the invasion of a rare type into a population composed of a resident type based on a pair-approximation method, in which the dynamics of both average densities and the nearest neighbor correlations were considered. Based on this invasibility criterion, we showed that, when there is a tradeoff between birth and death rates, the evolutionarily stable type is the one that has the highest ratio of birth rate to mortality. If these types are different species, they form segregated spatial patterns in the lattice model in which intraspecific competitive interactions occur more frequently than interspecific interactions. However, stable coexistence is not possible in the lattice model contrary to results from completely mixed population models. This clearly shows that the casual conclusion, based on traditional well mixed population models, that different species can coexist if intraspecific competition is stronger than interspecific competition, does not hold for spatially structured population models.
Key words: lattice model, invasion condition, pair approximation, evolutionary stable strategy, coexistence, spatial structure.
Maintenance of Diapause Variability in the Two-Spotted Spider Mite, Tetranychus urticae, in a Heterogeneous and Stochastic Environment
Yoshio Tsuda1), Akio Takafuji2),3) and Eiji Kuno2)
1) Institute of Tropical Medicine, Nagasaki University, Nagasaki 852, Japan
2) Graduate School of Agriculture, Kyoto University, Kyoto 606-01, Japan
3) [E-mail: takafuji@kais.kyoto-u.ac.jp]
Abstract. A simple nongenetic mathematical model analyzed the processes responsible for the variations in the diapause percentage among populations of the two-spotted spider mite, Tetranychus urticae Koch. This model incorporates the following assumptions. 1) Mites have diapause (DD), non-diapause (NN), and "plastic" (DN) populations (plasticity exists in the phenotypic expression of diapause in response to habitat conditions at the time of diapause induction). 2) A heterogeneous mite habitat consists of microhabitat L, in which all the non diapausing mites die during the winter due to the lack of winter host plants, and microhabitat O with winter hosts capable of supporting some of the non-diapausing mites overwinter. 3) Temporal fluctuation of winter conditions which affect the survival and reproduction of non diapausing mites. Using these assumptions, we compared the fitness functions of the three populations and analyzed the conditions under which each population is favoured over the other two, thereby elucidating the processes involved in the maintenance of variability in diapause. Our analysis revealed: 1) frequent mild winters are of primary importance for the non-diapause trait to be maintained, 2) the existence of winter hosts is also important for the non-diapause trait to be favoured, and this importance depends greatly on the degree of the adaptive diapause expression in the DN mites in response to habitat conditions, i. e., the better the phenotype-environment matching in DN, the higher the probability that DN will be favoured, 3) The combined effect of the temporal and spatial variation enhances the maintenance of variablity in the diapause trait of the mites.
Key words: diapause, phenotypic plasticity, environmental heterogeneity, stochasticity, mathematical model, Tetranychus urticae.
Shoot Growth Dynamics and Size-dependent Shoot Fate of a Clonal Plant, Festuca rubra, in a Mountain Grassland
Toshihiko HARA1) and Tom$B-@(J HERBEN2)
1) The Institute of Low Temperature Science, Hokkaido University, Sapporo 060, Japan and 2) Institute of Botany, Academy of Sciences of the Czech Republic, CZ-252 43 Pr$B[I(Jonice, Czech Republic
1) to whom all correspondence should be addressed (TEL: 011-706-5455; FAX: 011-706-7142; EMAIL: t-hara@orange.lowtem.hokudai.ac.jp)
Abstract. The relation of the within-season and between-season patterns of shoot growth were compared in a clonal grass with long-lived shoots, Festuca rubra, in a mown mountain grassland. The growth rate of shoot length from spring to summer in a year was almost constant for each shoot irrespective of spring shoot length each year. The annual shoot growth rate from spring to spring was negatively correlated with the shoot length in the first spring. Shoots of different length and age therefore tended to converge over time to a population of identical shoot size, suggesting an equalizing effect of growth pattern on size structure. Shoot size (shoot length and number of leaves) influenced the fates of shoots. Larger shoots showed an increased incidence of both flowering and formation of intravaginal daughter shoots and a decreased incidence of death in the subsequent time period. The fates of shoots were independent of their age. Although the negatively size-dependent spring-to-spring annual shoot growth rate acted to decrease shoot size variation, the remaining variation within the shoot population was still sufficient to generate different fates of shoots. These fates were not related to the previous life history of individual shoots. There was a significantly positive effect of the shoot size at initiation on its life expectancy. This was mainly attributable to the positively size-dependent survival rate of shoots in the early stage (< 1 year old) of shoot life history. Later on (> 1 year old), shoot size had little effect on the survival rate of shoots. Once small young shoots have survived this early stage (< 1 year old) in life history, they can grow vigorously, little affected by competition regardless of shoot size, and converge to a stable size structure of shoots of similar size. Only shoot size in the early stage (< 1 year old) of life history is important for the persistence of a shoot population.
Key words: daughter shoot formation, flowering, shoot demography, shoot survival, diffusion model, stability of shoot population.
Silique burst of Cardamine scutata (Cruciferae) as a physical inducible defense against seed predatory caterpillars
Shuichi YANO
Laboratory of Ecological Information, Graduate school of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan [E-mail: yano@kais.kyoto-u.ac.jp]
Abstract. Matured seeds of bitter cress, Cardamine scutata Thumb, are scattered by the bursting of siliques. They also burst in response to chewing by seed predatory caterpillars even when seeds are immature. In this case, the caterpillars are frequently expelled, or killed when their bodies become enswathed by the released pericarps. Consequently, many seeds escape from the attack. The plant trait is explained as an immediate induction of a physical defense against seed predators. The germination rate of scattered immature seeds was significantly lower than that of mature ones, which is a direct demonstration of trade-off between anti-herbivore defense and reproduction of the plant.
Key words: silique burst, herbivorous insect, cost of defense
induction, trade-off, Cardamine scutata