Shun Takagi

Host Plant Quality Influences Diapause Induction of Byasa alcinous (Lepidoptera: Papilionidae)

Abstract Using host plant quality as a diapause-inducing stimulus may be adaptive for herbivorous insects that use host plants whose phenology is unpredictable based on photoperiod or temperature. We examined the effects of leaf toughness of the woody vine Aristolochia kaempferi Willd. on larval performance and pupal diapause induction of the swallowtail butterfly Byasa alcinous Klug (Lepidoptera: Papilionidae). We also conducted field observations to assess whether human disturbance affects leaf phenology of A. kaempferi. Laboratory experiments showed that tough leaves decreased the survival rate of young larvae and lengthened the larval period, although pupal weight was not affected by leaf toughness. The incidence of pupal diapause was greater with tough leaves than with soft leaves and increased with larval duration. In the field, leaf age composition in A. kaempferi was greatly influenced by human mowing, which led to changes in leaf quality. These results suggest that B. alcinous has a plastic diapause strategy using host plant quality as a diapause-inducing stimulus, which may have evolved in frequently disturbed environments.

Facilitation of ground-dwelling wolf spider predation on mirid bugs by horizontal webs built by Tetragnatha spiders in organic paddy fields

Abstract Trait-mediated effects of predators can impact prey population dynamics by affecting prey behavior. The mirid bug Stenotus rubrovittatus (Matsumura) (Hemiptera: Miridae), a major insect pest in Japanese rice production, usually remains in the upper layer of paddies to feed on rice ears. However, the mirids are frequently trapped by horizontal webs of Tetragnatha spp. spiders, which are highly abundant in organic rice paddies, and fall to the bottom layers of paddies where they are preyed upon by ground-dwelling predators. It is hypothesized that Tetragnatha spp. spiders facilitate bug predation by wolf spiders through trait-mediated effects, in which their horizontal webs force the bugs onto or near the ground and thereby into the hunting zones of wolf spiders. Molecular gut-content analysis of 619 wolf spiders coupled with field measurements revealed that the number of wolf spiders that tested positive for mirid bug predation increased significantly with the density of Tetragnatha spp. spiders in the paddies. We also observed a positive relationship between Tetragnatha spp. abundance and total cover by their webs in paddies. We identified the potential for an unexpected interaction between an herbivorous insect pest and ground-dwelling spiders that usually inhabit different microhabitats in paddy fields by focusing on trait-mediated effects of webs built by Tetragnatha spp. Because spider webs occupy a certain proportion of the available space in terrestrial ecosystems, consideration of trait-mediated effects on interactions between flying insects and other predators may lead to a better understanding of local food webs.

Comparison of generalist predators in winter- flooded and conventionally managed rice paddies and identification of their limiting factors

Winter-flooding of rice paddies without the application of agricultural chemicals is attracting attention as a new agricultural method for enhancing the habitat conditions of wintering waterfowl in rice paddy ecosystems throughout Japan and east Asia. Conditions in these paddies are expected to result in restoration of not only the winter habitats of waterfowl but also those of other taxonomic groups during the rice growing season. In this study, we tested whether the diversity of summer spiders––ubiquitous predators in rice paddies––was higher in the winter-flooded paddies than in the conventional ones by conducting field measurements in 31 winter-flooded and 7 conventional paddies. Limiting factors of spiders in the winter-flooded paddies were then examined. Results revealed that both the density and species richness of spiders were significantly higher in the winter-flooded paddies than in the conventional ones both before and after the insecticide application against pecky rice bug Stenotus rubrovittatus (Matsumura)(Hemiptera: Miridae) to conventional paddies. In addition, spider density and species richness in the winter-flooded paddies correlated with the availability of two prey groups––chironomids and other nematocera. These findings suggest that in the winter-flooded paddies the diversity of generalist predators is higher than in the conventional ones during the rice-growing season and that the combination of management at both the landscape and field level is likely more effective for increasing spider abundance in winter-flooded paddies.

Time-scale dependency of host plant biomass- and trait-mediated indirect effects of deer herbivory on a swallowtail butterfly.

Despite recent attempts to quantify the relative strength of density- and trait-mediated indirect effects, rarely has the issue been properly addressed at the population level. Most research is based on short-term small-scale experiments in which behavioural and/or physiological responses prevail. Here, we estimated the time-scales during which density- and trait-mediated effects manifest, as well as the strength of these effects, using an interaction chain with three organisms (deer-plant-butterfly). A hierarchical Bayesian model was performed by using a long-term data set of deer density in the Boso Peninsula, central Japan (where local densities differ spatially and temporally) as well as densities of the swallowtail butterfly Byasa alcinous and its host plant Aristolochia kaempferi. The time-scale effect of deer on plant quantity and quality was estimated according to the degree of carry-over effects. The negative influence on leaf density showed a temporal saturation pattern over the long term, while the positive influence on leaf quality due to resprouting of leaves after deer browsing showed no clear temporal trend. The net indirect effect changed from positive to negative with time, with the negative density-mediated effect becoming prominent in the long term. Our novel approach is widely applicable in assessing the dynamic impacts of wildlife if the spatio-temporal variability of expansion and/or invasion history is known.

Utilization of emergent plants as an aestivation habitat by the Trapa -feeding leaf beetle ( Galerucella nipponensis ) in Lake Inba, Japan

Water chestnuts (Trapa spp.) dominate many shallow eutrophic lakes, and impact aquatic fauna. Use of Trapa beds by animals may vary with the growth stage of Trapa, but little is known regarding when and how animals use Trapa and nearby, non-Trapa habitat. To clarify seasonal habitat use by the Trapa-feeding chrysomelid beetle (Galerucella nipponensis), we examined seasonal changes in the density of G. nipponensis from Trapa beds and from adjacent emergent vegetation. Furthermore, to determine whether habitat use by beetles is associated with their physiological state, we measured their oviposition and foraging activities. The density of G. nipponensis in each life-stage changed seasonally, with a high density of adults inhabiting emergent vegetation in July, after beetle densities in Trapa beds had declined. Adults collected from Trapa beds showed active egg-laying and foraging activities, while beetles collected from emergent vegetation were in reproductive diapause. This is the first report of G. nipponensis leaving its host plant to initiate summer diapauses in another habitat type.

Invasion of the redback spider Latrodectus hasseltii (Araneae: Theridiidae) into human-modified sand dune ecosystems in Japan

Invasions of some areas of Japan by the exotic redback spider Latrodectus hasseltii Thorell (Araneae: Theridiidae) have been reported. While most of these invasions have occurred in urban areas, anthropogenic habitat modifications may provide an opportunity for L. hasseltii to invade semi-natural ecosystems, but the ecological impacts of L. hasseltii have only rarely been studied. We therefore examined the distribution of L. hasseltii in sand dune ecosystems and its potential impacts on other animals. In addition, we surveyed the occurrence of spiders on the exotic yucca Yucca gloriosa L. (Asparagaceae), another invader of sand dune ecosystems. Latrodectus hasseltii was observed in six of 18 sand dunes in the Chita Peninsula, central Japan, and was the dominant web-building spider at one site. The web contents of L. hasseltii consisted of various arthropod species, including the threatened ground beetle Scarites sulcatus Olivier (Carabidae). In all, 24 of 172 patches of exotic yucca were occupied by L. hasseltii, suggesting that colonization by exotic plants may facilitate the invasion of L. hasseltii into sand dunes. This is the first report of the invasion of L. hasseltii into semi-natural habitats in Japan, and these results suggest that L. hasseltii poses a threat to the conservation of coastal insects inhabiting human-modified sand dune ecosystems.

Seasonal changes in the biomass of floating leaved plant, Trapa spp., and its relation with a leaf beetle, Galerucella nipponensis, in Lake Inba, Japan

Trapa spp. dominate many shallow eutrophic lakes in Japan, which must affect the nutrient dynamics in lakes. Trapa spp. are utilized by several animals, in particular the leaf beetle, Galerucella. To quantify the trophic links and material transfers between Trapa spp. and G. nipponensis, their seasonal dynamics, the amount of leaves being consumed, and carbon and nitrogen stable isotope ratios (δ13C and δ15N) in Lake Inba were tracked. Trapa spp. began to increase in May, reached their highest biomass in September, and disappeared in October. G. nipponensis exhibited continuous reproduction from May until October. Grazing by G. nipponensis did not seem to affect the abundance of Trapa, as the average percentage of Trapa leaf consumed was 3%. Rather, G. nipponensis was influenced by Trapa, as δ13C and δ15N of Trapa spp. and G. nipponensis showed very similar seasonal fluctuations. The difference in δ15N was 3–4‰, which indicates that Trapa leaves were a main food source for G. nipponensis and a trophic linkage exists between them. The estimated nitrogen and phosphorus amounts in Trapa corresponded to 1% of nitrogen and 18% of phosphorus in the entire lake. If G. nipponensis migrate and die in a terrestrial environment, G. nipponensis can transport certain nutrients from lake to terrestrial environments. The nutrients from Trapa spp. are mostly regenerated, except for that biomass transported to terrestrial ecosystems by humans.