Shin-ya Ohba

Dietary items of predacious aquatic bugs (Nepoidea: Heteroptera) in Japanese wetlands

Some species of predatory aquatic bugs (Nepoidea, including Belostomatidae and Nepidae) are designated as threatened-vulnerable species in the Red Data Book of Japan and are regarded as effective predators of freshwater snails and mosquito larvae. To reveal the feeding habits of Nepoidea, we investigated their dietary items in wetlands and obtained data from the published literature. Lethocerus deyrolli (Vuillefroy) preys mainly upon frogs, and the major diet of the nymphs is tadpoles. Laccotrephes japonensis (Scott) adults prey mainly upon tadpoles. Lethocerus deyrolli nymphs and L. japonensis adults compete against each other as same-guild members because they are sympatric in wetlands. Both nymphs and adults of Appasus japonicus (Vuillefroy) feed on snails, and L. japonensis nymphs feed on aquatic insects, including mosquito larvae (39.3% of aquatic insects). These species are effective predators of medically important pests such as snails, and mosquito larvae.

Predators of Anopheles gambiae sensu lato (Diptera: Culicidae) Larvae in Wetlands, Western Kenya: Confirmation by Polymerase Chain Reaction Method

ABSTRACT Polymerase chain reaction analysis was performed to determine whether mosquito predators in wetland habitats feed on Anopheles gambiae sensu lato (s.l.) larvae. Aquatic mosquito predators were collected from six wetlands near Lake Victoria in Mbita, Western Kenya. This study revealed that the whole positive rate of An. gambiae s.l. from 330 predators was 54.2%. The order of positive rate was the highest in Odonata (70.2%), followed by Hemiptera (62.8%), Amphibia (41.7%), and Coleoptera (18%). This study demonstrates that the polymerase chain reaction method can determine whether aquatic mosquito predators feed on An. gambiae s.l. larvae if the predators have undigested An. gambiae s.l. in their midgut or stomach.

The Effect of Pyriproxyfen as a “Population Growth Regulator” against Aedes albopictus under Semi-Field Conditions

An insect growth regulator, pyriproxyfen, has been used for the control of a range of pest insects, including mosquitoes. Pyriproxyfen is effective in inhibiting adult emergence and sterilizing adult females. The Asian tiger mosquito, Aedes albopictus (Skuse), is an important vector of dengue and chikungunya, and is expanding its distribution throughout Europe and the Americas. In the present study, we evaluated the impact of pyriproxyfen-treated bed nets on population growth of Ae. albopictus under semi-field conditions, using 6 small microcosms. We created microcosms containing breeding sites to simulate the natural ecosystem of vector mosquito and installing miniature bed net treated with 350 mg/m2 pyriproxyfen in Experiment I and 35 mg/m2 in Experiment II. For each experiment, we also established microcosms installing untreated polyethylene net (untreated control). The installing nets were provided with artificially torn holes, to simulate damage and allow mosquitoes to penetrate. We released 100 pairs of Ae. albopictus into each microcosm, and allowed them to feed on a mouse under the bed nets at approximately 1-week intervals. In comparison with the untreated control microcosms, the number of eggs laid by the released adults in the pyriproxyfen-treated microcosms was significantly lower in both Experiment I and II. Moreover, egg hatchability was significantly suppressed and pupal mortality was increased. Our results indicate that tarsal contact with pyriproxyfen has been shown to suppress egg production and hatchability in adult females and the auto-dissemination of pyriproxyfen into larval breeding sites by adult mosquitoes, through contact with pyriproxyfen-treated polyethylene bed nets, may suppress the mosquito population density.

Relationships between length and weight of freshwater macroinvertebrates in Japan

Relationships between weight (W; dry weight) and length (L; head capsule width, total body length or head carapace length) were examined in 31 Japanese freshwater macroinvertebrate taxa, using the form W = aLb. The relationships were expressed as data of the lowest taxonomic level and data of higher taxonomic levels. The length–weight relationships obtained in this study were similar to those obtained in North America and Europe at the lowest taxonomic level, whereas they could be different from those obtained in North America and Europe at the higher taxonomic levels. We suggest that researchers should make their own regressions for a target taxon or use the regression for the same taxon as possible lower taxonomic level in the local area.

Predator–prey body size relationships when predators can consume prey larger than themselves

As predator–prey interactions are inherently size-dependent, predator and prey body sizes are key to understanding their feeding relationships. To describe predator–prey size relationships (PPSRs) when predators can consume prey larger than themselves, we conducted field observations targeting three aquatic hemipteran bugs, and assessed their body masses and those of their prey for each hunting event. The data revealed that their PPSR varied with predator size and species identity, although the use of the averaged sizes masked these effects. Specifically, two predators had slightly decreased predator–prey mass ratios (PPMRs) during growth, whereas the other predator specialized on particular sizes of prey, thereby showing a clear positive size–PPMR relationship. We discussed how these patterns could be different from fish predators swallowing smaller prey whole.

PREDATORY ABILITY OF ADULT DIVING BEETLES ON THE JAPANESE ENCEPHALITIS VECTOR CULEX TRITAENIORHYNCHUS

Abstract The predatory ability of adult Japanese diving beetles on 4th instars of the Japanese encephalitis vector mosquito, Culex tritaeniorhynchus, was assessed under laboratory conditions. To determine the differences in the predatory ability among 14 beetle species inhabiting rice fields, the following species were introduced to 10 Cx. tritaeniorhynchus 4th instars in a plastic cup: 5 small-bodied species (<9 mm in body length) comprising Hydroglyphus japonicus, Noterus japonicus, Laccophilus difficilis, Hyphydrus japonicus, and Agabus japonicus; 7 medium-bodied species (9–20 mm in body length) comprising Hydaticus rhantoides, Hydaticus grammicus, Rhantus suturalis, Eretes griseus, Hydaticus bowringii, Agabus conspicuous, and Graphoderus adamsii; and 2 large-bodied species (>20 mm) comprising Cybister brevis and C. japonicus. The average 24-h predation rate was highest in medium-bodied species (>90%), followed by small-bodied species (31%) and large-bodied species (19%). The functional responses to Cx. tritaeniorhynchus larvae of 3 medium-bodied species (H. grammicus, R. suturalis, and E. griseus) were estimated. Eretes griseus exhibited the highest attach rate and shortest prey-handling time, suggesting that medium-bodied diving beetles, especially E. griseus, may be efficient predators of mosquito larvae in rice fields.

Ontogenetic Dietary Shift in the Larvae of Cybister japonicus (Coleoptera: Dytiscidae) in Japanese Rice Fields

ABSTRACT A number of fragmentary reports suggest that the endangered diving beetle Cybister japonicus larvae feed on tadpoles, fish, and aquatic insects. However, no quantitative study on the feeding habits of C. japonicus larvae has been reported. In this study, field observations and rearing experiments were carried out to show the feeding ecology of C. japonicus larvae. Unlike previous commentaries, the first- and second-instar larvae of C. japonicus preyed on insects, mainly Odonata nymphs and Notonecta triguttata, irrespective of prey availability, but did not eat vertebrates such as tadpoles and fish in the field. On the contrary, the third-instar larvae fed on both insects and vertebrates. Rearing experiments showed that the number of Odonata nymphs consumed was significantly more than the number of tadpoles consumed by the first and second instars but third-instar larvae ate both the Odonata nymphs and tadpoles in the tadpole-Odonata nymph mixture experiment. The total body lengths of C. japonicus new adults in the Odonata nymph and tadpole-Odonata nymph mixture treatments were statistically equal. These results suggested that the first- and second-instar larvae of C. japonicus prey mainly on insects and do not eat vertebrate animals (insectivore), whereas the third-instar larvae fed on both insects and vertebrates (generalist).

The role of amphibian prey in the diet and growth of giant water bug nymphs in Japanese rice fields

Predatory insects that depend upon particular prey animals are commonly regulated by the prey animal’s abundance. Nymphs of the giant water bug Kirkaldyia (=Lethocerus) deyrolli (Heteroptera: Belostomatidae) are predators regarded as specialists in feeding on tadpoles. We studied the ontogenetic diet shift of aquatic nymphs by quantifying instar abundance and by analyzing captured prey and prey relative abundance during the period of rice irrigation in three localities. We also evaluated the contribution of major prey items (tadpoles, frogs, and Odonata nymphs) on specific growth rates of each nymphal stage in a rearing experiment. First to third-instar nymphs of K. deyrolli fed mainly on tadpoles, regardless of differences in prey availability. Nymphs of subsequent fourth and fifth instar stages shifted from tadpoles to other prey animals within each rice field. A rearing experiment demonstrated that giant water bug nymphs provided with tadpoles had greater specific growth rates at all nymphal stages, except for the final stage, than nymphs fed other prey (frogs and Odonata nymphs). The emergence of young K. deyrolli nymphs seemed to coincide with the period during which tadpoles became abundant in the rice fields. Consumption of tadpoles seems important to allow the nymph to complete its larval development in an unstable temporary habitat.

Food shortage affects flight migration of the giant water bug Lethocerus deyrolli in the prewintering season

The endangered giant water bug Lethocerus deyrolli (Vuillefroy) is frequently attracted in large numbers to artificial lights in Japan. To examine factors enhancing flight migration for L. deyrolli, we carried out field work in western Hyogo Prefecture, central Japan, in September during the nonreproductive and prewintering season. The body weight of specimens collected under flight migration (flight bugs) was significantly less than that of those collected in ponds (pond bugs). A field experiment using open cages in a rice paddy field was carried out with two treatments, with and without a food supply. The remaining rate of L. deyrolli for the food present treatment was significantly higher than that for the food absent treatment for the first two days. These results suggest that L. deyrolli would fly in search of food when the food supply of the present habitat becomes unsuitable.

Notes on paternal care and sibling cannibalism in the giant water bug, Lethocerus deyrolli (Heteroptera: Belostomatidae)

Males of the giant water bug, Lethocerus deyrolli, care for egg masses on vegetation above the water surface. They supply the developing eggs with water and guard them against predators. In the present study, mechanisms by which paternal care is extended were found. Males were found situated just below the water on the natal substrate (usually a stick), and the first instar nymphs were aggregated around the substrate. When disturbed, the males showed aggressive behavior, threatening the intruder with their forelegs. Nymphs up to 12 h old did not attack the offered sibling nymphs or anuran larvae, which are common prey in the field. The 24 h‐old nymphs attacked both prey animals; however, they preferred anuran larvae. Cannibalistic behavior in the nymphs was well developed 72 h after hatching, when the nymphs had already dispersed from the natal substrate. The suppression of sibling cannibalism in younger nymphs would promote the maintenance of tight nymphal aggregations and consequently extend male care in this predatory species.