Akio Takafuji

Infochemically mediated tritrophic interaction webs on cabbage plants

Abstract In response to damage by herbivores, plants are known to emit infochemicals that enhance the effectiveness of insect parasitoids. Studies on plant–parasitoid interactions mediated by such infochemicals have focused on the tritrophic systems in which plants are infested by a single herbivore species. In natural ecosystems, however, plants are often simultaneously infested by several herbivorous species. The present study focuses on two herbivorous species that simultaneously attack crucifer plants and their respective parasitic wasps. We first show the specific responses of the two specialist parasitic wasps [Cotesia plutellae and C. glomerata (Hymenoptera: Braconidae)] to infochemicals originating from cabbage plants (Brassica oleracea cv. Sikidori) infested by each of their respective host larvae [Plutella xylostella (Lepidoptera: Yponomeutidae) and Pieris rapae (Lepidoptera: Pieridae)]. We then coupled the two tritrophic systems on the same cabbage plants. These experiments demonstrated the presence of indirect interactions between the two species of herbivores. Overall, the results indicate the presence of infochemically mediated tritrophic interaction webs on a single plant.

Response of Predatory Insect Scolothrips takahashii Toward Herbivore-Induced Plant Volatiles Under Laboratory and Field Conditions

We studied the response of a predatory thrips, Scolothrips takahashii, towards herbivore-induced plant volatiles emitted by Lima bean plants infested by two-spotted spider mites Tetranychus urticae (green form). Tests were conducted with a Y-tube olfactometer in the laboratory and with traps under field conditions. The odor of artificially damaged and uninfested Lima bean leaves was not more attractive than clean air in the Y-tube olfactometer. The predatory insects showed a greater preference for Lima bean leaves infested by the two-spotted spider mites than for either clean air or uninfested bean leaves. They showed the same preference towards infested leaves from which all spider mites and their visible products had been removed. Neither the spider mites themselves nor their products attracted the predators. In a satsuma mandarin grove, two traps with infested Lima bean plants as an odor source attracted 42 adult S. takahashii in 55 days, whereas no S. takahashii were trapped in two control traps with uninfested Lima bean plants during the same period. No S. takahashii were found during this period in the vicinity of either the sample traps or the control traps (5-m radius of each trap). These data showed that S. takahashii use herbivore-induced plant volatiles in their foraging behavior in natural ecosystems.

The effect of the rate of successful dispersal of a phytoseiid mite, Phytoseiulus persimilis Athias-Henriot (Acarina : Phytoseiidae) on the persistence in the interactive system between the predator and its prey

Dispersal behaviour was studied on a predacious phytoseiid mite,Phytoseiulus persimilis Athias-Henriot in response to the density of its prey,Tetranychus kanzawai Kishida. And the effect of the change in the rate of successful dispersal of the predators among patches was tested on the persistence of the predator-prey system. The results of the study are summarized as follows: 1. With a severe decline in prey density available per individual predator, the predators exhibited a marked behavioural change and dispersed to other areas. 2. When two neighboring host plants did not touch with each other and the dispersal of the predators was possible only through a 20cm plywood “bridge” which connected the two plants, the rate of successful dispersal of the predators was only 20 to 25%, whereas it was 95% when the plants touched with each other. 3. In the system where 32 host plants touched with each other, the predators succeeded in immigrating into all the plants by the 16th day, and they completely eliminated all the prey by the 33rd day. 4. In another system, the 32 plants were equally divided into eight “patches” which were connected with each other with two of the bridges and the predators could move between them only through the bridges as mentioned above. In such a system the predators and their prey coexisted about three times longer than in the other one. 5. It is suggested that for a longer continued coexistence of the predators and prey, it would be necessary to introduce more physical barrier against the dispersal of the predators and to provide more chances for the prey to disperse. With a severe decline in prey density available per individual predator, the predators exhibited a marked behavioural change and dispersed to other areas. When two neighboring host plants did not touch with each other and the dispersal of the predators was possible only through a 20cm plywood “bridge” which connected the two plants, the rate of successful dispersal of the predators was only 20 to 25%, whereas it was 95% when the plants touched with each other. In the system where 32 host plants touched with each other, the predators succeeded in immigrating into all the plants by the 16th day, and they completely eliminated all the prey by the 33rd day. In another system, the 32 plants were equally divided into eight “patches” which were connected with each other with two of the bridges and the predators could move between them only through the bridges as mentioned above. In such a system the predators and their prey coexisted about three times longer than in the other one. It is suggested that for a longer continued coexistence of the predators and prey, it would be necessary to introduce more physical barrier against the dispersal of the predators and to provide more chances for the prey to disperse.

Inter- and intra-population variations in diapause attribute of the two-spotted spider mite,Tetranychus urticae Koch, in Japan

Populations of the two-spotted spider mite,Tetranychus urticae Koch collected from various localities and from various host plants in Japan showed wide variations in diapause attribute. Diapause percentages at 18°C/9L15D varied from nearly 100% in the north to 0% in the south-west. At intermediate latitudes the mites showed wide inter-population variations. Populations on herbaceous hosts in vinyl- or glass-houses gave significantly lower incidence of diapause than those on roses and deciduous fruit trees. Presence of winter hosts and better host quality under protected environments seemed to favour non-diapausing mites. The temperature threshold for diapause expression also varied widely among local populations. Northern populations consistently had higher and less variable thresholds than populations at intermediate latitudes with thresholds between 15 and 18°C. Inbred lines derived from a population in Kyoto exhibited a wide variation in diapause percentage at 18°C. These results show that diapause inT. urticae is a quantitative threshold trait and that populations in central Japan consist of a variety of genotypes with different diapause traits. This might provide a genetic source for adaptation to local and temporal variations in environmental conditions.

Effects of light on the tritrophic interaction between kidney bean plants, two-spotted spider mites and predatory mites, Amblyseius womersleyi (Acari: Phytoseiidae).

By analyzing the volatiles from Tetranychus urticae-infested kidney bean plants (Phaseolus vulgaris) at different times for two days, we found that they were mainly produced in the light. Tetranychus urticae showed a higher oviposition rate and spent more time feeding during the day (in the light) than at night (in the dark). Infested leaves placed in the light attracted the predatory mite Amblyseius womersleyi, whereas those that were placed in the dark for at least 2 h in daytime did not. This indicates that presence or absence of light affects the production of herbivore-induced plant volatiles. Amblyseius womersleyi dispersed more frequently and consumed more T. urticae eggs during the day (in the light) than at night (in the dark), whereas their oviposition rate did not differ between day and night. Presence or absence of herbivore-induced plant volatiles in the surroundings did not affect dispersal, predation or oviposition rates of A. womersleyi. These results show that A. womersleyi’s behavior coincides with the production pattern of herbivore-induced plant volatiles.

Spider Mites of Japan: Their Biology and Control

Spider mite biology and control in Japan were reviewed. Seventy-eight spider mite species of 16 genera (Family Tetranychidae) have been recorded in Japan. Several of the species recently described were separated from a species complex comprising strains with different ecological performance such as host range. These separations were first supported by crossing experiments and then confirmed by molecular genetic studies. Spider mite control in Japan is still dependent on heavy acaricide spraying in order to attain products of extremely high quality. The commercial use of natural enemies in spider mite management has just started.

Reproductive interference and its consequences for the competitive interactions between two closely related Panonychus spider mites

Two genetically incompatible Panonychus mites (Panonychus citri and Panonychus mori) geographically segregate their distributions in Japan. They occasionally co-exist on deciduous fruit trees at the intermediate latitudes, but their occurrences are unstable and P. mori is often excluded. Interspecific matings between the two species interfere with a subsequent effective intraspecific mating, to a greater extent for P. mori than for P. citri. Panonychus mori males show a strong preference for conspecific females, whereas P. citri males do not show any mating preference. This suggests that under the co-existence of the two species the deleterious effect through reproductive interference is more intense for P. mori than for P. citri. On peach trees where the two species co-exist with various density ratios, the proportion of ineffective females (females that did not produce any female offspring) was higher in P. mori than in P. citri and the proportion increased with increasing density of the other species relative to that of its own species. A simple differential equation model incorporating the above reproductive interference demonstrated that the stable co-existence of the two species is unlikely and that P. citri will have a much higher chance of excluding P. mori, due to a greater reproductive interference effect and higher reproductive rate than the latter. However, the outcome of competition depends on the initial density ratios, which will be influenced by various stochastic events, thereby reducing the probability of one-sided exclusion over a wider area.

Resource utilization pattern of two species of tetranychid mites (Acarina: Tetranychidae)

Life history and resource utilization pattern were compared between two closely related mite species of the genusTetranychus. 1. Tetranychus urticae developed more quickly and had a higher oviposition rate thanT. kanzawai. Consequently, the potential for population increase was greater forT. urticae than forT. kanzawai but the difference was slight. 2. T. urticae was more tolerant of a deteriorating food resource and/or over-crowding.T. kanzawai responded more sensitively to food deterioration and dispersed more quickly. 3. There was a considerable difference between injury patterns by the two species.T. kanzawai damaged host plants more severely, causing earlier and more extensive defoliation of the plants. 4. The maximum population size achieved byT. urticae was 2 to 3 times greater than that ofT. kanzawai on potted host plant. This showed that the former could utilize a food resource much more efficiently. Tetranychus urticae developed more quickly and had a higher oviposition rate thanT. kanzawai. Consequently, the potential for population increase was greater forT. urticae than forT. kanzawai but the difference was slight. T. urticae was more tolerant of a deteriorating food resource and/or over-crowding.T. kanzawai responded more sensitively to food deterioration and dispersed more quickly. There was a considerable difference between injury patterns by the two species.T. kanzawai damaged host plants more severely, causing earlier and more extensive defoliation of the plants. The maximum population size achieved byT. urticae was 2 to 3 times greater than that ofT. kanzawai on potted host plant. This showed that the former could utilize a food resource much more efficiently.

Induced response of tomato plants to injury by green and red strains of Tetranychus urticae.

We studied the induced response of tomato plants to the green strain and the red strain of the spider mite Tetranychus urticae. We focused on the olfactory response of the predatory mite Phytoseiulus persimilis to volatiles from T. urticae-infested tomato leaves in a Y-tube olfactometer. Tomato leaves attracted the predatory mites when slightly infested with the red strain, or moderately or heavily infested with the green strain. In contrast, neither leaves that were slightly infested with green-strain mites, nor leaves that were moderately or heavily infested with the red strain attracted the predators. We discuss the specific defensive responses of tomato plants to each of the two strains.

Genetic diversity and phylogeny of the Kanzawa spider mite, Tetranychus kanzawai, in Japan

Two types are known in the Kanzawa spider mite, Tetranychus kanzawai (K and T; see Gotoh et al., 1999), which differ in host range and have a unidirectional incompatibility. Prior to DNA analyzes, crossing between females of a known K type and males of each of 17 strains collected in Japan showed that six of the strains were of the K type, five were the T type and the rest consisted of a mixture of the two types. In order to elucidate the genetic diversity and phylogenetic relationship of T. kanzawai in Japan, we analyzed the DNA sequences of two regions – the internal transcribed spacer 1 (ITS1) of the nuclear ribosomal DNA (rDNA) and a fragment of the cytochrome oxidase subunit I gene (COI) of mitochondrial DNA – using 11 strains (six K-type strains and five T-type strains). Base substitutions were detected on 25 sites of COI (375 bp) and 19 sites of ITS1 (486 bp), resulting in eight and 17 haplotypes, respectively. The phylogenetic trees constructed using the DNA sequences failed to clearly distinguish between the two types. The results suggested that the T type was derived from the K type.