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Hayami Kasuga and Katsumi Togashi

Bursaphelenchus xylophilus causes pine wilt disease in susceptible pine species. To determine whether B. xylophilus persists in forests containing Abies sachalinensis and Picea jezoensis, we performed inoculation experiments using ten or 15 seedlings of A. sachalinensis, P. jezoensis, Pinus densiflora and P. thunbergii. Inoculation of 15 000 B. xylophilus caused 20 and 7% mortality in A. sachalinensis and P. jezoensis seedlings, respectively, in the nursery, indicating low susceptibility of A. sachalinensis and a lack of susceptibility of P. jezoensis. By contrast, B. xylophilus caused 40 and 70% mortality in P. densiflora and P. thunbergii, respectively. Bursaphelenchus xylophilus was recovered at extremely low densities from stems of externally asymptomatic seedlings of the first two tree species 9.5 months after inoculation. Inoculation of B. xylophilus on excised stem sections of A. sachalinensis and P. jezoensis seedlings showed a greater increase in population density in the bark than xylem after 3 weeks, whereas that on P. densiflora stem sections showed no difference in nematode density between the two tissues. The results suggest that B. xylophilus may persist in forests containing A. sachalinensis and P. jezoensis, when there are insect vectors with affinity for the nematode.

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Katsumi Togashi and Ichirow Kaihotsu

Monochamus alternatus adults carry the pine wood nematode Bursaphelenchus xylophilus. The nematodes are present in the tracheal system of vector beetles. It is known that carrying more than 10 000 nematodes causes drastic reduction in the lifespan and flight activity of beetles. To understand the effects of heavy nematode loads, 41 newly emerged M. alternatus adults were examined for lateral distribution of nematodes within the insect body. The nematodes were distributed unevenly between right and left sides of the beetle body at small nematode loads, whereas they were distributed evenly between them at heavy nematode loads. An analysis using Hagen-Poiseuille flow in a thin tube suggested that the rate of air flow decreased exponentially as the number of nematodes increased in a trachea. Thus, even distribution of nematodes between right and left sides of the vector body was considered to be responsible in part for the effects of heavy nematode loads on beetle activity.

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Katsumi Togashi and Takuya Aikawa

Abstract

To clarify the movement of Bursaphelenchus xylophilus in the tracheal system of adult Monochamus alternatus, 49 adult beetles were dissected 0, 15, 30, 45 or 60 days after emergence and the nematodes heading inwards to the distal end and outwards to spiracles in tracheae were counted. In adult beetles just after emergence, the majority of nematodes headed inwards. The proportion of tracheae with nematodes heading outwards was significantly higher for beetles aged 15, 30 or 45 days than for newly-emerged beetles. The proportion of nematodes pointing outwards tended to increase with beetle age and was significantly greater in 45-day old beetles than in newly emerged ones. A comparison of prediction with observation suggests that the nematodes move forwards in the tracheal system from spiracles and exit from spiracles after turning round in the tracheae. Le déplacement de Bursaphelenchus xylophilus (Nematoda: Aphelenchoididae) dans le système trachéen des adultes de Monochamus alternatus (Coleoptera: Cerambycidae) - Pour mieux comprendre le déplacement de Bursaphelenchus xylophilus dans le système trachéen de Monochamus alternatus 49 insectes ont été disséqués 0, 15, 30, 45 ou 60 jours après l'émergence et les nématodes se dirigeant dans les trachées vers l'intérieur - partie distale - et vers l'extérieur - stigmates - ont été comptés. Chez les insectes adultes, immédiatement après l'émergence la majorité des nématodes se dirigent vers l'intérieur. La proportion de trachées contenant des nématodes se dirigeant vers l'extérieur est significativement plus élevée chez les insectes âgés de 15, 30 ou 45 jours que chez ceux ayant émergé depuis peu. La proportion de nématodes se dirigeant vers l'avant a tendance à croître avec l'âge de l'insecte: elle est en effet significativement plus élevée chez les insectes âgés de 45 jours que chez ceux dont l'émergence est récente. La comparaison entre projection et observations suggère que les nématodes progressent dans le système trachéen à partir des stigmates et sortent par ces derniers après s'être retournés dans la trachée.

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Katsumi Togashi and Shigeru Hoshino

Abstract

Aphelenchoides besseyi is a seed-borne nematode and the causal agent of rice white tip disease. To determine the relationships between nematode numbers per seed, size of seed harbouring nematodes, and nematode mortality, 18 panicles were collected from 12 paddy fields showing three different levels of white tip disease. The panicles were investigated after storage for 30-82 days at 4°C. There was no nematode mortality within seeds during storage. Mean nematode number per seed increased and mean degree of seed swelling decreased as the paddy field infestation of white tip disease increased. For paddy field means, there was a significant, negative correlation between the mean nematode number per seed and the mean degree of rice seed swelling. Intriguingly, for individual seeds, the mean degree of swelling increased up to that typical of well-developed seeds with increasing nematode number per seed. Nematode mortality occurred in an inversely density-dependent fashion within individual seeds. The nematode exhibited a clumped distribution among seeds in each paddy field. Such ecological features of the nematode might contribute to its persistence in rice plant population.

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Koji Matsunaga and Katsumi Togashi

Abstract

Pine wilt disease by Bursaphelenchus xylophilus has been causing devastating damage to Pinus densiflora and P. thunbergii forests in Japan. Reforestation using resistant trees is one of the most effective control measures. In this study we examined the seasonal change in the number of nematodes passing through 5 cm long P. densiflora branch sections during 24 h, to determine the season of greatest susceptibility when resistant candidates can be identified easily. We inoculated 200 nematodes of each of four B. xylophilus and one B. mucronatus isolate on the cut end of individual branch sections. The overall seasonal change in the number of nematodes passing was similar among the five isolates. Peaks occurred in August and between December and February for four isolates. A virulent isolate, T-4, exhibited remarkable fluctuations, with the highest peak in August, suggesting that a test in August with T-4 is most informative. Another study showed that the passage ability, which is a measure of the dispersal ability in branches, was low for eggs and second-stage propagative juveniles and high for other developmental stages. This indicates that the composition of developmental stages in the inoculum is important when comparing the numbers of passing nematodes obtained on different occasions and in different places.

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Katsumi Togashi and Shota Jikumaru

Abstract

The transmission of Bursaphelenchus mucronatus by its vector beetle, Monochamus saltuarius, was investigated. Fortythree beetles were reared individually under outdoor conditions and characteristics of their complete transmission curves of B. mucronatus were determined. The averaged nematode-transmission curves exhibited by beetles carrying more than 10 000, 1000 to 9999, and 100 to 999 nematodes at their emergence had a peak of 568, 146 and 12 nematodes per 5 days, respectively. The peaks appeared during a period from 20 to 30 days after beetle emergence. Backward stepwise regression analysis showed that the initial nematode load, the nematode departure efficiency and the nematode transmission efficiency had significantly positive influences on the number of nematodes transmitted into pine twigs. Other statistical analysis showed that the initial nematode load was most important among the three factors to account for the among-beetle difference in the number of nematodes transmitted, followed by the nematode transmission efficiency and nematode departure efficiency in that order. There were significant, positive correlations between the nematode departure efficiency, the nematode transmission efficiency and the number of nematodes transmitted from beetle into pine twigs.

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Koji Matsunaga and Katsumi Togashi

Abstract

Differences among four isolates of Bursaphelenchus xylophilus and one of B. mucronatus in vulnerability to the inhibitory effect of Pinus densiflora on nematode dispersal were investigated by inoculating boiled and living branch sections 5 cm long. The intrinsic dispersal ability and vulnerability to the inhibitory effect of living branches differed among isolates. Additionally, the degree of branch inhibition of nematode dispersal was examined by inoculating the five isolates on 2.5 and 5.0 cm long, P. densiflora living branch sections. An increase in section length induced an 80% decrease in the number of nematodes of all isolates passing through the branch. The extent to which passing nematode number decreased as branch section length increased can be used to express susceptibility of pine species to pine wilt disease. Virulence had no relation to intrinsic and realised dispersal rates in B. xylophilus.

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Katsumi Togashi and Shigeru Hoshino

Abstract

The effect of nematode infestation on dispersal and survival of both host plant and parasitic nematode was investigated experimentally using rice plants, Oryza sativa L. and Aphelenchoides besseyi Christie, a seed-borne ectoparasite. Nematode-inoculated plants produced a greater proportion of light seeds floating on water than non-inoculated plants. Nematode mortality was greater in light seeds than in heavy seeds. In the case of light seeds, the mean degree of seed swelling increased as the number of nematodes harboured increased, whereas it was constant with increasing numbers of nematodes for the intermediate and heavy seed groups. A smaller proportion of light seeds germinated and these took longer for seminal roots to protrude than from heavy seeds, irrespective of whether they derived from nematode-inoculated or non-inoculated plants. These results indicate a trade-off for rice seed specific gravity between dispersal and competition of rice seeds and another trade-off between dispersal and reproduction of nematodes harboured in the seed.

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Katsumi Togashi and Shota Jikumaru

Abstract

Bursaphelenchus mucronatus is closely related to Bursaphelenchus xylophilus, the causative agent of pine wilt disease. Both nematodes are transmitted between host pine trees as the fourth-stage dispersal juveniles (JIV) by insect vectors. After the invasion of Japan by B. xylophilus, B. mucronatus, native to Japan, appears to have been replaced in the pine forests during the spread of the disease. To help understand this species replacement, the number of JIV carried by an insect vector (the initial nematode load) was compared between the two nematode species by using the beetle, Monochamus alternatus, in the laboratory. The initial load of B. mucronatus was significantly smaller than that of B. xylophilus although the number of third-stage dispersal juveniles (JIII) concentrated at the pupal chambers did not differ. Statistical analysis showed that the proportion of JIII moulting to JIV was the most important among three components explaining the difference in the initial load of B. mucronatus while the number of JIII concentrated at the pupal chamber was the most important for B. xylophilus. The phoretic affinity between the nematode and its vector is discussed in relation to its role in the species replacement.