Shota Jikumaru

Transmission of Bursaphelenchus mucronatus (Nematoda: Aphelenchoididae) through feeding wounds by Monochamus saltuarius (Coleoptera: Cerambycidae)

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.

Reproductive Traits and Diel Activity of AdultMonochamus saltuarius (Coleoptera: Cerambycidae) at Two Different Temperatures

To investigate the reproductive traits and diel activity ofMonochamus saltuarius, 14 pairs of the adults were reared and observed at each of 20°C and 25°C. Both sexes were more active at 25°C than at 20°C irrespective of the presence or absence of light. A diel change in behavior during 24 h was found on 33% of a total of 48 observations of three age classes of both sexes. No significant difference was found in the mean longevity between sexes or between the two temperature conditions. The time required for reproductive maturation showed no significant difference within the same sex when reared at 20°C and 25°C. The mean oviposition period was significantly longer at 25°C (28.8 days) than at 20°C (12.2 days). The mean lifetime fecundity was significantly greater at 25°C than at 20°C. There was no significant difference in the oviposition ratio (number of eggs deposited/number of oviposition wounds excavated), oviposition rate or hatchability for females between the two temperature conditions.

Evolutionary change in a pine wilt system following the invasion of Japan by the pinewood nematode, Bursaphelenchus xylophilus

Pinewood nematode (PWN), Bursaphelenchus xylophilus, is the causative agent of pine wilt disease (PWD) of pine trees and is transmitted by cerambycid beetles belonging to the genus Monochamus. PWN is believed to have been introduced into Japan from North America at the beginning of the 20th century. In this article, we first provide an outline of the PWD system and the range expansion of PWN in Japan and then review the literature, focusing on the virulence of PWN. Virulence is a heritable trait in PWN, with high virulence being closely related to a high rate of reproduction and within-tree dispersal. When two PWN isolates with different virulence levels are inoculated into pine seedlings, the more virulent nematodes always dominate in dead seedlings. In a laboratory setting, many more virulent nematodes board the insect vectors than avirulent ones. The age at which vectors transmit the most abundant PWNs to pine twigs changes during the course of a PWD epidemic. However, the relation between virulence and transmission of PWN remains as yet relatively unknown. Such information would enable ecologists to predict the evolution of the PWD system. In this review we also compare ecological traits between the PWN and the avirulent congener, B. mucronatus.

Boarding abilities of Bursaphelenchus mucronatus and B. xylophilus (Nematoda: Aphelenchoididae) on Monochamus alternatus (Coleoptera: Cerambycidae)

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.

Resistance of an indigenous biological system against expansion of the invasive nematode, Bursaphelenchus xylophilus , in cool areas of Japan

Bursaphelenchus xylophilus is the pathogen associated with pine wilt disease (PWD), an infectious disease of pine trees transmitted by cerambycid beetles of the genus Monochamus. Bursaphelenchus xylophilus is an invasive species, whilst B. mucronatus is a native congener and non-pathogenic to pine trees in Japan. To provide experiment evidence of the biotic and/or abiotic resistance to PWD expansion in a cool area of Japan, we inoculated B. xylophilus into healthy pine trees in a Japanese red pine (Pinus densiflora) stand with the indigenous pine tree-B. mucronatus-insect vector system before invasion of B. xylophilus in 1993 and 1994. Extremely cool air temperature with high precipitation and extremely high air temperature with low precipitation were observed in the summers of the 2 years of inoculation, respectively. The cool summer induced a low incidence of PWD and delayed disease development, resulting in the replacement of B. xylophilus by B. mucronatus within diseased trees and the emergence of Monochamus saltuarius beetles carrying B. mucronatus from the trees 2 years after the inoculation. The hot summer induced disease development in trees in the year of inoculation, but such diseased trees did not become infection sources because of the lack of M. alternatus, whose oviposition was synchronised with the period of disease development in pine trees. This study indicated that biotic factors were important in the inhibitory mechanism in a pine forest against the spread of PWD in the stand. Relevant biotic factors were the lack of M. alternatus and the presence of M. saltuarius carrying B. mucronatus.