Katsumi Togashi

MODELING THE SPREAD OF PINE WILT DISEASE CAUSED BY NEMATODES WITH PINE SAWYERS AS VECTOR

An epidemic of pine wilt disease has been spreading in wide areas of Japan for nearly a century. The disease is caused by the pinewood nematode, Bursaphelenchus xylophilus, with the pine sawyer, Monochamus alternates, as vector. The spread of disease is facilitated by an obligatory mutualism between the nematode and the pine sawyer: the pine sawyer helps the nematode transmit to a new host tree, while the nematode supplies the pine sawyer with newly killed trees on which to lay eggs. We present a mathematical model to describe the host-vector interaction between pines and pine sawyers carrying nematodes, on the basis of detailed data on the population dynamics of pine sawyers and the incidence of pine wilt disease at a study site located on the northwest coast of Japan. We used the model to simulate the dynamics of the disease and predict how the epidemic could be controlled by eradication of the pine sawyer. The main results are as follows: (1) There is a minimum pine density below which the disease always fails in invasion. However, even if the pine density exceeds this minimum, the disease fails in invasion due to the Allee effect when the density of pine sawyers is very low. (2) The minimum pine density increases disproportionately with increase in the eradication rate. (3) The probability that a healthy tree will escape from infection until the epidemic dies out decreases sharply with increase in the initial pine density or the initial density of pine sawyers.

Modeling the Expansion of an Introduced Tree Disease

Pine wilt disease is caused by the introduced pinewood nematode, Bursaphelenchusxylophilus, for which the vector is the pine sawyer beetle, Monochamus alternatus. Native Japanese pines, black pine (Pinus thunbergii) and red pine (P. densiflora), are extremely sensitive to the nematodes infection, and the parasite has been expanding nationwide in the last few decades, despite intensive control efforts. To understand the parasites range expansion in Japan, we modeled the dynamics of the pines and the beetle that disperses the nematode, using an integro-difference equation in a one-dimensional space. Based on field data collected in Japan, we investigated the dependence of the parasites rate of range expansion on the eradication rate of the beetle, the initial pine density, and the beetle dispersal ability. Our model predicts several results. (1) The Allee Effect operates on beetle reproduction, and consequently the parasite cannot invade a pine stand, once the beetle density decreases below a threshold. (2) The distribution of the dispersal distance of the beetles critically affects the expansion rate of the disease. As the fraction of the beetles that travel over long distance increases from zero, the range expansion accelerates sharply. (3) However, too frequent long-range dispersal results in a failure of the parasite invasion due to the Allee Effect, suggesting the importance of correctly assessing the beetles mobility to predict the speed of range expansion of the parasite. (4) As the eradication rate is increased, the range expansion speed decreases gradually at first and suddenly drops to zero at a specific value of the eradication rate.

Effects of spatio-temporal intervals between newly-hatched larvae on larval survival and development in Monochamus alternatus (Coleoptera: Cerambycidae).

The effects of distance between hatching larvae on survival and development were investigated inMonochamus alternatus. Two newly-hatched larvae were inoculated intoPinus densiflora bolts at a distance of 2.5 cm or 10 cm, simultaneously or at an interval of 2 weeks. Some larvae were inoculated singly as a control. When larvae were inoculated simultaneously, mortality of the closely-inoculated larvae was significantly higher than that of distantly-inoculated larvae. Such high mortality was identified as due to conspecific bites. When the two larvae were inoculated asynchronously, the first-inoculated larvae killed some second-inoculated larvae but were never killed by them. Consequently, mortality was higher in second-inoculated larvae than in first-inoculated larvae. In particular, there was a significant difference in mortality between them when the larvae had been inoculated closely. The mortality of second-inoculated larvae was higher in the closely-inoculated group than in the distantly-inoculated group although there was no significant difference between them. In the case of two simultaneously-inoculated larvae, the initial distance between them had no significant effect on the development and growth in the early larval stage. When the larvae were inoculated asynchronously, the first-inoculated larvae grew more quickly than singly-inoculated control larvae.

Different Developmental Responses of Virulent and Avirulent Isolates of the Pinewood Nematode, Bursaphelenchus xylophilus (Nematoda: Aphelenchoididae), to the Insect Vector, Monochamus alternatus (Coleoptera: Cerambycidae)

Abstract The pinewood nematode (Bursaphelenchus xylophilus) is known to be the causative agent of pine wilt disease, which is transmitted from wilt-killed to healthy pine trees by the insect vector Monochamus alternatus. The second-stage propagative juvenile develops into third-stage dispersal (JIII) or propagative juveniles. The JIII develops into the fourth-stage dispersal juvenile (JIV), a special stage for transportation, then the JIV enters the tracheal system of adult beetles in pupal chambers in the xylem. To determine the differences in some life history parameters related to transmission between a virulent and an avirulent isolate of B. xylophilus, progression to the different life stages was investigated using two nematode isolates of different virulence, M. alternatus larvae, and pine bolts with an artificial pupal chamber. The numbers of JIIIs and JIVs produced around the pupal chamber by the time of beetle emergence were much smaller in the avirulent isolate than in the virulent one. The proportion of JIIIs produced in a population around the pupal chamber (the JIII and JIV number/the total population) and the probability of JIIIs developing into JIVs (the JIV number/the JIII and JIV number) were also smaller in the avirulent isolate. Although the probability of JIVs boarding beetles (the initial nematode load/the JIV number) was equal between the two isolates, the mean initial nematode load was much smaller in the avirulent isolate. The smaller initial nematode load of the avirulent isolate was ascribed to its smaller rate of reproduction and to lower production rates of JIII and JIV. A trade-off between the virulence and transmission rate of B. xylophilus is discussed herein, taking into account the effect of the initial nematode load on the vector’s longevity and flight performance.

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.

Deterred oviposition response of Monochamus alternatus (Coleoptera: Cerambycidae) to oviposition scars occupied by eggs

1 The oviposition behaviour and response of Monochamus alternatus females to oviposition scars were investigated in the laboratory.

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.

A New Method for Loading Bursaphelenchus xylophilus (Nematoda: Aphelenchoididae) on Adult Monochamus alternatus (Coleoptera: Cerambycidae)

Abstract A new method was developed for loading the pinewood nematode, Bursaphelenchus xylophilus (Steiner et Buhrer) Nickle, on the beetle Monochamus alternatus Hope. Postdiapause beetle larvae were sterilized with 70 and 99.9% aqueous ethanol and placed singly in flasks where B. xylophilus reproduced on the fungus Ophiostoma minus (Hedgcock) H. et P. Sydow that had been grown on autoclaved barley grain and Pinus densiflora Sieb. et Zucc. wood chips. The fungus produced a large nematode population that developed to a high proportion of third-stage dispersal juveniles that molted to the fourth-stage dispersal juveniles. The survival rate was 80%, and the mean nematode load was 10,096. It took a mean of 5 wk to obtain the nematode-infested beetles after the initiation of nematode rearing.

Spread Of An Introduced Tree Pest Organism – The Pinewood Nematode

Invasions of exotic species sometimes cause devastating effects in ecosystems they have invaded. Nowadays worldwide economic activity increases the flow of people and trading materials within and between continents, resulting in increasing occurrence of invasion of exotic species. The invasion process generally involves four phases: arrival, establishment in the new habitat, range expansion, and saturation (Liebhold et al. 1995). The spread pattern of invading organisms has been studied empirically and theoretically by many authors (Andow et al. 1990, Shigesada et al. 1995, Veit and Lewis 1996, Yamamoto et al. 2000). Pinewood nematode, Bursaphelenchus xylophilus (Steiner et Buhrer) Nickle, is the causative agent of pine wilt disease (Kiyohara and Tokushige 1971). The infection of the nematode induces a rapid tree mortality of susceptible pine species such as Pinus densiflora, P. thunbergii, and P. sylvestris (Kiyohara and Tokushige 1971, Kondo et al. 1982). The nematode is inferred to be native to North America and introduced into Japan at the beginning of 20th century (Mamiya 1988). Since then it has spread to Korea, Taiwan, and China and devastated pine forests in East Asia. In Japan, for example, the annual loss of pines reached a maximum value of 2,430,000 m in 1979 (Mamiya 1988), and then was held at a level of about 1,000,000 m in the early half of 1990’s. It was also found in Portugal in 1999 (Mota et al. 1999). The objective of this article was to determine the spread pattern of pinewood nematode at within-stand, local, and regional levels in Japan. For this purpose, we summarized the biology of the nematode and its vectors first.

Distance effect of co-occurring tree species on pine wilt disease incidence in Pinus densiflora seedlings inoculated with Bursaphelenchus xylophilus

Field-grownPinus densiflora seedlings were inoculated withBursaphelenchus xylophilus and the incidence of pine wilt disease was analyzed with respect to the spatial relationships between pine seedlings and adjacent seedlings of other tree species in a pure pine stand and three stands mixed withAlnus sieboldiana, Sarothamus scoparius or naturally associated species. The disease incidence was 60.9% in a 0–40 cm distance class from the nearestA. sieboldiana seedlings and then decreased with increasing minimum distance. The mean minimum distance between pine seedlings andA. sieboldiana was significantly shorter in diseased seedlings than in healthy ones. The highest disease incidence (53.9%) was found in a 0–40 cm distance class from the nearestS. scoparius, although the relationship with minimum distance was blurred by a high incidence in the 80-cm distance class. The mean minimum distance between pine seedlings was significantly short in diseased seedlings in a pure stand, whereas it was not so short between pine seedlings and other tree species in the two stands mixed withS. scoparius and the naturally associated species. The distance effect on disease incidence was noticeable inA. sieboldiana andP. densiflora at high density. This was not so clear inS. scoparius and was not found in the naturally associated species.