Deciduous oak dieback in Japan has been known since the 1930s, but in the last ten years epidemics have intensified and spread to the island's western coastal areas. The symbiotic ambrosia fungus Raffaelea sp. is the causal agent of oak dieback, and is vectored by Platypus quercivorus (Murayama). This is the first example of an ambrosia beetle fungus that kills vigorous trees. Mortality of Quercus crispula was approximately 40% but much lower for associated species of Fagaceae, even though each species had a similar number of beetle attacks. It is likely that other oaks resistant to the fungus evolved under a stable relationship between the tree, fungus and beetle during a long evolutionary process. Quercus crispula was probably not part of this coevolution. This hypothesis was supported by the fact that P. quercivorus showed the least preference for Q. crispula yet exhibited highest reproductive success in this species. Therefore, P. quercivorus could spread more rapidly in stands with a high composition of Q. crispula. The present oak dieback epidemic in Japan probably resulted from the warmer climate that occurred from the late 1980s which made possible the fateful encounter of P. quercivorus with Q. cripsula by allowing the beetle to extend its distribution to more northerly latitudes and higher altitudes. Future global warming will possibly accelerate the overlapping of the distributions of P. quercivorus and Q. crispula with the result that oak dieback in Q. crispula will become more prevalent in Japan.

1 Flying populations of an ambrosia beetle, Platypus quercivorus (Murayama), a vector of an ambrosia fungus Raffaelea quercivora, which causes deciduous oak diebacks in Japan, were determined by sticky screen traps. 2 Platypus quercivorus beetles tended to move upwards along slopes. The highest concentrations of flying beetles usually occured at the upper forest margins. 3 During the period when the number of flying beetles was increasing, the incidence of newly infested trees spread from the epicentre into the forest.During the period when the number of flying beetles was decreasing, the epicentre shrank into the upper forest edge. Newly infested trees did not occur in this period because most trees had already been infested. 4 Near the upper forest edge, where many beetles were highly concentrated throughout the season, the number of new entry holes decreased greatly after the initial attack early in the season, although many adults were present throughout the period.

Field surveys and laboratory experiments were carried out to investigate the influence of light condition on the distribution and movement of Platypus quercivorus adults at the stand level. Both male and female adults showed a positive phototaxis. In the field, beetles tended to move upward and were highly concentrated near the upper forest edge along a road, although many adults were thought to have emerged from the lower middle portion of the stand. The positive phototaxis of newly emerged beetles appeared to be one of the causes of these phenomena. The light condition and the directional movement of adults were important factors determining the spatial distribution pattern of P. quercivorus adults in a stand.

An ambrosia beetle, Platypus quercivorus, is a vector of Japanese oak wilt, which causes massive mortality of oak trees in Japan. Previous studies have identified (1S,4R)-4-isopropyl-1-methyl-2-cyclohexen-1-ol ((-)-IMCH) as the major component of the aggregation pheromone of P. quercivorus. We tested the ability of synthetic (-)-IMCH to attract P. quercivorus adults. The synthetic compound attracted 14.4 times as many P. quercivorus, of both sexes, as control traps. Control and (-)-IMCH-baited traps both captured 3.32 times as many males as females, probably because of the nature of Lindgren funnel traps and of male-biased emergence sex ratio early in the season of adult flights. (-)-IMCH was proved to be a major component of the aggregation pheromone of P. quercivorus.

The within stand spatial distribution of flying Platypus quercivorus adults was quantified by sampling populations with four types of sticky screen traps. The greatest numbers of adults were captured in long traps located at lower positions (0.5-2.5 m height) both at the forest edge and in the forest interior. The numbers of adults captured in square screens hanging from anchored balloons were also greatest at a height of 1 m. Lower numbers were captured in square screens located at the top of the forest canopy and in square screens located above the canopy. Many adults were captured under the forest canopy near the forest edge, but only small numbers were captured inside gaps. We conclude that adult P. quercivorus is generally associated with the forest edge, a trait shared by several other bark and ambrosia beetles.