K. Furuta

Host preference and population dynamics in an autumnal population of the maple aphid, Periphyllus californiensis Shinji (Homoptera, Aphididae)

The autumnal population of the maple aphid, Periphyllus californiensis Shinji starts in October when aestivating larvae become active. The aphid utilized the key‐fruits of maple trees in October, the leaves in November and the twigs in December. Winged females appeared in November and dispersed. This dispersal spreaded the autumnal population over almost all host trees on which the aphid population had disappeared in the previous summer. Though the aphid is autoecious, some individuals on such trees that shed their leaves before mid‐November had to disperse to other trees in order to produce oviparae after mid‐November. Many winged females reproduced offspring on the trees whose foliage was orange‐yellow in hue, but while only a few reproduced on the trees whose foliage was red. The autumnal color of maple trees is an important factor which keeps some maple trees free from aphid infestation in autumn.

Phoretic mites and their hyperphoretic fungi associated with flying Ips typographus japonicus Niijima (Col., Scolytidae) in Japan

Flying Ips typographus japonicus from Hokkaido (Japan) carried 12 species of phoretic mites, three of which were not previously recorded in Europe. The mite biologies were diverse, including specialists feeding on microorganisms, beetle eggs, and nematodes which were common under beetle elytra. Hyperphoretic on these mites were seven distinct species of fungal spores, plus an undetermined number identifiable only as conidia. The spores stuck anywhere on the mite bodies with no special carrying structures evident. Ophiostoma bicolor was the most common species, with the pathogenic Ceratocystis polonicum present in small numbers.

A comparison of endemic and epidemic populations of the spruce beetle (Ips typographus japonicus Niijima) in Hokkaido

Ips typographus japonicus is the most devastating pest of Picea jezoensis and P. glehnii in Japan. In endemic condition, egg gallery density under bark seemed to have an upper limit at about 200–300. In outbreaks after a strong typhoon, beetles produced in windfalls attacked and killed trees. In those beetle‐killed trees, egg gallery density reached a high level of 700–1100 and both body size and reproductive rate of the population was remarkably reduced. The change in the upper limit of attack density, which must be caused by the behavioural changes of free‐living adult beetles at high density levels, seems to play an important part in bringing the population back down to an endemic level.

Effect of natural enemy exclusion on mortality of Ips typographus japonicus Niijima (Col., Scolytidae) in Hokkaido, Japan

The impact of natural enemies on the reproduction and survival of Ips typographus japonicus Niijima in Hokkaido was tested by sequentially caging off cohorts of a single beetle generation so as to investigate the effects of natural enemies at different developmental stages. A consistent increase in mortality of between 17–18% in the first 2–3 weeks of exposure to natural enemies was observed, after which mortality remained relatively constant. Natural enemies which were found to occur in logs exposed during this time were the predators Medetera sp., Thanasimus substriatus, and the adult parasitoid Tomicobia watanabei. Of these, only the first two were found to be significantly associated with beetle brood mortality.

The effect of host tree on the natural enemy complex of Ips typographus japonicus Niijima (Col., Scolytidae) in Hokkaido, Japan

Abstract: The effect of host tree on the ecology of the insect natural enemy complex of Ips typographus was investigated. Tree species was found to influence parasitoid composition and abundance, with pteromalids being more abundant in Picea jezoensis than in P. abies. Within trees, bark texture had a significant influence on pteromalid density with higher densities occurring in smooth bark. Bark hardness was also shown to affect density of both pteromalid and braconid parasitoids. Pteromalid parasitoids were found to respond density dependently to host density. Predators showed no consistent response to host tree. Only the clerid predator Thanasimus substratus showed a density‐dependent response to prey density. A predatory dolichopodid fly, Medetera sp., was shown to be a significant mortality factor for Ips typographus larvae. These results are compared to those of previous studies and their relevance to biological control commented upon.

Effect of tree host and beetle density on reproduction and survival of Ips typographus japonicus Niijima (Col., Scolytidae) in Hokkaido, Japan

Reproduction and survival of I. typographus japonicus at non‐outbreak densities in relation to two tree species, its natural host, Picea jezoensis, and an introduced host, Picea abies, was investigated in 1990 and 1991 in the natural mixed forest zone of Hokkaido, Japan. Beetles did not discriminate between logs of the two species during the attack phase, however, female beetles produced significantly longer egg galleries in P. jezoensis logs than in P. abies logs, particularly in 1990. Regression analysis showed that egg galleries were on average 2 cm longer in P. jezoensis in both years. A consistent trend toward higher larval survival in P. jezoensis logs was also observed. It is postulated that these differences were caused by a combination of differences in tree ages used in the studies and therefore their physiological status and inherent differences in the suitability of each tree species as breeding resource. Intraspecific density dependent effects on reproduction and survival were similar to those previously reported for outbreak or post‐outbreak populations, with the strongest effects being on reductions in egg gallery length and larval survival as beetle density increased. These factors were important even at the relatively low densities found in this study. The relevance of these results to the management of I. typographus in the spruce forests of Japan and the importance of native insects exploiting non‐native host plants as new resources is briefly discussed.

Annual alternating population size of the thuja aphid, Cinara tujafilina (Del Guercio), and the impacts of syrphids and disease

The thuja aphid, Cinara tujafilina (Del Guercio), is anholocyclic and lays larvae parthenogenetically throughout the winter in Tokyo. Aphids were reared individually in the outdoors from autumn to spring. And population dynamics in the field were observed for five years. Theoretical developmental threshold and reproductive threshold were estimated to be at 9.2 and 3.5 °C. The aphids born in winter required two months to develop and continued to lay larvae for 2–4 months. They had smaller intrinsic rates of increase but had greater longevity and laid more larvae than those born in spring. Thus, aphid numbers increased from autumn to winter at a fixed rate, and the larger populations in autumn were followed by higher peaks the next spring. At this latter time, the combined impact of syrphids and disease caused by Conidiobolus sp. greatly reduced the aphid populations. These populations thus had tended to have alternate abundant and scarce years. When syrphids were removed, however, a high peak number was realized every spring. Therefore, syrphids, which showed an aggregative response to large aphid populations, were shown to be responsible for the alternate abundant and scarce aphid cycles. Large number of aphids infested the same trees for at least three years, and host trees did not seem to have played an important role in the dynamics of the aphid populations. Disease epidemics seemed to kill the aphids before they caused severe adverse effects to host trees.

Between-tree distance and spread of the Sakhalin fir aphid (Cinara todocola Inouye) (Hom., Aphididae) within a plantation

The Sakhalin fir aphid, Cinara todocola, is one of the most destructive pests in young plantations of Sakhalin fir (Abies sachalinensis) in Hokkaido. The effects of increasing between‐tree distance on the spread of aphids within a plantation due to dispersal of wingless aphids were examined experimentally. It was found that increased spacing prevented/delayed the spread of the aphids and decreased the percentage of trees infested. Tree mortality was greater in plots with a high percentage of infested trees. About 85% of the trees heavily infested in summer died, with younger trees being more susceptible to death than older trees. High mortality was also thought to be the result of the combined effects of aphid infestation and competition among trees. More widely spaced planting may therefore be an alternative to parasitoid release in the integrated control system proposed by Yamaguchi and Takai (1977).

Seasonal difference in the abundance of the spruce beetle (Ips typographus japonicus Niijima) (Col., Scolytidae) within and outside forest in a bivoltine area

Abstract: In Hokkaido (Japan), the spruce beetle (Ips typograpaus japonicus) kills standing spruce trees after selection cutting. The seasonal change in the number of beetles captured in 18–21 traps within or outside forest was analysed, with reference to a control strategy for this species in a bivoltine area. The number of beetles captured in traps had two peaks: one, which was small, in early June, and the other, which was a large peak, in July. However, the number of beetles captured in traps outside the forest had only one small peak, in June. The difference between the inside and outside of the forest was caused by the difference in the number of newly emerged beetles captured in traps. Beetles were distributed contagiously within the forest and there was no remarkable change in distribution pattern throughout seasons. Beetles were distributed at random in June when beetle activity was high outside forest. Overwintered beetles must undertake long‐range dispersal irrespective of whether they are within or outside of forest for finding suitable breeding materials. Nevertheless, the most important point is that newly emerged beetles stayed inside the forest and only a few individuals dispersed outside the forest. This study recommends that different control methods are adopted according to the seasons for the control of spruce beetles.

Amounts of favourable feeding materials in spring for the maple aphid, Periphyllus californiensis Shinji, estimated from the phenological relations between the aphid and host trees

Phenological relationship between the maple aphid (Periphyllus californiensis Shinji) and 2 maple species, and population dynamics of the aphid on the maple trees were studied in spring. It was revealed that 1. most shoots of the trees were shorter than 10 cm and put forth 2–4 leaves and only a few shoots were longer than that and put forth more leaves; 2. stem mothers laid normal larvae on unfurling leaves of A. palmatum for about 9 days period after the leaves emerged; 3. winged females laid normal larvae on the leaves of A. amoenum for about 6 days period after the leaves emerged, and on inflorescences for about 8 days period from 3–11 days after the inflorescences emerged; 4. winged females on fully expanded leaves laid aestivating dimorphs.