コノテガシワオオアブラムシ Cinara tujafilina (del Guercio)の夏期における低密度個体群の解析
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In the nursery of University Forest Experiment Station at Tanashi, population density of the thuja aphid, Cinara tujafilina (del Guercio) increases from autumn to the next spring, then decreases abruptly and is kept at low levels throughout the summer season. To know the factors which keeps this aphid's density at low levels in the summer, (1) fertility of wingless females on potted seedlings of Thuja orientalis was estimated in the outdoors, and (2) the effects of natural enemies on the aphid population was analyzed in the field. In the outdoors, a negative correlation was found between the mean air temperature and the number of larvae laid per female (r=-0.998). The number of larvae laid per female was very small in the summer; few aphid laid larvae during the two months, August and September. In the field, three study plots 1-3 (each had 32 T. orientalis of about 25 cm high) were established, and the aphid numbers in the plots were estimated once a month from April 1984 to December 1985. The distances between the plots were about 5-15 m. Syrphid larvae were removed from the two plots 2 and 3 every week after 5 April 1985 (Table 3). All trees of plot 3 were covered with a sleeving cage (2 mm mesh) after 23 April of 1985, and dispersal of the aphids was restricted within the plot. The populations in the three plots showed similar trends until April 1985. However, the situation changed when the removal of syrphid larvae began. The densities of the aphids in the two plots 2 and 3 increased in May of 1985 and then decreased again in July. Large number of syrphid larvae were captured on 5 April when the first removing was done. The heavy decreasing in aphid density from March to April was considered to have been caused by these syrphids. No natural enemies except spiders and ants were found on the trees for eight months from May to December of 1985. In plot 3, where the trees were covered with the cage, aphids were exterminated in the summer. It was clear that the decreasing in aphid density in the summer was due not to moving of the aphids from foliage to root nor to the predation by syrphids or lady beetles, but to the decrease in fertility of the aphid population.
- 東京大学大学院農学生命科学研究科附属演習林,The Tokyo University Forests,東京大学農学部林学科,Department of Forestry, Faculty of Agriculture, University of Tokyoの論文
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