Host: The Japanese Forestry Society
In the agamic generation of a cynipid wasp, Aphelonyx glanduliferae, variation in the survivorship among 5 years (on a Quercus serrata tree, T1) and refuge from parasitoid attack in relation to gall size was examined. Furthermore, the survivorship of the cynipid was compared among 7 trees (T1-T7), although analyses were based on data sets of 4 tree categories: T1, T2, T3, and others.
On T1, less than 2% of the cynipid survived in the latter 4 years. Gall density on 11 Q. serrata trees around T1 (within 50 m in distance) was conspicuously lower than that on T1, suggesting that the cynipid adults scarcely immigrate from other trees to T1. Thus, the survival rate on T1 should be usually lower than 2%. The survivorship of the cynipid was mostly affected by parasitoids. Although gall abortion and disease was also main causes of death of the cynipids in some years, the mortality rates by these factors varied among years. Therefore, refuge from parasitoid attack is likely to be a factor of critical importance in maintaining population levels of the cynipid. More cynipids survived parasitoid attack in larger galls, indicating that large galls provide refuge from parasitoid attack. The proportion of the cynipids that had survived parasitoids was not significantly affected by the statistical interaction between year and gall size. This suggests that the effect of gall size on the refuge from parasitoid attack is relatively fixed among years.
The survival rate of the cynipids did not differ among trees. However, the main mortality factors differed among trees. On T1 and T3, ca. 80% of the cynipid had been attacked by parasitoids. On T2, 75% of the cynipid had died of gall abortion.
Gall abortion of the agamic generation of the cynipid is most likely to be caused by the degradation in the quality of host plant tissues. The differences in the mortality by gall abortion both among years and among trees would be induced by the variation in the quality of host plant tissues. Gall size, which showed the relatively fixed effect on the refuge from parasitoid attack, are known to be regulated by host plant traits. Therefore, if the size of the cynipid galls varies among trees, this could also induce variations in the survivorship of the cynipid.
