- 昆蟲 (ISSN:09155805)
- vol.41, no.2, pp.220-227, 1973-06-25
Some experiments and field observations were conducted in 1970 and 1971 in Ino, Kochi, to assess the effect of micryphantid spiders, mainly Oedothorax insecticeps, upon the larval colony of the tobacco cutworm, Spodoptera litura. In the taro field, the density of the micryphantid spider was high in the rainy season, i.e. June to early July, but it decreased rapidly in early July. The low density was maintained during the summer season, although the spider was very abundant in wet places overgrown with weeds. On the basis of the functional response of the spider, it was estimated that the maximum number of hatchlings which can be eaten by a spider was 96 individuals during the 1st instar. Survivorship curves of the spider-free larval colonies in cages of nylon gauze were compared with those of colonies exposed to the spider in the cages. Sixty and 18 per cent of mortality were observed by the next day of hatch in exposed and spider-free cages, respectively. Of 60 per cent mortality, only 4 per cent was accounted for by the actual predation which was assessed by the number of head capsules left on the taro leaf. Since 18 per cent of hatchlings were dead in spider-free cage, remainder, i.e. 60-18-4=38 per cent, was loss by larval dispersal out of the leaf induced by the spider's attack. Egg masses composed of various numbers (100-900) of eggs were placed on taro plants, and changes in number were followed during the 1st instar. In the period from late June to early July when micryphantid spiders were abundant, the survival rates of 1st instar larvae were 2-3% regardless of the number involved in the colony up to 900 individuals per colony. Taking consideration of the maximum number of prey could be eaten by a spider, i.e. 96 individuals, almost all of the hatchlings would be killed indirectly by the disturbing effect of spiders.