Abstract
Although they say the_recent outbreak of mite following by the use of new acaricides are generally attributed to the destruction of their natural enemies by those materials, little definite evidence has yet been presented to verify this supposition. Since the mite problem is quite serious in Hokkaido and since many unusual fluctuations in population of the pest had been observed, a field test was carried out in 1956 in the orchard of Hokkaido Agri ultural Experiment Station to ascertain the relationship between the reduction of their natural enemies and the increased number of European red mite.
For mite and predator experiment, five apple trees (of the same variety) were selected and each one of those sprayed with machine Oil, DDT, BHC and parathion respectively. Number of mite, egg, predator was counted under hand magnifier on 10 leaves per tree and population-data were based on the investigations which were made twice a week at intervals of three days.
The results of the test are shown in Figures 1, 2 and Table 1.
In the middle of July there was a heavy mite population in DDT plot and this continued until the middle of August. Then the leaves of the tree discoloured, because of severe mite injury and then there was a rapid fall in number of mite and egg. In September and October, an extremely small number of winter eggs were found on twigs of the tree.
Non-sprayed plot showed significantly lower population than DDT plot throughout summer, but it was observed that there were mites enough to cause foliage injury. In autumn a small number of winter egg was found in this plot.
On the contrary, in three plots where machine oil, BHC and parathion were used respectively, the population of mite and egg remained low from the beginning of July to the beginning of August and no visual discoloration of foliage was observed. But rapid increase in number of mites started in the middle of August in each and all of those three plots received heavy winter egg infestation.
Mite predators were very scarce in all plots until August, but in autumn Scymnus and others were counted in DDT and non-sprayed plots. It seems that this increase in number of predator came too late to have any appreciable effect on the summer mite population.
According to our observation, it is evident that the increase in number of mite in BHC, parathion and machine oil plots in autumn is mainly due to the fact that good condition of foliage was favorable for the mite, not from the decrease of their natural enemies caused by the application of those acaricides. The factors which are responsible for the great summer, increase of egg and mite in DDT plot are unknown.
