The prevention of insect pest injury to potatoes is always an important but difficult problem. As pointed out by the author in the preceding article (HUKUSIMA, 1956), insect pests were twice as abundant in monocrop field of potato as in intercropped field of potato and corn. Comparing the seasonal change of insect communities in these fields, it became apparent that the community of the intercropped field attained to a stable state earlier than that of the monocrop field and was poor in number. A few works have been done so far by several authors on the relationship of intercropping to insect populations, and it was reported that intercropping maintains insect population at low level. This decrease in population in intercropped field convinced that the tall intercropping plants which play an important role as a partition-wall are conducive mainly of a higher rate of prevention in the migration by small-sized insects. Except the large 28-spotted lady beetle (Epilachna vigintioctomaculata MOTSCHULSKY) which causes extensive damage to potato leaves, the insect fauna of the potato field is composed principally of small-aphids such as Aphis gossypii GLOVER, Aulacorthum matsumuraeanum HORI and Myzus persicae SULZER which widely distribute in northern Japan and the onion thrips (Thrips tabaci LINDEMAN). The high intercropped crops seem to serve to prevent the migration of these small insects. The work reported in this paper was undertaken with an aim to draw some definite conclusions on the causes which induce an increase or decrease of certain insects and to treat the difference between the insect communities of the potato field and the potatocorn intercropped field.<br>In 1956, the observations were made on two adjacent plots 6 by 12 feet in size. Potatoes and corns were sown on April 29th ridging in eastwest direction and with 75cm wide. The first sprouting was observed on May 20th. Insects were collected by hand from June 4th on 27 potato and 47 corn plants when both plants were 20cm in height in order to obtain the relative population densities in each plot. Emulsion of malathion was sprayed to clear the population off on July 2nd and 3rd when potato and corn plants attained to 57cm and 50cm in height respectively. After this treatment counts of insects were continued at weekly intervals to compare the faunistic compositions and their seasonal successions with those before the application of insecticides.<br>In 1957, about 0.1 acre was used under the similar design as in 1956. The field was divided into six equal sections of one twentieth acre, and these sections were used from northern one for potato alone, corn alone, potato alone, potato intercropping with corn, potato alone and potato intercropping with corn. Both potatoes and corns were sown on April 30th in each plot on six ridges in each plot. The first sprout was observed on May 22nd in this year. Insects were collected by hand periodically from June 2nd from the plants on four ridges in each plot. When potato plant attained to 45cm and corn 62cm in height, emulsion of BHC was sprayed on June 18th. After this treatment counts of insects were made at weekly intervals for the same purpose as in 1956. Observations were made twice on July 24th and on August 2nd on temperature, light, solar radiation and wind velocity between ridges at a height of 30cm above the ground-surface in each plot.<br>The counts made regularly during the entire period of observations of insects in potato fields under different cultural practices are shown in Tables 1 and 2. While both crops remained still low, the first appearance of insect was observed approximately at the same time in both monocrop and intercropped fields, and no significant difference in the increase of insects was found between potato and corn plants. As corn plant grew, however, this difference became significantly apparent in the two years studied, and as can be seen from the following rates