ANALYSIS O F PROCESSES I N CONTROL O F INSECTS
15
dependent on favourable conditions of rainfall and temperature, and
the population fluctuations caused by changes in these factors have been
demonstrated by earlier workers. But when Wallace made a close study
of small areas of field, he found local variations in abundance bearing
no relation to the prevailing moisture and temperature. There was a
changing mosaic of densities that could not be explained by migratory
movement. When he plotted densities early in the season (May) with
densities on the same patches late in the season (September), the points
were fitted quite well by a hyperbolic curve (Fig. 6). The coefficient of
10ol 0
1
,
I
1
0
50
100
150
200
Population density in Mow
FIG. 6. Relation between densities of lucerne flea populations at given p0htS early and
late i n the 1953 season. (From Wallace, 1957.)
correlation between the estimates of initial and final densities in the
year concerned (1953) was - 0-426 (significant at P < O - O O l ) . Wallace
remarked that “where densities exceeded approximately 20 per aquare
link in May they had decreased by September, and where they were
below this figure in May they had increased, there being only a few
minor departures from this relation. Also, in general, the greater the
departure from 20 in May, the greater was the departure, of opposite
sign, in September.’’ In observations over 4.successive years a significant
negative correlation was maintained between May and September
values, and the density mosaic was different each year, so that it could
not be related to soil differences. Later observations provided an
15
dependent on favourable conditions of rainfall and temperature, and
the population fluctuations caused by changes in these factors have been
demonstrated by earlier workers. But when Wallace made a close study
of small areas of field, he found local variations in abundance bearing
no relation to the prevailing moisture and temperature. There was a
changing mosaic of densities that could not be explained by migratory
movement. When he plotted densities early in the season (May) with
densities on the same patches late in the season (September), the points
were fitted quite well by a hyperbolic curve (Fig. 6). The coefficient of
10ol 0
1
,
I
1
0
50
100
150
200
Population density in Mow
FIG. 6. Relation between densities of lucerne flea populations at given p0htS early and
late i n the 1953 season. (From Wallace, 1957.)
correlation between the estimates of initial and final densities in the
year concerned (1953) was - 0-426 (significant at P < O - O O l ) . Wallace
remarked that “where densities exceeded approximately 20 per aquare
link in May they had decreased by September, and where they were
below this figure in May they had increased, there being only a few
minor departures from this relation. Also, in general, the greater the
departure from 20 in May, the greater was the departure, of opposite
sign, in September.’’ In observations over 4.successive years a significant
negative correlation was maintained between May and September
values, and the density mosaic was different each year, so that it could
not be related to soil differences. Later observations provided an