ANALYSIS O F PROCESSES IN CONTROL O F INSECTS
11
increase of x 45 over the succeeding 4 years, averaging x 2.9 per annum
(Gunn, 1960). Gunn and Symmons (Zoc. cit.) commented: “if . . . a
population exists in generally favourable conditions, such as outbreak
areas are now supposed to provide, and unfavourable factors are few
and powerful, then large fluctuations may be expected.” Glasgow and
Welch (1962) presented estimates of the annual abundance of ‘an insect
of unusually low fecundity, the tsetse fly Glossina swynnertoni Aust.,
in an area of 40-50 sq. miles of thorn bush in the Shinyanga District of
Tanganyika. In the first 5 years the range of the annual estimates was
approximately 4.5-fold, and in the following 5-year periods it was about
2.1-, 4.2- and 3.2-fold. Taking 10- instead of 5-year periods, the fluctuation was about 18-fold in the first period and 5.4-fold in the second.
Taking all together as a single 20-year period, the range is just over
18-fold. I have divided the run of years as above in order t o illustrate
the point that the amplitude of fluctuation depends partly on the
number of years or generations covered, and to provide a basis for comparing the values for Glossina with those quoted below, for some other
insects.
Richards and Waloff (1961) estimated the numbers of the beetle
Phytodecta olivacea on a more or less isolated patch of about two acres of
the leguminous shrub, broom (Sarothumnus scoparius), in southern
England. Over a period of 5 years, the estimated numbers of adults in
spring (some of them a year older than the others) ranged from 17 027
in the second year to 4 000 in the fifth, i.e. a 4.3-fold range of fluctuation.
For a number of species of oak-feeding caterpillars in Wytham Wood,
near Oxford, Varley and Gradwell (1963) have illustrated annual
estimates of population density for the years 1949-62. Over the first
10 of these years, their record is complete for 9 species. I have measured
the approximate ranges of fluctuation from their graph, and converted
the values from their logarithmic to a geometric scale. In the first
5 years, the values range from nearly x 3 (for Eucosma isertana) to
x 25 (for the winter moth Operophtera brumata). In the second 5 years,
they range from about x 2 (for Erannis defoliaria) to x 10 for Cosrnia
trapezina). Taking the two together as one 10-year period, the values
range from about x 3-4 (for Eucosma) to x 25 (for Operophtera).
Klomp (1962) shows graphically the annual density of caterpillars
of six species of Lepidoptera living in the foliage in a forest of Scots
pine in the Netherlands (Fig. 5). In the first 5 years of his 10-year period,
the approximate range of fluctuation varied from x2.9 (Panolis) to
x 28 (Thera), and in the second 5 years from x 5.3 (Thera) to x 33
(Ellopia). Taking the 10 years as one period, the range of fluctuation
varied from x 16 (PanoZis) to x 95 (Eupithecia). The similarity to the
11
increase of x 45 over the succeeding 4 years, averaging x 2.9 per annum
(Gunn, 1960). Gunn and Symmons (Zoc. cit.) commented: “if . . . a
population exists in generally favourable conditions, such as outbreak
areas are now supposed to provide, and unfavourable factors are few
and powerful, then large fluctuations may be expected.” Glasgow and
Welch (1962) presented estimates of the annual abundance of ‘an insect
of unusually low fecundity, the tsetse fly Glossina swynnertoni Aust.,
in an area of 40-50 sq. miles of thorn bush in the Shinyanga District of
Tanganyika. In the first 5 years the range of the annual estimates was
approximately 4.5-fold, and in the following 5-year periods it was about
2.1-, 4.2- and 3.2-fold. Taking 10- instead of 5-year periods, the fluctuation was about 18-fold in the first period and 5.4-fold in the second.
Taking all together as a single 20-year period, the range is just over
18-fold. I have divided the run of years as above in order t o illustrate
the point that the amplitude of fluctuation depends partly on the
number of years or generations covered, and to provide a basis for comparing the values for Glossina with those quoted below, for some other
insects.
Richards and Waloff (1961) estimated the numbers of the beetle
Phytodecta olivacea on a more or less isolated patch of about two acres of
the leguminous shrub, broom (Sarothumnus scoparius), in southern
England. Over a period of 5 years, the estimated numbers of adults in
spring (some of them a year older than the others) ranged from 17 027
in the second year to 4 000 in the fifth, i.e. a 4.3-fold range of fluctuation.
For a number of species of oak-feeding caterpillars in Wytham Wood,
near Oxford, Varley and Gradwell (1963) have illustrated annual
estimates of population density for the years 1949-62. Over the first
10 of these years, their record is complete for 9 species. I have measured
the approximate ranges of fluctuation from their graph, and converted
the values from their logarithmic to a geometric scale. In the first
5 years, the values range from nearly x 3 (for Eucosma isertana) to
x 25 (for the winter moth Operophtera brumata). In the second 5 years,
they range from about x 2 (for Erannis defoliaria) to x 10 for Cosrnia
trapezina). Taking the two together as one 10-year period, the values
range from about x 3-4 (for Eucosma) to x 25 (for Operophtera).
Klomp (1962) shows graphically the annual density of caterpillars
of six species of Lepidoptera living in the foliage in a forest of Scots
pine in the Netherlands (Fig. 5). In the first 5 years of his 10-year period,
the approximate range of fluctuation varied from x2.9 (Panolis) to
x 28 (Thera), and in the second 5 years from x 5.3 (Thera) to x 33
(Ellopia). Taking the 10 years as one period, the range of fluctuation
varied from x 16 (PanoZis) to x 95 (Eupithecia). The similarity to the