ANALYSIS O F PROCESSES I N CONTROL O F INSECTS
35
c
3
-0
-
'Or
1927
I
I
I
I
I
1
c
1955
e54
Adults per armful of plant, in autumn
FIG. 15. Estimated mortality of adult broom beetles from autumn to following spring,
in relation to their autumn population density. Data from Richarda and Waloff (1961).
4 . By Artijicial Reduction or Exclusion of a Regulatory Factor
If a particular factor is thought to play a part in regulating the
abundance of a population, this can often be tested by eliminating the
factor or reducing it, and assessing the consequent change in numbers.
Entomologists at Riverside, California, have developed this approach
for evaluating the effect of natural enemies on scale insects and mites
infesting fruit trees (DeBach et al., 1949, 1950, 1951, DeBach 1955,
1958). The methods used have included (i) enveloping small branches
in fine cloth sleeves, some open at the end, others closed to exclude
parasites, (ii) local treatment with a selective insecticide that will kill
the natural enemies and leave the host population little affected (see
also Huffaker and Spitzer, 1951), and (iii) allowing ants, artificially
fostered if necessary, to have access to certain trees where they greatly
reduce the effectiveness of natural enemies. The third method has
recently been used by Banks (1962) in experiments with Aphis fabae
Scop. on potted bean plants in cages. These were placed near nests of
the ant Lasius niger in a garden, but the ants were excluded from some
of the plants. He demonstrated that the ants drove away most predators, while the colonies of aphids not protected in this way were
eliminated or had their numbers persistently restricted by the predators.
Lack (1954) gives an account of some analagous experiments with
vertebrates.
D. INTERPRETING EFFECTS O F SUCCESSIVE MORTALITIES
A comparative study of the numbers in a population killed by
different factors is only a beginning in the investigation of its population dynamics, and, by itself, readily misleading. The building up of
35
c
3
-0
-
'Or
1927
I
I
I
I
I
1
c
1955
e54
Adults per armful of plant, in autumn
FIG. 15. Estimated mortality of adult broom beetles from autumn to following spring,
in relation to their autumn population density. Data from Richarda and Waloff (1961).
4 . By Artijicial Reduction or Exclusion of a Regulatory Factor
If a particular factor is thought to play a part in regulating the
abundance of a population, this can often be tested by eliminating the
factor or reducing it, and assessing the consequent change in numbers.
Entomologists at Riverside, California, have developed this approach
for evaluating the effect of natural enemies on scale insects and mites
infesting fruit trees (DeBach et al., 1949, 1950, 1951, DeBach 1955,
1958). The methods used have included (i) enveloping small branches
in fine cloth sleeves, some open at the end, others closed to exclude
parasites, (ii) local treatment with a selective insecticide that will kill
the natural enemies and leave the host population little affected (see
also Huffaker and Spitzer, 1951), and (iii) allowing ants, artificially
fostered if necessary, to have access to certain trees where they greatly
reduce the effectiveness of natural enemies. The third method has
recently been used by Banks (1962) in experiments with Aphis fabae
Scop. on potted bean plants in cages. These were placed near nests of
the ant Lasius niger in a garden, but the ants were excluded from some
of the plants. He demonstrated that the ants drove away most predators, while the colonies of aphids not protected in this way were
eliminated or had their numbers persistently restricted by the predators.
Lack (1954) gives an account of some analagous experiments with
vertebrates.
D. INTERPRETING EFFECTS O F SUCCESSIVE MORTALITIES
A comparative study of the numbers in a population killed by
different factors is only a beginning in the investigation of its population dynamics, and, by itself, readily misleading. The building up of