STUDIES ON THE CEREAL ECOSYSTEM
147
S . sterwrarium was therefore more likely to be found on those farms
in the study area which followed a leyfarming system; this was particularly so in 1972. In order to study this kind of effect in more detail, the
farms in the study area were divided into two categories: (1) those
which followed a ley farming system, and (2) those which either grew
cereals continuously or had adopted the newer break crops. The samples
obtained from the cereal fields were similarly separated into the two
categories and the total number of predatory Diptera in each sample
recorded. The frequency distribution of the number of predators per
sample is shown in Fig. 13.
More predatory Diptera were found in samples from cereal fields in
ley farming areas than in samples from farms growing continuous
cereals or adopting new break crops. This was true of each of the
families of predatory Diptera.
4. The relationship between numbers of predatory and non-predatory
Although many predatory Diptera are catholic in their taste, their
most important prey is probably other Diptera. The relationship
between the numbers of predatory and non-predatory Diptera was
therefore investigated in those samples taken in the middle of June
1971. For convenience, ley farming areas have been designated “ley”
and the other areas (continuous cereals etc.) “arable”, and the samples
were divided accordingly.
In “arable” areas there was a significant correlation (r = +0-33,
P < 0.05) between the number of predatory and non-predatory Diptera.
In the “ley” areas the correlation was significantly different (P < 0-05)
and negative (r = -0.06) (see Fig. 14).
These results suggest that under a ley farming system populations
of prey Diptera (including many pests) are balanced by dipterous
predators, whereas in areas which grow cereals continuously or which
have an arable system incorporating break crops such as beans,
kale, oil-seed rape etc., non-predatory populations are higher. Thus,
the predator:non-predator ratio was significantly higher (P < 0.01)
in the “ley” area (see Table XI).
It is relevant to note that Frydlewicz-Ciesielska (1961) also found that
there was an inverse correlation between the numbers of Dolichopodidae and Empididae and the numbers of phytophagous flies in meadows.
Fewer predators were found in his so-called artificial meadows (leys)
than in the natural meadows (permanent pasture).
These results suggest that the numbers of predatory Diptera can vary
considerably from area to area and that this will be reflected in the
stability of populations of prey Diptera.
Diptera
147
S . sterwrarium was therefore more likely to be found on those farms
in the study area which followed a leyfarming system; this was particularly so in 1972. In order to study this kind of effect in more detail, the
farms in the study area were divided into two categories: (1) those
which followed a ley farming system, and (2) those which either grew
cereals continuously or had adopted the newer break crops. The samples
obtained from the cereal fields were similarly separated into the two
categories and the total number of predatory Diptera in each sample
recorded. The frequency distribution of the number of predators per
sample is shown in Fig. 13.
More predatory Diptera were found in samples from cereal fields in
ley farming areas than in samples from farms growing continuous
cereals or adopting new break crops. This was true of each of the
families of predatory Diptera.
4. The relationship between numbers of predatory and non-predatory
Although many predatory Diptera are catholic in their taste, their
most important prey is probably other Diptera. The relationship
between the numbers of predatory and non-predatory Diptera was
therefore investigated in those samples taken in the middle of June
1971. For convenience, ley farming areas have been designated “ley”
and the other areas (continuous cereals etc.) “arable”, and the samples
were divided accordingly.
In “arable” areas there was a significant correlation (r = +0-33,
P < 0.05) between the number of predatory and non-predatory Diptera.
In the “ley” areas the correlation was significantly different (P < 0-05)
and negative (r = -0.06) (see Fig. 14).
These results suggest that under a ley farming system populations
of prey Diptera (including many pests) are balanced by dipterous
predators, whereas in areas which grow cereals continuously or which
have an arable system incorporating break crops such as beans,
kale, oil-seed rape etc., non-predatory populations are higher. Thus,
the predator:non-predator ratio was significantly higher (P < 0.01)
in the “ley” area (see Table XI).
It is relevant to note that Frydlewicz-Ciesielska (1961) also found that
there was an inverse correlation between the numbers of Dolichopodidae and Empididae and the numbers of phytophagous flies in meadows.
Fewer predators were found in his so-called artificial meadows (leys)
than in the natural meadows (permanent pasture).
These results suggest that the numbers of predatory Diptera can vary
considerably from area to area and that this will be reflected in the
stability of populations of prey Diptera.
Diptera