STUDIES ON THE CEREAL ECOSYSTEM
139
frequency of predatory insects in complex communities. The relationship between the proportion of insects which are predatory in samples
from cereal fields, and the faunal diversity indices of these fields was
therefore examined (see Fig. 12). There was a significant correlation
(r = + 0.66, P < 0.001) between a and the proportion of arthropods
which were predatory. Although many of these predators are known to
feed on aphids (see pp. 160-161), the feeding habits of some are not
yet known and we could not calculate the relationship between a and
the probable predator pressure on aphids. Nevertheless, high a values
seem likely to be associated with high aphid predation and this is now
being investigated. It is already clear that the classic aphid predators
such as the Coccinellidae, Syrphidae and Neuroptera were not generally
sufficiently numerous to be important in this respect (see pp. 131, 143
and 167).
7. Parasitism
Parasitism of the apterous aphids, mostly by Braconidae of the tribe
Aphidiinae, was generally low and, more important, not clearly
related to density. I n 1971 parasitism was about twice as high as in
1970, but still only 10% of the peak number of apterae. The mean
level of parasitism expressed in relation to the maximum number of
non-parasitized individuals did not vary significantly between peak
apterae densities of 100/m2 and 1600/m2. A considerable amount of
hyperparasitism by Chalcidae was noted but not quantified. The level
of parasitism did not vary consistently over the study area, was not
dearly associated with farming regimes, and over the three-year
period 1969-1971 did not appear important in the determination of
aphid numbers. However, this may not be the case in other years and
other areas, and does not mean that parasitism in cereal virus carrying
Hemiptera is not important. A high level of parasitism was noted in
the leaf hopper Javesella pellucidu (F.). I n 1970 13% of the adult
females were prevented from laying eggs by the Strepsipteran Elenchus
tenuicornis (Kirby) and much higher levels (up to 60-70%) have been
noted in Finland (Kanervo et al., 1967).
c. D I P T E R A
1. Introduction
In common with other insect groups, little is known about the
dipterous fauna of cereal crops. The few studies that have been made
were mainly concerned with pest species and their associated pamsites
and predators, and not with the complex as a whole. Consequently, in
this country particular attention has been paid to the cereal stem
borers (Oscinella frit L. and Leptohylemyia carrctata (Fall.)), leather
139
frequency of predatory insects in complex communities. The relationship between the proportion of insects which are predatory in samples
from cereal fields, and the faunal diversity indices of these fields was
therefore examined (see Fig. 12). There was a significant correlation
(r = + 0.66, P < 0.001) between a and the proportion of arthropods
which were predatory. Although many of these predators are known to
feed on aphids (see pp. 160-161), the feeding habits of some are not
yet known and we could not calculate the relationship between a and
the probable predator pressure on aphids. Nevertheless, high a values
seem likely to be associated with high aphid predation and this is now
being investigated. It is already clear that the classic aphid predators
such as the Coccinellidae, Syrphidae and Neuroptera were not generally
sufficiently numerous to be important in this respect (see pp. 131, 143
and 167).
7. Parasitism
Parasitism of the apterous aphids, mostly by Braconidae of the tribe
Aphidiinae, was generally low and, more important, not clearly
related to density. I n 1971 parasitism was about twice as high as in
1970, but still only 10% of the peak number of apterae. The mean
level of parasitism expressed in relation to the maximum number of
non-parasitized individuals did not vary significantly between peak
apterae densities of 100/m2 and 1600/m2. A considerable amount of
hyperparasitism by Chalcidae was noted but not quantified. The level
of parasitism did not vary consistently over the study area, was not
dearly associated with farming regimes, and over the three-year
period 1969-1971 did not appear important in the determination of
aphid numbers. However, this may not be the case in other years and
other areas, and does not mean that parasitism in cereal virus carrying
Hemiptera is not important. A high level of parasitism was noted in
the leaf hopper Javesella pellucidu (F.). I n 1970 13% of the adult
females were prevented from laying eggs by the Strepsipteran Elenchus
tenuicornis (Kirby) and much higher levels (up to 60-70%) have been
noted in Finland (Kanervo et al., 1967).
c. D I P T E R A
1. Introduction
In common with other insect groups, little is known about the
dipterous fauna of cereal crops. The few studies that have been made
were mainly concerned with pest species and their associated pamsites
and predators, and not with the complex as a whole. Consequently, in
this country particular attention has been paid to the cereal stem
borers (Oscinella frit L. and Leptohylemyia carrctata (Fall.)), leather