150
a. R. POTTS AND G . P. VICEERMAN
the Aphidiinae, which are primary parasites of aphids (see p. 139). The
remainder mostly belonged to the Alyssinae, which are primary
parasites of small Diptera. About 0.5% of the Braconidae were apterous.
One of the most numerous apterous Braconidae was Gelis sp.; this is
thought t o be a parasite of cocoons of other Hymenoptera and of the
eggs of spiders.
Thenext most important group, the Chalcidoidea, included many aphid
hyperparasites. The Ichneumonidae were generally scarce, probably
because they are mostly primary parasites of the Lepidoptera. Some
species are, however, parasites of Coleoptera, Tenthredinidae and
Araneae.
Formicoidea were usually found near grazed downland pasture and
these were mainly Lasius flawus (F.) (Potts, 1970a).
2. Parasite-host relationships
The investigations on aphid populations described earlier in this
paper concluded that parasitism was not very important in our study
area in 1970 and 1971. However, the general biological significance
of the other parasites in the cereal arthropod community requires
study.
The total number of parasites in the samples from cereal crops in
1971 was compared with the number of other insects (excluding
Collembola) ( = “hosts”). There was a clear and significant correlation
(r = +0-63, P < O . O O l ) (Fig. 15). The number of parasites increased
disproportionately to the number of hosts, and at high-host densities
the number of parasites approached the number of hosts. The biological significance of such a general relationship is not clear. However,
preliminary work suggests that there are important relationships
between the Diptera and their parasites. These are currently being
investigated and some scattered records that parasitism may be, under
certain conditions, important in the determination of numbers of certain
Diptera are given here.
The hymenopterous parasites of Oscinella frit L. have been described
by Imms (1930, 1932), Simmonds (1952) and Bhattacharyya (1957).
Estimates of parasitism of the tiller generation of 0. frit on oats
(Southwood and Jepson, 1962) ranged from 1.5-18.0%, and of the
panicle generation from 0-2-0-9%. According to Jones (1968) the
panicle generation of 0. frit is out of phase with the life histories of the
parasites. Although parasitism of the tiller and panicle generations
appeared to be unimportant, Bhattacharyya (1957) recorded a level
of parasitism of 30.3% in the overwintering generation. In the winter
1971-1972 the level of parasitism of the surviving larvae (12%) of the
overwintering generation of 0. frit, in a crop of winter wheat (var.
a. R. POTTS AND G . P. VICEERMAN
the Aphidiinae, which are primary parasites of aphids (see p. 139). The
remainder mostly belonged to the Alyssinae, which are primary
parasites of small Diptera. About 0.5% of the Braconidae were apterous.
One of the most numerous apterous Braconidae was Gelis sp.; this is
thought t o be a parasite of cocoons of other Hymenoptera and of the
eggs of spiders.
Thenext most important group, the Chalcidoidea, included many aphid
hyperparasites. The Ichneumonidae were generally scarce, probably
because they are mostly primary parasites of the Lepidoptera. Some
species are, however, parasites of Coleoptera, Tenthredinidae and
Araneae.
Formicoidea were usually found near grazed downland pasture and
these were mainly Lasius flawus (F.) (Potts, 1970a).
2. Parasite-host relationships
The investigations on aphid populations described earlier in this
paper concluded that parasitism was not very important in our study
area in 1970 and 1971. However, the general biological significance
of the other parasites in the cereal arthropod community requires
study.
The total number of parasites in the samples from cereal crops in
1971 was compared with the number of other insects (excluding
Collembola) ( = “hosts”). There was a clear and significant correlation
(r = +0-63, P < O . O O l ) (Fig. 15). The number of parasites increased
disproportionately to the number of hosts, and at high-host densities
the number of parasites approached the number of hosts. The biological significance of such a general relationship is not clear. However,
preliminary work suggests that there are important relationships
between the Diptera and their parasites. These are currently being
investigated and some scattered records that parasitism may be, under
certain conditions, important in the determination of numbers of certain
Diptera are given here.
The hymenopterous parasites of Oscinella frit L. have been described
by Imms (1930, 1932), Simmonds (1952) and Bhattacharyya (1957).
Estimates of parasitism of the tiller generation of 0. frit on oats
(Southwood and Jepson, 1962) ranged from 1.5-18.0%, and of the
panicle generation from 0-2-0-9%. According to Jones (1968) the
panicle generation of 0. frit is out of phase with the life histories of the
parasites. Although parasitism of the tiller and panicle generations
appeared to be unimportant, Bhattacharyya (1957) recorded a level
of parasitism of 30.3% in the overwintering generation. In the winter
1971-1972 the level of parasitism of the surviving larvae (12%) of the
overwintering generation of 0. frit, in a crop of winter wheat (var.