44
M. E . SOLOMON
c. FUNCTIONAL RESPONSES OF SOME INSECT PARASITES
Holling (1959b) went on to examine some published examples of the
functional responses of insect parasites to rises in host density. These
were from experiments with various Hymenopterous parasites, namely
Dahlbominus fuscipennis (Zett.) searching for cocoons of the pine sawfly
in laboratory cages (Burnett, 1951), on a lawn (Burnett, 1954), and
in a plot of woodland (Burnett, 1958d); Chelonw texanus Cress. searching for eggs of the flour moth Anagtgasta kuehniella (Zell.) (Ullyett,
1949a); Cryptus inornatus Pratt searching for cocoons of the beet webworm Loxostege sticticalis (L.) (Ullyett, 1949b) ; and Mormoniella
vitripennis (Walk.) searching for puparia of the housefly (DeBach and
Smith, 1941b); this last experiment, and Ullyett’s, were in the laboratory. In every case, the graph of the number of affected hosts against
host density, illustrated in Holling’s paper, was of the same general
form as Fig. 17. In a later paper, Holling (1961) cited two further
examples of the same sort, one unpublished, the other from the work
of Miller (1959, 1960) on two parasites of the spruce budworm, and
one could now add the experimental findings of Chant (1961) with the
phytophagous mite Tetranychus tehrius and a predatory mite (a
Typhlodromus). Varley and Edwards (1957) reinterpreted the highly
artificial experiments of DeBach and Smith (1941a,b, 1947) taking
account of the way in which the parasite’s behaviour and physiology
influence its response to availability of hosts. It can be inferred from
their conclusions that under more natural conditions the functional
response curve would probably have an early density-dependent phase
before assuming the form shown in Holling’s diagram.
On the whole, however, the adherence of insect parasites to the basic
curve of functional response, concave below, is impressive. We may
conclude that there is a general tendency for the number of hosts
affected to increase in less than linear proportion to the total number
or density of hosts. This is an inverse density relationship, one that
cannot by itself lead to regulation. When host density increases, percentage parasitism will generally decline, until perhaps the functional
response is followed by a delayed numerical response. The same
principles apply to the action of predators: up to a certain limit, they
can be expected to take more prey as prey density rises, nevertheless,
other things being equal (including the numbers of predators), the
percentage taken will be smaller.
D. MAMMALIAN PREDATORS OF THE P I N E SAWFLY
It might be expected that vertebrate predators would show more
complex responses to changes in the density of their prey than insects
M. E . SOLOMON
c. FUNCTIONAL RESPONSES OF SOME INSECT PARASITES
Holling (1959b) went on to examine some published examples of the
functional responses of insect parasites to rises in host density. These
were from experiments with various Hymenopterous parasites, namely
Dahlbominus fuscipennis (Zett.) searching for cocoons of the pine sawfly
in laboratory cages (Burnett, 1951), on a lawn (Burnett, 1954), and
in a plot of woodland (Burnett, 1958d); Chelonw texanus Cress. searching for eggs of the flour moth Anagtgasta kuehniella (Zell.) (Ullyett,
1949a); Cryptus inornatus Pratt searching for cocoons of the beet webworm Loxostege sticticalis (L.) (Ullyett, 1949b) ; and Mormoniella
vitripennis (Walk.) searching for puparia of the housefly (DeBach and
Smith, 1941b); this last experiment, and Ullyett’s, were in the laboratory. In every case, the graph of the number of affected hosts against
host density, illustrated in Holling’s paper, was of the same general
form as Fig. 17. In a later paper, Holling (1961) cited two further
examples of the same sort, one unpublished, the other from the work
of Miller (1959, 1960) on two parasites of the spruce budworm, and
one could now add the experimental findings of Chant (1961) with the
phytophagous mite Tetranychus tehrius and a predatory mite (a
Typhlodromus). Varley and Edwards (1957) reinterpreted the highly
artificial experiments of DeBach and Smith (1941a,b, 1947) taking
account of the way in which the parasite’s behaviour and physiology
influence its response to availability of hosts. It can be inferred from
their conclusions that under more natural conditions the functional
response curve would probably have an early density-dependent phase
before assuming the form shown in Holling’s diagram.
On the whole, however, the adherence of insect parasites to the basic
curve of functional response, concave below, is impressive. We may
conclude that there is a general tendency for the number of hosts
affected to increase in less than linear proportion to the total number
or density of hosts. This is an inverse density relationship, one that
cannot by itself lead to regulation. When host density increases, percentage parasitism will generally decline, until perhaps the functional
response is followed by a delayed numerical response. The same
principles apply to the action of predators: up to a certain limit, they
can be expected to take more prey as prey density rises, nevertheless,
other things being equal (including the numbers of predators), the
percentage taken will be smaller.
D. MAMMALIAN PREDATORS OF THE P I N E SAWFLY
It might be expected that vertebrate predators would show more
complex responses to changes in the density of their prey than insects