198
N. WALOFF
(years) and the two psyllid species in 5 generations. As in Phytdeiu,
the peaks of greatest abundance occurred on the bushea while they were
young. By 1967 more then half of the original bushee are dead or dying,
and even if some insect species will attain high densities again, it ia
improbable that their absolute numbers will be aa greet aa in the peak
years. Thk may not affect the abundance of the specia over a wide
area, as each year new breeding sites are colonised by dispersal by
flight. Many insect populations are fluid, but they am stebilieed in the
focal points formed by suitable breeding sites. Moreover, only in some
species on broom, e.g. in Bruchidius a & and in Piezodorus does the
whole adult population move out of the breeding site every sewon. In
many, a large part of the adult population remains in the original site
and in the extreme caees of species with polymorphic forms only the
alate individuals emigrate, while the brachypterours forms tend to stay
put. On broom this waa seen in aphids and the weevils, Sitona regensteinemis and Apion immune.
The cyclical nature of plant habitate was emphasized by the work of
Watt (1947), who described the gradual changes in the patterns of
plant communities, but the contributions of phytophagous insect species to these changes are not always fully appreciated. It is most probable that the total insect fauna on broom contributes to the weakening
of plant and its early death. Huffaker (1969) points out that in a new
country, in the absence of the usual insect fauna, many plants flourish
and may be checked when the fauna is intentionally introduced. In this
way, broom is flourishing on the Sierran foothills in California where
the fauna on it is poor in introduced species (Frick, 1964; Parker, 1964;
Waloff, 1966).
Many of the species show a preference for young bushes, or for the
current year’s green shoots, which tend to be denser on the younger
plants. There was evidence for this in Psyllidae (Watmough, 1963), in
Leucoptera (Agwu, 1967) and in Apion immune (Williams, 1966) which
was never found on old bushes. The dying broom is immediately attractive to two Rpecies of scolytid bark beetlea, which further weaken the
plant and precipitate its death. But on the whole, any succession that
occurs in this life system is mainly in the densities of populations on
young and old bushes rather than in their complete absence.
Although the changes within the habitat are of importance to the
phytophagous species, they are by no means the only factors that determine their abundance. However, it seems probable that aa the plants
get older they provide a less suitable diet whish reduces fecundity.
Laboratory experiments (Waloff and Richards, 1958; Donia, 1958) indicated that fecundity of Phytodecta wm lower on old than on young
broom, while in the field fecundity dropped by a factor of 2.4 w the
N. WALOFF
(years) and the two psyllid species in 5 generations. As in Phytdeiu,
the peaks of greatest abundance occurred on the bushea while they were
young. By 1967 more then half of the original bushee are dead or dying,
and even if some insect species will attain high densities again, it ia
improbable that their absolute numbers will be aa greet aa in the peak
years. Thk may not affect the abundance of the specia over a wide
area, as each year new breeding sites are colonised by dispersal by
flight. Many insect populations are fluid, but they am stebilieed in the
focal points formed by suitable breeding sites. Moreover, only in some
species on broom, e.g. in Bruchidius a & and in Piezodorus does the
whole adult population move out of the breeding site every sewon. In
many, a large part of the adult population remains in the original site
and in the extreme caees of species with polymorphic forms only the
alate individuals emigrate, while the brachypterours forms tend to stay
put. On broom this waa seen in aphids and the weevils, Sitona regensteinemis and Apion immune.
The cyclical nature of plant habitate was emphasized by the work of
Watt (1947), who described the gradual changes in the patterns of
plant communities, but the contributions of phytophagous insect species to these changes are not always fully appreciated. It is most probable that the total insect fauna on broom contributes to the weakening
of plant and its early death. Huffaker (1969) points out that in a new
country, in the absence of the usual insect fauna, many plants flourish
and may be checked when the fauna is intentionally introduced. In this
way, broom is flourishing on the Sierran foothills in California where
the fauna on it is poor in introduced species (Frick, 1964; Parker, 1964;
Waloff, 1966).
Many of the species show a preference for young bushes, or for the
current year’s green shoots, which tend to be denser on the younger
plants. There was evidence for this in Psyllidae (Watmough, 1963), in
Leucoptera (Agwu, 1967) and in Apion immune (Williams, 1966) which
was never found on old bushes. The dying broom is immediately attractive to two Rpecies of scolytid bark beetlea, which further weaken the
plant and precipitate its death. But on the whole, any succession that
occurs in this life system is mainly in the densities of populations on
young and old bushes rather than in their complete absence.
Although the changes within the habitat are of importance to the
phytophagous species, they are by no means the only factors that determine their abundance. However, it seems probable that aa the plants
get older they provide a less suitable diet whish reduces fecundity.
Laboratory experiments (Waloff and Richards, 1958; Donia, 1958) indicated that fecundity of Phytodecta wm lower on old than on young
broom, while in the field fecundity dropped by a factor of 2.4 w the