STUDIES ON TEE INSECT FAUNA ON SCOTCH BROOM
201
the smaller ones, either within their own gallery system or in the neighbouring systems. A few pupae were a h destroyed in this way.
The most striking examples of intra- and interspecific competition
were seen in the Psyllidae, and of intraapecifio competition in Leiqhon
h e t e r d y l i , the parasite of Heterocordylus. An account of competition
in the Psyllidae is given on pt~gea 135-137, and it will be remembered
that the nymphal stages of A. ~partii und A . genietue oclcupy the same
ecological niche and have the seme food requirements. A. ~parbii develop more rapidly in spring and is at an advantage, as it haa the first
choice of swollen buds. In 1961, in the year of the peak density of
A. ~ p r t i i ,
aa many aa fifty to seventy nymphs occasionally crowded
into a single bud and frequently they were drowned in their own aecretions while those that were displaced suffered from predators. The
average density throughout the plantation w u four nymphs per bud
and this degree of crowding wiw exceptional, i.e. occurred in only a few
buds, in the other years. Nymphal mortality was high and Watmough
(1963) estimated that overcrowding c a d death of 25-30% of the
population. The effects of crowding were a h investigated experimentally and are shown in Figs 6 and 7. Also there is little doubt
that the females of Lewphron heterocopdyli competed for oviposition
sites (i.e. for the firat and second instar nymphs of their host, Heterocordylus), at lemt in 1962, when 94% of the host population waa parasitised and 57% was superparaaitieed. As only one larva completea
development within the host and a11 the supernumerary larvae die, the
paraaite larvae also competed for requisitee within the host haemocoele.
A low degree of superparmitism occurred in the other years when parasitism was lower, but it did not exceed 9%.
In the Sierran foothills of California, Orthdylzce coltcdop ie extremely
abundant in the absence of the other four species of broom Miridae. In
England, where all the species occur together, it is usually the least
abundant of the five species. Similarly, 0. virescem occurred in enormous numbers near Victoria on Vancouver Island, where the populations of 0. concolor and Aaciodema were minute and the other two species absent (see Table LI). It is tempting to consider this aa indirect
evidence of interspecific competition, but the effects of climate are unknown and the “life systems” are different.
It is obvious that many factors interact to check the abundance of the
species that make up the fauna on broom, and frequently when one
type of pressure on a population is released another steps in and some
balance within the life system is temporarily maintained.
On th8 whole, the species living on broom appear to be well adapted
to the temperate conditions within which their host plant is distributed.
For instance, Apion immune, LeuCoptera and Piemdorzce lay their
201
the smaller ones, either within their own gallery system or in the neighbouring systems. A few pupae were a h destroyed in this way.
The most striking examples of intra- and interspecific competition
were seen in the Psyllidae, and of intraapecifio competition in Leiqhon
h e t e r d y l i , the parasite of Heterocordylus. An account of competition
in the Psyllidae is given on pt~gea 135-137, and it will be remembered
that the nymphal stages of A. ~partii und A . genietue oclcupy the same
ecological niche and have the seme food requirements. A. ~parbii develop more rapidly in spring and is at an advantage, as it haa the first
choice of swollen buds. In 1961, in the year of the peak density of
A. ~ p r t i i ,
aa many aa fifty to seventy nymphs occasionally crowded
into a single bud and frequently they were drowned in their own aecretions while those that were displaced suffered from predators. The
average density throughout the plantation w u four nymphs per bud
and this degree of crowding wiw exceptional, i.e. occurred in only a few
buds, in the other years. Nymphal mortality was high and Watmough
(1963) estimated that overcrowding c a d death of 25-30% of the
population. The effects of crowding were a h investigated experimentally and are shown in Figs 6 and 7. Also there is little doubt
that the females of Lewphron heterocopdyli competed for oviposition
sites (i.e. for the firat and second instar nymphs of their host, Heterocordylus), at lemt in 1962, when 94% of the host population waa parasitised and 57% was superparaaitieed. As only one larva completea
development within the host and a11 the supernumerary larvae die, the
paraaite larvae also competed for requisitee within the host haemocoele.
A low degree of superparmitism occurred in the other years when parasitism was lower, but it did not exceed 9%.
In the Sierran foothills of California, Orthdylzce coltcdop ie extremely
abundant in the absence of the other four species of broom Miridae. In
England, where all the species occur together, it is usually the least
abundant of the five species. Similarly, 0. virescem occurred in enormous numbers near Victoria on Vancouver Island, where the populations of 0. concolor and Aaciodema were minute and the other two species absent (see Table LI). It is tempting to consider this aa indirect
evidence of interspecific competition, but the effects of climate are unknown and the “life systems” are different.
It is obvious that many factors interact to check the abundance of the
species that make up the fauna on broom, and frequently when one
type of pressure on a population is released another steps in and some
balance within the life system is temporarily maintained.
On th8 whole, the species living on broom appear to be well adapted
to the temperate conditions within which their host plant is distributed.
For instance, Apion immune, LeuCoptera and Piemdorzce lay their