152
N. WALOFF
(v) No mriou~ effects of intra-specific Competition were observed, since
the oviposition &en, and hence the mining eitm, were in ample supply,
yet mme of the larvae were occccsionally obseved to die from cannibalism when their mines coalesced. As the life cycle and s&vd of
humpbra are intimately conneoted with its mining habits on broom,
the presence of other mines waa of interest a it could introduce interspecific competition. Besides the larvae of Apion immune, the larvae of
the moth Trifurcula immundeUa Zeller (Nepticulidac) formed mines on
the green shoots. The adult period of this nepticulid moth usually
coincides with the tail-end of that of Leucoptera (mid-August) when the
latter has laid most of its eggs. There was no evidence of interspecific
competition and only once in this study were larvae of Leucaptea and
Trifurczcla found dead when their mines met. However, the feeding
habits of Trifurculu may have helped to accelerate the progreasive
deterioration of the habitat and thus to contribute to the concentration
of the immature stages of Leucoptera in 1965-66. This concentration
may have been indirectly responsible for increased paraitism and
directly for increased predation. On the other hand, Trifurcula larvae
were also p a r a s i t i d by Necremnus sp. which also attacks L e w r a
and were preyed on by birds. Thus Trifurcula may have released Borne
of the pressure on Leumptera population.
5. Summary of the factors affecting abundance of LeuCoptera
The changes in the natural population of Lezccoptera in the 3 years
of this study were considerable and the adult population decreased
from 5 515 000 in 1964 to 89 000 in 1966, i.e. by a factor of 62. The fall
in the level of the population was linked with the deterioration of the
habitat and there is little doubt that LeuCoptera caterpillars precipitated the death of some of the ageing broom bushes.
Amongst the most striking factors that affected the abundance of
this species were the annual variations in fecundity and the high proportion of sterile eggs laid by the progeny of a very dense population.
The larvae suffered from a ( ? bacterial) disease and were heavily parasitised by eulophids and it is possible that the abundance of LeuCoptera
is regulatd by Tetrastichus. The early larval stagea in mines escaped
the effects of arthropod predators, but they were pecked out by birds.
Eggs and wandering larvas were attacked by predacious insects, while
web-spinning spiders were the most conspicuous predators of the adults.
In three consecutive years the numbers of webs in the habitat (Area 4)
were directly correlated with the peak numbers of the moth. Part of the
adult population emigrated out of the plantation every year and it is
not impossible that the process is a denElity dependent one.
N. WALOFF
(v) No mriou~ effects of intra-specific Competition were observed, since
the oviposition &en, and hence the mining eitm, were in ample supply,
yet mme of the larvae were occccsionally obseved to die from cannibalism when their mines coalesced. As the life cycle and s&vd of
humpbra are intimately conneoted with its mining habits on broom,
the presence of other mines waa of interest a it could introduce interspecific competition. Besides the larvae of Apion immune, the larvae of
the moth Trifurcula immundeUa Zeller (Nepticulidac) formed mines on
the green shoots. The adult period of this nepticulid moth usually
coincides with the tail-end of that of Leucoptera (mid-August) when the
latter has laid most of its eggs. There was no evidence of interspecific
competition and only once in this study were larvae of Leucaptea and
Trifurczcla found dead when their mines met. However, the feeding
habits of Trifurculu may have helped to accelerate the progreasive
deterioration of the habitat and thus to contribute to the concentration
of the immature stages of Leucoptera in 1965-66. This concentration
may have been indirectly responsible for increased paraitism and
directly for increased predation. On the other hand, Trifurcula larvae
were also p a r a s i t i d by Necremnus sp. which also attacks L e w r a
and were preyed on by birds. Thus Trifurcula may have released Borne
of the pressure on Leumptera population.
5. Summary of the factors affecting abundance of LeuCoptera
The changes in the natural population of Lezccoptera in the 3 years
of this study were considerable and the adult population decreased
from 5 515 000 in 1964 to 89 000 in 1966, i.e. by a factor of 62. The fall
in the level of the population was linked with the deterioration of the
habitat and there is little doubt that LeuCoptera caterpillars precipitated the death of some of the ageing broom bushes.
Amongst the most striking factors that affected the abundance of
this species were the annual variations in fecundity and the high proportion of sterile eggs laid by the progeny of a very dense population.
The larvae suffered from a ( ? bacterial) disease and were heavily parasitised by eulophids and it is possible that the abundance of LeuCoptera
is regulatd by Tetrastichus. The early larval stagea in mines escaped
the effects of arthropod predators, but they were pecked out by birds.
Eggs and wandering larvas were attacked by predacious insects, while
web-spinning spiders were the most conspicuous predators of the adults.
In three consecutive years the numbers of webs in the habitat (Area 4)
were directly correlated with the peak numbers of the moth. Part of the
adult population emigrated out of the plantation every year and it is
not impossible that the process is a denElity dependent one.