STUDIES ON THE INBECT FAUNA ON SCOTOH BEOOM
139
(d) J'adwa muuing changes in numbera of ad&
One of the main c a u w of changea in adult numbers of the two speciea
of psyllids is dispersal, which is a regular phenomenon in the life of both
species. In A. apartii and in the k t generation of A. genistae it occurs
soon after the peak density of adults has been reached. Only a few
individuah of A. genktae disperse in their second generation and none
at all after overwintering, in early spring.
In 1961 almost the entire emigration of A. apartii (i.e. 99% of the
total catch in the suction trap) occuITBcI in the fortnight when the
numbers of adults in samples fell from 204 to 29. It appears therefore
that emigration c a d the rapid reduction in adult population and
that 86% of the psyllids left the broom plantation. Watmough (1963)
considers that the loss by emigration together with the reduction in
fecundity, which both followed the heavy nymphal mortality caused
by overcrowding, caused the crash in numbers in 1961. It will be recalled that in 1961 the number of remuits (eggs) to the population w a ~
913 070 000 whereas in 1962 the recruits were only 8.8% of the 1961
figure. In 1962 a large proportion of the adult population of A. qurtii
emigrated and dispersal again occurred at the time when the peak
number of two adults per 100 g sample fell to 0-24.
In A . genisietae many adultr, also fly out of the habitat. It waa calculated that 86% of the first generation emigrated in 1960. In 1961 the
p r c e n t q e of the population that emigrated was 66, in 1962 it waa 45,
while in 1963 it appears fo have been negligible. Thus, the highest proportion of A. genistue population emigrated in its first generation in
1960 and the resulting~seoond generation wm very emall.
It msy be inferred that s denaity dependent effeot on dispersal by
flight ie detectable in A. genwtut?.
Further, experimentally it ww found that the fecun$ty of A. qeni.9b.e
decreaaed markedly with rise in density, but the densities in the field
in 1969-63 were never high enough for this effect to be manifestel. The
revem appears to be true for A. epartii. Experimentally it was shown
that there is a marked reduction in fecundity when there is less than
100 mm of suitable stem (oviposition site) per female. In 1961 when
growth of young shoots WBB stunted by the effect8 of numerous psyllid
nymphs, it mems likely that the density of females exceeded the level
of one female per 100 mm of suitable, soft, young stem, i.e. there was
restriction of suitable oviposition sites. Although lighter females are
produced in conditions of high nymphal denaities, no effects of weight
on fecundity could be detected in laboratory experiments.
6. Summary of tire factots causing population claanges
From the time when broom was planten in Area 4 in 1957, the
139
(d) J'adwa muuing changes in numbera of ad&
One of the main c a u w of changea in adult numbers of the two speciea
of psyllids is dispersal, which is a regular phenomenon in the life of both
species. In A. apartii and in the k t generation of A. genistae it occurs
soon after the peak density of adults has been reached. Only a few
individuah of A. genktae disperse in their second generation and none
at all after overwintering, in early spring.
In 1961 almost the entire emigration of A. apartii (i.e. 99% of the
total catch in the suction trap) occuITBcI in the fortnight when the
numbers of adults in samples fell from 204 to 29. It appears therefore
that emigration c a d the rapid reduction in adult population and
that 86% of the psyllids left the broom plantation. Watmough (1963)
considers that the loss by emigration together with the reduction in
fecundity, which both followed the heavy nymphal mortality caused
by overcrowding, caused the crash in numbers in 1961. It will be recalled that in 1961 the number of remuits (eggs) to the population w a ~
913 070 000 whereas in 1962 the recruits were only 8.8% of the 1961
figure. In 1962 a large proportion of the adult population of A. qurtii
emigrated and dispersal again occurred at the time when the peak
number of two adults per 100 g sample fell to 0-24.
In A . genisietae many adultr, also fly out of the habitat. It waa calculated that 86% of the first generation emigrated in 1960. In 1961 the
p r c e n t q e of the population that emigrated was 66, in 1962 it waa 45,
while in 1963 it appears fo have been negligible. Thus, the highest proportion of A. genistue population emigrated in its first generation in
1960 and the resulting~seoond generation wm very emall.
It msy be inferred that s denaity dependent effeot on dispersal by
flight ie detectable in A. genwtut?.
Further, experimentally it ww found that the fecun$ty of A. qeni.9b.e
decreaaed markedly with rise in density, but the densities in the field
in 1969-63 were never high enough for this effect to be manifestel. The
revem appears to be true for A. epartii. Experimentally it was shown
that there is a marked reduction in fecundity when there is less than
100 mm of suitable stem (oviposition site) per female. In 1961 when
growth of young shoots WBB stunted by the effect8 of numerous psyllid
nymphs, it mems likely that the density of females exceeded the level
of one female per 100 mm of suitable, soft, young stem, i.e. there was
restriction of suitable oviposition sites. Although lighter females are
produced in conditions of high nymphal denaities, no effects of weight
on fecundity could be detected in laboratory experiments.
6. Summary of tire factots causing population claanges
From the time when broom was planten in Area 4 in 1957, the