STUDIES ON THE INSECT FAUNA ON SCOTCH BROOM
199
plants were rapidly ageing. Yarnell (1966) ale0 found that in Brmhidiw
fecundity was higher on young than on old bushes, and in Sitolsa
(Danthanarayana, 1966) the numbers of eggs per female were 691 and
262, i.e. differed by a factor of 2.6 in two successive generations.
Whether the condition of the plant waa responsible for this drop in
fecundity is not known, but this instance also servee to emphaaise that
recruitment to a new generation is not determined by the numbers of
adults alone. An interesting feature in the study of LeuCoptem (Agwu,
1967) was the high sterility of eggs (49%) which occurred in the offspring of a parental generation of very high density. Klomp and Gruys
(1965) and Klomp (1966) have evidence of an inverse relationship of
viability of eggs and larvae in the pine looper, Bupalus piniarius L.,
and larval density of the preceding generation. It is possible that some
such effect is also manifest in Leucopteru, but the subject is now open
to experimentation. Similarly, we now need better understanding, probably of a biochemical nature of the factors that induce changes in
fecundity when the insects feed on old and on young broom shoots.
Amongst the other factors that affect abundance of the various members of the broom fauna, are predation and dispersal, but these topics
have already been discussed in Sections VIII and IX. Large parts of
populations may be lost by emigration and few apecies eacape the
impact of the general arthropod predators, except those in the specialised microhabitats such as the broom pods, stem mines or root nodules.
On the other hand, many of these species are heavily parasitised and
it is probable that parasitism plays the dominant part in checking their
abundance. Over 80% of Apion fwcirostre larvae can be killed by parasites, mainly by Habrocytus s q m t e r , while up to 46% of Bruchidiw
larvae were parasitised, mainly by Triaepis. I n Leucoptera, 58% of the
late larvae and pupae were dwtroyed by parasitism in a generation that
followed one of great density. Agwu (1967) suggests that larval parasitism, especially by Tetrastichzcs sp. nr. quhctops may have a regulatory effect on the populations of this moth. There is also a very large
complex of parwites around the scolytid bark beetles PHwopWthorms
and Halystinue, but its full impact is not yet understood. Further, it is
highly likely that the Leiophron spp. regulate the populations of two
broom mirids, Heterocordylw, and Aaciodellea. Heterocordylw and its
nymphal parasite L. lreterocordyli both emerge early in the spring. This
temporal isolation results in a simplified relationship and their populations appear to fluctuate in a density - delayed manner. At the other
extreme, Smith (l966a) considers that the effects of parwitism on the
populations of aphids are negligible and Watmough (1963) did not find
any parasites at all in the two species of psyllids.
All the above instances refer to hymenopterous parasites, but the
199
plants were rapidly ageing. Yarnell (1966) ale0 found that in Brmhidiw
fecundity was higher on young than on old bushes, and in Sitolsa
(Danthanarayana, 1966) the numbers of eggs per female were 691 and
262, i.e. differed by a factor of 2.6 in two successive generations.
Whether the condition of the plant waa responsible for this drop in
fecundity is not known, but this instance also servee to emphaaise that
recruitment to a new generation is not determined by the numbers of
adults alone. An interesting feature in the study of LeuCoptem (Agwu,
1967) was the high sterility of eggs (49%) which occurred in the offspring of a parental generation of very high density. Klomp and Gruys
(1965) and Klomp (1966) have evidence of an inverse relationship of
viability of eggs and larvae in the pine looper, Bupalus piniarius L.,
and larval density of the preceding generation. It is possible that some
such effect is also manifest in Leucopteru, but the subject is now open
to experimentation. Similarly, we now need better understanding, probably of a biochemical nature of the factors that induce changes in
fecundity when the insects feed on old and on young broom shoots.
Amongst the other factors that affect abundance of the various members of the broom fauna, are predation and dispersal, but these topics
have already been discussed in Sections VIII and IX. Large parts of
populations may be lost by emigration and few apecies eacape the
impact of the general arthropod predators, except those in the specialised microhabitats such as the broom pods, stem mines or root nodules.
On the other hand, many of these species are heavily parasitised and
it is probable that parasitism plays the dominant part in checking their
abundance. Over 80% of Apion fwcirostre larvae can be killed by parasites, mainly by Habrocytus s q m t e r , while up to 46% of Bruchidiw
larvae were parasitised, mainly by Triaepis. I n Leucoptera, 58% of the
late larvae and pupae were dwtroyed by parasitism in a generation that
followed one of great density. Agwu (1967) suggests that larval parasitism, especially by Tetrastichzcs sp. nr. quhctops may have a regulatory effect on the populations of this moth. There is also a very large
complex of parwites around the scolytid bark beetles PHwopWthorms
and Halystinue, but its full impact is not yet understood. Further, it is
highly likely that the Leiophron spp. regulate the populations of two
broom mirids, Heterocordylw, and Aaciodellea. Heterocordylw and its
nymphal parasite L. lreterocordyli both emerge early in the spring. This
temporal isolation results in a simplified relationship and their populations appear to fluctuate in a density - delayed manner. At the other
extreme, Smith (l966a) considers that the effects of parwitism on the
populations of aphids are negligible and Watmough (1963) did not find
any parasites at all in the two species of psyllids.
All the above instances refer to hymenopterous parasites, but the