STUDIES ON THE INSECT FAUNA ON SCOTCH BROOM
127
in the two preceding years (i.e. 70% of the population in 1956, 67% in
1956 and only 36% in 1957). The greater proportion of apteroue adults
in 1967 waa undoubtedly responsible for the lerger population, since
alatae rapidly leave the area without reproducing. Clearly a later season
with a higher proportion of alatae is both an advantage to the dispersal
of the speciea and ie an important factor in the decline of a 1 - 1
population.
(b) Predation by b i d . It was not possible to determine precisely the
importance of predation by birds relative fo all the other factors, but
in an experiment where bushes were protected from birds, but not from
invertebrate predatora and parasites, the abundance of A. sprtii wm
reduced to 58% in 20 days, oompared with 98% in uncaged bushes
(see Table XX). Birde fed preferentially on the largeat reproductive
TABLE XX
Ccnnprieon of rates of decline in aphid nzctnbcrs on a caged
and uncaged broom bush (Smith, 1966a)
6 June
15 June
26 June
Caged h u h
Total Acyrthosiphon on 6 ehoota
630
374
234
Totel apteroua edults
47
31
14
Uncaged bush
Toid Acyrthoeiph on 6 shoots
564
11s
11
Total apteroue adu1t.a
51
8
1
aphids and, in the same way as emigration of alatee, reduced the reproductive rate of the population immediately. In 1957 more birds were
attracted by the higher population of aphids and they gtayed on the
bushes longer. The periods when birds were p m n t on broom were
checked by counts of “droppinge” on sheets spread beneath the bushse.
The most constant speciea in the area were (Pacus coerulew (Prazak)
(blue-tit), Fringilla coelebs geneferi (Kleinschmidt) (chaffinch) and to a
lesser extent PruneUa mw&?, u& Occidentalia (Hart.) (hedge sparrow) and
8yZuk.z cuwucu cuwuca (L.) (leaeer whitethroat). All these speciea perched
on the tops and sides of buehea and ate the aphids rapidly and
systematically along the shoots. Close range observationa showed that
alatae were preferentially eaten by blue-tits, and after that, the
largest aphids.
( 0 ) Preddim by inue-.
A f b r the major decline of A~ydwaiphim
the population rebsined at a oomparetivdy low level in all the 3 years.
127
in the two preceding years (i.e. 70% of the population in 1956, 67% in
1956 and only 36% in 1957). The greater proportion of apteroue adults
in 1967 waa undoubtedly responsible for the lerger population, since
alatae rapidly leave the area without reproducing. Clearly a later season
with a higher proportion of alatae is both an advantage to the dispersal
of the speciea and ie an important factor in the decline of a 1 - 1
population.
(b) Predation by b i d . It was not possible to determine precisely the
importance of predation by birds relative fo all the other factors, but
in an experiment where bushes were protected from birds, but not from
invertebrate predatora and parasites, the abundance of A. sprtii wm
reduced to 58% in 20 days, oompared with 98% in uncaged bushes
(see Table XX). Birde fed preferentially on the largeat reproductive
TABLE XX
Ccnnprieon of rates of decline in aphid nzctnbcrs on a caged
and uncaged broom bush (Smith, 1966a)
6 June
15 June
26 June
Caged h u h
Total Acyrthosiphon on 6 ehoota
630
374
234
Totel apteroua edults
47
31
14
Uncaged bush
Toid Acyrthoeiph on 6 shoots
564
11s
11
Total apteroue adu1t.a
51
8
1
aphids and, in the same way as emigration of alatee, reduced the reproductive rate of the population immediately. In 1957 more birds were
attracted by the higher population of aphids and they gtayed on the
bushes longer. The periods when birds were p m n t on broom were
checked by counts of “droppinge” on sheets spread beneath the bushse.
The most constant speciea in the area were (Pacus coerulew (Prazak)
(blue-tit), Fringilla coelebs geneferi (Kleinschmidt) (chaffinch) and to a
lesser extent PruneUa mw&?, u& Occidentalia (Hart.) (hedge sparrow) and
8yZuk.z cuwucu cuwuca (L.) (leaeer whitethroat). All these speciea perched
on the tops and sides of buehea and ate the aphids rapidly and
systematically along the shoots. Close range observationa showed that
alatae were preferentially eaten by blue-tits, and after that, the
largest aphids.
( 0 ) Preddim by inue-.
A f b r the major decline of A~ydwaiphim
the population rebsined at a oomparetivdy low level in all the 3 years.