STUTJIES ON THE INSECT F A I X A ON SCOTCH BROON
101
weds. Although there is no direct conclusive evidence, it is considered
that the later hatching Apion larvae compete with the already overcrowdtd larvae of 13ruchidiu.s..
D. (Tables V apd V I ) Mortality of the late larvae and pupae in the
pods is mainly caused by parasites. In 1960 less than half of the
Bruchidiua larvae wliirh entered seeds emerged aa adults. However,
over a million beetles emerged, an increase on the parental generation
by a factor of 32 within one generation. In 1961 the same degree of
mortality was seen at this stage. As a contrrmt, over 80% of the total
number of Apion larvae in Area 1 were killed by ectoparasites. Various
species were involved but the main one was Habroqtus sequester. In the
young plantation (Area 4) only 45.595 Bruchidiua larvae were killed by
parasites, mainly by Triaspia and to a lesser extent by Habrocytus.
Many Hdrocytus were found to be hyperparasitised by Mesopolobus
mediterraneus and occasionally by Aprostocetus tibialis. Two minor
hyperparasites Torymus sp. nr. micropterm and Eupelmus urozonus also
occurred. In a few instanceu there was evidence of superparasitism
when Habrocytus females paralysed and oviposited upon larvae of their
own species.
As a contrast to parasitism, predation was not important because
of the enclosed nature of the microhabitat.
Yet another mortality factor was the ejection of Apion when the
pods dehisced. Bruchidius larvae can complete development and pupate
in ejected seeds, but Apion larvae and their ectoparasites are more
exposed and have little chance of survival after they are ejected.
E. The overwintering mortality of adults was estimated experimentally. About one third of Bruchidius adults die but a much lower
proportion of Apion.
F. In both these species of beetles marked changes in number occur
as a result of dispersal to hibernation sites. Many Apion adults hibernate under broom, but those of Bruchidiw away from the host plant.
Of one million beetles that emerged in 1960 only 7% (68 400) congregated in the plantation in the following epring.
This limited study indicated some of the factors that bring about
changes in number from generation to generation. The relative contributions to different mortalities varied greatly in 1960 and 1961. Thus
there were twice as many ovipositing Bruchidius in Area 4 in 1961 as
in 1960, yet the numbers of emerging beetles of thc next generations
were similar in the 2 years. In thc 2 years, mortality caused by parasitism and by unknown causes was similar in Bruchidiw and these
factors arc probatbly reasonthly predictable. The catastrophic changes
in the habitat causchd by spring frosts and which resulted in intraapecific competition of larvae for seeds was completely unpredictable.
101
weds. Although there is no direct conclusive evidence, it is considered
that the later hatching Apion larvae compete with the already overcrowdtd larvae of 13ruchidiu.s..
D. (Tables V apd V I ) Mortality of the late larvae and pupae in the
pods is mainly caused by parasites. In 1960 less than half of the
Bruchidiua larvae wliirh entered seeds emerged aa adults. However,
over a million beetles emerged, an increase on the parental generation
by a factor of 32 within one generation. In 1961 the same degree of
mortality was seen at this stage. As a contrrmt, over 80% of the total
number of Apion larvae in Area 1 were killed by ectoparasites. Various
species were involved but the main one was Habroqtus sequester. In the
young plantation (Area 4) only 45.595 Bruchidiua larvae were killed by
parasites, mainly by Triaspia and to a lesser extent by Habrocytus.
Many Hdrocytus were found to be hyperparasitised by Mesopolobus
mediterraneus and occasionally by Aprostocetus tibialis. Two minor
hyperparasites Torymus sp. nr. micropterm and Eupelmus urozonus also
occurred. In a few instanceu there was evidence of superparasitism
when Habrocytus females paralysed and oviposited upon larvae of their
own species.
As a contrast to parasitism, predation was not important because
of the enclosed nature of the microhabitat.
Yet another mortality factor was the ejection of Apion when the
pods dehisced. Bruchidius larvae can complete development and pupate
in ejected seeds, but Apion larvae and their ectoparasites are more
exposed and have little chance of survival after they are ejected.
E. The overwintering mortality of adults was estimated experimentally. About one third of Bruchidius adults die but a much lower
proportion of Apion.
F. In both these species of beetles marked changes in number occur
as a result of dispersal to hibernation sites. Many Apion adults hibernate under broom, but those of Bruchidiw away from the host plant.
Of one million beetles that emerged in 1960 only 7% (68 400) congregated in the plantation in the following epring.
This limited study indicated some of the factors that bring about
changes in number from generation to generation. The relative contributions to different mortalities varied greatly in 1960 and 1961. Thus
there were twice as many ovipositing Bruchidius in Area 4 in 1961 as
in 1960, yet the numbers of emerging beetles of thc next generations
were similar in the 2 years. In thc 2 years, mortality caused by parasitism and by unknown causes was similar in Bruchidiw and these
factors arc probatbly reasonthly predictable. The catastrophic changes
in the habitat causchd by spring frosts and which resulted in intraapecific competition of larvae for seeds was completely unpredictable.