138
IT. WALOFF
artd also becaune mortality of A. i3padii w m much greater than that of
A. genistae. Thus, with 2 nymphs of each specie8 per bud there waa no
mortality of A. genit~tae, but 46% of A. sprtii died.
The experiment indicated better survival of A. geniatm at the expense
of A. e p r t i i , but this is likely to be more important in limiting A. qartii
in warm climates, where A. genistae can increase rapidly in early spring.
In Britain this is not likely to occur in the field, where A. e p r t i i hatchea
a fortnight in advance of the other species and where its nymphs occupy
the swollen buds first.
In the field the effect.8 of psyllids on the plants were obvious. Bushes
with the highest densities of psyllids per bud were usually stunted with
little shoot growth, while the leaves and stems were covered with sticky
honey dew and fungus. Shoota with shortened internodes were espcielly
common in 1961.
(c) Effects of natural enemies
It is remarkable that in contrast to all the other species studied on
broom no parasites were found to attack the two psyllid species at
Silwood in 1961-63. Parasites are known to have an important effect
on psyllid populations (Clark 1962) and a large number of hymenopterous species are known to attack psyllid nympha and adults. A t
Silwood, on two occasions only, 0. W. Richards found two mummified
fifth instar nymphs of A. genistae from which adulte of the encyrtid,
Priornitus mitratus Dalm. emerged.
As a contrast, the effects of predators were considerable, although
in the field the removal of nymphs by the predacious species must have
reduced the effects of overcrowding in both species of psyllid. Psyllids
are the most abundant prey available to predators on broom and for
this reason t,hey are likely to be the most important food source. The
effects of predation were studied serologically and they are discussed
in Section VIII and only a brief outline is given here. It is of interest
that Miridae were the most important single group of predators because
of their large numbers, and Anthocoridae because collectively they are
numerous and two of the species feed preferentially on psyllids
(Drmpster, 1963, 1968). In 1961, the year when A. 8 p r t i i crashed, there
wcre more predators than in the following years and this was entirely
due to the higher numbers of Miridae and Anthoooridae, the numbers of
the other predators remaining similar in the 3 years of this study.
The number8 of psyllids taken, obviously have a relation to the
abundance of the &her alternate prey. The prey that is the next most
abundant on broom is the green aphid AcyrtMpkon ~partii, but as can
be seen in Table LV (Section VIII) it formed only a small fraction of the
total food material. However, the aphids are f a y exposed, wherecle
the psyllid nymphs do attain a measure of protection within the buds.
IT. WALOFF
artd also becaune mortality of A. i3padii w m much greater than that of
A. genistae. Thus, with 2 nymphs of each specie8 per bud there waa no
mortality of A. genit~tae, but 46% of A. sprtii died.
The experiment indicated better survival of A. geniatm at the expense
of A. e p r t i i , but this is likely to be more important in limiting A. qartii
in warm climates, where A. genistae can increase rapidly in early spring.
In Britain this is not likely to occur in the field, where A. e p r t i i hatchea
a fortnight in advance of the other species and where its nymphs occupy
the swollen buds first.
In the field the effect.8 of psyllids on the plants were obvious. Bushes
with the highest densities of psyllids per bud were usually stunted with
little shoot growth, while the leaves and stems were covered with sticky
honey dew and fungus. Shoota with shortened internodes were espcielly
common in 1961.
(c) Effects of natural enemies
It is remarkable that in contrast to all the other species studied on
broom no parasites were found to attack the two psyllid species at
Silwood in 1961-63. Parasites are known to have an important effect
on psyllid populations (Clark 1962) and a large number of hymenopterous species are known to attack psyllid nympha and adults. A t
Silwood, on two occasions only, 0. W. Richards found two mummified
fifth instar nymphs of A. genistae from which adulte of the encyrtid,
Priornitus mitratus Dalm. emerged.
As a contrast, the effects of predators were considerable, although
in the field the removal of nymphs by the predacious species must have
reduced the effects of overcrowding in both species of psyllid. Psyllids
are the most abundant prey available to predators on broom and for
this reason t,hey are likely to be the most important food source. The
effects of predation were studied serologically and they are discussed
in Section VIII and only a brief outline is given here. It is of interest
that Miridae were the most important single group of predators because
of their large numbers, and Anthocoridae because collectively they are
numerous and two of the species feed preferentially on psyllids
(Drmpster, 1963, 1968). In 1961, the year when A. 8 p r t i i crashed, there
wcre more predators than in the following years and this was entirely
due to the higher numbers of Miridae and Anthoooridae, the numbers of
the other predators remaining similar in the 3 years of this study.
The number8 of psyllids taken, obviously have a relation to the
abundance of the &her alternate prey. The prey that is the next most
abundant on broom is the green aphid AcyrtMpkon ~partii, but as can
be seen in Table LV (Section VIII) it formed only a small fraction of the
total food material. However, the aphids are f a y exposed, wherecle
the psyllid nymphs do attain a measure of protection within the buds.