Pest management - Ecological Applications: Toward a Sustainable World - page 156

Fig. 6.8 Mean numbers
(±SE, log scale) (on
continuously monitored
20-cm-branchlets of 30
randomly selected
gumwood trees) of the
pest scale insect
Orthezia insignis and its
biological control agent,
the ladybird Hyperaspis
pantherina . Mean scale
insect numbers dropped
from more than 400
adults and nymphs (in
September 1993) to
fewer than 15 (in
February 1995) when
sampling ceased. Mean
ladybird numbers
increased from January
to August 1994,
coinciding with an
obvious decline in scale
insects, before ladybird
numbers declined
again. The highest
recorded numbers of
ladybirds were 1.3
adults and 3.4 larvae
per 20-cm branchlet.
(After Fowler, 2004.)

2.8

0.8

2.4

0.6

2.0

1.6

0.4

1.2

0.8

0.2

0.4

0 0

May
1993

Sept
1993

Jan
1994

May
1994

Sept
1994

Jan
1995

Sampling date

estimated that all remaining individuals of this rare tree would be dead by 1995.
Again a ladybird beetle was the savior. Hyperaspis pantherina was cultured and
released on St Helena in 1993 and as its numbers increased there was a correspond-
ing 30-fold decrease in scale insect numbers (Figure 6.8). Since 1995 no scale out-
breaks have been reported and culturing of the ladybirds has been discontinued
because the population is maintaining itself at low density in the wild, as good
importation biocontrol agents should.
A quite different kind of biological control agent proved successful in Hawaii
against the South American climbing vine - banana poka ( Passifl ora tarminiana ) -
considered the most serious threat to Hawaii's unique high-elevation forests. In 1991
the fungus Septoria passifl orae was found on banana poka seedlings in its home
range in Colombia. After containment laboratory studies in Hawaii showed the
fungus to be specifi c to banana poka, spore suspensions were sprayed onto leaf
surfaces during 1996-97 and produced major epidemics that severely defoliated the
vines. The biomass of the weed was reduced by 40-60% within 1 year and by more
than 95% 5 years later (Figure 6.9). By 2003, banana poka had been eliminated from
most forests (Trujillo, 2005).
It should not be supposed that biological control campaigns always involve just
a single agent. For example, in response to the invasion of Australia by the aggres-
sive weed Mimosa pigra from tropical America, which forms impenetrable 6-m-high
thickets beside rivers and fl oodplains, so far 11 species of insect and two pathogenic
fungi have been released without much evidence of bringing the weed under control
(di scussed furt her in Section 6.5.2).

6.3.2 Conservation
biological control -
get natural enemies
to do the work

Many pests have a diversity of natural enemies that already occur in their habitat.
In the case of the aphid pests of wheat (e.g. Sitobion avenae ), predators that specialize
on aphids include ladybirds and other beetles, heteropteran bugs, lacewings (Chrys-
opidae), fl y larvae (Syrphidae) and spiders. Many of these natural enemies spend
the winter in grassy boundaries at the edge of wheat fi elds, from where they disperse