Environmental Engineering Reference
In-Depth Information
Fig. 6.9
Reduction in
banana poka biomass
(mean ± SE) after
application of the
fungus
Septoria
passifl orae
at Laupahoe-
hoe in the Hilo Forest
Reserve, Hawaii.
Biomass before
application (at time '0')
is taken to be 100%,
and values from
subsequent surveys are
shown in relation to
this. (After Trujillo
et al., 2001.)
100
80
60
40
20
0
0
2
2.5
3
4
Years after inoculation
to reduce aphid populations around fi eld edges. Farmers can protect grass habitat
around their fi elds and even plant grassy strips in the interior to enhance
these natural populations and the scale of their impact on the pests. This example
of conservation biological control parallels our earlier discussion of how grassy
strips can provide a refuge for natural enemies against pesticide use in cornfi elds
(Section 6.2.6).
Urban settings also have their pest control problems and conservation biological
control sometimes works here too. Bagworms, caterpillars of the moth
Thyridopteryx
ephemeraeformis
, are among the most important pests of ornamental trees and
shrubs in the eastern USA. The caterpillar constructs a bag-shaped shelter of silk
and leaf fragments that it never leaves. Adult males (which have no wings) mate by
inserting their genitalia into bags of the wingless adult females, whose eggs hatch
to release fi rst instar larvae that disperse by 'ballooning' on the wind. Natural
enemies include a diversity of wasp parasitoids, such as
Itoplectis conquisitor
, whose
larvae consume their caterpillar hosts. The adult parasitoids, on the other hand,
often visit fl owers to eat pollen and nectar, resources that can increase longevity
and reproduction. Ellis et al. (2005) tested the hypothesis that the provision of
fl owerbeds (planted with fl owers in the aster family) in the vicinity of shrubs (
Thuja
occidentalis
) would increase the effects of natural enemies and reduce the bagworm
population on the shrubs. Figure 6.10 shows how rate of parasitism of bagworms
increased with proximity to a fl owerbed, emphasizing the potential of this conserva-
tion biological control strategy.
Finally, you should note that vertebrates are occasionally used as biological control
agents, and great tits (
Parus major
) in apple orchards (
Malus domestica
) provide a
Fig. 6.10
Percentage
parasitism (±SE) (by
Itoplectis conquisitor
and
other parasitoid wasps)
of bagworm pests on
shrubs, in relation to
distance to the nearest
fl owerbed (which
provides food resources
for the adult parasi-
toids). Bagworms were
collected from the wild
and 20 were placed on
each experimental
shrub at the beginning
of the 3-month
experiment. (After Ellis
et al., 2005.)
80
70
60
50
40
30
20
0
6
12
18
24
Distance from flowerbed (m)
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