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Objectives
Background
Tree loss from repeated insect defoliation (eg. Christmas
and pasture beetles) is affecting the quality of the Australian
environment. Tree planting in pasture areas to redress the problem
is costly and risky with no assurance they will survive in the
long term.
Anecdotal evidence suggests maintaining or recreating the natural biodiversity (plants and animals) would retard the tree mortality rate by allowing natural control agents of scarabs (parasitoids and predators) to function normally. Scientific data on the potential parasitoids, their hosts and an understanding of their relationship with vegetation and environmental parameters is lacking.
Research
Parasitoids were sampled by Malaise traps on two properties at twenty sites up to 400 m from remnant vegetation. Scarab larvae were obtained from transect sampling while scarab beetles were obtained by light trapping. Sampling occurred during severe drought conditions.
Outcome
Parasitoid wasp numbers declined with distance from
the remnant vegetation and increasing drought severity. The genera
Prosena and Senostoma increased under drought conditions.
Some tachinids are better able to handle the conditions of open
pasture than others.
Larvae of tree-feeding scarabs (Christmas and pasture
beetles) peaked in the open pasture adjacent to the remnant vegetation;
non tree-feeders attained their maximum well into the pasture.
The abundance of parasitoid pupae depended on soil characteristics
and/or the occurrence of tea tree (a major nectar source for adult
parasitoids).
Movement of adult parasitoids and scarab beetles
is strongly seasonal, and then subject to meteorological conditions
(particularly rainfall and wind speed).
The low parasitoid populations (adults and pupae) taken away from the remnant vegetation suggests they have limited influence on the scarab larval complex damaging pasture in the open. In the open, in moister areas, mermithid nematodes and bird predators (ravens, ibis, magpies and currawongs) appear to have a greater impact on scarab numbers than the potential parasitoids.
Implications
The tiphiids, scoliids and selected tachinids are
beneficial on farms for pest control. Their distributions are
highly dependent on remnant vegetation, particularly the shrub
component. The wasps and larger tachinids remained close to, and
were most frequently taken from, their energy sources. These energy
sources, particularly Leptospermum, Baeckea and potentially
Eucalyptus spp., are a small subset of the available flowering
plant species. Planting Leptospermum and Baeckea
on farms would increase available energy sources and activity
of these adult parasitoids.
The regular spacing of windbreaks across pasture
(intervals <800 m) would maximise biological control by these
parasitoids. Windbreaks should be interconnected with existing
remnant vegetation to provide corridors to aid parasitoidal dispersal.
RIRDC Project No: DAN-68A
RESEARCHER: A J Campbell
ORGANISATION: NSW Dept of Agriculture
Tropical Fruit Research Station PO Box 72PHONE: 066 28 0604
FAX: 066 28 0209
Last updated: 10 October 1996
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