August 17, 2004
A
CSIRO scientist is a step closer to solving the mystery of
how insect pests survive in low oxygen environments such as
grain silos and how to reduce multi-million dollar reliance on
toxic fumigants to kills such pests.
CSIRO
entomologist Victoria Haritos says she has found clues as to how
weevils, beetles and storage moths change their breathing
patterns and metabolism to survive in low oxygen levels and high
concentrations of fumigants.
Fumigants
such as phosphine and methyl bromide are widely used to kill
insects in grain silos and pests of horticulture. But these are
highly toxic treatments and some beetles have developed
resistance to phosphine. It also makes fumigation treatment
substantially more expensive.
Alternative
treatments to fumigants include "controlled atmospheres" which
use either low oxygen or high carbon dioxide to kill insect
pests. These treatments are desirable because they are
non-chemical and leave no residues, and in the case of low
oxygen, result in no harmful emissions to the environment.
However, they are not cost competitive because they take up to
28 days to control insects and require a high level of
atmosphere control.
So why are
some insects (mostly beetles and moths) able to survive
incredibly low levels of oxygen, like zero per cent for periods
of time up to 20 days? Dr Haritos says that when there is oxygen
present, even down to as low as two per cent, some of the stored
grain insects can compensate by breathing significantly more
regularly, and they can maintain this for many days.
"We have
monitored their breathing patterns and exhalation of carbon
dioxide and can see that they are keeping up with their oxygen
requirements through more frequent ventilation," she says. "This
tells us why two per cent oxygen is not low enough to really
impact on these insects - because they can adapt their
physiology to the low oxygen situation."
In a low
oxygen environment, insects reduce their metabolic need and
increase ventilation by fully opening tiny valves called
spiracles located on their sides. Spiracles regulate the
exchange of gases.
"This
protective response is triggered by the insects sensing low
levels of oxygen. How they do this is not yet known," Dr Haritos
says. "It is this immediate protective response that we want to
override because this will leave the insect very vulnerable to
low oxygen and it should quickly die."
"We have
found clues to the signalling that is involved in the low oxygen
response but we have not been able to prove it yet. Once this is
known with certainty, we will be able to explore chemical,
physical or genetic means by which we can override it. Our
ultimate goal for this part of the research is to enable low
oxygen to be a faster and more cost competitive treatment to
toxic fumigants."
Dr Haritos
has also examined the effect phosphine on the respiration of
insects that are resistant to the gas.
"Resistant
insects exhale carbon dioxide in the same pattern and maintain
the same metabolic rate in a phosphine atmosphere that would
normally be very toxic to them" she says. "By contrast,
respiration patterns in insects that are susceptible to
phosphine become irregular followed by a sharp decrease in
carbon dioxide release indicating a toxic response."
"It was
thought that the resistant insects avoided phosphine uptake by
slowing or stopping respiration but we now understand that the
resistant insects breathe normally in a phosphine atmosphere and
take up substantial amounts of phosphine."
Dr Haritos
is a member of the Product Protection Stream. Her insect
respiration research is being funded by Australian grain
handling and marketing companies. These results are from the
first year of research with funding for another two years. |