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Unlocking the ways insects pests survive without air

CSIRO : 15 October, 2006  (Technical Article)
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.
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