If the staff have to spray pesticides on grass or shrubs or roses, the airconditioning system has to be closed down and tourists kept off the grass ramps and roof.

The change to non-chemical means of controlling insects has overcome these problems, along with saving money.

Head gardener Paul Janssens says all the grass at Parliament House had to be poisoned and replaced just a few years after the building opened when it was found it was being eaten by the Argentinian stem weevil.

"We used to spend about $20,000 a year trying to spray that in October and February,'' Mr Janssens says.

The change to a more insect-resistant variety of grass has been a success, but it hasn't solved another problem - infestation by the Argentinian scarab.

Mr Janssens says lawns were sprayed to kill those bugs, until a new method of control was developed by Robin Bedding of the CSIRO.

He uses nematodes, tiny worm-like creatures that act like microscopic missiles to seek out their insect targets, which are concealed in trunks, stems or the soil, safe from most predators or poisons.

That method is being credited with saving a million hectares of pine forest in Australia and a million hectares of apple trees in China from insect pests.

Mr Janssens says the nematodes are sprayed onto grass on Parliament House's ramps and roof at the rate of half a million per square metre.

They are applied in the evening and washed into the grass and soil by sprinklers.

''For us that has been one of our big success stories,'' he says. ''The beauty of this research is it came from just over at Black Mountain so we were involved in the early trial work that Robin Bedding did.''

Mr Janssens is also eliminating the use of pesticides on rose and other bushes by using predatory insects.

This relies on one experienced gardener spending hours every day examining trees and bushes for signs of trouble.

''Although it takes up one full-time salary, it is a lot cheaper than spraying,'' he says.

An additional benefit is not needing to annoy the occupants of the building by shutting down the airconditioning system for half a day.