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How do insects choose their favourite weed? Scientists hoping to control weeds want to know if these insects will eat other plants or only their absolute favourite weed.

Weeds usually come here from overseas. In their home country they live with insects and fungi which eat them. Scientists study these insects and fungi to see how targeted they are and if it would be safe to release them in Australia to stop the spread of weeds here.

Michelle Rafter is a CSIRO scientist who studies plant-insect interactions and runs the tests where we give the insects different choices and see which plants they really like to eat. Her testing makes sure the new insects won’t eat native plants or food plants.

“We’re looking for specialised insects that only eat one type of plant. We don’t want generalists that like to eat lots of different types of plants,” says Michelle. “We start with their favourite weed snacks and then we offer them other plants related to their favourite foods,” she adds. Sometimes Michelle’s team even sends some of our native plants over to the country the insects come from to see if they will eat these plants out in the environment.

The chemicals the plants release help the insects to recognise their favourite plants. But other related plants can have a similar chemical blend, even if it’s not exactly the same as the host plant. “We do this testing in special glasshouses so the insects can’t escape,” says Michelle. “If they only eat their favourite plants, and don’t munch on other plants, then they pass the test!”

Some insects that have passed the test and been released here are the Cabomba weevil from South America, and the Paterson’s Curse root weevil from Europe. Once they’re released out in the environment, they find the weeds by following their chemical signals. When they eat the weeds, it helps reduce their growth and stop their spread across the Australian landscape.