Key points
- Invasive weeds impact our environment by altering ecosystems and threatening native plants and animals.
- Biocontrol agents are usually insects or fungi which originate in the home range of the weed.
- Candidates go through multistage testing to make sure they won’t spread to other plants.
Fortunately for us, weeds have enemies. Biocontrol harnesses the power of these natural adversaries, limiting the otherwise unchecked growth and spread of weeds. Biocontrol methods offer safe, cost-effective, landscape-scale weed control.
Host-specificity testing is our special sauce. It makes sure these biocontrol agents are safe for release – and ready to work as weed-busting warriors.
We put these potential agents through multiple rounds of host-specificity testing. It’s like a whole series of really tough interviews and exams before they’re hired.
First past the host
Michelle Rafter is an entomologist researching plant-insect interactions.
“We’re looking at how an insect finds and recognises plants as a potential host, and then how the insect completes its lifecycle across host plant species,” Michelle said.
“Even if an insect seems like a generalist in that it can use multiple plant species, we investigate further. We usually find there is a primary host plant and then a series of less preferred secondary hosts they might use less frequently.”
Plant chemical signatures allow the insect to recognise host plants. Alternative host plants might have chemical blends that are similar to, but not quite the same as, the primary host plant.
“Ideally we are searching for insects and fungi that are specialists,” Michelle said.
“They’re locked into recognising the chemical signatures from one specific plant species – the target weed – as their host.”
There’s no place like home
The journey starts overseas wherever the weeds have come from – South America for the Cabomba weevil, and France for the sea spurge fungus. Researchers look for insects that feed on the weed. Then they check if these insects are also feeding on close relatives of the target plant in the native range.
If the insect is found only on the target weed, that indicates its likely pretty host-specific already in its natural environment. If the insect is found on a host plant and closely related plants, it’s less specialised. This insect could still be a potential biocontrol agent if those related plants aren’t present in Australia.
Weed it to me
For each weed we need to figure out the crucial part of the life cycle to target.
“If you have an insect that eats the leaves, but the plant can drop its leaves to persist through droughts, that agent won’t have as much impact as a seed feeder that burrows into the seeds and stops them reproducing,” Michelle said.
The insects undergo initial screening to see how they react to different test plants that are close relatives to the target weed.
“We have to be very stringent with host testing because these insects have never encountered our native plants before,” Michelle said.
“If any of our native plants are close relatives of the target weed, they may have overlapping chemical signals for the insects.”
Meal or no meal
Once we’ve imported the potential insect agents, we put them through a full risk assessment in quarantine.
It starts with a ‘no choice’ test, which is our fundamental and toughest test. The insects only have access to the test plants. These plants come from a host test list based on detailed plant family trees showing closely related species.
Researchers monitor their feeding and whether they’re mating and laying eggs. They also observe whether those eggs are developing into larvae or nymphs, eventually becoming the next generation of adults.
“If they're doing all those things across different plants then they’re probably not sufficiently host specific and they won’t pass the test,” Michelle said.
But if they're in a cage with a particular plant and they’re not feeding or laying eggs then that plant is obviously not a host for them.
Buffet indecision
The second stage of testing is called ‘choice testing’. The insects are given a choice between the target weed and the plants which they fed on in the first ‘no-choice’ tests.
“We’re looking at what they do when they have options, and with all the chemical cues of different plants,” Michelle said.
We’re trying to get an indication of what would happen in the field when they're presented with a choice. If the insect or fungi only feeds and develops on the target weed despite the presence of alternative test plants, it’s unlikely to use that test plant in the environment.
From here to paternity
If we see larvae or nymphs completing development in a no-choice test, we need to check if they can sustain a population over multiple generations. We offer the next generation that same test plant species to see if they can continue to feed, breed and reproduce.
“Sometimes what we see is the population crashes and they're not able to sustain multiple generations,” Michelle said.
This shows that the insect couldn't sustain a population in Australia using just that test plant.
Back to the future
If we still need more information to get a complete picture of any risk to our native plant species, there’s one last test we can do. We send our test plants overseas to the native range of the insect, with all the correct permissions, for open field testing.
The plants are put out into the field to see if the insects can recognise them as a potential host in an open field environment – even if they’re not their preferred host. If there’s damage and egg laying, clearly the insects can use those plants as a host, and it would be too risky to release them in Australia.
The cane toad: unscientific biocontrol gone bad
The case study that’s always front of mind for biocontrol is the cane toad. But the cane toad example wasn’t a science-based biocontrol program.
“To start with, there was no testing done for the cane toad,” Michelle said.
“It doesn’t even feed on the cane beetles it was meant to control.”
The cane beetles live higher up on the cane stem. The cane toads can’t even reach this area of the plant let alone feed on the beetles there. There's a complete mismatch in ecologies.
The cane toad would never have passed all the detailed host testing that we do today.
Future of biocontrol
Weed biocontrol remains a landscape scale, sustainable and safe weed management tool.
“I really think weed biocontrol is a tool that's still going to be needed in the weed management toolbox,” Michelle said.
“It’s not a silver bullet, and it won’t eradicate weeds. But it can effectively complement integrated weed management approaches.”
Our rigorous host testing makes sure our modern biocontrol agents are safe, targeted and effective – before being approved for release under Australia’s stringent biosecurity regulations.