Francesca Noakes You might’ve heard (we shouted pretty loud) about our amazing Research Scientist, Dr Elia Pirtle, winning the best presentation award at the Plant Biosecurity Research Symposium last week!
Elia spoke about how Australia has had some unexpected help in our fight against exotic insect pests. Enter stage right, our bad guy: the vegetable leafminer (Liriomyza sativae, VLM). Vegetable leafminer is a tiny ‘agromyzid’ fly with a taste for a wide variety of fruit and vegetable crops, threatening our vegetable, nursery and melon industries.
Vegetable leafminer can cause severe yield losses due largely to destructive feeding by the larvae, which create serpentine mines in the leaf. This mining not only looks unseemly (and could reduce marketability of crops), it also hinders photosynthesis, which can stunt and even kill plants.
Having already perpetrated havoc overseas, vegetable leafminer reached the Australian mainland in 2015 and has established in Seisia (one of the northern-most communities of the Cape York Peninsula).
Luckily, vegetable leafminer populations within the Torres Strait have so far not reached the problematic levels witnessed overseas and they remain contained to its only mainland hold-out in Seisia. This is in large part thanks to an insect community that can easily slip right under our nose: tiny little native predators called parasitoid wasps.
Female parasitoid wasps lay their eggs on the leafminer larva. When the eggs hatch, the newborn wasp eats the fly larva before completing its development and emerging as a fully formed adult. As Elia says, you can think of them as ‘tiny biosecurity officers’ – on the lookout to feast on any vegetable leafminer invaders.
Dr. Peter Ridland of University of Melbourne has been conducting an extensive review of parasitoid wasps in Australia. He is finding that our native leafminer species are abundant, and so too are their parasitoid wasps. In fact, Australia has at least 95 species of parasitoid wasps that attack our native leafminer flies! Of these, 70 of those species are already known to attack vegetable leafminer overseas.
We know what you’re thinking – if there are so many tiny biosecurity officers on call to help out, why has vegetable leafminer become a problem elsewhere?
Well unfortunately, the parasitoids might be deadly to leafminers but they are extremely sensitive to chemicals (much more so than their vegetable leafminer prey).
Consequently, vegetable leafminer have become a much larger problem on farms overseas where pesticides are sprayed frequently as parasitoid populations are inadvertently destroyed, allowing the flies to breed and multiply, no longer hindered by all those little predators. In the most extreme overseas examples, this has contributed to crop losses as high as 70%.
Fortunately, this hasn’t occurred in the areas of the Torres Strait and in Seisia where vegetable leafminer has established and where we have found at least six species of parasitoids attacking the leafminer (sometimes killing up to 80% of fly larvae).
And it’s not just the wasps that are important.
The presence of such a large community of parasitoids ready to respond to the vegetable leafminer invasion was primarily due to their other favourite food, native leafminer species, already being in abundance. The native leafminers found in the region rarely cause damage to horticultural crops, preferring to feast on common garden weeds, but they do provide a tasty snack to attract and maintain hungry wasps.
So, it was thanks to these native flies that Australia already had a ‘primed’ community of wasps, none of which are particularly fussy eaters, that were very happy to attack vegetable leafminer as soon as it arrived. This has no doubt helped to slow the spread to the south.
However, vegetable leafminer could still spread into agricultural areas, and if that happens, it becomes paramount that Australia maintains its parasitoid wasp communities. Now it is time to share this story with growers across Australia, to ensure everyone is ready to manage vegetable leafminer in a wasp friendly way.
To that end, cesar and our project partner, AUSVEG, are now preparing a workshop series in high risk areas to share what we’ve learned about these little wasps, as well as vegetable leafminer risk potential, effective surveillance and chemical options. Stay tuned for more info about these workshops, and the other work we are undertaking to support our tiny biosecurity friends.
Acknowledgements
Elia would like to acknowledge the help and support of the Northern Australia Quarantine Strategy teams in Cairns, Thursday Island and Seisia, as well as support and participation from the community in the Torres Strait and Northern Peninsula area, including representatives of the Kaurareg Native Title Aboriginal Corporation, the Torres Shire Council, The Apudthama Land Trust, the NPA Regional Council, the Torres Strait Regional Authority, myPathways, and Seisia Enterprises. Community support has given us access to valuable study sites, including school farms, community gardens, and back yards in the search for wasps.
Cover image: Photo by Elia Pirtle, Cesar Australia
