Elise Oakman 
When leafminer (Liriomyza spp.) biosecurity threats become established in Australia, management strategies are often complicated as these species are prone to evolving insecticide resistance. Supporting parasitoids has shown to be one of the best ways of managing such threats.
However, these types of beneficial species are easily wiped out via chemical applications. To address this problem, a new report has been released compiled by Cesar Australia researchers outlining how to monitor leafminers to reduce chemical applications and support parasitoids to aid management.
Establishment of polyphagous leafminer in Australia
There are three species of “leafminer flies” which have long been on the Australian federal government’s 40 ‘high risk’ biosecurity species, that eventually established in Australia between 2015 and 2020. These include the vegetable leafminer (VLM, Liriomyza sativae), the American serpentine leafminer (ASLM, Liriomyza trifolii) and the serpentine leafminer (SLM, Liriomyza huidobrensis).
In 2008, VLM was detected for the first time throughout the north Australian islands of Torres Strait and then on the Australian mainland at Seisia in far north Queensland in 2015 (IPCC 2017). There has not yet been detections in any other regions of Australia, despite ongoing surveillance efforts.
In late 2020, SLM was detected in the Sydney region and eradication was subsequently deemed unfeasible (IPCC 2021a). The following year ASLM was detected in northern Western Australia and within the Torres Strait. Final considerations on technical feasibility of eradication are still underway (IPCC 2021b), but eradication is unlikely.
The damage
Referred to generally as the polyphagous Liriomyza leafminer, these flies are part of a well-known group (family Agromyzidae) of small, morphologically similar flies whose larvae feed internally on plants, often as leaf and stem miners.
The majority of damage caused by polyphagous Liriomyza leafminer occurs during larval feeding between the upper and lower leaf surface, which curtails photosynthetic ability and reduces marketability of some crops. Fortunately researchers had been preparing for the arrival of these species to Australian production regions since 2017 as part of a Hort Innovation funded project in response to the establishment of L. sativae in Far North Queensland.
Managing polyphagous leafminer
Hort Innovation’s subsequent project is MT20005 (Management Strategy for serpentine leafminer) is in response to the L. huidobrensis and L. trifolii incursions into production regions in NSW and QLD. As part of this project, Cesar Australia researchers are now working together with partners from the Queensland, New South Wales, Western Australia and Northern Territory state governments, as well as the University of Melbourne and AUSVEG to support appropriate management of these pests.
Polyphagous Liriomyza leafminer are secondary pests, meaning they only reach damaging levels after severe reductions in parasitoid populations. This can occur after insecticide-based control has disrupted beneficial populations. When issues arise, a complicating factor is that Polyphagous Liriomyza leafminer are prone to evolving insecticide resistance that makes control and eradication difficult.
We know that one of the most important foundations underpinning successful programs managing leafminer overseas is to conserve parasitoid wasps by avoiding broad-spectrum insecticides and using economic thresholds to delay and reduce sprays. Our researchers have been developing plans to help growers and agronomists monitor leafminer populations with the goal of making informed pest management decisions and reducing chemical reliance.
Report to aid management
The report titled “Monitoring to support integrated pest management of Liriomyza spp. pests in Australia: a mini review of global monitoring plans” compiled by Cesar Australia researchers, reviews monitoring programs that have been developed overseas and can be used to guide our management of leafminer flies in Australia.
There are four sampling techniques covered in the report, each aimed at estimating population sizes of leafminer to support the use of Economic Thresholds. These techniques are:
- Counting of infested leaves (see Figure 1)
- Counting of live Liriomyza larvae within leaf mines (aided by a hand lens) (see Figure 2)
- Counting of Liriomyza pupae (caught in ‘pupal trays’ or rearing bags)
- Counting of Liriomyza adults on yellow sticky traps
There are benefits and limitations to each technique, however they are intended to be used in combination to effectively monitor leafminer populations. The report outlines how much sampling needs to be done to be confident you have an accurate representation of the population size in your crop. Making accurate estimates of leafminer populations is a prerequisite to using economic thresholds aimed at reducing unnecessary chemical costs and unwanted toxicity effects on beneficials.
Figure 1 (Left): Damage caused by adult leafminer. A) SLM stippling damage to choy sum (Shannon Mulholland, NSW DPI); B) SLM damage to cucumber (Shannon Mulholland, NSW DPI); C) SLM damage to celery (John Duff, DAFF)
Figure 2 (Right): A) Live larvae (VLM pictured) can be seen feeding via a hand lens; B) Holding the leaf up to the sun can increase visibility of larvae inside mines; C) Inactive mines may be empty; or D) may contain a dead larva. (Elia Pirtle, Cesar Australia)
Contact
Acknowledgments to Dr. Elia Pirtle (Cesar Australia) as lead author. If you have any questions please contact epirtle@cesaraustralia.com.
More information
AUSVEG – Learn more about leafminer management
Cesar Australia PestFacts – Learn more about the recent serpentine leafminer incursion
A short guide to monitoring for serpentine leafminer in Australia, based on this report, is available on the AUSVEG MT20005 project webpage
Acknowledgments
Management strategy for serpentine leafminer, Liriomyza huidobrensis (MT20005) is a strategic levy investment under the Hort Innovation Vegetable, Potato, Onion and Nursery Funds. This project has been funded by Hort Innovation using the vegetable, potato, onion and nursery research and development levies and contributions from the Australian Government. Hort Innovation is the grower-owned, not-for-profit research and development corporation for Australian horticulture.
Cover image: Photo by Elia Pirtle, Cesar Australia
