R&D | EXOTIC PEST PROFILE L. trifolii damage to chrysanthemum. American serpentine leafminer: A threat to horticulture The American serpentine potato industries, and neither species is L. trifolii larvae feeds internally on living leafminer (Liriomyza trifolii) is a present in Australia. plant tissue, particularly plant leaves, small fly belonging to the family The following is an overview of reducing photosynthetic activity and Liriomyza trifolii (L. trifolii), commonly causing premature leaf drop. Unlike many Agromyzidae. It infects plant known as the American serpentine other leafminers, the American serpentine species from 29 plant families leafminer, focusing on the pest’s biology, leafminer is polyphagous (it has a broad including many vegetable, distribution, effect on horticultural host range from a number of families). ornamental and legume crops. industries worldwide, and chemical and It also has a high reproductive rate and Currently, Australia remains free biological control strategies. has developed resistance to several of this leafminer species but it is classes of broad-spectrum insecticides. Pest overview Ornamental crops, legumes and now well-established in nearby vegetables are affected by the pest. countries, including Indonesia. Adult L. trifolii can grow between 1-1.7 Host families include Apiaceae (celery), However, if the pest does millimetres. The thorax and abdomen are Asteraceae (chrysanthemum, gerbera, establish in Australia, it could grey/black with patchy yellow regions, lettuce), Brassicaceae (broccoli, threaten our horticulture industry. while the head is completely yellow. cauliflower), Cucurbitaceae (cucumber, AUSVEG Biosecurity Officer Females will use their ovipositor (a tubular melon, pumpkin), Fabaceae (beans, organ which can rasp through the leaf lentils), and Solanaceae (potato, tomato, Madeleine Quirk reports. surface) to insert eggs just below the leaf capsicum). surface that hatch between 2-5 days after Damage from this pest reduces both The Research, Development and being laid. Eggs are too small to be seen marketability and crop yield, and has Extension program for control, eradication by the naked eye, so a seemingly healthy resulted in economic losses to growers and preparedness for vegetable leafminer plant may be harbouring the pest without globally. Examples include: (MT16004) was developed in recognition us knowing. • Between US$18 and $21 million of the extensive impact that vegetable Inside the leaf tissue, colourless damage per year to Californian leafminer (VLM; Liriomyza sativae) could larvae grow to 3 mm, and as they move greenhouse ornamental growers in have on the vegetable, nursery and through three instar phases, the larvae the early 1980s. melon industries if it were to move into become yellow-orange and exit the leaf • $93 million in damage to the production areas with no management at the third instar phase. Larvae pupate Californian chrysanthemum industry plan in place. Project partners include externally to the leaf, usually in the soil between 1981 and 1985. Cesar, Plant Health Australia, Northern below the plant, from which adults • 80 per cent losses to celery growers Australia Quarantine Strategy (NAQS), emerge 7-14 days later. in Florida in 1980, estimated at the University of Melbourne and AUSVEG. The duration of L. trifolii’s life cycle US$9 million. The project has recently expanded varies with temperature. At 28 degrees to investigate two closely related Celsius (°C), a single cycle can be Risk of spread and establishment leafminers, Liriomyza trifolii and achieved in 14-15 days. At lower in Australia Liriomyza huidobrensis. These pests have temperatures, the cycle takes longer. been recognised as potential threats to Research has shown that adults will not Major risk entry pathways for leafminers Australia’s vegetable, nursery, melon, and feed or lay eggs at temperatures as low into Australia are by importation of as 12°C. infested ornamental host plants, cut 34 | Vegetables Australia Adult American serpentine leafminer. Images courtesy of Central Science Laboratory, Harpenden, British Crown, Bugwood.org. flowers, leafy vegetables and seedlings. keep leafminer populations in check. Further Reading Human-assisted entry can also occur, Overseas, several insecticides are used e.g. illegally on plant material. for the control of L. trifolii, including but • Capinera JL. 2017. American Globally, L. trifolii dispersal and not limited to abamectin, azadirachtin, serpentine leafminer, Liriomyza trifolii establishment has occurred rapidly, with chlorantraniliprole, cyromyzine, (Burgess). University of Florida. populations found on most continents, indoxacarb, spinetoram and spinosad. including Europe, Asia, Africa, Central Currently, Plant Health Australia (PHA) is America, the Caribbean, North America, seeking to obtain insecticide permits for South America and Oceania (American control of L. trifolii and other Liriomyza Samoa, Guam, Fiji and Tonga). It has leafminers in Australia as a preparedness recently been identified in Indonesia. measure for growers. PHA will be Movement of cut flowers and infested releasing a contingency plan in coming chrysanthemum cuttings have played a months, which includes further details on major role in the spread of L. trifolii from these applications. • Reitz SR, Gao Y & Lei Z. 2013. its original range in the Americas to other Nevertheless, many species of Insecticide use and the ecology regions in the world. parasitoid wasps have been recorded of invasive Liriomyza leafminer While L. trifolii is not yet present in attacking the pest. MT16004 project management. Australia, many horticultural production partners from Cesar and the University regions in Australia have climatic of Melbourne are currently reviewing conditions similar to locations overseas international literature to investigate the where it has already established, suite of parasitoids that attack L. trifolii and therefore could be at risk of worldwide, and they will cross-reference establishment. It is a heat-tolerant this list with those that already exist species, meaning it will thrive in in Australia. tropical locations, potentially tolerating They have already identified possible temperatures near 35°C. It does not parasitoid species for future control of adapt as well to cool-climate regions as L. sativae and have identified several other leafminers, but it could maintain wasp species attacking Torres Strait populations year-round in protected and Seisia populations. Some of these environments such as greenhouses. wasps are also found in other regions of Australia, which is promising for future Chemical and biological control management of vegetable leafminer. It Find out more is highly likely that the same, or similar, Any unusual plant pest should be reported L. trifolii can rapidly develop resistance suite of parasitoids that might control L. immediately to the relevant state or territory agriculture agency through the Exotic Plant Pest to various chemical groups, particularly sativae populations in Australia could also Hotline (1800 084 881). organophosphates, carbamates, diamides assist in potential control of L. trifolii. For further information, please contact AUSVEG’s and pyrethroids, which can make Extension and Engagement Team on 03 9882 0277 or email [email protected]. control difficult. Application of broad- This project has been funded by Hort Innovation spectrum insecticides often results in using the vegetable, nursery, melon and potato research and development levies and larger leafminer populations as these contributions from the Australian Government. insecticides reduce the reservoir of natural Project Number: MT16004 enemies (parasitoid wasps as well as other generalist predators like spiders), which Summer - 2019/20 | 35 R&D | PROJECT UPDATE Parasitoid wasp (Diglyphus isaea) compared against common wasp (Vespula germanica). Source: Elia Pirtle (parasitoid wasp), Shutterstock (common wasp). How parasitoid wasps are helping to fight the leafminer battle In 2017, a Hort Innovation Vegetable Fund project was established to recognise the damaging impact that vegetable leafminer could have on some of Australia’s fresh produce industries if it were to move into production regions. AUSVEG Biosecurity Officer Madeleine Quirk explores the role of parasitoid wasps in controlling vegetable leafminer populations in far-north Queensland, the importance of parasitoid wasps overseas, and research conducted into reservoirs of parasitoid wasps within Australia’s non-pest leafminers. Vegetable leafminer (Liriomyza sativae; numbers of leafminer larvae by directly News from the incursion front VLM), potato leafminer (Liriomyza feeding on host larvae to acquire nutrients huidobrensis) and American serpentine for the development of additional eggs in Cesar Research Scientist Dr Elia Pirtle has leafminer (Liriomyza trifolli) are exotic the wasp. undertaken extensive field research in the flies that cause significant damage to Dr Peter Ridland from the University Torres Strait and Cape York Peninsula, horticultural commodities overseas, and of Melbourne has conducted a studying parasitoid wasps and their potentially threaten Australia’s vegetable, literature review of the wasps that relationship with VLM. nursery, melon and potato industries. parasitise leafminer flies in Australia and “Some of our Australian species already VLM established itself in the Torres Strait internationally. show signs of very effective vegetable in 2008 and was subsequently