Seasonal occurrence and life cycle of the allium leaf miner (Phytomyza gymnostoma) in Belgium

The allium leaf miner, Phytomyza gymnostoma (Diptera: Agromyzidae), is a damaging species to Alliaceae plants including leek, chives, onion, garlic and shallot. P. gymnostoma has spread quickly throughout Europe and North America. Adult flies emerge in spring from overwintered pupae and create feeding punctures in leaves and feed on the sap. However, economic losses are caused by the larvae which mine leaves, stems and bulbs and lower the marketability of the crop. Plants affected by P. gymnostoma larvae tend to rot due to secondary infections by fungi and bacteria in damaged tissues. Although P. gymnostoma has caused considerable damage to leek production in Flanders (Belgium), there is lack of information on its distribution, life cycle and seasonal phenology under Belgian climate conditions. Therefore, this research aims to 1) investigate the occurrence and distribution of P. gymnostoma in Belgian leek fields, and 2) study the life cycle and the emergence of developmental stages in the field, and at different constant temperatures in the growth chamber to develop a degree-day model.
Monitoring was performed weekly during the period 2016-2017 using yellow sticky traps and yellow bowls containing soapy water and a preservative. P. gymnostoma adults were reared on leek or chives plants and maintained at 15°C, 68% relative humidity and a photoperiod of 16h:8h (L:D). To study the life cycle, leek plants were placed in nylon cages and exposed to 2-day-old adults for 3 days. Then, plants were incubated at five constant temperatures (10, 15, 20, 25 and 30°C) and the emergence and duration of different developmental stages were determined. P. gymnostoma was detected in 16 organic and conventional leek fields in Flanders. The pest developed two generations in leek fields, the first in early June and the second in October. The duration of egg and larval development of P. gymnostoma decreased with increasing temperature. The pupal development was delayed at 10 and 25°C. The time required for completing the life cycle was 123 days at 15°C. Except for 30°C, at which no pupa turned to adult, P. gymnostoma completed its life cycle at all tested temperatures. The highest reproduction rate was observed at 15°C. Based on the thermal requirements of P. gymnostoma and the annual temperature of Belgium, maximum two generations can be completed per year. The degree-day model obtained from this study will help to develop an integrated management strategy against the allium leaf miner.
Key words- Agromyzidae, leek, degree-day model, phenology, temperature.