Distribution, life cycle and overwintering ability of the allium leaf miner (Phytomyza gymnostoma) in Belgium

The allium leaf miner, Phytomyza gymnostoma (Diptera: Agromyzidae), is a destructive pest of Allium crops including leek, chives, onion, garlic and shallot. This is a European species which was first described from Poland and is now distributed throughout Europe and the United States. Adult flies emerge in spring from overwintered pupae and create feeding punctures in leaves using their ovipositor and feed on the sap. Economic losses, however, are caused by the larvae which mine leaves, stems and bulbs and lower the marketability of the crop. Damaged plants by larvae are very sensitive to fungi and bacteria. Although P. gymnostoma has caused considerable damage to leek production in Belgium, there is lack of information on its distribution, life cycle and overwintering under Belgian climate conditions. Therefore, this research aims to 1) investigate the occurrence and distribution of P. gymnostoma in Belgian leek fields, 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 and 3) to explore the overwintering ability of the pupae.
Monitoring was performed weekly during the period 2016-2019 using yellow sticky traps. To study the life cycle, leek plants were placed in nylon cages and exposed to adults for 3 days. Plants were then incubated at five constant temperatures (10, 15, 20, 25 and 30°C) and the emergence and duration of different life stages were determined. To study the overwintering ability, the lower lethal temperature (LTe50) was determined by subjecting the pupae for 1 min to low temperatures (-5 to -20°C). The pupae were then kept at 15°C and the number of emerged adults was recorded. In another experiment, pupae were exposed to four temperatures (5, 0, -5 and -10°C) for several periods (0, 4, 8, 16, 32 and 64 days). After cold exposure, survival was measured by counting the number of emerged adults from the pupae at 15°C.
P. gymnostoma was detected in several organic and conventional leek fields in Flanders, Belgium. The pest developed two generations per year, one in April and one in October. The duration of egg and larval development decreased with increasing temperature. The pupal development was delayed at 10 and 25°C. The time required for completing the life cycle was 110 days at 15°C. Except for 30°C, at which no pupa developed to adult, P. gymnostoma completed its life cycle at all tested temperatures. The highest reproduction rate was observed at 15°C. The predicted LTe50 for pupae was calculated -14.81°C. Based on the thermal requirements of P. gymnostoma and the annual temperature of Belgium, maximum two generations can be completed per year. Our results indicated that approximately 80% of pupae can survive Belgian winter conditions. The developed degree-day model and knowledge on the thermal tolerance of P. gymnostoma will help in implementing integrated management strategies against the allium leaf miner.