Allium leafminer (ALM) Phytomyza gymnostoma, an invasive insect from Europe, was discovered in Lancaster County, Pennsylvania in December 2015 and has since spread to at least five additional states. Larvae feed in leaves, stems and bulbs of all vegetable alliums causing plant damage, secondary infections and/or market rejection. The purpose of our research was to develop management recommendations for ALM. We determined the flight periods of ALM and created a degree day model to predict emergence to know when control measures were needed.  We also evaluated the efficacy of insecticides labeled to control native leafminers in allium on ALM. Research from Austria showed two flights of ALM (spring and fall) lasting from 3 to 4 weeks; our observations from population studies beginning in fall 2016 found similar emergence times but flight periods of 5 to 7 weeks. Research trials began in fall 2017 to evaluate the efficacy of various insecticides for this pest on leeks and continued with trials on sweet onions in spring of 2018 and 2019 as well as additional leek trials in fall of 2018 and 2019. Trial results with spring-planted onions indicate that insecticide applications may not be necessary as minimal damage will occur from ALM on this crop in Pennsylvania. Conventional insecticides that were most effective for ALM control on leek included dinotefuran, cyantraniliprole, and spintoram and organic options included spinosad and azadirachtin. Spring and fall flight periods were monitored during these seasons to advise growers when control measures were necessary. The use of colored sticky traps for monitoring ALM emergence in 2016 and 2017 was not as accurate as visually scouting fields for leaf damage. A spring-emergence degree-day model was developed in 2019 to more accurately determine when scouting should begin and will be validated in 2020. Work continues on developing a fall emergence degree-day model. Fall emergence is has been observed to start with cool temperatures but then stop with subsequent warmer temperatures making model development more challenging. Growers following our recommendations have reported successfully control of ALM in their crops.