A method to predict the beginning of the flight of apple sawfly, Hoplocampa testudinea (Klug) (Hymenoptera, Tenthredinidae) is presented. Adults were monitored during 8 years at the experimental orchard ‘De Schuilenburg’, Kesteren, The Netherlands, with visual sticky traps. Temperature sums, accumulated until the day of the first capture of sawfly, were calculated for different starting dates of the summations. Beginning on 1 January, variation of the temperature sums, calculated in each of 8 years, decreased when later starting dates for temperature summation were chosen, reaching a minimum on 1 April for soil temperatures (−5 cm). The soil temperature sum, accumulated from 1 April until the first capture, was on average 134 day-degrees above a threshold of 4 °C. So this was the most suitable starting date to predict the beginning of sawfly flight activity. As use of the summation method based on air temperatures is in practice more appropriate for timing of trap installation by extension services and apple growers, trap deployment is recommended at an adapted value of 157 day-degrees from 15 March.