## 2 Materials and Methods  
### Strawberry Sampling and Fly Collection  
Strawberries were collected from commercial fall-planted fields grown on raised beds in California's major production areas near the end of the season when fruit is more susceptible to drosophilid fly infestation. These areas represented 98.6 percent of California production and 86.7 percent of total U.S. production during this period [12]. Four fields (OXN 1, OXN 2, OXN 3, and OXN 4) near Oxnard, Ventura County were sampled on 30 May 2019. A second collection was made from three of these fields (OXN 1\({}^{\prime}\), OXN 2\({}^{\prime}\), and OXN 3\({}^{\prime}\)) on 18 June 2020, and a nearby field (OXN 5) was also sampled on that date since the processing strawberry harvest in OXN 4 had already been terminated. Due to COVID-19 and associated travel restrictions, no samples were collected in spring 2020. In the Santa Maria area (Santa Barbara and San Luis Obispo Counties), fruit samples were collected from three fields (SAT 1, SAT 2, and SAT 3) on 12 August 2019, and from two fields (SAT 4 and SAT 5) on 12 August 2020. In the Monterey Bay area (Monterey and Santa Cruz Counties), samples were collected from three fields on 17 October (MON 1, MON 2, and MON 3) and 5 November (MON 1\({}^{\prime}\), MON 2\({}^{\prime}\), and MON 3\({}^{\prime}\)) in 2019, and from four fields on 27 October (MON 4, MON 5, MON 6, and MON 7) and 10 November (MON 4\({}^{\prime}\), MON 5\({}^{\prime}\), MON 6\({}^{\prime}\), and MON 7\({}^{\prime}\)) in 2020.  
On each sampling date, 20 to 40 rip (suitable for fresh market) and 13 to 28 overripe (suitable for processing) fruit were randomly collected from each site and returned to our laboratory at the University of California, Davis to assess the levels of larval infestation as well as the relative abundance of _Drosophila_ species causing infestation. To rear _Drosophila_ adults from the samples, strawberries were placed in plastic containers (\(16\times 28.5\times 9.5\) cm) (The Container Store Group, Inc., Coppell, TX, USA) containing a thin layer of cotton topped with sand as a substrate for pupation. The lids of these containers had large openings (\(10\times 12\) cm) covered with fine mesh to ensure ventilation. The containers were then placed in a walk-in growth chamber at \(23\pm 1\)\({}^{\circ}\)C, 55-65% RH and a 14L:10D photoperiod (Percival Scientific Inc., Perry, IA, USA) and checked daily until fly emergence. Emerged flies were aspirated into Fisherbrand drosophila bottles (Fisher Scientific, Inc., Portsmouth, NH, USA) containing Bloomington standard drosophila cornmeal diet, then plugged with cotton. These flies were then anesthetized using CO\({}_{2}\) to facilitate their identification as SWD versus non-SWD _Drosophila_ using morphological characteristics.  
For samples collected from the Monterey Bay area in 2020, larvae were extracted from fruits immediately following collection from half of the samples. To extract larvae from the samples, strawberries were gently crushed and floated in a standard salt-water solution (125 g salt in 4 L of lukewarm water) for a minimum of an hour. The solution was then strained through a fine strainer, and the extracted _Drosophila_ larvae from each sample were collected with a fine brush and transferred to vials containing 70% ethanol for storage before being processed for DNA extraction and identification as SWD vs non-SWD _Drosophila_ using molecular diagnostic markers. The remaining fruit from each sample were used to rear out adults as described above.