I found another species of Sawfly in our above ground pool today. This one is entirely black. Size is approximately 7-8mm. This looks to be in the genus Dolerus. From what I’ve found about host plants, it looks like the larvae feed on most grasses and horsetail. Females deposit eggs into the plant tissue where they hatch and the larvae will feed for approximately one month before exiting and pupating overwinter in the surrounding soil. There is one generation (univoltine) per year and adults emerge in early spring. Adults are recorded as feeding on tree sap from Maple (Acer), Apple (Malus), and Pear (Pyrus). They also take nectar from Willow (Salix) flowers, and from Cherry and Plum (Prunus) flowers. This means they do provide some pollination activity.
Looks like WWU Biology Department is working on a web page for Sawfly identification, but it’s not up and running yet. You can find their link below and bookmark it to check out at a later date.
References and Further Reading
Baine Q, Looney C (2019) Plant associations for three sawfly species (Hymenoptera, Tenthredinidae) in the Pacific Northwest. Journal of Hymenoptera Research 74: 27–33. https://doi.org/10.3897/jhr.74.46795
Looney C, Smith DR, Collman SJ, Langor DW, Peterson MA (2016) Sawflies (Hymenoptera, Symphyta) newly recorded from Washington State. Journal of Hymenoptera Research 49: 129–159. doi: 10.3897/JHR.49.7104
WWU Biology PNW Sawflies http://pnwsawflies.biol.wwu.edu/accounts/login/