Insect and Spider Identification: CLOSED: What are these killing our lillies?, 1 by Abb6

Abb6 wrote:
Thousands of these criiters have appeared on all three varieties of our lillies and are causing die-off. The picture is not great, but they have soft yellow bodies, six legs and red triangles on thier backs. The largest ones have developed long skinny wings at the tail. They appear to still crawl and flit rather than fly. I have not found them on any other plant in our yard. They are causing no visible (to my completely untrained eye) damage but have definitely overwhelmed the plants and prevented any flowers from opening. What are they and how can I prevent them next season? This year's plants are already a total loss.