The water lily aphid has a vector. It is trees in the family Prunus. This includes mainly cherry trees, especially the wild cherry. The aphid over winter on the cherry trees and crawl down to find the water lily leaves when they come out in the spring. If you have any cherry trees within a hundred feet or so of your pond you can control water lily aphids by either cutting the trees down or by spraying them each year in February (before the buds come out) with dormant oil spray. Be sure not to get too much spray in your pond, however. Barring tree removal or spraying, you can try spraying the aphids with a mixture of water and dish detergent (joy) at a ratio of 1 tsp per gallon). Again, be careful with the spray, just spray the lily leaves with the aphids, don't spray the whole pond.