The question is what effect does it have on beneficial wildlife? There is the danger that you end up killing off the predators as well as the pests, so you end up with a big food source with no predators, and new pests come in, with no predators to knock the numbers down, they multiply in bigger numbers than before. This locks you into a vicious cycle of spray, plague, spray, plague etc. I would rather check regularly for blackfly and run my fingers up the stems squishing them as I find them.