I have a bed of canna in my garden . There are about 14 plants.So far there were no flowers. Now one of them has developed a bunch of buds out of which one flower has bloomed today. But the flower was drooping and when I went near and checked it I saw two green worms with little spikes allover their body on the flower. There was brownish residue on the leaves near the buds.I don't know what pests these are and what pesticide I should use. Can anybody help me?

These sound like caterpillars rather than worms. I haven't heard of any worms with spikes on them before. I find that an environmentally friendly plant feed and pesticide combined, sprayed regularly on to plants keeps most bugs at bay. The one I use is SB Plant Invigorator which can be bought on the web at www.sbplantinvigorator.com. Works a treat for me!