Aphids are tiny oval light green (usually) bugs that particularly enjoy feasting on new growth. They suck out the juices of leaves and stems. They also secrete a sticky substance that is attractive to ants. You can knock off aphids by spraying them with water. Or you can use Lady bugs
In their book, Green Thumb Wisdom: Garden Myths Revealed!, Doc & Katy Abraham say: [The Lady bug/beetle] is unusual because it overwinters in massive mountain clusters in California, allowing collectors to harvest and package large numbers for sale in nurseries and through garden supply catalogs and other outlets through the country. In case someone asks you, its name is Hippodamia convergens, order Coleoptera and family Coccinellidae -- a real friend to have in the home or the garden. When they run out of aphids (plant lice) they either feed on themselves or move on.
Robert Osborne, of Corn Hill Nursery, writes in his book, Hardy Roses: An Organic Guide to Growing Frost- and Disease-Resistant Varieties: Aphids are perhaps the most common pest of roses. They are small, soft-bodied, usually lime green creatures that puncture the soft new growing tissues with their mouths and suck the leaf juice... [Osborne goes into detail about aphids in the book, their life cycle, etc., which is good to know if you've got a problem with them]... Each aphid is theoretically capable of producing millions of aphids by the end of its cycle. The reason we are not swimming in aphids is because so many other insects and birds consume aphids... [Osborne calls lady bugs "the most efficient enemy of the aphid"...] When aphids begin to multiply, adult ladybugs arrive at the aphid colonies and lay their eggs. After about two weeks, tiny, opaque and ravenous larvae hatch and begin feeding. They hold the aphids in their large mandibles and suck their insides out. In only a few days these larvae grow to nearly twenty times their original size and eliminate the colony of aphids...
[Here's what Osborne recommends to tackle aphids:] It is imperative that you do nothing when aphids first appear on your roses. Spraying at this stage, even with soap or similar nontoxic substance, is a tragic mistake, for the ladybug's eggs or larvae, as well as other predators, will be killed... Once the ladybugs establish a presence, the aphids will be kept to minimal levels...
Roses vary tremendously in their attractiveness to aphids. One hardy old favorite, 'F.J. Grootendorst', is notorious as a gourmet treat for aphids. At the nursery we used to spray this variety often to try to keep the aphid population down, with only limited success. Once we let the predators do our work, we found that, after the required waiting period, our 'Grootendorst' becomes a wonderful place to study predators in action. Now our 'Grootendorst's stay relatively clean all season. In contrast, a new variety, 'Champlain', must be last on the aphid's list of restaurants, because we never see the pests on this rose.
[The first year I grew roses, the garden was overrun with aphids and I had a tough time controlling them with pesticides. The following year, I dispensed with the pesticides and released some lady bugs in the garden. Ever since then, and as far as I can see, the aphid population has been negligible. I feel it's due to a combination of predatory insects, like lady bugs, and blasting those aphids I do see with the water hose! -- Alex Sutton]