Black Aphids, Rabbits Of The Bug World
One day you spot a few black aphids in your garden. A week later they're all over the place, or at least seem to be. Black aphids are in a sense the rabbits of the bug world, except they breed even more prodigiously. A pair of black aphids, once established, can become 80 or so in a week's time, and repeat the process several times.
Black aphids are not necessarily the Darth Vader of the aphid world in spite of their black coloring (sometime black species are more hairy than others and appear almost grey). Black aphids aren't really any more difficult to control than aphids of other colors, though a few will disagree with that contention.
Suckers And Disease Spreaders - Aphids don't eat plants in the same sense that many other insects do. Aphids, including black aphids are sucking insects. They pierce leaves and flowers and suck out the juices. In doing so, they sometimes spread disease from one plant to another. Because of this disease-spreading characteristic, a colony of black aphids can cause a great deal more damage to a garden than would normally be the case if they simply sucked the life out of a few plants.
Once an aphid population gets out of control they can obviously constitute a problem. Still, they are for the most part not to difficult to get rid. One approach, though not favored by many, is to do nothing. The aphids will soon eat themselves out of house and home and eventually die off. Letting this happen isn't usually a sound approach, especially if you have some prize rose bushes you'd want to protect.
Ladybird, Ladybird - Another approach is to purchase some predictors, ladybugs (ladybirds) for instance. Ladybugs and ladybug larvae will happily feast on any aphid within striking distance. If you purchase a box or packet of ladybugs they sometimes will simply fly away, but if there are aphids about, enough of them will stick around to clean things up. The lacewing fly is another super aphid killer, and like the ladybug is not itself in any way a pest.
Spray Away - The garden hose is another weapon one can use against black aphids. Blasting the little pests away from plants with a stream of water sometimes is all that needs to be done. Even though aphids may travel around somewhat freely from plant to plant, or on a single plant, once on the ground they seem to have a problem finding their way back to plants. If you spray away some aphids and see more aphids a few days later, they very likely are not the same aphids.
Aphids can also be sprayed with a weak detergent solution (which penetrates the shell and causes dehydration) or an oil solution (which messes up their ability to breathe). Harsher insecticides, while effective, can be overkill. When spraying anything on your plants besides water, the well-being of the plant needs to be taken into account as well. When spraying for black aphids, especially with a compound containing oil, one should avoid spraying during a hot sunny afternoon, as the collection of drops on the leaves can create small magnifying lenses which can result in the foliage becoming burned. Controlling aphids should not become a search and destroy effort, where not only are the aphids killed off, but your prized plants are as well.
A couple of final thoughts. When controlling black aphids, spread around a little ant powder as well if ants are about. Ants raise aphids like ranchers raise cattle. Aphids provide ants with honeydew, and in return, the ants carry aphids to places where the grazing, or sucking, is especially good. The other thought is, if there are not too many black aphids around, and ladybugs make an appearance, leave the aphids alone. The ladybugs will deal with them, and may just stick around for the summer.