You are admiring the roses when you notice what appear to be flecks of white cotton. Closer inspection reveals the stems crawling with green bugs: aphids. The white flecks are their moulted skins.
The roof of your car, parked under shade trees, is covered with a sticky coating. Lick your finger and the substance tastes sweet: aphids. The substance is called 'honey dew.' You've been shat upon from a great height.
Where do they come from? While they can migrate naturally great distances on winds it could be that your neighbour has failed to maintain a
sanitaire cordon . . . or you have yourself. Any newly-purchased or gift plant should receive a precautionary spray of insecticidal soap. Apply a dormant spray of horticultural oil to trees and shrubs in late winter or very early spring. This will destroy the overwintering eggs of aphids along with many other pests.
Look again before spraying the roses. They may be preyed upon by ladybeetles, hoverfly larvae, parasitic wasps, aphid midge larvae, crab spiders, and lace wings, as well as entomopathogenic fungi which you won't see. Spray and you're going to kill all these friendly bugs as well.
There are 4,400 species known, of which perhaps 250 species constitute serious pests. In North America they are usually called aphids or aphis. Confusingly, in Brit-talk, they are greenflies and blackflies. But they are not members of the true fly order of Diptera. Instead they are in the order Hemiptera (formerly classed in Homoptera--entomologists are an unsettled lot), and the superfamily Aphidoidea.
Aphids feed by thrusting their proboscis into plant tissue and siphoning off the sap. This is often so sweet that excess passes through their bodies to be excreted as the aforesaid honey dew. This attracts ants which may actually "farm" the aphids, moving them to new plants and fighting off predators. Meanwhile the loss of sap causes leaves to curl, buds to drop and, in severe infestations, plant death. Worse yet, aphids serve as vectors of plant diseases, particularly viruses.
The number of aphids can be astounding. This may be ascribed to their sexual habits--or lack thereof. This involves parthenogenesis or asexual reproduction. Wingless females emerge from overwintering eggs and produce living young females rather than eggs. Every few weeks the process is completed. It is known for already formed aphids to be inside those yet unborn. In late summer or early fall, triggered by change in day-length, the females produce a mixed generation of winged males and females, which may fly off to alternate host plants. These mate, the females produce overwintering eggs and the whole process is repeated to horticultural distress.
Spray if you must but more savvy solutions exist in your hands. They are called fingers. Run them up infested stems, squishing the little blighters to death. Better yet, in dying they release a warning scent to other aphids that death and destruction lurks nearby, advising them to clear off. It is a cheap, elegant and environmentally safe solution.
