Life can take some very extraordinary forms, and the aphid is just one, albeit familiar, example. The sight of the first one or two greenfly in spring and early summer does not evoke a recognition of impending serious plant destruction. It is only when the few become thousands and even millions, usually within the space of a few days, when buds and shoots disappear under a seething mass, that it occurs to many gardeners that they must remember to get something to deal with them - next time they go to the shops!
There are several insecticides that will quickly polish off greenfly; preparations based on lindane and malathion have been available for several years and are quite reliable. Having regard to the method by which the tiny insect sucks out the sap, you would expect it to seek new and soft tissue into which to sink its tube, which explains why you find them concentrating on new soft shoots, buds and on the softer undersides of leaves.
These are very soon blown on the wind to other plants when they all quickly begin to give birth, repeatedly, to living young wingless females, which within a short time each begin to give birth repeatedly to wingless females, and so it goes on generation after generation, multiplication upon multiplication. Literally, millions of offspring are produced in a short time, and not a single male is involved - they do not exist!
This reproductive process, called parthenogenesis, is not at all uncommon in the insect world, and by a long, long way is nothing like as incredible or bizarre as some examples. It does, however, explain how a plague can appear almost overnight. The multiple birth continues through the summer months, encouraged by mild weather and suitable host plants like your roses.
Overcrowding is one condition - there are others - that triggers a hormone that leads to the production of more winged females which are soon blown away by wind.
The healthy plants remained healthy. However, as soon as they were exposed to greenfly from the infected plants, they quickly showed all the signs of virus infection.
Viewed 134 times by 44 viewers
