Pest of the month: Aphids

With the coming of spring pests are becoming more apparent on bonsai plants. Last week, I saw a couple of winged aphids on a Japanese Box and on closer examination of its young shoots found wingless individuals as well. I was sure wingless aphids were just immature stages of the winged adults. In the lab, I was surprised to find that the aphids were three different species: the Potato Aphid (Macrosiphum euphorbiae, left image), the Rose-grain Aphid (Metopolophium dirhodum) and the Black Citrus Aphid (Toxoptera aurantii, right image). As their names suggest all three species are common pests of roses, grasses, potato and many other plants. I have roses and lime in my garden, but the aphids were feeding on a Japanese Box, probably because it had lots of new shoots. All three species give birth to live young and they don’t even need to mate to reproduce. The Black Citrus Aphid adults can be with and without wings. One of my colleagues told me that adults of this species can make an audible noise by rubbing their hind legs against their body, but I have difficulty believing that. Jokes aside, aphids multiply very quickly and it is good to nip them in the bud by spraying plants with a pesticide. They feed on plant sap, transmit plant viruses and their excreta on leaves encourages the growth of sooty mould fungi.