Fortunately for our orchids, they all die of cold, especially in frosty weather. But if that’s so, how can aphids reappear in spring? It’s because the crafty devils make sure that they lay a good supply of eggs in nooks and crannies before they die, that’s why. These eggs, unlike their parents, are unaffected by the cold and remain dormant until the warmer days of spring arrive.
When the eggs hatch, all of the young aphids are wingless and female. After a few days they begin to bear live young, with no intervention or assistance from male aphids (there are none at this stage!). The rate of reproduction is incredible – it has been calculated that if all survived, the progeny of a single aphis after ten generations would weigh more than the entire human population of China!! Actually, that calculation was done many years ago – it may take eleven generations today!
Fortunately for both us and our plants, aphids have many enemies, as well as being sensitive to the cold. They are eaten by birds and also by a variety of other insects. Ladybirds, in particular, are particularly fond of aphids and can eat 30-40 per hour! What greedy little devils ladybirds are, which explains why they’re sold commercially as predators of aphids.
Aphids are sap suckers. The aphis drills a hole in the soft new foliage with her beak and then sucks the sap, which is freely available at that stage. She has greater difficulty as the growth ages, because it’s harder for her to drill a hole, and there is also less sap in the older growth. Many aphids break their beaks and then die of starvation.
As the cooler days of autumn approach, there is a change in the life cycle. Both male and female aphids are born at this time. Both are born with wings, presumably to provide greater mobility for their sexual adventures. These females lay eggs, not live aphids, and so the species prepares itself to survive another icy winter.
Orchids are readily infested by aphids (except in winter), which soon reach plague proportions unless control measures are implemented. Some growers squirt them off the foliage when watering as a control measure. But this practice fails to dislodge all of them and the survivors soon breed replacements. It’s often argued that “they don’t eat much” but this isn’t strictly true – they probably suck sufficient sap to retard the development of the new growths.
More important, however, is the possibility that they can transfer virus from one plant to another – the virus recently found in native dendrobiums is almost certainly transmitted by aphids. Maybe the aphids are wingless in spring and summer but they can certainly fly about in autumn. Also, if you hang your plants from vertical wire mesh, as I do, the aphids can fall from one plant to lower ones, regardless of the season. Therefore I spray my plants with an insecticide as soon as any aphids appear. Almost any insecticide is effective. For small outbreaks I use one of the many pressure pack sprays available at plant nurseries, taking care to hold the can at least 300 mm from the plant (to avoid frost burn caused by rapid evaporation of the propellant). For larger outbreaks I use Orthene®, a wettable powder, applied as an aqueous solution; unfortunately, it is not available from nurseries and is only sold in kilogram amounts at horticulturalist supply outlets.
Much of the information in this article was gleaned from an article written by the late Crosbie Morrison over fifty years ago. Many readers of my vintage will remember Crosbie Morrison for his fascinating nature talks on radio in the 1940s and 1950s. At that time one referred to one aphis, but two aphids. However, newer dictionaries give the option of aphid or aphis for a single individual, while a group of them can be called aphids or aphides.