Friday, January 29, 2010
The prolific aphids
So, your rose bushes look lovely, loaded with blossoms and you glory on their beauty. Then, disaster strikes, almost out of nowhere an invasion of vicious sap suckers starts draining the life out of the bushes: Aphids! You engage in a losing battle, hosing them, spraying soapy water, squashing them between your fingers; you refuse to use pesticides because you see yourself as ecologically responsible, besides pesticides wouldn't do much good anyway. The trouble is that when the infestation reaches a certain point it is already too late.
This is the rose aphid (Macrosiphum), a member of the very large and very odious family Aphididae and here we have an extremely heavy infestation. There are adults and youngsters, called nymphs, struggling for a free spot where to pierce the skin of the stem and feed on the plant juices. Notice that there are two kinds of adults, wingless and winged ones. Most aphids are wingless; they are little machines devoted to feeding, growing and reproducing, nothing else; they dispense with any other activities, even sex. But when conditions get too crowded some of them grow to be winged adults and fly off in search of better feeding areas.
So, what can you do to protect your roses? Prevention and foresight are the most effective tools against this stealthy enemy. It helps to know a little about their complex life cycle. In the fall or early winter rose aphids give birth to both males and females; they mate and they lay eggs which can overwinter safely. All the adults die with the onset of cold weather and the absence of juicy fluids in the plant stems; but the eggs can survive until next spring.
The eggs are pretty tiny, dark, shiny buttons scattered along the bush stems. You will have to try really hard to see them because of their minuscule size, less than 1/16"! Ah! But, if you could eliminate them or at least reduce their numbers considerably your worries would be almost over. In the spring each egg produces one female, but what a female! It is a supermom (professionals call her a foundress). It doesn't need to mate with a male, and it produces only live born females, which in turn do nothing but produce more and more babies. Since it doesn't spend any energy searching for food and since it doesn't need males to reproduce, the numbers can grow at an accelerated rate.
The colony shown above consists of descendants of one of those supermoms, a huge clone of identical twins, all of them feeding on your prized rose bush. That is why eliminating those nearly invisible eggs would protect your bush better than any later action.
The other line of defense is the wide array of aphid enemies, which under the right circumstances can keep the aphid populations from getting out of hand. Their enemies are our friends; who are they? Ladybugs are the first to come to mind; we all know that they are the gardener's helpers. There are a few others, just as helpful or perhaps even more so. Tiny parasitic wasps that lay their eggs inside the aphids; their larvae eats the growing aphid from the inside; gruesome, but an effective pest control. Syrphid fly larvae are gluttonous aphid eaters; each one can eat a few dozen aphids in the couple of weeks that it takes to reach maturity. Lacewing larvae, with formidable jaws, can attack aphids easily. All these are considerably smaller and less conspicuous than ladybugs, perhaps this is why they usually go unnoticed but they all play important roles in the control of aphid populations.
The trouble with pesticides is that they also kill all these aphid enemies; unfortunately the restoration of their populations takes considerably longer than the build up of aphid populations. So the immediate consequences of using pesticides are good but the long term consequences are counterproductive because the enemies of aphids aren't there when needed.
List of articles