Posted on Saturday 28 February 2009
against the hop-fly, the turnip-fly, and the phylloxera. The smaller andthe more insignificant our enemy, viewed individually, the moredifficult is he to cope with in the mass. All the elephants in the worldcould have been hunted down and annihilated, in all probability, withfar less labour than has been expended upon one single little all butmicroscopic parasite in France alone. The enormous rapidity ofreproduction in the family of aphides is the true cause of ourhelplessness before them. It has been calculated that a single aphis mayduring its own lifetime become the progenitor of 5,904,900,000descendants. Each imperfect female produces about ninety young ones,and lives long enough to see its children’s children to the fifthgeneration. Now, ninety multiplied by ninety four times over gives thenumber above stated. Of course, this makes no allowance for casualtieswhich must be pretty frequent: but even so, the sum-total of aphidesproduced within a small garden in a single summer must be something veryextraordinary.
It is curious, too, that aphides on the whole seem to escape the noticeof insect-eating birds very tolerably. I cannot, in fact, discover thatbirds ever eat them, their chief real enemy being the little lizard-likelarva of the lady-bird, which devours them everywhere greedily inimmense numbers. Indeed, aphides form almost the sole food of the entirelady-bird tribe in their earlier stages of existence; and there is nobetter way of getting rid of blight on roses and other garden plantsthan to bring in a good boxful of these active and voracious littlegrubs from the fields and hedges. They will pounce upon the aphidesforthwith as a cat pounces upon the mice in a well-stocked barn orfarmyard. The two-spotted lady-bird in particular is the determinedexterminator of the destructive hop-fly, and is much beloved accordinglyby Kentish farmers. No doubt, one reason why birds do not readily seethe aphis of the rose and most other species is because of theirprevailing green tint, and the close way in which they stick to theleaves or shoots on whose juices they are preying. But in the case ofmany black and violet species, this protection of imitative colour iswanting, and yet the birds do not seem to care for the very conspicuouslittle insects on the broad bean, for example, whose dusky hue makesthem quite noticeable in large masses. Here there may very likely besome special protection of nauseous taste in the aphides themselves (Iwill confess that I have not ventured to try the experiment in person),as in many other instances we know that conspicuously-coloured insectsadvertise their nastiness, as it were, to the birds by their owninteguments, and so escape being eaten in mistake for any of their lessprotected relatives.
On the other hand, it seems pretty clear that certain plants haveefficiently armed themselves against the aphides, in turn, by secretingbitter or otherwise unpleasant juices. So far as I can discover, thelittle plunderers seldom touch the pungent ‘nasturtiums’ or tropsaelumsof our flower-gardens, even when these grow side by side with otherplants on which the aphides are swarming. Often, indeed, I find winged