THE PLAGUE OF CATERPILLARS.
[To THE EDITOR OF TUB " SPROTATOR.1 doubt if the interesting old records quoted by your correspondent throw much real light on this matter. Tilllately, there were no trained observers of natural phenomena, and there was so little knowledge of their causes, that few could have been competent to judge whether one cause was more pro- bable than another. Take, for example, the statement that,
In 1499, in Germany, vegetation was destroyed by blight and Caterpillars together." I should ask (if there were any one to answer), What blight and what caterpillars ? Of caterpillars, there are hundreds of different kinds that prey on vegetation, many with widely different habits ; and as to " blight," that may be described as a term used by practical gardeners to make their employers believe they know all about it, when anything has injured vegetation. It is analogous to the railway guard's answer to the passenger who asks why the train is stopped, and is told, " Because the signals are against us." If the tender shoots curl up, or the rosebuds, instead of opening, turn brown, or the leaves look mildewed and spotted, or there is a great abundance of aphides, or hairy, black flies, the gardener has one confident explanation, " It is a blight," and he probably adds something in a mysterious tone about the east wind having brought it. As to Schenkius's statement that the winter, having been so severe as to kill nearly all the brute creation, the sunshine next summer was so hot as• to set the trees on fire,—I do not believe Schenkius. I think it will be by such skilled and careful observations as those of Miss Eleanor Ormerod that we when arrive at adequate knowledge on the subject. In the mean- time, may I venture to offer the following general considerations ?
1. Since the number of individuals, on the average, remains the same, an enormous majority (say, ninety-nine hundredths) die without continuing their species. 2. Every insect has four stages to pass through in a year, sometimes in half a year or less, viz., egg, caterpillar, pupa, and winged insect ; it is in the caterpillar stage that all the eating is done. 3. The caterpillar and the winged state, being the stages of exposure, are those in which most of the thinning takes place ; and the latter is the more important stage in this respect, because by the time a caterpillar has grown large enough to be made a satisfactory meal of, it has already done much mischief. 4. In the cater- pillar stage, the thinning effected by birds, tree-bugs, carnivor- ous caterpillars, &c., is enormous. Ichneumon flies are not to be included, for though they kill, death does not take place till the caterpillar is full fed; and, in the meantime, the young, parasitic family they provide it with has probably increased the cater- pillar's voracity. 5. In the winged stage, also, immense numbers are destroyed by birds and by bats and carnivorous insects. 6. But none of these checks on multiplication, nor all together, appear to be sufficiently variable to account for that sudden in- crease in caterpillars in some years that is spoken of as a plague ; the question arises whether we do not find such a check in the weather. I doubt whether any one who has not been a practical entomologist has an adequate conception of the thinning effect exercised by unfavourable weather on insects in the winged state, or the correspondingly quickening effect of weather that is favourable. The effect is concentrated, because the existence of insects in this state is brief—a few days for an individual, a few weeks for a whole species—and it is strong, because in this stage they are so sensitive to it. It is not merely that rough weather destroys them, and that in windy weather they do not fly, but the mere absence of sunshine is enough to prevent many species from moving at all. Like the "pale primroses," but in a different sense, they,— " Die unmarried, ere they can behold
Bright Phoebus in his strength."
The sunless summer of 1860 nearly exterminated from some of the Sussex and Hampshire woods several species which had formerly been plentiful there. A rainy fortnight may produce such an effect on a species ; on the other hand, a hot fortnight at the right time may foster an immense increase in the number of fertile eggs laid. Even when the species does not require sunshine, greater or less warmth makes a vast difference in the numbers on the wing; iu hot weather, the lightest touch of the beating-stick brings them out in swarms, while in ordinary weather they are difficult to rouse.
I need scarcely add that the circumstance that a principal cause of insect plagues—the weather—is beyond our control, affords no argument against the use of such means of keeping them under as are within our power to influence. Chief among these means seems to be the preservation of those vivacious in- habitants of our woods and gardens, some shy, some impertinent, but all (except sparrows) delightful, our wild birds.—I am, Sir, Sze.,
[What's the matter with the poor sparrows fl—En. Spectator.]
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