THE PLAGUE OF CATERPILLARS.
[To THE EDITOR OF TEE " SPECTATOR."]
Sta,—Some years since, I had considerable opportunity of observing, and indulged in some speculation as to the causes of, the variation in the abundance of leaf-destroying insects,—not, I must own, from the arboriculturist's point of view, but rather from that of Sir Joseph Banks, as represented by Dr. Wolcot, when he complains that the great plague of flies was wasted on the unscientific Pharaoh,—and I then came to the conclusion that a much less constant cause must be found for this than the relative abundance of insectivorous birds.
The caterpillars that make most havoc with the first leaves of the oak are those of some of the common Lepidoptera (moths). I do not think it is any exaggeration to say that each pair of these produces, on the average, 200 eggs, from which it follows that 99 out of every hundred die before filling up the numerical place of the species in nature. With such a margin as that between 1 and 100, it is easy to see how great a difference a favourable season might make in the abundance of a species. But this is by no means all, for several such seasons- may and not seldom do come in succession, and then the margin is enormously widened. I believe the variation in the weather to be the chief cause of the variation in the number which live to complete their lives. The winged stage rarely lasts more than a few weeks, often only a few days ; and if these happen. to be wet and stormy, myriads of such fragile creatures must die before they have had much opportunity of flying. The risk of the complete extinction of a species from this cause is so far recognised in the scheme of nature, that an old writer on natural history (Haworth) long since called attention to the circumstance that in one species, the " small eggar," which is "doomed to a regular appearance at the termination of the cold and ungenial month of February," many individuals will lie in the pupa state for two, three, or more years, instead of all coming out their first year. One of the most singular exemplifications of this habit is afforded by a rare species rejoicing in the quaint old name of the "Kentish Glory." The pupa of this species has to work
its way through its light cocoon of silk and moss before the moth is disclosed, and it is odd to see those individuals which seem to have " made up their minds " to come out in their first spring season wriggling out in this way, while the rest lie quietly entombed for another period of one or more years.
If I were to be asked to conjecture the chief cause of the abundance of oak-destroying caterpillars this spring, I think I should be inclined to ascribe it to the remarkably mild and dry winter we have had, one that must have been most favourable to the various oak-feeding species (Defoliaria, &c.) which are at that season in the winged state.—I am, Sir, &c., [But the oak caterpillar was almost as destructive last year in Windsor Park and many other parts of the country as it is this; and last year's winter was severe in the extreme.—ED. Spectator.]
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