Many thanks for your letter received yesterday morning, which is very interesting indeed to me, and which I hope to reply to very soon, but now I am replying to your note accompanying the caterpillars from the Peckforton Hills, though not so fully as I could wish, for disasters befell the letter, and it arrived by special messenger from the Post Office, with the announcement that the things had got loose, and were creeping all about! Any way but little remained to judge by, so I report on what was visible. Most of the caterpillars were loopers (fig. 30), and the largest proportion of these, though differing so much in colour, appeared to me to be the _Cheimatobia brumata_. As you know there may be every variety of shade in these Winter moth caterpillars, from pale green down to smoky brown or almost black. Another kind of which I only find two specimens (small and very small, respectively), look as if when grown they would be the Mottled Umber moth, which is so injurious this year. There are just single specimens of a few other non-looper kinds, but at this present time all the kinds come under only one method of (feasible) treatment, and I am afraid this (even if feasible) would be much too costly on such a great scale. Washing with Paris-green or London-purple, or with kerosene emulsion, would be the right thing, or our British form of the emulsion, made by Messrs. Morris, Little and Son, Doncaster, and sold, I believe, at a very low price (consequent on the large demand for it), under the trade name of “antipest.” This only needs diluting. But when we come to dealing with great areas like the Peckforton Woods, I believe that the only really practicable way of, in some degree, lessening the evil, and counteracting its effects, is throwing water from some large engine. If a fire engine and a supply of water were available this might do a great deal of good.
I was consulted by the late Sir Harry Verney about “an ancestral oak” at Clayden, which appeared nearly cleared of leafage, and I advised playing the house fire engine on it—and the plan succeeded. The moisture falling around the tree pushed on the second leafage and (conjecturally) saved the tree. But with woods it is most difficult to manage application. I am afraid I am only able to say what would be best, if it could be done.