Dear Sirs,—I would very gladly help you about the moth-caterpillar attack on your potatoes, but I am afraid that without caterpillar or moth I cannot name it. There are very many infestations to potato of caterpillars, nearly allied to what you will, I think, very likely know well as the “Turnip grub.” These are so numerous that it would be quite hopeless for me to endeavour to name merely from description and the chrysalides; and even with the caterpillar it would have been difficult (though I would with pleasure have tried), on account of some of these pests greatly resembling each other, and also some (identical grubs) altering their colours completely as they moult. I should have been glad to help you, but as these creatures are now turning to chrysalides the attack is presumably nearly over for the present.
P.S.—For general use in an attack of this kind the spray that you have been using, which is very nearly equivalent to the U.S.A. kerosene emulsion, is probably about as good as you could try; for I conjecture that you might not like to try “Paris-green”? Possibly this would not answer, and for various reasons—it being a ground crop as well as the tuber a food crop—it might not be desirable; still, I just name it.
August 4, 1900.
I am obliged by the fresh specimens of caterpillars received this morning from your agent, Mr. Carswell, and from these and the moths coming out to-day from the chrysalides previously sent me, I am able to say that the larvæ are those of the Plusia gamma moth, popularly known as the Silver Y-moth. I am not aware of these caterpillars having been recorded as injurious to potato leafage, excepting in the year 1892, when I had information of two attacks to this crop, in both instances from caterpillars migrating from clover. It is too late to-night to give you a detailed account, but I write now, as you will be interested to have the identification as soon as possible.
1, Eggs; 2, caterpillar; 3, chrysalis in cocoon; 4, moth.
FIG. 40.—GAMMA OR SILVER Y-MOTH, PLUSIA GAMMA, LINN.
August 5, 1900.
Your potato attack is, as I mentioned last evening, caused by the caterpillar of the Silver Y-moth, so named from a small bright mark on the fore-wings, in shape like the English Y or the Greek Gamma. The moth is about half an inch in the spread of the fore-wings, which have a satiny lustre and are varied with rich coppery, as well as grey and brown, marks. The hinder wings are greyish, with a brown border. The caterpillars are fairly recognisable by being what are called “half-loopers.” Having only two pairs of sucker feet beneath the body (besides the customary claw feet) they form a slight arch when they walk. The attack is occasionally very destructive and is one of those which we have proof of having been blown to us, in moth condition, from the Continent; and, from some information which has come to my hands since I received your letter, I think it is not at all unlikely such may be the case now, with another kind of crop. The caterpillars feed on many plants, those of the cabbage and turnip kind especially; also on Leguminosæ, as peas and beans. Sugar beet they are destructively partial to. I should not at all think that the attack was likely to recur to potatoes, or that, as the infestation is now past its destructive stage, it was worth troubling yourselves about. If you should desire more about it than I can easily condense into a moderate letter space, you would find a careful account of the attack, with a good figure, in my sixteenth Annual Report on Injurious Insects. Hoping, however, that my few notes may be all you require, yours truly,
Eleanor A. Ormerod.