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Ichneumon Flies — Part 2

This important but widespread parasite of caterpillars occurs in California, Texas, Iowa, Illinois, Missouri, Ohio, New York, New Hampshire, Massachusetts, Virginia, District of Columbia, and is probably to be found in all parts of the United States except possibly in the very coldest portions. It is parasitic upon a large number of different kinds of larvae of moths feeding upon such concealed spinners as the tent caterpillar of the orchard and the army worm of the forest (Clisiocampa americana and C. disstria) and upon certain stalk borers and gall makers, leaf folders and certain of the larger leaf miners. It is by far the most abundant of the parasites of the white marked tussock moth (Orgyia leucostigma), a famous shade tree enemy of the northeastern United States. The adult Pimpla is shiny black in color and has a wing spread of, on the average, three-quarters of an inch, the length of its body being about one-half an inch, but it varies greatly in size, the adults issuing from well fed larvae exceeding these measurements and under-fed specimens transforming into much smaller adults. Upon the tussock moth caterpillar and upon the army worm of the forest the female Pimpla will lay her eggs when the larvae are full grown or just after they have spun their cocoons, the latter time being seemingly preferable. When either species is abundant their newly spun cocoons are the rallying points of large numbers of these Pimplas. One of the females after running about for a few seconds will suddenly thrust its ovipositor through the cocoon into the body of the caterpillar which will writhe with pain. The Pimpla, however, will remain apparently undisturbed and push its ovipositor in to its full extent, remaining motionless in this position for about five minutes during which time the abdomen is slightly moved in a peristaltic manner as if forcing the eggs out and down through the ovipositor. She will then turn round, take a step or two, and finally give the caterpillar what is apparently a coup de grace with two or three thrusts in quick succession. Sometimes the same operation is repeated. The eggs are not thrust into the body of the caterpillar when the latter is enclosed in its cocoon but sometimes are to be found loose between the cocoon and the caterpillar and sometimes not firmly attached to the skin of the caterpillar standing upright on end. The eggs are elongate, somewhat wider at one end than the other, pure white in color and perfectly smooth appearing highly polished. The average length is one mm., and the greatest width about one and eight-tenths mm. The duration of the egg state must be very short and is probably only a matter of but a few hours. After hatching the Pimpla larvae when at work upon a just spun up tussock moth caterpillar feed externally upon the body of the caterpillar, the mouth-parts closely applied to the skin and in fact obviously sucking blood through a minute orifice. Their growth is rapid and there is no perceptible casting of the skin. In mid-summer the larvae will become full grown in four days, when they are nearly ten mm. long, by three mm. in greatest diameter, fusi-form in shape, and slightly curved. The color is yellowish white. They soon begin spinning cocoons. These cocoons, at first white and afterward turning gradually to a pale yellow brown, becoming much the same color as the cocoon of the tussock moth, are denser in structure and are composed of a considerably finer quality of silk. They are long oval in shape and are closely applied together adhering so firmly that it takes some little force to separate them. They are applied side by side and so closely that their oval outline becomes more or less angular at the point of application. Two days after the spinning of the cocoon in midsummer the larva changes to pupa and adults issue sometimes as early as six days later, making the entire life round of the species about fifteen days.

It sometimes happens that a tussock moth caterpillar is stung when it is just on the point of transforming, but in such a case the transformation to pupa is occasionally accomplished. The recently formed pupa is also occasionally stung. Contrary to the general rule holding when caterpillars are eaten out by the Pimpla larvae the latter seem to feed within such pupae and evidently to spin their cocoons within the caterpillar's pupa skin so that one will frequently find an apparently perfect pupa of the tussock moth within its cocoon which, however, contains four or five cocoons of the Pimpla packed close together and completely filling it.