to-day eclipsed by its congener, and it is only in these later years that it has taken an importance of the first order, tending even in certain districts to take the first rank over the gipsy moth.
The caterpillars of these two species are extremely common in Europe, their original home. They are injurious and from time to time appear in great number. It is to be remarked that in a year following their large multiplication, the caterpillars of these insects become quite rare, and that they remain so for a long time. They are, then, very far from being responsible for damage similar to that which they cause every year on the other side of the Atlantic. With us their presence is tolerable, and they do not cause notice since they do not threaten the vitality of the trees. In Massachusetts, on the contrary, they constitute a permanent plague which has commenced to invade neighboring states.
The difference in these conditions appears to be that in Europe the insects are held in check by parasites, which are much more numerous than in the United States.
Some American parasites have adapted themselves to destroying the gipsy moth. There are 5 hymenopterous and 6 dipterous parasites, without counting several predaceous species which attack it. But this is small in comparison with the 27 hymenopterous and 25 dipterous parasites of the gipsy moth in Europe. While the parasites of the brown-tail moth are less known, it is perfectly sure that in Europe this insect is kept in check much more efficaciously by its natural enemies than is the case in America. On account of these considerations it was only natural to seek to introduce into Massachusetts the original parasites of these two insects. For a long time it was not judged wise to undertake the enterprise. A law obliging the systematic destruction of the gipsy moth and the use of insecticidal mixtures seemed to render it inadvisable. Moreover, there was confidence in the fact that the native parasites would increase. Now the conditions have changed. In 1900, the appropriations were stopped, at a time when the insect was well in hand. In five years, however, it has spread over a territory four times as great as that which it occupied in 1900 and has commenced to spread into the neighboring States of New Hampshire and Rhode Island.
On the other hand, in the 36 years that the insect had infested the country about Boston, American parasites, if efficacious, would have manifested it in an appreciable way. The same considerations applied to the brown-tail moth.
Americans resolved, then, to attempt a last and great effort to master the plague against which a long struggle had given insufficient results. In the appropriation bill of the Federal Congress, in 1906, $2,500 were appropriated to begin the importation of parasites of these two insects