resulting in the formulation of the general principle of the utility of stomach poisons. This has led to the investigation of many materials from the standpoint of their value as insecticides, together with determinations of their relative efficiency in different cases, how to control their effects and how they may best be applied, resulting in the development of spray pumps, nozzles and spraying apparatus in general.
Closely following the discovery of stomach poisons as insecticides came that of contact poisons for sucking insects, for though Harris had suggested soap solutions in one or two instances the general principle had until this time failed to be formulated. Here, too, investigation progressed rapidly, developing different materials as contact insecticides varying in strength and in their range of application until this field may now be considered to have been well explored.
Fumigation, as a method of control, during all this time remained almost unnoticed, its limitations being apparently so great, and the fumigants themselves being so mild as to give little promise of results of value. But during the last twenty years the utilization of gas-tight tents, and of hydrocyanic acid gas and carbon disulfid has shown that this method of control has a far wider range of applicability than was formerly supposed, and fumigation is now perhaps as well developed and its possibilities as thoroughly understood as is the case with stomach and contact poisons.
During the last three quarters of a century the ravages of insects have so greatly increased as to attract much attention to the subject, and many persons have become specialists in economic entomology. Numbering less than half a dozen in 1850, we now find more than five hundred workers, each year publishing thousands of pages on the results of their investigations. Large societies now hold regular meetings at which the problems of economic entomology are discussed; and the subject, once of little importance and of which almost nothing was known, has now become a large and important branch of applied science, with more positions waiting than there are competent persons to fill them.
The rapid increase in the losses caused by destructive insects, which has focussed so much attention on economic entomology is difficult to state accurately in figures, but was estimated in the report of the U.S. Commissioner of Patents (then in charge of the agricultural work of the government) in 1850, to be at least twenty millions of dollars, while other estimates of that period made in terms of the total crop value placed the loss at about ten per cent. Since that date conditions have changed materially and are continuing to change for the worse. The development of speedy commerce has enabled many of the most serious pests of foreign lands to reach and establish themselves here, till in addition to our own native insects we have also one hundred or more from abroad, many of them developing destructive