A certain planter found that the amount of sugar obtained from his cane was decreasing yearly, though he could see no good reason for it. The land looked right; he ploughed deeply, harrowed well, kept the weeds down, gave the cane plenty of water, could find no reason to complain of climatic conditions, but still did not get satisfactory results. Finally the head of the experimental station was consulted and an agricultural chemist was sent to the plantation. This chemist, after careful investigation, took samples of the soil from various parts of the land; these were analyzed and the source of the trouble was found to be the lack of potash. Just here it may be explained that when the same crop is taken from the land many years in succession, without adequate fertilization, some of the essential properties of the soil become exhausted. Speaking generally, these are lime, soda, potash, phosphates and nitrogen. In this particular instance, as has been said, the land had been gradually drained of its potash. The experimental station recommended the planter to scatter a certain fertilizer over his fields. This advice was followed and the next crop showed remarkable improvement, the yield of cane and sugar per acre being greater than ever before. At one time the sugar industry of the Hawaiian islands was threatened with annihilation by a little insect called the “leaf-hopper.” The harm done by this pest was so enormous that one plantation having an average yearly crop of 19,000 tons was so severely affected that the yield dropped from 19,000 to 12,000, and then to 3000 tons in three successive crops. All the plantations on the islands suffered to a greater or lesser extent, and the entire sugar industry of Hawaii was jeopardized. The hoppers punctured the stalks and leaves of the young cane, and in the holes thus formed laid their eggs by thousands. When the young hoppers hatched out, they fed on the juices in the stalk and in the leaves, thus destroying the leaves and depriving the cane of its protection and principal means of absorbing nourishment from the air. As soon as the leaf-hopper by its ravages made itself known in the islands, the entomologists were consulted, and they were confronted with the task of studying the life and habits of the hopper for the purpose of finding, if possible, some other insects that would attack and exterminate it. It is well known to entomologists that every insect pest has natural enemies; the vital question in this case was—what were the natural enemies of the leaf-hopper and where were they to be found? Obviously, too, the problem was to discover insectivorous enemies that would not themselves attack the cane after they had destroyed the hopper.