An Insect Olfactometer

Abstract

Briefly stated, the insect olfactometer consists of a specially constructed Y-tube through which insects pass from a dark chamber, being attracted by a light suspended near the free ends of the forks of this tube. The dark chamber is attached to the base of the Y-tube, while a suction apparatus to draw odors through the forks is attached at the point where the base and forks unite. The principle involved is to attract the insects equally toward the entrances of the forks by the light stimulus, but when ready to enter these forks they are influenced unequally by the odors drawn through the forks, one fork serving as an attractive or repellent side and the other fork as the control side. The whole apparatus is so constructed and manipulated that the interfering factors are practically controlled, leaving only the olfactory responses to be recorded. It seems that so far as the insects are concerned, the key to success in work of this kind depends upon the proper physiological state of the species which is being tested. The proper state perhaps includes many factors, but particularly the proper age and sexual condition, and a hungry condition when food odors are being used. In this investigation it was shown by a large series of experiments that potato beetles can be induced to respond to their food odors by subjecting them to the odors from water extracts and steam distillates of their host plants, and it was further shown by another large series of experiments that these insects can be induced to respond to the emanations from their living host plants, giving experimental proof for the first time that plants attract insects by emitting odors. Emanations, drawn through the apparatus from living potato plants, could not be detected by the writer, but in every one of the 24 experiments conducted the potato beetles gave attractive responses, the highest percentage being 76.7 per cent. After aerating the steam distillates of potato foliage, potato tubers, horsenettle, henbane, and tomato plants for some time, the odors from all of these distillates became attractive to insects, and the writer could detect a common odor among them. This odor closely resembled that from boiling potatoes or from the juice of freshly cut potatoes, and might be called a potato odor. Since all these plants belong to the potato family (Solanaceas), it would be interesting to know whether they possess a particular constituent or constituents which emit a common odor, as is indicated by the responses of the beetles and by the writer's nose. If so, it is suggested that the chemist tell us what they are, so that we may use them in poisoned baits or in traps, thereby perhaps reducing the cost of controlling potato beetles which must now be done by dusting and spraying the potato plants.