The effect of climatic conditions upon populations of insects

It is clear that many of the periodical changes which may be observed in the abundance of insects are due to climatic factors. The factors which limit geographical spread of species are also frequently climatic. In the present state of knowledge, two climatic factors—temperature and humidity—have been investigated. With regard to their effects on insects of medical importance, a considerable amount of information exists, some of which is fragmentary. Some of the effects, especially of humidity, are quite unexpected and could only have been discovered by investigations in the laboratory. Certain general laws are now known, but very great differences exist between different species of insects. It is possible that, as knowledge increases, it will be found that some parts of solar radiation are at least as important as temperature and humidity in controlling insect populations. The view is put forward that observations made in the field serve to ask questions which can be answered by precise physiological work in the laboratory. There is need for a new type of field work; in order that the bearing of the physiological work on natural phenomena may be elucidated, large collections of critical data are required from the spots in which the insects actually live. It is thought that, if such facts were available, control of certain harmful insects might be achieved by the intelligent modification of climate.