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by which man's artificial cultivation of crops disturbs the deli- cate balance of life, and produces those fluctuations or " waves " of insect abundance which the entomologist recognizes as pest outbreaks that he has to deal with. If he can he has to trace to their origin these waves of increase, and the modern science of entomology is to do this as far as may be. In many cases the problem is the simple one of determining if the insect is an introduced one; if it is, it has probably been introduced without the natural enemies that checked it in its original home; and it is due to the work of an Englishman, R. C. L. Perkins, that this principle was applied in the Sandwich Is. against the insect so seriously destructive to the sugar-cane crop, the Cane-leaf-hopper (Perkinsietta saccharicidd). Its natural home is Australia. Study of the insect there showed it to be kept in check by a variety of insects, some of which were introduced to the Sandwich Is., eventually reducing the insect to the status of an ordinary insect, not a pest. This principle has been followed in other cases, notably the Fluted Scale of Orange (leery a purchasi), the Gipsy Moth in the United States (Porthetria dispar), the Brown-tail Moth (Euproctis chrysor- hoea) in the United States, the Sugar-cane Cockchafer of Mauritius (Phytalus Smithii); it is the natural principle to pro- ceed upon in the case of introduced insects and it has been developed especially in the United States, which owes to importa- tion so many of its principal pests.

In other cases the disturbance and consequent occurrence of pests is due to violent alterations in the proportions of the crops grown. Where there has been, as in England, a fairly uniform mixed cultivation of many crops over a long period, a balance of life has been established as under natural condi- tions; but where the variations in supply cause fluctuations in prices and large areas are put under other crops, this balance is disturbed, and there are waves of insect pests; naturally this manifests itself far more quickly under tropical conditions, where there are several broods of an insect a year, than under temperate conditions, where there is only one brood a year with a long resting period, and it is partly this factor which makes for the very great loss from insect pests in the tropics.

Having obtained any data as to the factors producing an outbreak, it is to be seen whether there is any way of quickly restoring the balance of life, or of so modifying cultural prac- tice as to avoid the outbreak, and it is to this that the ento- mologists' attention is specially directed. The nature of the rotation of crops, the time of sowing, the use of early or late maturing varieties, these are important points; a further point is on what food plants or under what conditions the insect spends the time when the crop is not on the ground or is not fruiting, and modern entomology emphasizes more definitely the value of the old maxim of clean cultivation, of growing only crops, with nothing on the land besides no weeds, no alternative food plants, no " volunteer " plants. This is par- ticularly the case with permanent cultivation such as fruit or such tropical crops as tea, coffee, cacao, rubber and the like.

A point of great interest, which has as yet been scarcely touched, is that of " immunity," whether natural or induced. The discovery that American vine stocks were immune to Phylloxera, since they had always been exposed to it in America, whence Phylloxera came, did much to save the European vine industry; there are stocks of apple which are apparently immune to the root forms of woolly aphis (Schizoneum lanigera) ; vigor- ous plants growing under good conditions are frequently " im- mune " to attacks of such sucking insects as Leaf-hoppers (Jassidae), Scale Insects (Coccidae), Psyllidae and White Fly (Aleurodes), This question is far more developed in the case of Fungi (e.g. rust in wheat) than in that of insects; but it is a question which has come more definitely to the front during the past few years. It is not at present possible to induce immunity, but it may soon be possible to do so.

The fourth point in the entomologists' plan is that of the utilization of direct remedies, such as insecticides, fumigants, and soil insecticides. There was a considerable modification in opinion during 1910-20, and but for the large propaganda by

insecticide firms, the use of these artificial methods would have considerably declined. In the actual practice of spraying there have been few improvements and no radical changes. The arsenates, nicotin, lime, sulphur, soaps, these are still the principal insecticides; heavy oil emulsions have replaced paraffin, and the present tendency is to seek farther afield for new and more toxic substances. But progress has been small, there has been little systematic investigation, and there are only differ- ences in detail in the use of insecticides and spraying-machines. A feature of the development of the subject has been the broadening of knowledge among farmers, fruit-growers and even the general public, particularly in the United States, but also in Europe. In England, publicity and propaganda cam- paigns have been mainly concerned with the house fly, and there is still great ignorance about other pests; but this is passing with the increase of nature-study and the greater devel- opment of natural-history societies in schools.

Legislation. Experience of the value of legislation in regard to the spread of plant pests and to their destruction has resulted in a very definite simplification of the laws and enactments relating to insect pests, and a conference of delegates of 26 nations in Feb. 1914 at Rome formulated the Rome convention, which still further simplifies the principles governing the regulation of plant traffic from country to country.

Almost every country now seeks to protect itself against new pests, which, introduced without natural checks, become immensely active and destructive in a new habitat. These measures were very varied and are now simpler, and when a convention is estab- lished finally, it will probably rely upon one definite principle; but there is still some variety. Countries seek to protect themselves by prohibiting the import of plants from a specified locality, of specified plants, of anything likely to cause disease, of insects, or of packing with infested articles ; so, for instance, a sugar-growing country prohibits the importation of canes, and also, perhaps, of all plants from an area in which a virulent insect pest is active.

An alternative is to permit importation under conditions; ports of entry may be designated at which alone plants come in, the plant imports may be limited to special times, or must be in new packages, or only in postal packets. The most general condition is inspection on arrival by a competent officer, who may order their destruction if infested with a pest, or the plants must be fumigated on arrival, this being done usually with hydrocyanic-acid gas gen- erated from potassium cyanide and sulphuric acid, the amount used being about 2 oz. cyanide per 100 cub. ft. space for half an hour.

Formerly great stress was laid on the value of quarantine; all plant imports were grown in a quarantine ground under the super- vision of a Government botanist until it was certain that they had no disease. The objection was that if the plant was diseased the disease was brought into the country and, whatever the supervision of the botanist, might get loose and spread ; an alternative to quar- antine was "following up"; the importer notified the arrival of plants and the exact spot where he planted them ; an inspector saw them at intervals in order to destroy them if any disease developed.

A more recent principle is that of admitting plants without delay if they were accompanied by a certificate that they had been in- spected by an official of the Agricultural Department of the export- ing country, and were declared free of pests scheduled by the importing country, or that they had been grown in an area declared free of the scheduled pest. The latter applied particularly to Phylloxera of vine; the former applies to the schedule of pests drawn up in each country, and this is the principle accepted by the Rome convention of 1914. Each country is to draw up a schedule of pests which are not epidemic in that country but which are recognized pests, and any country sending plants will have an ade- quate service of trained inspectors to give certificates that either the nursery sending the plants, or the actual consignment, is free of pests. This cannot be done without a " Phytopathological Service," so that the consenting countries bound themselves to create this, and this has to some extent been done in Great Britain, France, Italy, Holland, Belgium. The United States refused even to discuss the question, and has adopted drastic inspection and certificates of freedom from pests, but experience has shown it that few certificates are reliable and it is not willing to receive consignments from any country whose certificates are not really of established value (e.g. those of Holland are accepted).

A further difficulty is that plants may come from countries not able to give certificates, e.g. Tibet, and these plants are very liable to introduce disease. Inspection on arrival is the usual method but an effort has been made to enable such plants to be grown under quarantine when imported by a firm of established reputation in Great Britain, so as to avoid the risks of unpacking at the port.

Internal legislation, prescribing action on the part of growers, has made progress during recent years, and has been very much simplified. In Great Britain the sale-of-diseased-plants order of 1921 puts a penalty on the sale of plants " substantially attacked "