_Phylloxera_ has been treated by plunging into the soil near the roots small blocks of some slowly-soluble medium, such as gelatine, impregnated with carbon-bisulphide, the volatile fumes of which kill the insect, and even more drastic remedies have been tried along similar lines. In America orchard trees infested with insects or fungi have been covered one by one with light tents, and the vapours of prussic acid, burning sulphur, and other poisons allowed to act inside the tent. In all such cases it must be remembered that uncontrolled ignorance of the properties of poisons on the part of the operator may lead to disaster, and the same applies to the much easier treatment of greenhouses, and cases where poisoned food is laid about for insects or vermin. Attempts, not altogether unsuccessful on the small scale, have also been made to introduce epidemic diseases among rats, mice, and locusts and other insects, by inoculating some of them with parasitic bacteria or fungi (_Empusa_, _Isaria_, etc.), and then allowing them to run loose in the hope that they will communicate the disease to their fellows. The introduction of lady-birds into districts infested with Coccideae and similar pests which they devour, is also recorded as successful, as also the importation of birds into forests plagued with caterpillars. It must not be over-looked, however, that man's interference with the existing balance of events in the natural struggle for existence is occasionally disastrous, as witness the results of importing rabbits into Australia, goats into the Canary Islands, and sparrows in various countries. Darwin's well-known illustration of the inter-relations between clover, bees, field-mice, and cats (_Orig. of Species_, 6th ed., 1876, p. 57), which shows the astounding probability of the dependence of such a plant on the number of cats in the neighbourhood, well illustrates the situation. Mere mention must be made of other special treatments. Caterpillars and larger animals are often picked by hand or their natural enemies--_e.g._ birds, are encouraged in forests. Locusts are caught in nets, trenches, etc., and buried. Woodlice, slugs, etc., are often trapped by laying attractive food such as carrots and overhauling the traps daily: similarly with earwigs. Rings of tar round tree stems have been employed to prevent caterpillars creeping up them. American Blight has been treated by rapidly flaming the stems. Syringing with hot water has also been employed for vines affected with mildew, mealy bug, etc. With regard to the alleged immunity from devouring insects of certain poisonous plants, it has been pointed out that _Pangium edule_, which abounds in prussic acid, is infested with a grub, and ivy is occasionally eaten by caterpillars.