The amount of carbon-bisulphide to be used varies, according to circumstances. For soil fumigation a special type of “gun” is on the market for injecting the bisulphide into the soil, but for ordinary garden purposes it is sufficient to make holes in the ground with a stick, pour in the fumigant, and close up the holes. When holes are made about 18 in. apart, half an ounce of bisulphide to a hole is sufficient, the depth of the hole varying according to the depth of the pest to be controlled.

For the fumigation of seeds, bulbs, potatoes, etc., an airtight chamber is necessary. This is also of value in the control of pests of potted plants. The dimensions of a chamber will vary according to the demands made upon it. Carbon-bisulphide gas being heavy, the containers (shallow dishes) should be placed on a shelf near the top of the chamber during fumigation. The proportion of fumigant to the air space varies according to the plants and insects to be fumigated.

For lawn-infesting insects, carbon-bisulphide can also be used in an emulsion as a spray prepared as follows:—​Fifty grams of powdered resin are gradually added to 135 cc. of a 7 per cent. solution of sodium hydroxide, previously warmed; 450 cc. of hot water is now added, and the whole agitated until the resin is completely dissolved, when 50 cc. of oleic acid is also added. To prepare the emulsion, three parts of this soap solution are thoroughly agitated with seven parts of carbon-bisulphide until emulsification is complete, which can be gauged by the creamy-white colour and viscosity. For use dilute in the proportions of 18 pints of the emulsion with 50 gallons of water, applying by means of a watering-can or spray-pump at the rate of one gallon to every square foot of lawn.

Calcium cyanide, on being exposed to the atmosphere, gives off hydrocyanic acid gas, the evolution of the gas being governed by temperature and humidity. Calcium cyanide has replaced the old method of generating the gas by the action of sulphuric acid on potassium cyanide, and is sold in the form of dusts or granules. In the use of this material very great care is necessary, since the gas is highly poisonous, and also scorching of the foliage of plants results if atmospheric conditions are not considered carefully. With ordinary care, however, calcium cyanide can be safely handled. It is extremely effective against all kinds of pests, and can be used to fumigate soil, glass-houses, or as a dust on plants in the open.

(c) Influence of Natural Enemies.

As stated in the first chapter, plants are to be looked upon as the primary producers of life (since all animals are directly or indirectly dependent upon them), and the animals as the consumers. Many of the latter are destructive to crops grown by man, and become pests, but others, fortunately, exist upon these pests, and are classed as beneficial animals; it is the purpose of this section to deal with the more important of these from a horticultural viewpoint. In New Zealand such beneficial animals are insects, birds, and the hedgehog.

Insects.

There is a wide range of insects that live at the expense of their fellows, and without these plant production would be impossible, either by Nature or by man. These so-called beneficial insects or parasites are the greatest factor in maintaining within reasonable bounds the insects that destroy vegetation; they are of much greater value in this respect than birds. In recent times the utilising of beneficial insects as a means of pest control has developed as one of the most important branches of entomological research.

From a general viewpoint, the beneficial insects are to be found mainly among the groups, including wasps, beetles, flies (two-winged insects) and lace-wings. The following are some examples:—

Common examples of parasitic insects are the ichneumon wasps ([Fig. 14]a), chalcid wasps ([Fig. 14]b), and ensign wasps, the first being the most conspicuous, the others less so owing to the minute size of many of them. A characteristic feature of these forms is the stalk-like attachment of the abdomen to the thorax and the sting-like ovipositor of the female, which may be of short or moderate length, sometimes projecting as a tail-like appendage beyond the end of the abdomen. Parasites deposit their eggs either upon or within the body of their victims or hosts, which are eventually destroyed by the larvæ hatching from the parasites’ eggs. Destructive caterpillars and their pupæ, and also aphides, are attacked by these wasp-like parasites, which in many cases restrict their depredations to one or a limited number of host species, while others are more general in their selection. Another group, the predaceous wasps, should be mentioned here. These insects in the adult state are hunters, and capture and paralyse by stinging such insects as caterpillars and flies, as well as spiders, which are stored in nests or cells for the nourishment of the predators’ offspring.