One of the best means of control is to dust the plants with powdered tobacco. Another method is to treat infested plants with soot or lime, but this must be done at night, and the material used must come into actual contact with the pests. An effective poison bait, but one that requires to be carefully handled, owing to its poisonous nature, is a mash made of 6lb. of bran mixed with 1lb. of arsenate of lead and an equal weight of treacle; this is made into a stiff paste, water being added if necessary. Lumps of this mash are placed about the plants to be protected. As a barrier to prevent the inroads of slugs and snails, plants may be surrounded by a belt of calcium cyanide; this would have to be replaced each night, and the utmost care taken in handling, since the substance and the gas evolved from it are highly poisonous; out of doors, however, the gas, being diluted with air, would not be very injurious as long as one did not stand over the treated ground longer than was necessary for laying the cyanide.
Apart from the above methods, the key to the control of slugs and snails is “clean farming”—that is, the removal of all places, such as rubbish and rank vegetation, where the animals will find shelter; the compost heap is a favourite breeding place, and this should be turned over at intervals and dressed with lime.
Eelworms.
Eelworms are minute, unsegmented worms, related to the parasitic thread-worms of animals, and are abundant in soil and water; it is usually the surface layers of the richer soils that are inhabited by them. Of the long list of species, only a few are destructive to vegetation, but these constitute one of the greatest problems of the horticulturist. It is thought that the injury caused to plants by eelworms is toxic rather than mechanical, and some plants apparently are capable of producing anti-toxins, which neutralise the toxins of the eelworms; such plants possess an immunity. There are three important species in New Zealand.
The so-called bulb-eelworm (Anguillulina dipsaci) attacks more than two hundred kinds of plants, but is of especial interest to the horticulturist on account of its attacks upon hyacinths, daffodil, narcissus, and gladiolus, causing deformity and rotting of the tissues ([Fig. 13], 16). It has been found that this eelworm develops from egg to adult within a period of between three and four weeks; the eggs are capable of lying dormant in the soil for as long as seven years. Infested bulbs and corms should be treated by immersion for three hours in water heated to 110 deg. Fahr.
Potatoes are often damaged by the beet eelworm (Heterodera schachtii), which causes what is known as “potato sickness,” when the growth is retarded, and wilting takes place; the root-system shows an abnormal development of secondary or “hunger-roots.” The eggs are retained in the body of the female, which forms a protective sack or cyst ([Fig. 13], 17 and 18), and in this state the eggs pass the winter in the ground, where they are known to remain dormant for a period of ten years; under favourable conditions in the spring, the larvæ emerge from the eggs and attack the rootlets of suitable host plants, entering them at the extreme tip. Satisfactory methods of control have not yet been developed under field conditions, but a four-year crop rotation following potatoes is suggested; seed potatoes from infested ground should not be used.
The roots of tomatoes are often found to be a mass of galls, due to attack by the root-knot eelworm (Heterodera radicicola), which also infests tobacco roots as well as other plants ([Fig. 13], 19 and 20). All stages of this species are to be found in the root galls; the female lays her eggs in a gelatinous egg sack, which remains attached to the parent. The larvæ, on hatching, either remain within the parent gall or leave it and enter the soil, where they seek out and attack the roots of another plant. In tomato gardens steam sterilisation of the soil is the most effective means of control.