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. CHAPTER X. Principles of Pest Control. In dealing with the control of plant pests, the objective is to prevent attacks, or, when the attacks have established, to check them as much as possible. In the latter case the term “exterminate” is in too frequent use; it is not usually practicable to exterminate a pest, and the best that can be done is to check or control it. In the control of animal pests, it should be borne in mind that the pests are usually associated with other factors inimical to plant life, such as unthrifty plants, due to injury or malnutrition, and fungous and bacterial diseases, any one of which might be either the primary or secondary cause of plant injury.