THE REGULATION OF NUMBERS OF CYSTS AND EGGS PER CYST PRODUCED BY GLOBODERA-ROSTOCHIENSIS AND GLOBODERA-PALLIDA ON POTATO ROOTS AT DIFFERENT INITIAL EGG DENSITIES

The relation between initial egg density Pi of Globodera rostochiensis pathotypes Ro 1 and Ro 3 and of G. pallida pathotype Pa 3 and the number of cysts producted per g dry haulm (considered to be proportional to size of the food source of the nematodes) Pf on various potato cultivars in ten pot experiments with ranges of initial egg densities was in accordance with the equation Pf = M(1 - e(-x)) with x = aPiM-1. This indicates a distribution of feeding sites of equal size for female juveniles in the roots according to Nicholson's competition model and adjustment of the population size in the roots by elimination of excess female juveniles by an active mechanism, thus preventing reduction of the size of the surviving females and their mortality by crowding, which would be an inefficient use of the food source. However, relative numbers of eggs per cyst y(e) were reduced according to the equation y(e) = m(e) + (l - m(e)) 0.9z with z = PiT(e)-l-l for Pi > T(e) and y = 1 for P(i) < T(e), because of the adverse effect on the quality of the food source by invasion of the roots by much larger numbers of juveniles than could survive. T(e) varied between pathotypes, cultivars and experiments and was generally larger than T in the equation for the relation between Pi and final total plant weight. Therefore, the mechanism reducing the average number of eggs per cyst operates independently of that reducing plant size.