Movement of eelworms vi. The influences of soil type, moisture gradients and host plant roots on the migration of the potato-root eelworm heterodera rostochiensis wollenwebermovement of eelworms

The optimum crumb sizes for movement of potato-root eelworm larvae in a sandy loam, a heavy clay and a peat soil were 150-250 and 250-400 mu. Mobility was very similar in clay and sandy loam, in both of which there was an optimum suction for movement. In the peat, however, mobility increased with suction and no optimum suction was established. Larvae may be able to move in peat at high suctions because friction between the larvae and the peat crumbs is less than between clay or sand crumbs. Larvae moved to the wet end of a moisture gradient in sand, the number increasing with the steepness of the gradient. The rate of spread of larvae in sand 150-250 mu diameter varied between 2 and 3 cm. a day, depending on suction. As pore size increases, any upward movement in a moisture gradient is opposed by falling under gravity. Larvae do not respond to a moisture gradient or fall under gravity in sand where the width of the pore approximates to the diameter of the larva. The presence of host roots also counteracted the response to a moisture gradient; the degree of orientation to the roots increased with the time the roots were in the sand. Direct observation on larvae, newly emerged from cysts, in the presence of host plant roots, suggests that larvae orientate themselves at a distance from the root and do not reach the root by random movement. Many of the movements of eelwonns are explicable by considering the relationship between pore size, eelworm diameter and water distribution, and a diagram relates movement and various soil factors.