The Usefulness of Some Larval Dimensions To Distinguish Heterodera Rostochiensis and H. Pallida

Abstract

More than 98 % of Norwegian populations of round-cyst nematodes attacking potatoes are H. rostochiensis pathotype A. Others having yellow females are similar or identical to Dutch pathotype B and C. Most populations of H. pallida have white females and are Dutch pathotype D, British E. One population with cream females is evidently British pathotype B. The discrimination of H. pallida populations on the base of white females showed to be reliable. On average larvae from populations of this species were larger than from populations of H. rostochiensis for all characters measured. A ratio (R) of the component of variance between species to the total variance for measure of six larval dimensions, are tabulated. Particularly high value of R is shown for stylet length. For this measurement little is gained by measuring more than two specimens to identify populations of pure species. The distance from the tip of the head to the base of the stylet knobs is a slightly poorer discriminant and that from the medium bulb plates to the excretory pore is poorer still for a single measure. If more than ten specimens are measured within each population, however, the latter is the most reliable measure of all the characters investigated. Measurements of the length of the shaft and the basal knobs, the length of the clear tail area, and especially body length are all unreliable in distinguishing between species.