There are many different species of plant-parasitic nematodes that feed on corn, and the different species vary greatly in their ability to cause damage. For example, the damage threshold for spiral nematode (Helicotylenchus) on corn is greater than 1,000 per 100 cc (a little less than a half cup) of soil, but only one needle nematode (Longidorus) per 100 cc soil can be damaging. Also, some nematodes that feed on corn are endoparasites--that is, they feed from within the root tissue. But most corn nematode species are ectoparasites, living in the soil and feeding from outside roots.