Helicotylenchus species The Spiral Nematodes Damage to Corn.  Helicotylenchus pseudorobustus  is a common and well-known parasite of corn, especially in the mid-western U.S.  Crop losses are hard to measure because this nematode often occurs with other species of parasitic nematodes. Biology.   At least three species of spiral nematodes are known as parasites of corn.  These nematodes have a characteristic arcuate to spiral body shape when at rest. Ecology.  Spiral nematodes are seemingly ubiquitous with corn, and can occur in large numbers in a wide variety of soils. Symptoms.   Mild stunting and reduced yields occur where populations are large.  Small light to dark brown lesions may also occur on infected plants. Damage to other crops.  Control.   Chemical control may be of value when spiral numbers are high and other species are contributing to root damage.