Nematodes are not insects, but minuscule worms less than 1/16 inch long. Some are beneficial, parasitizing plant pests--but others are pests that eat roots and, occasionally, foliage. They infest a wide variety of plants, including woody types as well as flowers and vegetables. Symptoms are similar to those of overwatering: plants become stunted and yellow, deteriorate slowly, and usually die. Diagnosis is not simple; a professional soil test is usually needed.
Nematodes are most common in sandy, moist soils in warm-weather climates such as the Southeast. Control is difficult; soil solarization may help. Contact your local Cooperative Extension Office for advice.
Complete soil fumigation will control nematodes, but the job can be done only by a professional certified to work with the required pesticides.