Gary Vos, age 12, of St. Catharines, Ont., Canada, for his question:
WHAT IS THE NEMATODE'S LIFE CYCLE?
The earth's most plentiful animals are the single¬celled protozoa, too small for the human eye to see. A protozoan may multiply by dividing itself into a pair of newborn twins and become more or less immortal. The nematodes are the second most plentiful animals. Also, most of them are too small to be seen without a microscope. As a rule, their life cycle is limited to a few years, and it may be cut short by changing temperatures.
Multitudes of mini nematodes teem all over the earth, from hot sandy deserts to cold mountaintops and even in the oozy floor of the sea. There may be as many as 20 million of these midgets in one square yard of your garden soil, and some types attack the roots of your vegetables.
Under the microscope, the average nema resembles a little white thread,lashing around like a whip. It may be called a threadworm, a roundworm or an eelworm, depending on its shape. Some types live independent lives in the soil. Others are parasites that feed on the living cells of plants, animals or people.
The average life cycle begins with a batch of 300 to 600 minuscule eggs. The female may keep them inside her body until the larvas are ready to hatch. Or she may lay them in a jellified mass and leave them to hatch in the warm weather.
A newly hatched nema is a lazy larva inside a tough coat called a cuticle. He grows by molting, shedding his tight old cuticle for a larger one. Through the first three molts, there is no known difference between the male and the female larvas. Then a change takes place.
The male grows a new cuticle under the old one. He coils up inside like a pretzel, emerges and comes forth looking like a skinny little worm. At this stage, the female grows a new outer cuticle and begins to bulge. She fixes herself to perhaps a root and stays there the rest of her life. The male also finds a food supply but later he may move around.
Near the surface, soil dwelling nemas are wiped out by frosty winters and by hot dry summers. However, others survive from one crop season to the next at deeper levels. Naturally they multiply in the growing season, when food is plentiful. Garden nemas are pests because they attack our favorite plants. Naturally, we hate to wipe them out with strong chemicals, which also may destroy other creatures. However, it seems that nematodes cannot abide marigolds. These bright tangy¬smelling flowers may not do much to help this year's crops. But their roots produce a chemical in the soil that often discourages the pesky critters through several future seasons.