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Monica's Weekly Gardening Tips: Nematodes - the Good, the Bad, and the Ugly

Nematodes or roundworms are the most numerically abundant animals on Earth; in fact, four out of every five animals on Earth is a nematode. They are obviously a diverse species, but for our purposes we will focus on the Good and the Bad for landscapers and gardeners. As for the Ugly, see for yourself.

Beneficial nematodes are nature's defense against plant eating insects. The beneficial nematodes seek out and kill over 200 species of pest insects in the soil and will have no detrimental affect on species such as ladybugs, earth worms and other helpful beneficial insects. When they sense the temperature and carbon dioxide emissions of soil-borne insects, beneficial nematodes move towards and enter the pest or larvae through its body openings. The nematodes carry a bacterium that kills insects within 48 hours. The bacteria is harmless to humans and other organisms and cannot live freely in nature. The nematode then feeds on the tissue of the dead insect or larvae.

Specifically, the beneficial nematodes are effective against grubs and the larval or grub stage of Japanese Beetles, Northern Masked Chafer, European Chafer, Rose Chafer, Fly larvae, Oriental Beetles, June Beetles, Flea beetles, Bill-bugs, Cut-worms, Army worms, Black Vine Weevils, Strawberry Root Weevils, Fungus Gnats, Sciarid larvae, Sod Web-worms, Girdler, Citrus Weevils, Maggots and other Dip-tera, Mole Crickets, Iris Borer, Root Maggot, Cabbage Root Maggot and Carrot Weevils. They are also effective against other pests such as termites, German cockroaches, flies, ant, and fleas. You can purchase packets of the beneficial nematodes to augment those that are most likely already in your gardens.

That's the good. Now to the bad. There are also root-feeding nematodes, particularly root knot nematodes, which attack a wide range of plants: many common vegetables, fruit trees, and ornamentals. Examples are boxwood, hibiscus, hydrangea, juniper, mulberry, oak, and roses. Nematodes that infect plants cause distinctive swellings, called galls, on the roots of affected plants where they feed and develop. They are difficult to control and can spread easily from garden to garden on tools and boots or on infested plants.

Proper control of root knot nematodes starts with proper hygiene. Wash tools, gloves, and shoes after working in infested areas and try to purchase nematode-free plants. There are also nematode resistant varieties and root-stocks. For tomatoes, look for VFN (Verticillium, Fusarium, Nematodes) code on the seed packet or label. These are resistant to common root knot nematode species. The best chance for controlling root knot nematodes is to increase the water content of the soil, since nematodes most often damage plants that are water-stressed. Thus add soil amendments such as peat, manure, and composts to increase the water and nutrient holding capacity of the soil.

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