Green onions (Allium spp.) are biennials hardy in U.S. Department of Agriculture plant hardiness zones 3 through 9, but generally grown as an annual. Onion varieties differ in flavor, color, shape and storage methods, and green onion are traditional onions harvested before the bulbs fully form. Also known as scallions, spring onions and salad onions, they share the same cultural conditions as traditional onion, and the same pests. Even the most pungent onion has its pests, and green onions are no exception.
Thrips are tiny; you most likely won't notice them on crawling on your green onions. These pests thrive during the hot, dry days of spring and summer, and can cause extensive damage to onions. Using their sucking mouth parts, thrips feed on plant juices, often feeding beneath leaf folds and near the leaves close to the bulb. Signs of thrips include distorted leaves with white or silver streaks or blotches. You can blast thrips off plants with the water hose.
Adult onion flies lay their eggs, the onion maggot, at the stem of onion plants on the soil or on the neck of new leaves. Onion maggots are white, tapered and small, only about 0.04 inch long. These pests feed on onions at all stages, beginning with seeds and continuing through root development of green onions. If onions are left to grow until maturity, maggots also feed on developing bulbs. Symptoms of onion maggot infestation include poor stand and slow plant development.
Nematodes are wormlike pests that attack the bulbs and stems of green onions. The pests enter through the roots, and infect plants with a toxic substance that causes yellowing and wilting leaves along with die-back of leaf tips. Infection in older plants can cause stunted growth, limpness and swollen bulbs at the plant base. When a nematode infection is suspected, uproot and dispose of all infected plants immediately. Do not plant green onions in the same area where they were grown the previous year.
Wireworms are the larvae of click beetles, varying from 1/2 to 1 1/2 inches long, and living in garden soil. These pests can reduce the number of healthy green onion plants, because they feed on seeds during and after germination. Wireworms are boring insects that penetrate roots and stunt plant growth. To control wireworms, inspect the soil before planting, and flood the area if the pests are present. Once plants are infested, remove both the infected plants and the soil.