Common Onions pests

Onion maggots are the most damaging insect pest; the small white larvae of a fly tunnel into stems and developing bulbs, causing seedlings to wilt and collapse and leaving rot channels in mature bulbs. Onion thrips are tiny, slender, pale yellow to tan insects that rasp the leaves and hide in the neck and leaf folds, producing silvery streaks and blotches that reduce bulb size. Leek moth and allium leafminer are increasingly common in some regions and leave winding tunnels or rows of white feeding dots in the leaves. Cutworms may sever young transplants at the soil line, and rabbits or other animals rarely bother onions because of their strong scent.

Common diseases

Downy mildew is a fungal-like disease that shows as pale, elongated patches on leaves that later turn fuzzy and gray to purple in humid weather, causing leaves to yellow and die back. Botrytis leaf blight, or blast, produces small white spots with pale halos on the foliage, while botrytis neck rot softens and browns the bulb neck in storage. Purple blotch causes sunken tan lesions that develop purple centers with concentric rings on leaves and stalks. Onion white rot is a serious soilborne fungus that yellows and wilts plants and leaves fluffy white growth and small black bodies on the bulb base, and bacterial soft rots can turn the inner bulb scales slimy and foul smelling.

Prevention

Rotate onions and all alliums so they do not return to the same bed for at least three to four years, since many diseases and the onion maggot build up in the soil. Give plants generous spacing and keep beds weeded so leaves dry quickly, which is the single best defense against the leaf diseases. Start with certified disease-free sets or transplants, choose thrips-tolerant or disease-resistant varieties when available, and water at the base in the morning rather than wetting the foliage. Cure bulbs thoroughly with dry necks before storage, and clean up and remove all allium debris at the end of the season rather than composting it.

Organic control

Cover the bed with floating row cover from planting until the plants are established to exclude egg-laying onion maggot flies, leek moth, and allium leafminer; seal the edges since these pests emerge from the soil. Knock thrips populations down with a strong spray of water, and if numbers climb use insecticidal soap or a neem-based product applied to the leaf folds and necks where thrips hide, following the product label. Encourage natural enemies such as lacewings, minute pirate bugs, and predatory mites that feed on thrips by keeping some flowering plants nearby. For downy mildew, purple blotch, and botrytis, an approved copper product used preventively before wet weather can help; always read and follow the product label for the crop, timing, and application.

Scouting & monitoring

Scout weekly once plants are up. In cool spring weather watch for wilting or collapsing seedlings that signal onion maggot, and check the base of plants. As the weather warms, inspect the inner leaf folds and necks for thrips by gently spreading the leaves, and look for their silvery feeding streaks. After rainy or humid spells, examine leaves for the pale patches of downy mildew and the ringed lesions of purple blotch, and at harvest check necks and bulb bases for the soft, moldy signs of neck rot and white rot.

Tips

Airflow is everything with onions, so resist the urge to crowd the rows and always water the soil rather than the leaves. Let bulbs cure with fully dry necks in a warm, well-ventilated spot before storing, and cull any bulb with a soft neck so one rotting onion does not spoil the rest.

General home-garden IPM guidance, focused on prevention and least-toxic control. Always read and follow the product label for any pesticide, including organic products. Page updated Aug 4, 2026.