Onion and Seed Corn Maggots

Whether in adult form or as small maggots, several species of fly, including the onion fly, can contaminate and destroy onion crops. These flies lay their eggs close to the bulb of the plant, near the ground. When the eggs hatch, small maggots bore into the onion plant at its base, cutting the leaves of the plant from the bulb and stopping photosynthesis. As they mature, onion flies eat the leaves of the onion plant. Onion maggots are much more likely in the cooler northern states.

Maggot Solutions

Spinosad and Neem Oil are natural substances that can be toxic to the insects or at least a deterrent to them eating on the onions. Onions that are susceptible to onion flies are protected by physically covering the bulb area with a fleece to prevent flies from landing.

Rotating your crop or sprinkling the area with ground cayenne pepper, ginger, dill, or chili powder will repel females and prevent them from laying eggs.

Onion Thrips

Thrips feed on the leaves of onion plants, causing the leaves to turn white. High-level infestations cause significant leaf damage that results in a reduction of photosynthetic area and the plant’s ability to produce food for the developing bulb. Infestations that develop during the early stages of bulb formation have the largest impact on bulb size and quality. Infestations later in the season are less problematic, as onions can tolerate higher populations of thrips when they are closer to harvest. Thrips feeding damage affects the leaf quality of green onions because of the formation of feeding scars.1  Thrips are also a vector of the Iris yellow spot virus on onions.

Thrip Solutions

Weekly sprayings of Neem Oil or Spinosad will control their spread. To check for thrips, cup your hands around the leaves and see if you see any on your hands.

Slug and Snail Control

If you reduce watering your onions or water them early in the day, the garden will be less hospitable to snails and slugs, which need high levels of humidity to survive. Beer traps — small containers holding beer and buried to the soil level — are common methods to control snails and slugs. They work well, provided they are cleaned daily and replenished with fresh beer.

Slug Solutions

Scattering a slug bait containing iron phosphate around the base of onions at a rate of 1 teaspoon per 1 square yard should take care of slugs.

Grasshoppers

Grasshoppers favor onions and often appear in onion patches during summer. Although many grasshopper species feed during the day, their camouflage may prevent them from being seen in an onion stand until they’ve caused considerable damage. Some grasshoppers cut ragged holes in leaves while others consume whole leaves at a time. Adult grasshoppers can travel more than 15 miles per day during migration, making control of them extremely difficult.

Grasshopper Solutions

Using Neem Oil will help prevent them from eating on the foliage.

Cutworms

Cutworms are nocturnal caterpillars that hide in soil during daytime and crawl up plants at night to feed on foliage. When it comes to plants such onions, they may simply snip the long, graceful leaves and drag them into their underground burrows to feast. These 1- to 2-inch long, brown, gray and black caterpillars prefer seedlings but clip whole plants as well. When the caterpillars’ numbers are large, they can devastate an onion crop in a single night.

Cutworm Solutions

If their number is high, an option is to apply Bacillus thuringiensis (Bt) mixed at a rate of 1 teaspoon per 1 quart of water to foliage that hasn’t been consumed. It will destroy the cutworm population quickly. Reapply Bt every three or four days; it breaks down quickly when exposed to sunlight. Read Bt’s label carefully before applying the product.

Varmints

Fortunately, very few animals eat onions. Squirrels, rabbits, and groundhogs as well as birds will sometimes bite into the foliage or pull a young plant out of the ground but very rarely do they cause major damage.

Varmint Solution

Spreading Epsom salts around your foliage or covering the crop with a fleece will help prevent damage early after planting. Once the onions get established, these varmints will generally not bother them.