Vegetable gardens are often just as beautiful as many flower gardens and also deliver delicious food at harvest time. Unfortunately, several species of animals find our vegetable gardens just as delectable as we do. Onions developed important defenses such as a harsh taste and an acrid odor that activates tear ducts, but these advantages do not stop some hungry herbivores.
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. Unlike other pests, no chemical repellent has proven effective against onion flies. Onions that are susceptible to onion flies are protected by physically covering the bulb area with a fleece to prevent flies from landing.
Grub shaped and yellow in its pupate form, thrips grow into small flying grayish brown insects by feeding on the leaves of onions. Onion thrips eat the leaves of onions their entire life and breed up to twice per year in great numbers. Great numbers mean that a growing thrip infestation can work quickly through an onion crop. These insects thrive in warm moist temperatures above 85 degrees Fahrenheit and are often killed by long periods of cold. Even small numbers of onion thrips are dangerous to onion crops, since they also transmit yellow spot virus, a plant infection that can render onions inedible.
Affecting many different plants including onions, leafminers begin their lives on the leaf of an onion plant. Once hatched, they shield themselves from predators by folding a part of the leaf over onto themselves. Now protected, leafminers begin to munch the portion of leaf beneath them. They continue this behavior until autumn, when they mature into flies to reproduce. Like the onion flies, leafminers such as the Allium leafminers are not susceptible to chemical deterrents.
Larvae of moths such as cutworm and armyworm will attack the roots and bulb of an onion. Feeding on the fruit itself, these larvae bore large holes and gashes in and along the side of the plant, contaminating it and sapping its nutrients.