Among the onion parasites, we have the onion fly (Delia antiqua). It is an insect that belongs to the order Diptera, family Anthomyiidae, and is a very common parasite found throughout the Italian territory. In addition to onion cultivation, the larvae of this fly also cause damage to other similar crops, such as garlic, leeks, shallots, which are all horticultural crops that produce underground bulbs (Liliaceae family). The insect can be controlled with appropriate agronomic practices and, in cases of severe infestation, by using pest control products approved for organic farming.
In this article, we describe Delia antiqua, its life cycle, the damage it causes to plants, and agronomic prevention and biological defense techniques.
Description of the Onion Fly
The adult Delia antiqua is a gray-yellow fly with 5 poorly defined dark bands on the thorax. It measures between 5 and 7 mm in length. The difference between males and females lies in their legs, which are black in both cases. In males, the hind tibia is equipped on the inside with a series of 6-8 setae of equal length, regularly distributed. The wings and halteres are yellowish.
The eggs of the onion fly are very small (1.5 x 0.6 mm), elongated, almost kidney-shaped, with longitudinal striations, and white in color.
Fully mature larvae of Delia antiqua measure between 6 and 8 mm in length. They have a conical shape with a truncated posterior end, lack legs, and are white-yellowish in color.
Damage to Crops
As mentioned, the onion fly develops on onions, leeks, shallots, and garlic. The damage is caused by the larvae, which attack the tissues of the underground bulb, causing serious damage. On young seedlings that have just been planted, the damage can be irreparable and fatal. This pertains to direct damage. However, indirect damage is equally severe. The larvae are carriers of bacteria, especially soft rot bacteria (Erwinia carotovora). In practice, bulbs attacked by fly larvae are no longer edible, and damage can occur through rot even after harvesting, in storage warehouses.
When Does Onion Fly Infestation Occur in the Fields?
The onion fly, as we will see, can complete up to 4 generations in a year. The most at-risk transplants are those in spring and autumn. This is due to the milder temperatures during these seasons, which extend the period of egg-laying by adult flies.
This fly overwinters in the soil, burying itself at a depth of 5-10 cm in the vital pupal stage of diapause. The first flight of adults occurs in April when the soil temperature is around 10 °C. The adult onion fly lives for about 50 days and feeds on flower nectar, reaching sexual maturity in 2-3 weeks. After mating, females lay between 150 and 200 eggs in cycles of 12-15 days. The eggs are laid in the soil, in groups of 15-20, usually near the base of the plants or directly in the outer scales of the bulb.
Development of Larvae
The embryonic development of onion fly larvae is completed in about 3-8 days, giving rise to onion fly larvae. From the beginning, the larvae penetrate the bulb and complete their development there, with the time required varying depending on climatic conditions. With external temperatures between 25 and 30 °C, it takes about 20 days for the complete development of the larva. Once they reach full maturity, the larvae emerge from the damaged bulbs and pupate in the soil to give rise to new adults after 15-30 days. As mentioned, the onion fly can complete up to 4 generations a year, with the last one usually arriving in November.
Agronomic Prevention Against the Onion Fly
The first step in eliminating the onion fly from gardens is undoubtedly the adoption of appropriate agronomic practices, starting with crop rotation. Let’s take an example. If in one season, there was an infestation of Delia antiqua in the plot of land where onions were grown, in the following year, it is imperative to avoid replanting onions in the same spot. It is evident that by not following this basic rule, we would allow the parasite to reproduce effortlessly, providing the first overwintering adults with an ideal substrate.
As mentioned, the onion fly overwinters in the soil in the pupal stage, just a few centimeters deep. In winter, by performing medium-depth soil cultivation, such as plowing, we expose the insect to freezing temperatures, effectively interrupting its cycle.
Use of Mature Fertilizers
Onion flies are attracted to immature organic fertilizers, especially manure. This is one of the reasons why we always recommend using well-matured fertilizers to avoid attracting harmful insects.
There are some plants that, when grown together, are very compatible. In this case, we are talking about onions and carrots, precisely because of their respective pests. The onion fly cannot tolerate carrot leaves, and conversely, the carrot fly cannot tolerate the proximity of onions. By intercropping these two vegetables, we have, with little effort, established effective biological agronomic prevention against two significant pests.
Monitoring Onion Fly
To check for the presence of the onion fly, we recommend monitoring using yellow chromotropic traps, which are available for sale in specialized stores or can be homemade with simple yellow plastic plates and vinyl glue. By using chromotropic traps, you will have a clear idea of the presence of the parasite and, at the same time, conduct mass capture.
Biological Products Against Onion Fly
Among the products allowed in organic farming and purchasable without the need for a license, the most effective against the onion fly is azadirachtin, the active ingredient of neem. If monitoring reveals a significant presence of adult flies, azadirachtin (mixed with water) should be generously applied to the soil to reach and eliminate the larvae. This type of treatment should always be done in the evening, allowing more time for the active ingredient to take effect.