Leek miner fly and onion fly – harmful maggots on onions and leeks

Macro of the leek eaten by the lavas of the leek leaf miner fly

The Leek miner fly and the onion fly can cause considerable damage in the vegetable garden. Both fly species infest all bulbous plants, including chives, but leeks, garlic and onions are particularly affected in the vegetable garden.

How can I recognize the infestation of the leek leaf miner fly and what is the difference to the onion fly? Can I control the vegetable flies or protect my onion vegetables in the garden as a preventive measure?

Description of the two vegetable flies

The Onion fly (Delia antiqua) belongs zoologically to the family of flower flies – a very flattering name for the damage this insect can do in the garden! The family name of the Leek miner fly (Napomyza gymnostoma) is more meaningful: leaf miner flies. The beet fly (Pegomyia betae) is another annoying fly species that preys on chard, spinach and beet (read more here in the garden blog : Beet fly).

The flies themselves will hardly be seen in the garden. Both are similar to the house fly and just as erratic. Leek leaf miner fly and onion fly, however, have clearly distinguishable pests due to their different lifestyles.

Damage pictures of leek leaf miner fly and Onion fly

Like all flies, vegetable flies develop from the egg through the larval stage (maggots) and pupation to the finished fly. The maggots of the leek leaf miner fly and onion fly pupate in so-called "barrels". These brown, wheat kernel-shaped formations are easily recognizable and a clear indication of vegetable fly infestation (see picture above).
Only where the maggots feed and the barrels are formed is different and accordingly different is also the damage pattern of the two vegetable flies.

Infestation of the Leek leaf miner fly

From the beginning of April the leek leaf miner flies hatch and start laying their eggs on the leeks. The young hatched maggots bore into the stem and eat their way down the leaves (minify). These feeding tunnels from top to bottom often burst open and the leeks begin to rot. Even already strong leek shoots become weak and fall over. Another sign of infestation of the leek leaf miner fly is a crippled and twisted growth of the leek. (see pictures below).

After about three weeks the larvae pupate and the brown barrels are found in parts of the plant close to the ground.

After the flight of the first generation in April, the leek leaf miner fly remains in a summer dormant phase. The second generation hatches in late summer from August/September and infests – as happened to me – the winter leek. Depending on the weather, a third generation can develop and infest all leeks and leek plants that have been spared so far. Infestation of leek plants and making them inedible.

Leeks burst open due to infestation with leek leaf miner fly

Typical for the leek leaf miner fly: small barrels in the shank

Burst open thick leek due to infestation with leek leaf miner fly

Even thick leeks are infested in autumn

cracked and twisted leek bulb infoge infestation with leek leaf miner fly

twisted leek leaves indicate early infestation

Leek miner fly and onion fly - harmful maggots on onions and leeks

Leek miner fly on spring onion

Infestation of the onion fly

The onion fly, which hibernates in the soil as a pupa, hatches from the end of April, always at the time of dandelion flowering, as the flies feed on its nectar and pollen. The onion fly lays its eggs at the shoot base of young onion plants. The maggots that hatch from them feed at the base of the bulb plant and hollow it out from the inside. The onion plants wilt, rot at the base of the stem, and die. and can be easily pulled out of the ground. Sometimes the bright maggots of the onion fly can be found there on more vigorous plants.

In contrast to the leek leaf miner fly, the onion fly does not pupate in the plant, even in the first generation, but in the soil. When the second or even third generation hatches, the onion and leek plants are already stronger and more robust and are not damaged as much. This is also a difference to the leek leaf miner fly: the damage is sometimes greatest on winter leeks.

Control and protection

As different as all three vegetable flies are – leek leaf miner fly, onion fly and beet fly: a chemical-free measure can safely protect all onion crops in the garden if used correctly: close-meshed crop protection nets! These also help at the same time against the leek moth (Acrolepiopsis assectella), another leek pest.

On the Internet there are numerous tips for the natural control of the leek leaf miner fly, such as the installation of Mixed crops or special Herbal broths. In my experience, none of this really helps. Even with the often recommended very early attachment my spring onions were infested, even if the damage was limited and a harvest – with cutbacks – was still possible.

Less close to nature, but also permitted in organic farming, are Neem-based plant protection products for spraying and working into the soil. The seeds of the neem tree contain an oil that is toxic to insects. The active ingredient from neem oil is effective against sucking and biting insects.
As a preventive measure to strengthen the plants, it helps to work natural fertilizer based on neem – so-called neem press cake – into the soil. The crops take up the neem active ingredient and are rather avoided by the pests.

For the control of the onion fly there are also Nematodes in the trade. Nematodes are filamentous micro-soil creatures and the natural enemies of the onion fly.

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