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Species Page

onion fly

Delia antiqua
This information is part of a full datasheet available in the Crop Protection Compendium (CPC);www.cabi.org/cpc. For information on how to access the CPC, click here.

Distribution

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Host plants / species affected

Main hosts

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Allium cepa (onion)
Allium porrum (leek)

List of symptoms / signs

Leaves - abnormal colours
Leaves - wilting
Roots - external feeding
Roots - internal feeding
Stems - internal feeding

Symptoms

Onion seedlings are particularly susceptible. Maggot feeding on the roots or on the swollen base of the plant usually kills the plant. Early symptoms of onion fly attack appear as yellowing and wilting of the outer leaves, or even the collapse of the entire plant.

Damage occurs mainly from June to August in Europe. There are two, rarely three, generations during the summer, and maggots of the second and third generations will tunnel into the onion bulbs as well as attack the roots.

Prevention and control

Cultural Control

Crop rotation gives some control, and onion fields should be kept well separated. Onions grown in the year following an attack by D. antiqua should be sown as far away from infested land as possible.

In gardens and allotments, infested plants should be carefully removed together with larvae and burned. The soil should be turned to destroy puparia.

The application of animal manure attracts adult flies and increases infestation levels.

Host-Plant Resistance

The relative susceptibility or resistance of various Allium cultivars has been investigated (McFerson et al., 1996). Partial resistance has been identified in onion cultivars, but attempts to increase the levels of resistance have met with difficulties (Galmarini, 1997). Some differences in ovipositional response are due more to plant size than to plant chemistry. The cues triggering oviposition are discussed by Harris and Miller (1991), and host-finding cues by McDonald and Borden (1997).

Chemical Control

Chemical control methods need to be applied against the eggs or young larvae as they are useless once the larvae are inside the onions. Seed dressings using a varied arsenal of chemicals give good protection against D. antiqua, and are the most economic form of applying insecticides for control. Resistance to certain pesticides is now being detected.

Oviposition deterrence using pine oil constituents gave up to 85.3% deterrence (Ntiamoah et al., 1996).

Foliar spraying or dusting is not effective in controlling infestations.

In North America, ovipositional deterrents have been analysed and identified.

Biological Control

Semi-sterile mutants have been studied in the USA but have not yet been used in practical control systems. In the Netherlands, the sterile-male technique was so successful in field trials that it was put into commercial practice although found to be both complex and labour-intensive.

Laboratory experiments have shown that D. antiqua larvae are highly susceptible to the entomophilic nematode Neoaplectana carpocapsae [Steinernema carpocapsae]. Several species of parasitic Hymenoptera and Coleoptera (Staphylinidae) have proved to be effective control agents of immature D. antiqua.

A simulation model for D. antiqua population dynamics has been developed and can be used to optimise the timing of control measures against a specific life-stage of the fly (Otto and Hammes, 2000).

An integral management involving crop rotation, potentially resistant onion genotypes and chemical treatment at planting gave promising results <2% plant losses) (Walters and Eckenrode, 1996).

Impact

D. antiqua can cause serious damage especially in small-scale operations. The worst damage occurs in June and July, with some in August, and small plants suffer the most. A single larva can kill a young plant (up to three leaves). When present in large numbers, larvae can destroy a large proportion of young plants.

In Iran, infestation of onions by D. antiqua can be 80-90% after heavy spring rain and treatment with animal manure. In Romania, infestation levels range from 15 to 62%. In Poland, first-generation D. antiqua destroy 24.6-83.7% of onion plants, whilst the second generation only destroys 1.2-5.2% of plants (Szwejda, 1982). In Michigan, USA, first-generation larvae caused twice as much damage as second-generation larvae.

In plants that are bulbing up or are grown from sets, the larvae bore into the bulb from the base or side. A single bulb may contain up to 30 larvae and usually rots away.