Maggots

Delia sp.

Diptera: Anthomyiidae

Synonyms for the genus Delia is Hylemya. Important species in this genus are Delia antiqua Meigen (onion maggot), Delia radicum (L.) (cabbage root maggot), and Delia platura Meigen (bean seed fly). For all three species are several synonyms known. Delia species are worldwide present. D. antiqua and D. radicum are more common in Europe and North America, while D. platura is also established in tropical regions. There are few reports about D. antiqua present in the topics.

Host plants and damage

D. antiqua is a pest of the genus Allium. D. radicum is a serious pest of cole crops such as Brassica chinensis (cabbage) and Brassica oleracea var. gemmifera (brussel sprouts). D. platura is polyphagous and major host plants are Phaseolus (beans), Zea mays (maize), Allium cepa (onion), Cucumis sativus (cucumber), and Lactuca sativa (cabbage).

Plant growth stages affected

Seedling and vegetative growth stages are attacked by Delia species.

Affected plant parts

Maggots feed mainly on the roots or on the basal plates of their host plants, but they are also found on seeds.

Symptoms

Early symptoms of attack by adults appear as yellowing and wilting of the outer leaves. Suffering plants may be stunted and yields reduced.

Morphology & biology

Adult flies can overwinter if conditions are not favorable. White eggs about 0.5 – 1.5 mm in length and are laid on leaves or necks of host plants or in the soil, usually within 20 cm of the host plant. Females prefer older plants for oviposition.

Larvae hatch after 3-7 days and immediately burrow through the soil to find host plant roots where they begin to feed. They attack the roots or the tissue at the base of the plant. There are three larval instars lasting a total of about three weeks. Larvae are elongate, creamy-white maggots. Fully-grown larvae can be up to 10 mm in length.

Pupation takes place in the soil, up to a depth of 10 cm. Pupae are brown, hard, and oval shaped. Adult flies emerge after 7-22 days. They are slender flies, and 6-7 mm in length.

Monitoring

Look for maggots feeding inside plant roots and seeds.

Pest management

Biological control

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

An important natural enemy of D. platura is the parasitoid Eucoila tanebae. In Newfoundland the beetle Aleochara bilineata kills large numbers of D. platura pupae.

Cultural control

Crop rotation can reduce Delia infestation. The distance between neighboring fields is important for crop rotation systems. Adults can fly up to 2 km. Infested plants should be carefully removed together with larvae, and burned.

The use of less susceptible crop varieties is another option in reducing damage.

Chemical Control

Only eggs and young larvae are susceptible to chemical control. Once larvae enter onion bulbs they come no longer in contact with insecticides. Many synthetic insecticides have been reported to be effective against D. antiqua.