Root Maggots (Delia spp.)
Root maggots are the immature, damaging phase (larvae) of adult flies (Diptera: Anthomyiidae). Cabbage rootfly maggots (D. radicum) feed on all cole crops, but prefers cauliflower, brussel sprouts, and winter or spring-planted brassicas such as turnip and radish, respectively. In the PNW, there is a root maggot ‘complex’ which contributes to the successful management of these pests. Click here for a comparison table of related species.
Identification and Life Cycle Adults look similar to house flies but are smaller (5mm), dark ash grey color with a dark stripe along the top of the abdomen, and covered with black hairs and bristles (setae). The reddish purple eyes on males nearly touch in the centre of the head while female eyes are separated. Identification of rootflies is difficult, and depends on minute characteristics such as thorax bristles and setae on legs.
Overwintering occurs in the pupal stage in soil or crop residue. In the PNW, adult flies emerge in early spring, between 300-600 GDD (growing degree days) after January 1st. Female flies lay eggs directly at the base of host plants or in nearby cracked soil, and prefer maturing plants (5-7 leaves) rather than seedlings. Maggots hatch in 4 to 10 days and migrate below ground where they feed continuously for approximately 3 weeks. They then leave the roots and pupate on or just below the soil surface.
Depending on weather conditions, pupae will either emerge as adult flies 2 weeks later or overwinter as pupae. There are multiple generations per year. More information about Delia spp. lifecycles can be found at this post.
Damage is caused by immature flies (maggots) as they tunnel into developing feeder roots and the established taproot. This tunneling directly reduces uptake of water and nutrients, and more importantly, can increase pathogens such as blackleg and soft rot.
Monitoring for adults is done by predicting adult flights based on degree-day models, and setting pan traps (a.k.a water traps) at field edges in early spring. Yellow sticky traps also can be effective. Adult flies are attracted to yellow because it simulates a flowering brassica crop. If adults are found, scout the field for wilted plants or areas of reduced growth that may indicate damage by root-feeding maggots. Affected plants may have a blue or purple hue to the foliage, especially on lower leaves.
Look for maggots on the root and in the surrounding soil. When checking for eggs, gently move the soil around with a small paintbrush. It is not necessary to count the eggs. Knowing that eggs are present is enough to consider the plant infested. If the root has empty holes, but no maggots are found, they have left the roots to pupate and an insecticide treatment would be ineffective.
Although there is not an official threshold, we have noted damage in direct-seeded radish when pan traps exceed two flies per day.
Cultural control: Crop rotation is integral to controlling cabbage maggot. Direct-seeded crops are at risk because because female flies use moisture as an oviposition cue. Residue should be disked immediately after harvest, and repeated at least once thereafter.
Biological control: Cabbage maggot may be suppressed by natural enemies such as carabid beetles, staphylinid (rove) beetles, and parasitic wasps. Providing habitat and other conservation techniques to preserve natural enemies are recommended. However, biological controls alone are not an effective means to control this pest, especially once a population has established.
Chemical control: Chlorpyrifos banded in-row at time of planting or transplanting may be used, but may cause stand reduction if applied in summer. Seed treatments are only marginally effective against maggots, because they rarely last long enough to provide adequate control. In-furrow treatments may provide some control during establishment, but are not registered for all brassica crops. Insecticide is not recommended to attempt to control adult flies.
Consult the PNW Handbook for current insecticide labels and rates per crop.
View our research report from 2016 trial in fresh-market radish
Use USPEST.org to track growing-degree days for your location