How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

Larvae are small, legless, white maggots usually less than 0.33 inch (8 mm) when full grown; their head end is pointed and the rear is blunt with a dozen short, pointed fleshy processes arranged in a circle around two brown, button-like spiracles. They are found feeding on feeder roots or boring into the taproot. Adults are dark gray flies about half the size of the common housefly; they lay their eggs in cracks in the soil near plant stems, and hatching larvae burrow beneath the soil surface to invade the roots. After feeding 3 to 5 weeks, larvae pupate in roots or surrounding soil. Adults may emerge from pupae within 2 to 3 weeks, or the pest may overwinter as pupae when conditions are unfavorable for development. There are at least two to three generations in cool, moist climates along the coast.

DAMAGE

Cabbage maggots damage and destroy root systems of all cole crops, riddling roots with tunnels when infestations are heavy. Tunnels provide entryways for pathogens that cause blackleg and bacterial soft rot. Young plants between seedling emergence until about a month after thinning or transplanting are most vulnerable; healthy plants attacked after they are well established can usually tolerate moderate infestations. Cauliflower and Brussels sprouts may be more susceptible than hybrid cultivars of broccoli; crops planted in winter and spring suffer more damage than summer-planted crops.

MANAGEMENT

Cool, wet spring weather is favorable to the population development of these pests. Where maggots are a perennial problem, grow seedlings for transplants in fumigated soil in the greenhouse or under frames of clear plastic or organdy. Avoid hardening transplants near infested fields. Direct-seeded crops may avoid some injury when a set of drag chains is attached behind the planter to eliminate the moisture gradient in the seedrow. Adult flies are believed to be able to locate the seed row for egglaying by honing in on the higher moisture levels created when the soil is overturned for planting. Older plants may outgrow moderate cabbage maggot populations if maintained with a careful irrigation schedule. Always disk under crop residues immediately after harvest. Maggots can survive for some time in crop residue. Do not follow susceptible crops with susceptible crops unless sufficient time has passed for the residue to dry or decompose completely.

Monitoring and Treatment Decisions
Every year, treat spring planted or transplanted crops in areas where cabbage maggot causes economic injury with a band of insecticide at the base of the plant at the time of planting or transplanting. Later sprays can not be relied upon to effectively control the pest. Seedbeds for transplanted crops should also be fumigated or treated with an insecticide. Once the crop emerges, watch for wilting, lighter green plants, or reduced growth that may indicate a maggot infestation; pull up affected plants and check roots and soil to confirm presence of maggots. If several rows of seedling plants are infested, plants may be removed and rows replanted; drenching with insecticide is also an option but such treatments are difficult, costly, and may not be adequate. If roots are tunneled but no maggots are present, maggots have left roots to pupate and insecticide treatments would be of little value.

PUBLICATION

UC IPM Pest Management Guidelines: Cole Crops
UC ANR Publication 3442
Insects and Mites
E. T. Natwick, UC Cooperative Extension, Imperial County
Acknowledgments for contributions to Insects and Mites:
W. J. Bentley, UC IPM Program, Kearney Agricultural Center, Parlier
W. E. Chaney, UC Cooperative Extension, Monterey County
N. C. Toscano, Entomology, UC Riverside

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