Agriculture: Cole Crops Pest Management Guidelines

Bulb Mites

  • Bulb mites: Rhizoglyphus spp., Tyrophagus spp.
  • Description of the Pest

    Bulb mites are shiny, creamy-white, bulb-shaped mites that range in size from 0.02 to 0.04 inches (0.5–1 mm) and look like tiny pearls with legs. They generally occur in clusters and inhabit damaged areas of the crop. Females lay about 100 eggs individually or in clusters. Larvae have three pairs of legs, while nymphs and adults have four pairs of brown legs.

    Bulb mites have a wide host range, and feed on bulbs, roots, and tubers of various plant species. They can survive on decaying vegetation in the field until it is completely decomposed.

    Damage

    Bulb mites can stunt plant growth and reduce stand density. They can attack germinating seedlings and deform their leaves, but are most commonly secondary pests that colonize damaged crop tissue. Primary damage from Rhizoctonia diseases, clubroot, or cabbage maggot can attract bulb mites.

    This pest is most damaging in cool, wet weather, especially when there is undecomposed plant material in the soil. Though their damage is not widespread in California, they can be a problem for direct-seeded cole crops (especially broccoli) in the Salinas Valley. Bulb mites are not a pest of cole crops in the southern desert due to the hot, dry weather that occurs in that region.

    Management

    Cultural Control

    Rapid rotation from one crop to the next fosters the survival of mites on leftover crop residue in the soil. Decaying cole crops, especially cauliflower, may harbor very high bulb mite numbers. To reduce mite damage:

    • Allow complete decomposition of organic matter by leaving fields unplanted (fallow) between the harvest of the previous crop and the start of the next crop.
    • Avoid consecutive plantings of cole crops and other Brassica crops, particularly those that are direct-seeded.
      • Except in the southern desert, where summer temperatures are too high to favor bulb mites.

    Monitoring and Treatment Decisions

    There are currently no specific monitoring methods for bulb mites. Use a stereoscope to examine fragments of undecayed vegetation in the soil for the presence of the mites.

    The key to preventing bulb mites and their damage is to allow organic matter to completely break down before planting. No action thresholds currently exist, and pesticide applications for this pest are generally not necessary in cole crops. However, fumigation for other pests will also control bulb mites.

    Text Updated: 12/20