Agriculture: Lettuce Pest Management Guidelines

Bulb Mites

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

    Bulb mites are shiny, creamy white, bulbous-appearing mites that range in size from 0.02 to 0.04 inches (0.5–1 mm) long. They have four pairs of short brown legs and look like tiny pearls with legs. Bulb mites have a wide host range, feed on many kinds of bulbs, roots, and tubers, both, in storage or in the field. These mites can survive on decaying vegetation in the field until it is completely decomposed.

    Damage

    Bulb mites damage lettuce at germination only by penetrating the seedcoat as soon as germination begins. This pest is most damaging when emergence is slowed by cool, wet weather. Bulb mites can drastically reduce plant stands, especially when lettuce follows cole crops.

    Management

    Cultural Control

    Rotation from one crop to the next without proper cultivation will foster mite survival on the leftover vegetation in the soil from the previous crop. Decaying cole crops, especially cauliflower, may harbor high bulb mite numbers. Fallow fields to allow complete decomposition of organic matter; this will in turn reduce mite numbers. Flood irrigation or heavy rains during the winter may reduce mite levels in the soil.

    Monitoring and Treatment Decisions

    No specific monitoring methods are available. Use a microscope to examine fragments of non-decayed vegetation in the soil for the presence of the mites.

    Treatments are generally preventive and should be considered for fields that are high in organic matter or that have had previous bulb mite problems. No treatment thresholds exist. Proper vegetation management and timing of planting is key.

    Common name Amount per acre** REI‡ PHI‡
    (Example trade name) (hours) (days)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.
    PREPLANT
    A. METAM SODIUM*
    (Vapam) 50–75 gal see label NA
    COMMENTS: Metam sodium injected into the soil at 6 inches deep before planting has provided fair to good control of bulb mites living on organic matter in the field. However, it will not control mites brought into the field on infested bulbs. Fumigants such as metam sodium are a source of volatile organic compounds (VOCs) but are minimally reactive with other air contaminants that form ozone.
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
    * Permit required from county agricultural commissioner for purchase or use.
    NA Not applicable.
    Text Updated: 04/17
    Treatment Table Updated: 04/17