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How to Manage Pests

UC Pest Management Guidelines


Adult bulb mite, Rhizoglyphus robini.

Lettuce

Bulb Mites

Scientific Names: Rhizoglyphus spp., Tyrophagus spp.

(Reviewed 8/07, updated 10/09)

In this Guideline:


DESCRIPTION OF THE PESTS

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. They have a wide host range, feed on many kinds of bulbs, roots, and tubers, and can infest bulbs in storage or in the field. Bulb 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
Rapid rotation, from one crop to the next, fosters survival of mites on the leftover vegetation in the soil from the previous crop. Decaying cole crops, especially cauliflower, may harbor very high bulb mite populations. Fallow fields to allow complete decomposition of organic matter; this reduces field populations of the mite. 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 undecayed vegetation in the soil for the presence of the mites.

Treatments are generally preventive and should be considered for fields that are high in vegetative 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/Acre R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
When choosing a pesticide, consider information relating to the impact on natural enemies and honey bees and environmental impact. Not all registered pesticides are listed. Always read label of product being used.
 
PREPLANT
A. METAM SODIUM* 50–75 gal 48 NA
  (Vapam)
  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. Fumigate only as a last resort when other management strategies have not been successful or are not available.
 
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) 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.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Lettuce
UC ANR Publication 3450
Insects and Other Arthropods
E. T. Natwick, UC Cooperative Extension, Imperial County
Acknowledgment for contributions to Insects and Mites:
W. E. Chaney, UC Cooperative Extension, Monterey County
N. C. Toscano, Entomology, UC Riverside

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