Onion Pests - Bulb Mites
Adult: Shiny, creamy white, bulb shaped, and about 0.03 inch (.8 mm) long. Wingless with four pairs of short brown legs. Mouthparts and legs are purplish-brown. Often described as tiny pearls with legs.
Egg: White, minute, and laid singly on bulbs.
Immature Stages: White to brown, oval, 0.15 to 0.4 mm long, with three pairs of legs initially then four pairs as the mite matures. Stages include larva, protonymph, and deutonymph.
Bulb mites have a wide host range and overwinter on decaying vegetation such as weeds or plant debris from previous crops. Males die shortly after mating but females may live for about a month. Females will lay 50 to 100 eggs in a lifetime (about six to eight per day). Eggs hatch in 2 to 7 days. One generation can be completed in 2 to 4 weeks under favorable conditions. Bulb mites are slow moving and generally occur in clusters deep in the crevices between the roots and stem plate.
Bulb mites feed on the roots, basal plate, and outer skin layers of onion bulbs. Feeding injury provides openings for soil-borne fungal pathogens such as Pythium, Rhizoctonia, and Fusarium. Bulb mites can reduce plant stands and vigor. Injury typically occurs during early vegetative growth stages of onion. Symptoms resemble those of damping-off caused by Pythium. Infestations affect onion bulbs both in field and in storage.
Cool, wet weather that retards plant growth favors bulb mite injury, and cultural practices that promote rapid growth can allow plants to outgrow injury
- Allow crop residues to fully decompose prior to planting onions. This will discourage bulb mites.
- Use clean seed and transplants. Examine transplant seedlings for presence of mites prior to planting and discard any that are soft when squeezed.
- Rotate onions. Mite populations will increase in soil following successive plantings of onion.
- Store bulbs under cool temperatures and low relative humidity. Storing onion bulbs under the appropriate conditions minimizes diseases and reduces build-up of bulb mite populations
- Use hot water treatments. For bulbs to be planted as onion sets, dip bulbs in hot water before planting. Note: Hot water treatment can weaken bulbs.
Pre-plant soil fumigation can be used to control mites that are found in the soil prior to planting. Soaking bulbs in a miticide before planting can help prevent bulb mite injury.
Bulb mite populations may be suppressed by the soil-dwelling predatory mite Hypoaspis aculeifer.