Description of the Pest
There are two groups of bulb mites that can infest onion and garlic:
- Mites in the family Acaridae, which are round and look like tiny pearls.
- Mites in the family Eriophyidae, called dry bulb mite and wheat curl mite. These mites are elongate and banana-shaped or wormlike.
Both groups of mites can infest onion and garlic both in the field and in storage. They can survive on decaying vegetation in the field until it is completely decomposed. These pests are not currently a problem in the low desert region.
Bulb mites in the family Acaridae are shiny, creamy-white, and bulb-shaped. They are between 0.02 to 0.04 inches (0.5–1 mm) long and have brown legs. These mites generally occur in clusters and inhabit damaged areas under the root plate of onion bulbs or garlic cloves.
Females lay eggs singly or in clusters of up to 100 eggs on damaged or decaying tissue on the bulb surface. Larvae have three pairs of legs, while nymphs and adults have four pairs of legs. All stages feed on the crop.
These mites have a wide host range, and feed on germinating seeds, bulbs, roots, and tubers of various plant species.
Eriophyid Mites: Dry Bulb Mite and Wheat Curl Mite
The dry bulb mite and wheat curl mite are smaller and more elongate than the other bulb mites, which are globular. These mites are microscopic, white, wormlike organisms of about 0.01 inch (0.25 mm). They have four legs that are located near the head.
In addition to onion and garlic, the dry bulb mite also feeds on liliaceous bulbs such as tulips. The wheat curl mite primarily feeds on cereal grains and wild grasses but occasionally feeds on onion and garlic as well.
Bulb mites can stunt plant growth and reduce the onion or garlic stand. These mites also promote the rot of bulbs in storage by penetrating the outer layer of bulb tissue and allowing rot-causing pathogens to gain entry.
Bulb mites in this group are most damaging in cool, wet weather, when plant growth is slow. In direct-seeded onion, they can cut off the radicle before the plant becomes established, reducing the stand and creating gaps in the seed lines where weeds may grow.
Dry Bulb Mite and Wheat Curl Mite
Feeding by the dry bulb mite and wheat curl mite also causes stored onions and garlic to desiccate. Feeding injury during storage produces sunken brown spots and streaks on garlic cloves. Heavy field infestations in the field may also cause streaking and twisting of garlic leaves and reduce both the plant stand and yield. Damage from these mites on stored bulbs is rare in California, and the dry bulb mite more commonly infests onion and garlic than the wheat curl mite.
Culturally manage these mites with the following methods:
- Use only clean seed cloves when planting garlic.
- Avoid planting successive onion or garlic crops and rotate with crops that are less susceptible to bulb mite damage.
- Avoid planting onion or garlic immediately after Brassica species, corn, grain, sudangrass, or grass cover crops.
- Consider treating garlic seed cloves with hot water (130ºF for 10–20 minutes) before planting. Note: although this practice may reduce mite infestations, it can also decrease germination.
- Flood irrigation or heavy rains during the winter may also reduce mite numbers in the soil.
- The normal drying process before storage can destroy light to moderate infestations.
- In other states, soaking seeds in 2% soap (not detergent) and 2% mineral oil for 24 hours before planting has successfully reduced mite infestations; however, this method has not been evaluated in California.
In addition to the cultural methods listed above, allow complete decomposition of organic material by leaving fields unplanted (fallow) between the harvest of the previous crop and the start of the next crop. 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 numbers of Acarid mites.
Monitoring and Management Decisions
No specific monitoring methods or action thresholds are available. Use a microscope to examine fragments of undecayed vegetation in the soil or volunteer onions or garlic for the presence of the mites. For dry bulb mite and wheat curl mite, examine the surface of garlic seed cloves under a microscope to determine if these mites are present. See Identifying Pests of Onion and Garlic and Their Damage for mite photos and identification tips.
Pesticide applications for Acarid mites are generally preventive and should be considered for fields that are high in vegetative matter or that have had previous bulb mite problems. However, allowing complete decomposition of organic matter in the soil before planting is the key to managing these pests.
Pesticide applications are generally not necessary for dry bulb mite and wheat curl mite because damage from these mites rarely occurs in California.
|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.|
|(Vapam HL)||37.5–75 gal||See label||NA|
|COMMENTS: 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 the two intervals is the minimum time that must elapse before harvest.|
|*||Permit required from county agricultural commissioner for purchase or use.|
|§||Do not exceed the maximum rates allowed under the California Code of Regulations Restricted Materials Use Requirements, which may be lower than maximum label rates.|