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Adult- Mature bulb mites vary from 0.5 to 0.9 mm long and have four pairs of legs. Their bodies are shiny, white, somewhat transparent, and smooth with reddish brown appendages.
Egg- The egg is white and translucent, 0.12 mm long, and ellipsoidal.
Larva- Shortly after hatching, the larva is 0.15 to 0.2 mm long and when well developed is 0.25 mm long. White and oval, larvae have only three pairs of legs and lack genital suckers.
Protonymph- The protonymph has four pairs of legs; it is oval and approximately 0.4 mm long. This stage can be distinguished from the tritonymph by having two genital suckers, whereas the tritonymph has three or four suckers.
Deutonymph or Hypopus- This quiescent stage is oval, convex on top, flattened below, brown, and 0.2 to 0.3 mm long. The mouthpans are absent. On the back lower side is a conspicuous sucker plate.
Tritonymph- The tritonymph is about 0.5 mm in length and has not yet developed a distinct genital aperture.
Fig. 112: Bulb mite,
Rhizoglyphus echinopus (Fumouze and Robin). Other important species include
Rhizoglyphus robini Claparede. Rhizoglyphus hyacinthi Banks, and
Rhizoglyphus engeli v. Eijndhoven, Acaridae, ACARI A-Female, B-Egg, C-Larva, D-Protonymph, E-Hypopus, F-Tritonymph, G-Damaged bulb.
Distribution- From their original discovery in Europe, bulb mites have now been found throughout the United States, Canada, Japan, and the Bermuda Islands. These mites are easily transported in shipments of infested bulbs.
Host Plants- Bulb mites have been recorded feeding on forced iris, lily, narcissus, Gloriosa, Hippeastrum, Eucharis, orchid, hyacinth and tulip bulbs, dahlia tubers, and freesia and gladiolus corms. These mites also infest vegetable bulbs.
Damage- Bulbs infested with bulb mites may rot and fail to produce new growth, or new growth may be off color, stunted, and distorted. Although the bulb mite is not considered a primary pest of bulbs, it is often responsible for serious losses; the slightest injury to a bulb will allow bulb mites to enter and become established. Once the mites are inside the bulb, they rapidly turn the bulbs into rotten pulp. Infestations of the bulb mite generally indicate that the bulbs have already been injured. This damage could have been caused by other pests, such as the bulb scale mite or bulb flies, mechanical injury, or improper storage. The root primordia of freesia and gladiolus may be bruised at planting. Apparently bulb mites can attack healthy new roots and corms, especially in greenhouses. The mites may penetrate into lily stems which become brittle. Infested lilies are often dwarfed, distorted, and the stem roots are suppressed. Bulb mites attack the young root tips of Hippeastrum and Eucharis. When bulb mites attack Gloriosa, the new tubers get numerous small cavities and tunnels and become distorted. Bulb mites may enter prematurely opened tulip buds (due to high storage temperatures or ethylene generated by diseased bulbs in storage) and cause bud necrosis.
Life History- Bulb mites are rarely noticed as isolated individuals, but rather as large colonies. All stages of the mite can be found throughout the year. Development may occur in five or six stages (a hypopal stage is sometimes produced). In those forms with six stages, the life cycle proceeds from egg to larva to protonymph to hypopus to tritonymph to adult.
Females lay up to 700 eggs each depending on the host. R. robini tends to form relatively small colonies on narcissus and tulips whereas R. echinopus forms large colonies on a greater range of bulb crops. These mites can survive by feeding on paper and other sources of organic matter. The eggs mature in 5.1 to 27 days. The total life cycle from egg to adult could be as short as 12.2 days (at 25°C) for R. robini or 13.9 days (at 25°C) for R. echinopus depending on the host bulb, temperature, and relative humidity. Adults live longer at lower temperatures (up to 121 days) and males tend to live twice as long as females. These mites can survive at 35°C, but they cannot lay eggs at that temperature. On the other hand these mites cannot develop at temperatures below 11.8°C. The length of development is greatly dependent on temperature, relative humidity (100 percent is best), and available food. Hypopi form when the population becomes crowded, or the substrate becomes too polluted by decay. The hypopal stage attaches to insects visiting the bulbs and may be carried to other bulbs. Hypopi do not feed (no head), and they are resistant to starvation and desiccation during adverse conditions. The ratio of males to females varies from 1 to 1, to 1.9 to 1, depending on relative humidity, diet, and perhaps other factors. Besides their direct feeding, bulb mites are a threat because they carry pathogenic fungi.
It is very important to avoid rough handling of bulbs to prevent injury that might afford an entry point for fungi and bulb mites. Bulb mites cannot withstand drought and dry bulbs in storage are usually not attacked (unless mites are already deep inside tissue). Bulb mites are very tolerant of a number of synthetic pesticides apparently due to active oxidases, esterases, and transferases that detoxify such chemicals. Flooding gladiolus corms for 5 days gave 96.1 percent mortality; 14 days gave 100 percent mortality. A predaceous mite, Cosmolaelaps claviger, feeds readily on R. echinopus and other soil organisms. R. robini has an alarm pheromone, citral, which although not toxic to the mites, was definitely repellent. These mites left bulbs treated with citral at 100 ppm, and a mixture of citral and a miticide gave significantly better control of R. robini than the miticide by itself. Evidently the alarm pheromone made the mites more active and increased their contact with the pesticide. Soaking bulbs in a miticide before planting has been shown to prevent bulb mite injury. Steam sterilization and methyl bromide at low concentrations eliminated the mites from soil. For specific chemicals and control methods see your county Extension Agent or consult your state's management guide for ornamental plant pests.
University of Florida/IFAS Pest Control Guides