Bulb mites (Rhizoglyphus spp. or Tyrophagus spp.) (Photo 1) are known to attack spinach, onion and garlic but recently I have observed them feeding on germinating seeds of broccoli (Photo 2). Cool and wet soil conditions especially during early spring, trigger rapid multiplication of these mites in the soil and they infest germinating seeds. These mites often feed on the internal content of the seeds leaving behind only the seed coat. Severe infestation will result in inconsistent crop stand (Photo 3, and 4). Those affected seedlings would have distorted shoot and/or delayed germination (Photo 5). Cotyledon leaves of those affected seedlings would have damaged margins (Photo 6). Unlike the damage from garden symphylan which appears in hot spots in a field, bulb mites affect the entire crop stand (Photo 3). 

Bulb mites seem to be composed of several species of soil mites including crown mites. Bulb mites are shiny, spherical and clear or off-white in color. These mites often have distinctive long hairs on their abdomen. These mites survive on the decaying plant material in the soil.

For further reading:http://www.ipm.ucdavis.edu/PMG/r584400111.html   http://www.ipm.ucdavis.edu/PMG/r732400111.html