Petrobia latens (Müller)
Common name: Brown wheat mite.
Geographical distribution: This mite has an almost cosmopolitan distribution,
Host plants: The brown wheat mite attacks over 200 host plants of economic importance, including a variety of grains and onion, garlic, asparagus, strawberries, cucumber, spice crops, clover and other legumes.
Morphology: Body about 0.5 mm long, red-brown to black, legs long and yellow, forelegs distinctly longer than the other legs. Claws pad-like; empodium hooked and with than 2 pairs of tenent hairs. With 2 types of eggs, bright-red non-diapause eggs and the white, capped diapause egg.
Life history: This parthenogenetic mite can complete a life cycle within a fortnight, raising three annual generations. Adults live for about 3 weeks. In autumn they lay up to 30 diapause eggs, which may remain in this state for long periods, hatching when wetted, usually in the following spring. The non-diapause females lay about 50-90 eggs each; all eggs are placed on soil next to vegetation.
Economic importance: The brown wheat mite is a pest of small grains in most parts of the world, but may also damage onion, carrots and many other crops. The mites feed only on leaves, which results in leaf yellowing and mottling, in unhealthy grain formation and in wilting and dying of plants. An entire field can appear yellow-brown during heavy infestations. Most damage is seen during periods of dry and hot weather. Extensive mite injury is sporadic, usually occurring when winter rains are followed by a dry period and the plants are stressed. Petrobia latens is a vector of barley yellow streak mosaic virus, which has been reported to cause yield losses to spring-planted barley in excess of 30%. The virus also infects wheat, reducing yields. In addition, the mite may sometimes be a nuisance in houses.
Monitoring. Brown wheat mites feed during the day, and the best time to scout for them is in mid-afternoon. Being dark-red, they are easily seen and accurate counting can be done by tapping plants over white paper and counting the dislodged mites. Volunteer wheat is a reservoir for the mites and should be examined. The presence of 30 or more mites/plant calls for control measures.
Horticultural Control: Removal by burning or deep plowing of volunteer cereal stands and other nearby susceptible host plants, crop rotation and deep plowing.
Chemical control: Organophosphates and some fungicides have provided good control in the field. Decisions on controlling the pest are difficult because of its, sporadic, weather-dependent occurrence and uneven rates of damage.
Biological control: Various predators attack the mite, but none are known to affect the extent of its damage.
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Kaneda, T. (and 6 co-authors). 2012. Susceptibility of the brown wheat mite, Petrobia latens (Müller) (Acari: Tetranychidae) to agricultural chemicals in carrot crops. Journal of the Acarological Society of Japan 21: 21-29.
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Sharma, A. and Nandan, D. 1966. Screening wheat genotypes against Petrobia latens (Muller). Annals of Arid Zone 35: 165-166.
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