Brown Wheat Mite
Petrobia latens (Muller)
The brown wheat mite is a sporadic pest of winter wheat in the western plains. Mite impact is most severe when drought conditions persist through the winter and spring. Cropping practices can increase incidence, but impact on wheat will largely depend on the moisture status of the wheat. In the northern Great Plains, the brown wheat mite also can transmit barley yellow streak mosaic virus.
Identification / Life Cycle
The brown wheat mite is about 1o f an inch (0.5 mm) in length with a dark brown to black body and lighter colored legs (Figure 7.13). The front legs are about twice as long as the others and are often held straight in front of the body. Brown wheat mites are parthenogenic (all females) and over-summer as dormant white eggs (Figure 8.14). In the fall, when they are exposed to lower temperatures and rainfall, white eggs will hatch. Multiple generations occur from fall through spring. Eggs laid from the fall through early spring will be red in color (Figure 7.14) and will hatch in about 7 days at 72°F (22°C). Mite populations increase more rapidly under dry conditions. Populations peak in early spring (April) then decline with the onset of continuous warm weather. The final spring generation produces dormant white eggs.
Figure 7.13 Brown wheat mite.
Figure 7.14 Brown wheat mite eggs.
Plant Damage and Response
The mites spend nights in the soil and among the leaves near the soil, and move up to feed on the leaves during the day. Feeding causes stippling or yellowing of the leaves, especially at the leaf tips. Extensive damage will result in bronzed or brown plants that appear drought stressed. The impact of brown wheat mite feeding will be most severe when plants are stressed by drought.
The greatest risk of brown wheat mite infestation occurs in continuous winter wheat or when volunteer wheat was present the previous spring. These situations can result in large populations of over-summering white eggs that hatch and infest the new crop in the fall. Control volunteer wheat and avoid continuous winter wheat to reduce the risk of large mite populations.
Decisions on the need to control brown wheat mite infestations are difficult because infestations mostly occur when the wheat is severely drought stressed. If no rainfall is received, mites remain active and plant damage increases, but yield potential will be reduced due to drought stress. However, if rainfall greater than ¼ to ½ inch is received, mite populations will be reduced along with plant stress. Treatments may only buy time for the plant to catch a critical rainfall event. Treatments should only be considered if mite populations exceed several hundred mites per row foot, damage symptoms are evident, and females are still depositing primarily red eggs. When sampling, mites are most active on the foliage in the early afternoon of warm days. As the proportion of white eggs increases, adult population densities begin to decline.