Ashleigh Faris, Cropping Systems Entomologist, IPM Coordinator
Meriem Aoun, Wheat Pathologist
Department of Entomology & Plant Pathology,
Oklahoma State University

Wheat Curl Mite (WCM) activity has been confirmed in Washita County, located in western Oklahoma. While the mites themselves are difficult to see, they can have a considerable impact on wheat health, primarily due to their role as vectors for several viral diseases such as wheat streak mosaic virus (WSMV). The Plant Disease and Insect Diagnostic Laboratory (PDIDL) has confirmed WSMV in the sample where WCM were detected in Washita County. This week, the PDIDL has also confirmed infection by WSMV in Blaine County (Canton, OK), McCurtain County (Garvin, OK), and Cleveland County (Noble, OK).

Identification

The Wheat Curl Mite is nearly invisible to the naked eye. At approximately 1/100 of an inch long, these pests require a 10x – 20x hand lens for proper identification.

• Appearance: They are white or cream-colored, cigar-shaped (cylindrical), and possess only four legs located near the head (Figure 1).
• Behavior: They are typically found in the protected areas of the plant, such as developing, youngest leaves or the furrows of the leaf surface. As the leaf unfurls, the mites migrate to the next emerging leaf.

Figure 1. Wheat curl mites and eggs on a wheat leaf (A, B), and mites on a maturing wheat kernel (C). Images courtesy G. Bauchan and R. Ochoa, USDA-ARS.

Biology and Life Cycle

Understanding the WCM life cycle is critical for preventative management:

• Rapid Reproduction: Under optimal temperatures (75° – 85°F), a WCM can complete its life cycle in 7 to 10 days. This allows populations to explode rapidly during warm autumns or springs.
• Dispersal: WCMs cannot fly; they rely entirely on wind currents to move from plant to plant or field to field. They crawl to the tips of leaves and hitchhike on the wind.
• Survival (The Green Bridge): WCMs are obligate parasites, meaning they require living green tissue to survive and reproduce. They persist through the summer on volunteer wheat and various perennial or annual grasses. This is known as the green bridge. If this bridge is not broken, mites move into the newly planted crop in the fall.

Damage and Virus Transmission

WCMs cause two types of damage:

1. Direct Feeding: Mites suck sap from the leaf cells. This causes the edges of the leaf to roll inward (the curl part of WCM) (Figure 2). This curling provides a protected microclimate for the mites to reproduce. Heavy infestations can cause stunting and a slowed appearance in growth.
2. Viral Vector (Primary Concern): The WCM is the sole vector for Wheat Streak Mosaic Virus (WSMV), High Plains Wheat Mosaic Virus (HPWMoV), and Triticum Mosaic Virus (TriMV).
   • Symptoms: Infected plants show yellowing, mottled or streaked leaves, and severe stunting (Figures 3 & 4).
   • Impact: If infection occurs in the fall, yield loss can be up to 100%. Spring infections are generally less damaging.

Scouting Techniques

Because the mites are so small, scouting focuses on leaf symptoms and having a hand lens:

• Check your Fields: Examine the youngest leaves of the wheat plant. Look for the characteristic inward rolling of the leaf edges (Figure 2).
• Use Magnification: Slowly unroll a suspect leaf and use a hand lens to look for tiny, white, slow-moving specks in the leaf furrows.
• Pattern of Infestation: Wind-dispersed mite infestations often start at the edge of a field (particularly edges adjacent to volunteer wheat or CRP land) and move inward in the direction of prevailing winds. Areas with infestations may show signs of yellowing and appear as patches distributed at random across the field (Figure 4).

Figure 2. Infestation of wheat curl mites on wheat results in tightly curled leaves and entrapment of subsequent leaves within the curl (A). After full leaf emergence, a tight curl at the leaf edge remains (B). Images courtesy of UNL Extension.

Figure 3. Wheat streak mosaic virus (WSMV) symptoms includeyellowing, mottled or streaked leaves. Image courtesy of Meriem Aoun, Oklahoma State University.

Figure 4. Plants at field margins, neighboring a wheat curl mite source, are the first to become infected with viruses of the Wheat Streak Mosaic Virus (WSMV) complex and develop symptoms, such as yellowing and streaking. Notice the gradient in color from the field edge (left) toward the center of the wheat field. Image courtesy of UNL Extension.

Management Recommendations

Currently, there are no effective rescue chemical treatments for WCM once symptoms appear in the field. Miticides generally do not reach the mites hidden inside the curled leaves. Management must be proactive:

1. Manage volunteer wheat and grassy weeds: This is the most effective management tool to break the green bridge. Ensure all volunteer wheat and grassy weeds are completely dead (via tillage or herbicide) at least two weeks prior to planting the new crop. WCMs will starve within days without a living host.
2. Delayed Planting: Planting wheat later in the fall reduces the window of time that mites must migrate into the crop and slows their reproduction rate as temperatures drop.
3. Variety Selection: Some wheat varieties offer resistance or tolerance to WCM or WSMV. Consult the latest OSU variety trial data to select adapted varieties for north-central Oklahoma that carry these traits. Currently, Breakthrough is the most resistant OSU variety, which carries the WSMV resistance gene, Wsm1.