By J.P. Michaud, Extension Entomologist, and Dallas Peterson, Extension Weed Management Specialist
Where volunteer wheat has emerged, producers should consider beginning control measures soon, if possible, rather than waiting until closer to wheat planting time. This is especially important on fields where wheat was hailed out and volunteer wheat emerged at the time of harvest, or shortly afterward.
Controlling the first flush of volunteer wheat now may mean you’ll need one more field pass than normal later in the summer to control volunteer wheat, but will help prevent even bigger problems down the road. It should be noted that grazing volunteer is not an effective option because there is green wheat material left that wheat curl mites can survive on.
Where wheat suffered hail damage after heading, volunteer often emerges even before the existing field is harvested — as much as two to three weeks or more earlier than it would normally emerge after harvest. This volunteer wheat is especially likely to become infected with wheat curl mites and lead to problems later in the season if left uncontrolled.
Wheat curl mites will move off growing wheat as the green tissue dries down and dies. After moving off the existing wheat at or near harvest time, the mites need to find green tissue of a suitable host soon or they will die of desiccation.
Research has found that the mites can live quite a few hours off the plant, and up to 24 hours under low temperature conditions, so significant numbers of mites may be blown in from farther away than previously thought.
If there is young, volunteer wheat growing at the time the current wheat crop is being harvested in the nearby region, the mites can quickly infest those volunteer plants and survive.
If volunteer has emerged and is still alive shortly after harvest in hailed-out wheat, wheat curl mites could easily build up rapidly and spread to other volunteer wheat that emerges later in the season.
On the other hand, if this early-emerging volunteer is controlled shortly after harvest, that will help greatly in breaking the green bridge. However, if more volunteer emerges during the summer, follow-up control will still be needed.
Volunteer wheat is not the only host of the wheat curl mite. Recent research has evaluated the suitability of wild grasses as hosts for both the curl mite and the wheat streak virus. Barnyardgrass topped the list in terms of suitability for both virus and mites, but is fortunately not that common in wheat fields. In contrast, green foxtail, although a rather poor host, could be an important disease reservoir simply because of its abundance. Take note of significant stands of these grasses in marginal areas and control them as you would volunteer wheat.
If volunteer wheat and other hosts are not controlled throughout the summer and are infested with wheat curl mites, the mites will survive until fall and could infest newly planted wheat at that time. Wheat curl mite infestations are the cause of infections of wheat streak mosaic, High Plains mosaic virus, and triticum mosaic virus.
Wheat varieties with the wsm2 gene for resistance to wheat streak mosaic (Oakley CL, Joe, and Clara CL) remain susceptible to High Plains mosaic virus and triticum mosaic virus, so controlling volunteer wheat is still important even if you plant one of those varieties.