Where volunteer wheat has emerged, producers should begin control measures as soon as 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 require one more field pass than normal later in the summer to control later volunteer, but will help prevent bigger problems down the road. Producers should note that grazing volunteer wheat is not an effective option because there is green wheat material left that will enable wheat curl mite survival.
Where wheat suffered hail damage after heading, volunteer wheat often emerges even before harvest – as much as two to three weeks earlier than it would normally emerge. This volunteer wheat is especially likely to become infested with wheat curl mites and lead to problems later in the season if left uncontrolled. Wheat curl mites move off wheat at or near harvest time, and need to quickly find green tissue on a suitable host plant or they will soon die of desiccation. 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 disperse on winds for greater distances than previously thought. Young, volunteer wheat growing in close proximity to current wheat harvest will be easily infested by the mites.
If volunteer has emerged and is still alive shortly after harvest in hailed-out wheat, wheat curl mites can build up rapidly and move from there to other volunteer that emerges later in the season. Eliminating this early-emerging volunteer shortly after harvest will help break 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 mosaic virus. Barnyard grass topped the list 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.
By Ryan Flaming, County Extension Agent, Agriculture & Natural Resources