Late-emerging volunteer wheat
Recent rains in much of Kansas have brought on a flush of volunteer wheat. Any field of volunteer should be killed as soon as possible at this time of year so that it is completely dead at least two weeks before any wheat is planted within a half-mile of it.
The main threats from late-emerging volunteer wheat, however, are a little different than the main threats from volunteer that emerged in early or mid-summer.
Normally we think of wheat curl mites and the diseases vectored by these mites (wheat streak mosaic, High Plains Virus, and triticum mosaic virus) as being the main threats from volunteer wheat. The threat of wheat streak mosaic is greatest from volunteer wheat that emerges early. That’s because wheat curl mites leave their main host, wheat, as the green tissue on the host is dying. In Kansas, that typically occurs in June. Wheat curl mites can also live on corn and a few other summer grasses. The mites on corn will leave as the corn plants begin to die.
Once the wheat curl mites leave their hosts, they can spread to other areas in the wind, normally within about a half-mile radius. But they only survive for about a week unless they find another living, green host plant on which they can survive. Volunteer wheat that emerges early in the summer provides an ideal oversummering host for mites. The mites can land on volunteer and multiply over the summer until the volunteer is either killed or they leave for other reasons. If there are fields of newly planted wheat nearby when the mites leave the volunteer, these new fields have a good chance of being infested with wheat curl mites – and becoming infected with wheat streak mosaic or a similar mosaic disease.
What about late-emerging volunteer? It is much less likely that volunteer wheat that came up after the rains on August 24-26 will get infested with wheat curl mites, unless there are other fields of volunteer within a half-mile that had come up earlier in the summer.
That doesn’t mean there are no problems associated with volunteer that just came up, however. It is still several weeks before most fields of wheat will be planted. That is plenty of time for this late-emerging volunteer wheat to become infested with either Hessian fly or greenbugs and oat bird cherry aphids.
The heavy rains some areas of Kansas received recently will not only bring up the volunteer, they will also cause adult Hessian flies to emerge from the oversummering pupae found in the old crowns and residue of last season’s wheat crop. The Hessian fly adults and volunteer wheat will both have emerged at about the same time due to the same rain events, meaning the volunteer stands a chance of becoming infested with Hessian fly where there were fields with wheat residue nearby. If newly emerged volunteer becomes infested with Hessian fly, the life cycle of the Hessian fly population will continue and could re-infest nearby planted wheat later this fall or early winter. Killing this newly emerged volunteer now will minimize the chances that Hessian fly can gain a foothold in the area.
Finally, newly emerged volunteer will be an ideal landing site for early waves of greenbugs and bird cherry oat aphids blowing in during September from southern areas. If these aphids find fields of volunteer wheat to land on, they will likely become established and multiply. They can then move on to infest planted wheat after it emerges in October. Greenbugs and bird cherry oat aphids can be vectors of barley yellow dwarf virus – an increasing problem in much of Kansas. By controlling any new stands of volunteer now, producers can help prevent these insect pests from becoming established in the immediate areas of those fields of volunteer. This will not guarantee that planted wheat will not become infected with barley yellow dwarf from aphids coming in from other locations, or at other times of the year, but it will help.