Throughout the Texas High Plains and the Rolling Plains and beyond from about late September to the end of November this question is asked every year. AgriLife Extension entomologists and agronomists working with wheat and other small grain have gotten so used to this annual question we have to be careful to not assume the cause. Our questions will be:
• “Are the spots getting larger? (Yes)
• Is the field in continuous wheat? (Yes)
White grub worms, most often of May or June beetles (the larval stage of the June bug),are the culprit. Due to lack of rotation or any cultural practices like lack of tillage or failure to control weeds (which provide the grubs an ongoing food source outside the season of wheat growth), grub worms move through their lifecycle. Wheat and other small grains planted into fields with continuous small grain typically show many dead spots, and they are getting larger (Fig. 1).
In two instances this fall based on phone discussions I thought maybe grubs were actually not the problem. No, the farmer just had to look more and dig a little deeper in the soil. Typically I like to dig plants in the “transition zone” between bare spots, then dying or poor plants, and what looks like normal, healthy wheat (Fig. 2). That is where you will most likely find the worms. Later in the season after significant cooling you may need to dig deeper, but it is not uncommon to turn up at least 1 grub per shovel full of soil in the transition zone.
The worst damage–a dead plant–occurs when a grub is feeding near the wheat seeding depth and severs the root system right below the crown. This seedling is left with no root and will die. If grubs feed a little lower on the root system, the plant will be small and its productivity much reduced (Fig. 3). This plant will have much less root than a normal plant (Fig. 4), but it is often enough to sustain it until cooler weather when the grubs move further down in the soil.
If I have white grubs in my wheat can they be controlled?
No. Once the wheat is planted you can’t get at the grubs. Though chemigation of Lorsban (chlorpyriphos) has been discussed by some, it is not labeled in wheat. (Occasionally ask your entomologist if there are any new options.) Often I recommend that growers wait until cold weather sets in, which means the grubs move further down in the soil, then re-seed your wheat in the blank spots.
What should I do if I anticipate grubs will be a problem in my next crop?
Here are some considerations:
• Remember that rotation with a non-host crop does help. This may be necessary though it is difficult for those in small grains grazing programs to rotate out regularly.
• The above wheat insect guide notes that Gaucho (imadacloprid) and Cruiser (thiamethoxam) seed treatments might be effective. Remember, though, that grub worms can be large (0.5″ or more), and these are more difficult to control.
• Control weeds to eliminate potential food sources for grubs during the off-season.
• Delayed planting may reduce the chance grubs will completely kill early seedings. This may not be an option in a grazing system, but if even some of your seedings can be delayed that may help.
For further information on grub worms in wheat and other small grains, and additional possible strategies for control visit with your area integrated pest management (IPM) Extension agent or your regional extension entomologist.