Crop & Pest Report

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Grasshoppers Increasing (7/12/12)

With the hot and dry weather, grasshopper populations are increasing, especially in western North Dakota. Wheat fields in McKenzie County are reporting large numbers of grasshoppers resting on wheat heads.

Grasshopper mapHowever, little to no feeding injury has been observed yet on wheat heads or flag leaves. It is important for the growers to scout field edges and within the field to determine if the field needs to be treated for grasshoppers. The action threshold for nymphs (immature grasshopper without wings) is 50-75 per square yard in field margins and 30-45 per square yard within the field. For adult grasshoppers, the action threshold is lower:  21-40 in field margins and 8-14 within the field. Four 180 degree sweeps with a 15-inch sweep net can be used to estimate one square yard.

We feel that the grasshopper situation has the potential to get worse, especially if the weather continues to be hot and dry. Flowering or heading cereal crops are vulnerable to grasshopper feeding injury on kernels and leaves (leaf stripping). As wheat matures and the only green tissue is just below the head, severe head cutting can occur. As wheat fully ripens and matures, grasshoppers will move out of wheat in huge numbers and into other late-season crops, such as corn, sunflower or flax that are still green.

Adult grasshopperA pyrethroid, such as Baythroid XL, Warrior II, or Mustang Max (or their generic equivalents) is recommended for control of grasshoppers in wheat. HOWEVER, it is important to note that not all of these pyrethroids have the same pre-harvest interval. Of these, Mustang Max has the shortest PHI at 14 days. Always read and understand the label (e.g., application rates, PHI). It is extremely important to observe the PHI when selecting and using any pesticide as we get closer to harvest.

Dimilin 2L (diflubenzuron) has good performance against grasshoppers, but is a growth regulator and only works on 1st, 2nd, and 3rd instar nymphs. In some areas, it may be too late to use this product if large nymphs (>½ inch) and adults are appearing. Dimilin 2L has a long PHI of 50 days, and in North Dakota Dimilin 2L can only be used west of US Highway 281.

 

Janet J. Knodel

Extension Entomologist

janet.knodel@ndsu.edu

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