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1. Root damage is greatest when the majority of larvae have completed the 3rd instar (larval stage). Often, this is around tasseling and falls in July or August. It can occur as early as June with above average temperatures in spring and early summer.
2. Digging roots too early can underestimate the amount of damage that can be present.
3. Roots dug after the majority of the adults emerge are often more difficult to wash and rate due to root regrowth.
4. Usually there is a 2 to 3 week window that is optimum for digging roots.
1. In 3 random locations per treatment, 5 consecutive root balls should be dug.
2. Digging roots from an area with no biotech trait to control CRW and/or no soil insecticide can help determine the overall CRW pressure.
3. If the CRW larvae pressure is light in this ‘untreated’ area, it is likely that feeding differences will be minimal in the other treatments and further digging may not be warranted.
4. Removing the upper portion of the plant about 1 to 2 feet above the root mass can help make handling (hauling out of the field, soaking, etc.) much easier.
5. Label the root masses with the appropriate treatment. Permanent marker on duct tape placed a few inches below where the stalk was cut works fairly well, but there are several other methods.
6. Instead of digging the plants individually, it is often easier to dig in a large oval shape around all five plants. Loosening the soil around the 5-plant area can help keep the root masses intact and still allow separation of the plants.
1. Washing roots immediately after digging may remove some of the smaller roots with the soil; therefore, roots should be soaked in a tub of water for 30 minutes or more to help remove soil from the roots before washing.
2. If rootworm larvae or pupae (Figure 1) are present in the 13;10;13;10;root ball, they may float to the top of the water while soaking.
3. Roots should be washed carefully using a pressure washer or strong hose. To avoid unwanted injury, the pressure washer should be on low. After washing, roots are ready to be rated.
1. When rating roots, carefully pull back at each node to allow for easier inspection of rootworm scarring and root pruning.
2. Three nodes should be evaluated, starting with the uppermost node which has all of the roots at least 1.5 inches into the soil.
3. Later in the season, it is likely that brace roots will be the first one or two nodes that will need to be evaluated. When evaluating brace roots, the 1.5 inch parameter becomes relative to the soil line instead of the crown.
4. To assign a damage rating, assess the root pruning and scarring using the 0 to 3 NIS scale (Table 1 and Figure 2).
5. If regrowth is extensive, consider removing it to more accurately assess damage to original root system.
1. Generally under good growing conditions, an NIS rating of 1.0 is when considerable economic damage is likely to occur. Under adverse conditions, especially drought conditions, an NIS rating of 0.25 can be enough to cause economic damage.
2. It is important to consider the average NIS rating and consistency when evaluating options for managing CRW.
3. Consistency of protection refers to the percentage of NIS ratings that are less than 0.25, the economic threshold under droughty conditions.