Rootworm larvae have been hatching and seeking corn roots for several weeks.

Immediately after hatching, the very small larvae crawl through the soil until finding corn roots. This is when the newly hatched larvae are most vulnerable to either desiccation or drowning if soils are extremely dry or wet. Once contact is made with the root, the larvae immediately begin feeding and enter the root. As they increase in size, so does their damage. They will feed both inside and outside the roots, causing tunneling and root pruning.

Several have asked if the rootworm mortality would be high this year because of the dry soils. From initial evaluations of our rootworm plots and reports we have seen, it is obvious that rootworm larvae have been successfully finding roots. In addition, the damage may be greater than normal because any root regrowth will be inhibited by hot and dry soils. This forces the larvae to feed more extensively on the “juicy” nodal roots, which are critical to keep the plant upright during pollination and ear fill.

Rootworm development has been progressing rapidly with the warm soils, as beetles are beginning to emerge throughout the state. One lonely male beetle was found in Carroll County on June 18, about a week earlier than normal. Many pupae have been found during root digs.

However, this just represents the “tip of the iceberg” – because rootworm hatching and feeding extends over several weeks, there is still time to sample roots of plants in high-risk fields, especially where insecticide efficacy is in question.

To sample for rootworms, use a shovel and lift out the root mass and surrounding soil and place on a dark surface (black plastic garbage bags work well). Carefully break up the clods and sort through the soil. Look for 1/4 to 1/2 inch long, slender, creamy-white larvae with brownish-black heads and “tails.” Once the soil has been separated from the root mass, inspect it for root scarring and pruning.

You may find the larvae under the leaf collars that are close to nodal roots; tear these leaves away to check. You may also observe the rootworms sticking out of roots. Repeat this process with several plants representing different areas of a field. An average of two or more larvae per plant represents a rootworm population that signals the need for a rescue cultivation application.

Insecticides applied after planting should be directed toward the base of plants. It is also important to throw soil up around plants to incorporate the insecticide and promote the establishment of brace roots. A good brace root system will help prevent plant lodging and reduce losses due to rootworm feeding.

If a no-till field has an economic population of larvae, placing the insecticide on top of the ground will normally not be effective. The only exceptions might be if the soil insecticide is watered in through irrigation or rainfall (ideally a half inch or more). Two liquid soil insecticides, Furadan 4F and Lorsban 4E, are labeled for post-directed applications. If one decides to mix the insecticide with a liquid nitrogen source for a sidedress application, compatibility checks should be made. Broadcasting the insecticide will greatly diminish rootworm efficacy.

SOURCE: Purdue Pest & Crop Newsletter.

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