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13;10;One sign that corn rootworm (CRW) eggs might be hatching is the presence of lightning bugs or fireflies. However, the presence of these insects lighting up the night sky should not be accepted as fact that CRW eggs are hatching, but the implied relationship has been established over time.1,2
The accumulation of growing degree days (GDDs) is a more accurate determination of CRW egg hatch. Research has shown that about 50 percent of CRW eggs may hatch from 684 to 767 accumulated GDDs (base 52 °F).2 Local GDD information can generally be accessed from university Extension weather specialists and/or websites.
13;10;9;9;9;9;9;Each larvae (Figure 1) of CRW passes through three developmental stages or instars before pupating to become an adult CRW beetle. Each instar period lasts from seven to ten days. The larvae are slender, cream-colored and have brown heads and a dark plate on the top side of the tail. Mature third instar larvae are about 1/2 inch long. Newly hatched or first instar CRW larvae are very small (less than 1/8 inch long) and may go unnoticed. First identification may occur when they reach the second instar. 13;10;13;10;9;9;9;9;9;
After the third instar, the pupa forms. The pupa is white, somewhat translucent, and is dormant or non-feeding. Rescue insecticidal treatments applied during the pupa stage will be of no value for control.13;10;9;9;9;9;9;
13;10;9;9;9;9;9;After CRW larvae have hatched and are actively feeding on corn, walk through a field in a “W�? formation and dig two root balls at the end of each W leg for a total of 10 corn root balls. Carry the roots out of the field to a convenient location and place them into a bucket of water for washing. If CRW larvae are present they should float to the surface. Early instars are small and can be easily mistaken for debris.13;10;13;10;9;9;9;9;9;
There is no established economic threshold for CRW larvae; however, agronomists have advised applying rescue treatments if there are 2 or 3 CRW larvae per plant.313;10;13;10;9;9;9;9;9;
CRW protection methods should be utilized in corn fields at-planting when CRW pressure is expected to be high. Potential high-pressure fields include: 1) continuous corn operations, 2) fields subject to northern corn rootworm (NCRW) extended diapause populations, and 3) fields subject to western corn rootworm (WCRW) variant populations.
At planting control measures include rotating to non-host crops, use of a dual mode of action (MOA) bacillus thuringiensis (B.t.) protected corn product, or use of a soil-applied insecticide (SAI) with a conventional corn product or in combination with a single MOA B.t. protected corn product. Depending on planting date, the efficacy of a SAI may not be sufficient to protect corn roots when CRW larvae hatch. Conventional corn products without CRW protection are the most vulnerable to CRW larval damage. If root digs indicate that CRW larvae populations are potentially damaging, farmers with the ability to irrigate or cultivate may apply a potentially effective rescue SAI treatment.13;10;13;10;9;9;9;9;9;
13;10;9;9;9;9;9;CRW is best controlled with preventative measures but rescue treatments do exist. Insecticides can be applied through an irrigation pivot (preferred) or by making lay-by cultivator applications (least effective). Regardless of application method, ample water by irrigation or rainfall is required to move the applied insecticide into the root zone. Lay-by cultivator-applied insecticide granules or spray should be applied to both sides of the corn row ahead of the cultivator equipment to allow for 2 to 3 inches of soil to cover the insecticide. If insecticides are applied through sprinkler irrigation equipment (chemigation), apply enough water to wet the root zone to the depth CRW control is needed. If soils are wet, allow enough soil drying to occur such that an application using a minimum amount of water will not produce surface runoff. Labeled rescue insecticides for CRW control include those outlined below and on the next page and possibly others. ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS.13;10;13;10;9;9;9;9;9;
Cobalt® Insecticide or Cobalt® Advanced Insecticide (Restricted Use Pesticides)
13;10;9;9;9;9;9;Counter® 15G or Counter® 20G (Restricted Use Pesticides)
Force® 3G Insecticide (Restricted Use Pesticide)
13;10;9;9;9;9;9;Lorsban® 4E, Govern® 4E, Hatchet®, Warhawk®, Yuma® 4E (Restricted Use Pesticides)
13;10;9;9;9;9;9;Thimet® 20-G (Restricted Use Pesticide)
13;10;9;9;9;9;9;Fields that require a rescue treatment are probably candidates for adult management as well. Be sure to continue scouting and managing adults as the season progresses to help ensure lower CRW pressures not only for this season, but the next growing season as well.13;10;13;10;9;9;9;9;9;