Rootworm Egg Hatch has Begun in Southeast Nebraska - UNL CropWatch, May 16, 2012

Rootworm Egg Hatch has Begun in Southeast Nebraska - UNL CropWatch, May 16, 2012

 Photo of corn rootworm larvae

(Left) On a recent Market Journal segment UNL Extension Entomologist Bob Wright talked with host Jeff Wilkerson about the need to move up the scouting timetable for insects, particularly rootworms and stalk borers in the early season. (Above) Figure 1. Rootworm larvae (Photo by UNL Department of Entomology)


May 16, 2012

Update

On May 21 a CropWatch reader from Hiawatha, Kan., reported finding adult western corn rootworm beetles in a corn field 9 miles southeast of Rulo, Neb. The beetles were causing leaf damage in V4 corn. This was the earliest date in 20 years that the seed company representative had found rootworm damage. He also found cloverworm in V2 soybeans.

 


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Lance Meinke, UNL entomology professor, found 1st instar western corn rootworm larvae in corn roots dug on May 7 at the ARDC near Mead in Saunders County. Rootworm larvae were also detected on May 10 at UNL’s South Central Ag Lab near Clay Center, the first day they were sampled. This is the earliest recorded initial egg hatch at the ARDC since recording began in 1985. Based on Meinke’s records since 1985, the previous earliest initial hatch was May 22 and the latest was June 16.

Entomologists at Purdue University reported similar findings for this year, with rootworm egg hatch beginning May 4-6 in Indiana. Egg hatch may begin somewhat later in northeast Nebraska and in western Nebraska.

Egg hatch can occur over an extended period (five to six weeks) and some continuous corn could be attacked by larvae at very early growth stages in 2012. Weather patterns in May will determine how spread out the hatch period will be and how early adults will emerge. We could see some adults by mid-June this year. If corn has not silked when adults begin emerging, adults will feed on corn leaves, scraping off the surface green tissue, producing window pane-type injury. See Table 1 for temperature requirements for rootworm growth and development.

Southern corn rootworms, which overwinter as adults and annually migrate into Nebraska, were observed fairly early this year and gravid females were  collected in April at the ARDC. These females may lay eggs in the soil before corn emerges around winter annual weeds. In previous years hey have been found damaging first year corn in southern Nebraska, so it is possible that southern corn rootworm larvae are also present in some areas (could be in first-year or continuous corn). Southern corn rootworm larvae are very difficult to distinguish from western and northern corn rootworm larvae when small, but produce similar damage to corn roots when present.

Robert Wright
Extension Entomologist, Lincoln
Lance Meinke
UNL Professor of Entomology

Table 1. Duration of immature stages of western corn rootworm at constant temperatures. (Source: Jackson and Elliot. 1988. Environmental Entomology 17: 166-171.)
Stage Days to complete stage (male/female) at different constant temperatures (°F) Degree days to complete stage (48.2°F base)
  64.4 69.8 75.2 Males Females
1st instar larva  8.1/8.6  5.6/6.2  4.8/5.3  70.4  77.7
2nd instar larva  6.8/7.1  4.9/5.4  4.3/4.9  61.7  70.6
3rd instar larva  15.0/15.5  11.2/11.9  9.4/10.4  140.5  149.2
Pupa  13.5/13.8  10.1/10.1  7.8/8.4  122.2  125.1
Egg hatch to adult emergence  43.4/45.0  31.8/33.6  26.3/28.9  394.8  422.6