Corn Borers, Rootworms, Other Pests: 2008 Season Summed Up, 2009 Eyed

By Jane Fyksen Crops Editor
Thursday, November 27, 2008 8:37 PM CST

The fall corn borer population in Wisconsin is the lowest in 10 years n and one of the lowest in the history of the state surveys. Surveys from 1942 to 2008 show this pest has, for the most part, become increasingly scarce since 1997, with a few exceptions, says Krista Hamilton, entomologist with the Wisconsin Department of Agriculture, Trade and Consumer Protection.

Hamilton also reports on corn rootworms, cutworms and the western bean cutworm.

“Flights of spring moths developing from overwintered larvae in May and June were distinctly lower than in the previous year, as egg and larval survival was apparently influenced by wet conditions in the southern half of the state. The magnitude of the second flight of moths was nearly four times lower than in 2007,” she says of European corn borer in the state this past season.

Results of the fall survey of second generation larvae showed a sizeable decrease in population densities from 2007 to 2008. Populations were well below normal, averaging 0.09 borer per plant (i.e. 9 borers per 100 plants). Hamilton says that’s the lowest average since 1998. The state average is about one-third that of last season and the 10-year average of 0.31 per plant, and far below the 50-year average of 0.46 per plant. Counts in individual districts were also extremely low. The most drastic reductions were noted in the west central, central and south central districts.

Injury to corn from larval feeding wasn’t found in 64 percent of the grain corn fields examined, most of which were presumed to be Bt hybrids. Roughly 2.5 percent of the fields had populations exceeding 0.50 borer per plant and only 1 percent had populations above the economic threshold of 1.0 borer per plant.

“The fall abundance survey in 230 fields registered the third lowest population since 1942,” she reports.

As for corn rootworm, DATCP field data from the annual survey of adult corn rootworms in August reveals an increase from 2007 populations in six of the nine agricultural reporting districts, including the southwest, southeast, west central, northwest, north central and northeast. Rootworms decreased in the remaining districts. The state average was one beetle per plant, the same as in 2007.

Average populations by district were as follows: Southwest 1.1; south central 1.5; southeast 1.6; east central 1.0; central 0.5; west central 0.7; northwest 0.5; north central 0.9; northeast 0.6.

Hamilton says the western specie constituted 52 percent of the state average population, while the northern species made up about 48 percent.

Research entomologists consider an average of 0.75 beetle per plant to indicate an elevated risk for root injury in continuous corn the following year if some form of control is not used; 38 percent of 229 fields in the major corn growing counties had such a count or higher.

“The obvious conclusion from these results is that there is a high potential for rootworm damage to continuous corn next season,” Hamilton reports.

As for cutworms, cool temperatures and spring flooding resulted in abundant growth of grass in many fields - conditions highly conducive for egg deposition and early larval development, according to this entomologist. The first migrant moths dispersing from the south central U.S. appeared in low numbers in Grant and Rock counties April 7, and intense captures of 8 to 9 male moths in two nights were registered from April 18 to 24. The primary cutting interval was projected to begin during the week of May 21 to 28 in the southern counties and continue into June in the central and northern areas. Problems due to this pest were not as prevalent or severe as expected, says Hamilton.

The first western bean cutworm moths were captured in a pheromone trap July 2 in Fond du Lac County, with the peak of the moth flight occurring from July 25 to Aug. 6, Hamilton details. Egg masses were noted near Westfield in Marquette County on July 29. Severe larval injury to corn from this flight was reported or observed in Adams, Columbia, Door, Green Lake, Juneau, Lafayette, Marquette and Sauk counties, where exceptional fields had 50 to 72 percent of the ears infested in late August. Moth activity declined to low levels by Aug. 21. A cumulative high count of 327 moths for the July-August monitoring period was registered near Princeton in Green Lake County.

She says the 112 Wisconsin pheromone traps captured a total of 2,433 moths in 2008, a minor increase from the 2,178 moths captured in 110 traps in 2007. “Although the annual flight was comparable to last year, late season larval infestations were more prevalent and much heavier,” Hamilton concludes.

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