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Corn
Volume 60 Number 20 Date 12/03/2015 CORN ROOTWORM - Beetle counts increased from 2014 across the eastern half of the state and decreased in western Wisconsin in 2015, a striking reversal from last year's survey trend. The August beetle survey found substantial population increases from 0.3 to 0.8 beetles per plant in the south-central and east-central crop districts and low to moderate increases in the southeast, central, north-central and northeast areas. District averages in the west-central and northwest were low at less than 0.3 beetles per plant, while counts in the southwest decreased but remained above the 0.75 beetle-per-plant threshold considered to indicate root damage potential for next summer. The 2015 state average count of 0.6 beetles per plant compares to 0.4 per plant in 2014. Although the overall Wisconsin corn rootworm population increased in 2015, counts of the western corn rootworm beetle were down considerably. The survey found a total of 1,372 beetles on 2,290 plants, only 324 (24%) of which were the western species. The overwhelming majority (1,048 specimens or 76%) of beetles observed were the northern corn rootworm species. One individual was the southern corn rootworm. The significant use of pyramided Bt-rootworm hybrids or the combination of soil insecticides with Bt-traited seed are both possible contributing factors to the low western corn rootworm counts noted in August. Results of the survey suggest a greater threat of larval rootworm damage to non-Bt continuous corn in 2016, with the highest risk in the southwest, south-central and east-central districts where economic averages of 0.8 beetles per plant were recorded. EUROPEAN CORN BORER - Larval populations declined to just 0.02 borer per plant this fall, the lowest state average in the 74-year history of Wisconsin European corn borer surveys. Minor population reductions from 2014 were found in four of the state's nine agricultural districts, while negligible increases were noted in the southwest, south-central, central, east-central and northeast areas. Eighty-six percent of the fields examined (196 of 229) showed no evidence of corn borer infestation. Based on the fall survey results, it is apparent that that the extensive use of transgenic Bt corn continues to be a major suppression factor on the European corn borer population. WESTERN BEAN CUTWORM - On the basis of pheromone trap counts, the annual moth flight peaked one week later and was 24% larger than that of 2014. The 2015 cumulative capture of 644 moths in 96 traps (seven per trap) was an increase from the 521 moths in 108 traps collected last year (five per trap), but still extremely low in comparison to the survey record of 10,807 moths in 136 traps (79 per trap) set in 2010. Larval infestations resulting from the flight were light for the sixth consecutive year, and the western bean cutworm was not a major pest of concern for most Wisconsin corn producers this season. Trapping surveys from 2005-2015 show that moth counts have been decreasing since 2010. BLACK CUTWORM - Migrants began arriving in the state by April 1. The first significant flight was registered on April 20-21 and the primary corn cutting window opened in southern Wisconsin by May 27. Much of the state's corn acreage was at low risk of infestation this spring as a result of early planting and a comparatively small moth migration. The April-May black cutworm trapping survey yielded only 361 moths in 43 traps, a marked decline from 1,068 moths in 2014 and the lowest cumulative count since prior to 2010. Economic damage to emerging corn was not observed in June. CORN EARWORM - Below-average moth populations in mid-south and southern U.S. source regions in 2015 resulted in fewer corn earworm moths arriving in Wisconsin. A two-week migration event recorded from August 20-September 2 brought large numbers of moths (3,437 moths in 15 traps) into the state, but the flights were too late to produce widespread earworm infestations since most sweet corn was well past the silking stage. TRUE ARMYWORM - Substantial flights of moths occurred from May 7-21 and again from June 11-16, providing an early warning of potential armyworm problems. Larvae and leaf damage became apparent in corn by early June and minor infestations were observed throughout the month. By July 2, small armyworm larvae were common and a few cornfields were showing larval populations above the 25% treatment threshold (for armyworms ¾ inch or shorter). Control measures were applied on a limited basis in 2015. Reports from the east-central area indicated that some winter wheat fields there were also treated for armyworms. CORN WILT DISEASES - Corn leaf samples from seed corn production fields were tested at the Plant Industry Laboratory for the bacterial diseases Goss's wilt and Stewart's wilt. Goss's wilt was confirmed in 15 of 39 samples from Adams, Dane, Eau Claire and Rock counties, for a 38% positive rate. This represents a marked increase from 2014 when only 9% of samples tested positive. Results for Stewart's wilt were negative for the fifth consecutive year. NORTHERN CORN LEAF BLIGHT - Northern corn leaf blight was particularly widespread in 2015 and severe on susceptible hybrids. Development of this disease was favored by cool, wet spring weather and high levels of inoculum from the 2014 corn crop. SEED CORN VIRUSES - Lab testing of corn leaves from 39 seed corn fields inspected in 2015 found three Dane County fields to be positive for sugarcane mosaic virus (SCMV) and maize dwarf mosaic virus (MDMV), both considered to be viruses of export significance. All fields were apparently free from maize chlorotic mottle virus (MCMV) and High Plains virus (HPV). TAR SPOT - Reported for the first time in the U.S. in September (in Indiana and Illinois), this new fungal disease of corn was not observed in Wisconsin in 2015. -- Krista Hamilton, DATCP Entomologist 
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