Pest Wrap-Up For Major Crops Revealing

By Jane Fyksen, Crops Editor
Wednesday, November 7, 2007 4:01 PM CST

The 2007 cropping season will soon draw to a close. The Wisconsin Department of Agriculture, Trade and Consumer Protection pest-watchers last week issued their final season summary of the insects and weeds that plagued producers this year, and made some predictions about next year’s potential problems.

The ag department’s fall European corn borer (ECB) survey of 230 fields around the state registered a minor increase in population densities compared to the fall of 2006. Corn borers entering the winter of 2007-08 have increased to 0.31 per plant, compared to 0.29 at the start of last winter. The state average, however, is equivalent to the 10-year average of 0.31 per plant and below the 50-year average of 0.48 per plant, reports entomologist Krista Hamilton.

She notes increases in the southwest crop-reporting district (0.20 to 0.28 per plant), as well as west central (0.42 to 0.52 per plant), east central (0.11 to 0.21 per plant), and north central (0.16 to 0.35 per plant). Decreases were documented in the remaining five crop-reporting districts in the state.

“Approximately 53 percent of the fields of grain corn had no detectable larval population; some of these unquestionably were Bt hybrids,” says Hamilton. “Roughly 23 percent of the fields had populations exceeding 0.50 borer per plant and 8 percent had populations above the economic threshold of one borer per plant.” Three counties had average densities greater than one borer per plant - Eau Claire (1.7 per plant), Pierce (1.14), and Waupaca (1.9). The state mean percentage of corn plants infested with second generation larvae was 28 percent.

“Many instances of ear shanks being infested with corn borer were noted during the fall survey. This feeding behavior, which resulted in hollow, weakened shanks and eventual ear drop, may have been due to borers finding the shanks more desirable than the saturated, rot-infected stalks,” Hamilton points out, noting that stalk rot was widely prevalent this fall because of stress from drought early in summer followed by late season rains.

This fall’s survey suggests another light first flight of moths next spring. Planting Bt hybrids for ECB management in 2008 may be unwarranted in many areas, considering that borer pressure is expected to be very low.

Stalk rot prevalent

Examination of many broken and downed corn stalks showed this condition usually wasn’t due to corn borer tunneling. In most instances stalk rot is to blame; it was present at high levels in many cornfields throughout the state. An unofficial fall survey of damaged stalks in 136 grain corn fields in October found that 10 percent of the fields were completely infected with stalk rot; 36 percent had more than half of the plants infected with stalk rot; and 20 percent had a quarter of the plants infected with stalk rot.

Fewer rootworms

The annual corn rootworm beetle survey reveals a state average population of one beetle per plant, which is less than the 1.4 per plant in 2006 and 1.6 in 2005. Averages by agricultural reporting district are: northwest 0.4 per plant; north central 0.7; northeast 0.5; west central 0.4; central 0.8; east central 1.4; southwest 0.4; south central 2.2; southeast 1.0. “The western species was dominant on a statewide basis, while populations of the northern species were higher in the cooler and more northern counties, including Barron, Chippewa, Door, Dunn, Clark, Green Lake, Juneau, Lincoln, Marathon, Marinette, Oconto, Pepin, Polk, Portage, Rusk, Shawano, Taylor, Vernon, Waupaca, Winnebago, and Wood,” Hamilton lists.

About 39 percent of the 222 cornfields surveyed have economic populations of 0.75 or more beetles per plant. The largest increase from 1.7 to 2.2 beetles per plant is in the south central district, while the largest decreases from 2006 to 2007 are in the southwest (2.2 to 0.4 per plant), northeast (1.8 to 0) and east central (2.2 to 1.4). An average of 0.75 or more beetles per plant indicates the potential for feeding injury by corn rootworm larvae in multi-year corn, warns Hamilton.

“The use of transgenic Bt corn rootworm hybrids was also measured this season. The percentage of survey sites that were Bt corn rootworm fields nearly doubled from 14 percent in 2006 to 27 percent in 2007,” she reports, noting that for the second year, Monsanto’s YieldGard was the more prevalent of the two technologies. A total of 23 percent of the fields tested positive for the YieldGard Bt-Cry3Bb1 protein, while just 4 percent tested positive for the Herculex Bt-Cry34/35Ab1 protein. More Bt-rootworm corn was planted in the southwest and south central districts in 2006 and 2007 relative to the other districts.

A network of 103 pheromone traps in 27 counties provided data on the distribution, emergence, peak flight, and abundance of the western bean cutworm in 2007 corn. Moths first emerged June 20 near Arcadia in Trempealeau County. Captures peaked between July 12 and Aug. 7. Black light trap captures escalated around the same time and registered a similar flight period. The highest single nightly capture of 83 moths on July 13 was at Princeton in Green Lake County. This location also registered the highest seasonal cumulative capture of 448 moths between July 12 and Aug. 9, with peak activity between July 12 and 21 when nightly captures averaged 29 moths. The second and third highest seasonal cumulative captures were 131 moths at Lancaster and 78 at Randolph. The 103 pheromone traps captured a total of 2,178 western bean cutworm moths during the 2007 season, and roughly 21 percent of these were reported from Princeton. A total of 17 traps, primarily those in the east central counties of Brown, Manitowoc and Sheboygan, captured no moths.

“The presence of this late-season corn pest in Wisconsin and the Midwest since the late 1990s is evidence of an eastward expansion in its geographic range, which was once limited to Colorado and Nebraska,” Hamilton explains. Consecutive years of trapping have not found populations of adults comparable to those registered in Iowa where cumulative captures have numbered as high as 2,069 moths per trap.

No heavy larval infestations were reported this season, and “western bean cutworm does not yet represent a significant threat to corn in Wisconsin,” says Hamilton.

As for variant western corn rootworm, the trapping network monitored 45 soybean fields this season and found no fields with populations above the economic threshold of five beetles per trap per day (BTD) for the four-week sampling period. Of those soybean fields in Columbia, Dane, Dodge, Green, Jefferson, Rock, Sauk, Sheboygan, Walworth, Washington and Waukesha counties, the highest averages of 3.05 and 2.17 BTD were found in Columbia and Rock counties, respectively.

“The trapping network results indicate that first-year corn planted after soybeans in the areas monitored is at a low risk for economic damage from larval rootworm feeding in 2008,” she reports, adding that eight soybean fields were also monitored in Buffalo, La Crosse, Monroe and Vernon counties using a modified survey protocol. All of the fields trapped well below the economic threshold at 0.01 to 0.70 BTD.

Turning attention to forage, Hamilton says alfalfa weevil larvae from overwintered eggs began appearing in sweep net collections by May 5 this season. Despite a late hatch, abnormally high temperatures during the first two weeks of May greatly accelerated larval development, and surveys found the first economic injury between May 10 and 16. Growers in the south and west were alerted to the heightened potential for alfalfa weevil damage on May 18; some applied chemical treatments, while others opted to adjust their harvest schedules. Damage in the east central counties peaked around May 22, at which time alfalfa was either cut or being cut. Chemical controls were initiated by May 10 and continued through the end of the month. At the start of June, any unprotected hay that had not been harvested in the southern three tiers of counties incurred heavy feeding injury.

Pupation began in the southern districts around June 8. Larval numbers decreased to less than 0.6 per sweep in all areas by June 22, and were very low during the balance of the summer.

The first wind-blown potato leafhopper migrants were detected May 9, but counts didn’t escalate to any appreciable degree until June 1. Populations were sufficient to justify treatment by mid-June, with 2.0 adults per sweep common in the southern and central districts. Numbers escalated throughout June to reach the highest levels of the season by July 13, when averages climbed to 4.3 leafhoppers per sweep in the central and southwest counties, 2.2 per sweep in the east central counties, and 16.8 per sweep in the west central counties.

“In the northwest, a crop consultant recommended treatment on nearly every acre of alfalfa, both established stands and new seedings. Heavy populations in apples, beans, and potatoes at this time coincided with a large dispersion from cut alfalfa,” says Hamilton. “During August when counts are perennially the highest, leafhoppers ranged from 1.0 to 12.1 per sweep in untreated fields to about 0.8 per sweep in those fields with spray programs.” Populations began to decrease over the state by mid-month due to heavy rainfall although residual populations remained into September, particularly in the dry northeast and north central counties. Potato leafhoppers were a persistent problem in 2007, she stresses.

More soybean aphids

The department looks for soybean aphids during the R2 to R4 stages to detect peak seasonal densities and assess fields while treatment may still be beneficial. Examination of 227 soybean fields between July 12 and 31 found non-economic aphid populations at 82 percent of the sites. Treatable or economic populations were detected at 18 percent of the sites, principally in the west central district and portions of the southwest, south central and central districts. Individual fields with high populations were found in Columbia, Crawford, Richland, Vernon, La Crosse, Marquette, Monroe, Juneau, Jackson, Buffalo, Trempealeau, Walworth, and Wood counties. Average soybean aphid densities in these areas ranged from 253 to 1,071 per plant. Moderate populations were detected in the central and north central districts, and low populations were found over much of the southeast, east central, northwest and northeast districts.

The 2007 statewide average number of soybean aphids per plant was 164. This compares to 69 in 2006, 108 in 2005, 11 aphids per plant in 2004 and 618 aphids per plant in 2003. The highest average number of aphids per plant recorded was 3,250 in a Columbia County field.

SCN in 44 counties now

Established populations of the Soybean Cyst Nematode (SCN) have been confirmed in 44 Wisconsin counties. Twenty-eight soil samples were collected at random this year from soybean fields in counties not known to be infested with SCN. Two samples from Fond du Lac County were positive for SCN in 2007, marking the first detection of SCN on soybean in that county. Soybean cyst nematode is the most serious economic pest of soybeans in the U.S.

Soybean virus symptoms were detected in 14 of 227 soybean fields. Results were: bean pod mottle virus (one field positive in Grant County); soybean dwarf virus (seven fields positive in Vernon, La Crosse, Walworth, Grant and Crawford counties; potyvirus group (one field positive in Dane County); cucumber mosaic virus (all negative); alfalfa mosaic virus (five fields positive in Crawford, Marathon, Monroe, Racine, and Sauk counties).

The fifth annual spring survey found the highest number of overwintered bean leaf beetles since surveys began in 2003. The department sampled 183 first-crop alfalfa fields in 44 counties between May 9 and June 13, and collected 509 beetles from 86 of the sites. Counts ranged from 0 to 26 beetles per site, with the greatest numbers swept from fields in Lafayette, Rock and Walworth counties.

“Although fewer beetles were found per site as the survey progressed northward, this insect appeared to have wintered successfully across much of central Wisconsin. Overwintered adults were detected in Buffalo, Jackson, La Crosse, Manitowoc, Outagamie, and Trempealeau counties for the first time in the history of the survey,” notes Hamilton. “Ordinarily very few bean leaf beetles survive the winter months north of the southern three or four tiers of counties.”

Subsequently, beetles were tested for Bean Pod Mottle Virus (BPMV). Overwintered beetles from 11 alfalfa fields in Iowa, Lafayette, Racine, Rock, and Walworth counties carried BPMV. This is the most sites with bean leaf beetles carrying BPMV documented since 2003. Despite the higher number of surviving beetles, no increase in the incidence or severity of BPMV was noted this season. A follow-up soybean virus survey in August found BPMV in a single Grant County field (of 220 fields tested).

Clarissa Hammond with the department reports that seed production field inspections this year totaled 750 acres, including roughly 588 acres of corn (74 fields), 157 acres of soybeans (10 fields), and 5 acres of cucumber seed production (7 fields). For the third year, all inspected cornfields were sampled for Pantoea stewartii, the causal agent of Stewarts wilt. It was detected in one of 74 fields.

Soybean fields were sampled for Anthracnose, bacterial pustule/blight, bacterial tan spot, bean pod mottle virus, brown stem rot, Cercospora blight and leaf spot, frogeye leaf spot, sudden death syndrome, pod and stem blight, southern bean mosaic virus, soybean cyst nematode, tobacco ringspot virus, tomato ringspot virus, and white mold. Four soybean fields in Rock County tested positive for SCN, and one field had trace levels of frogeye leaf spot. Inspection results were negative for all other diseases.

Potato pests targeted

Since 1953, when potato rot nematode was first detected in Wisconsin, approximately 3,014 acres (108 fields) have been quarantined to prevent its spread to non-infested areas. To date, 992 acres (30 fields) still remain under state quarantine. In 2007, seven fields were inspected and potato rot nematode was detected in two of seven fields in Langlade County. These two fields, totaling 190 acres, were placed under quarantine. Two additional fields were released from quarantine. One was released for certified seed potato production after fumigation and two successive potato crops showed no evidence of potato rot nematode. The other was released for table stock potatoes, says Sara Ott with the Pest Bulletin team.

A national survey for pale potato cyst nematode and golden nematode was started in 2006 by the USDA Animal and Plant Health Inspection Service (APHIS) in response to finds of pale potato cyst nematode in Idaho and golden nematode in Quebec, Canada, earlier that year. The objectives were to detect possible infestations by PCN in potato fields and certify that U.S. potatoes were grown in areas free of the cyst-forming nematodes.

As of Oct. 24, the department has collected and processed a total of 1,350 soil samples from Wisconsin for PCN (over four tons of piler dirt and soil), representing 75 percent of the goal of 1,800 samples by April 2008. Wisconsin has conducted surveys for golden nematode periodically in the past. Neither the pale potato cyst nematode nor golden nematode has been found in the potato growing areas of the state. The continued export of potatoes relies on the certification of potato fields and tubers as being free from these regulated pests.

Fall weed survey

Estimates of five major weed species were made during fall surveys in corn - foxtail, giant ragweed, velvetleaf, wild proso millet, and woolly cupgrass.

Of the 229 cornfields surveyed, 61 percent contained foxtail, 15 percent contained giant ragweed, 14 percent had velvetleaf, 5 percent contained woolly cupgrass, and 4 percent contained wild proso millet. Foxtail (giant, green and yellow) was the most prevalent of the weeds assessed. Giant ragweed occurred most often in the central and south central districts; velvetleaf was most common in the south central and southeast districts. Wild proso millet was noted in seven of the nine agricultural districts but occurred most often in the northeast, and woolly cupgrass was most prevalent in the southwest, particularly in Grant County. Woolly cupgrass was not found in any field in the southeast, east central, northeast or north central districts.

“Results of the survey suggest that some weed management programs were not entirely effective against foxtail this season, but overall weed control was good,” says Hammond.

Comments »

Comment on this story

Comments will be approved within 48 hours

Name:

Email: (optional)

Comments:

Image Verification: (Case sensitive)