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Infestations by crop pests taking up and down turns

Aug. 8, 2013 | 0 comments

Soybean aphids, grasshoppers, and corn earworms jumped into the picture for the Wisconsin Department of Agriculture, Trade, and Consumer Protection's last weekly Wisconsin Pest Bulletin (WPB) for July.

Soybean aphid populations topping an average of 250 per plant were found in one field each in Waupaca and Waushara counties on July 22.

The Extension Service also had its soybean trial plot east of Seymour in northern Outagamie County treated with an insecticide because of aphid populations exceeding 250 per plant.

That density of the insect is considered to be the threshold for economic loss (decreased yield) for the soybean crop, the WPB explained.

The weekly survey found aphid populations running at averages of 1-172 per plant in the other 86 fields surveyed during the reporting week.

The WPB noted, however, that the presence of many winged adults suggested the possible dispersal of populations to less infested soybean fields.

It pointed out that aphid numbers building rapidly during the daytime temperatures of 70-80 degrees, which prevailed for most of this week.

Grasshoppers were mentioned for the first time in the weekly reports for 2013. They were found in what the WPB called "moderate to high" numbers in some sandy soil fields in the Central Sands counties.

In a few fields, 9-10 grasshoppers were caught per net sweep, prompting the WPB to advise alfalfa growers to monitor the edges of fields for possible movement of the pest from nearby grassy areas.

Moths and Beetles

Pheromone traps caught between 20 and 47 corn earworm moths at monitoring sites near Chippewa Falls, Sun Prairie, Wausau, and Watertown during the reporting week.

For sweet corn, the WPB indicated the catch of 5-10 moths per trap on three consecutive nights as a basis for considering an insecticide treatment of a nearby crop while the tolerance with tomatoes is up to seven moths per week.

Japanese beetles were found in 42 percent of the soybean fields surveyed during the third week of July.

The WPB stated that 20 percent or higher leaf defoliation at the early reproduction growth stage of soybeans warrants an insecticide treatment. It noted that defoliation rates seen in the survey were running at 5-10 percent.

Green cloverworm larvae were being in low numbers in soybean fields all the way from Kenosha County to Chippewa County, the WPB reported. It suggests monitoring for the pest through the month of August.

Pea aphid numbers were climbing in alfalfa fields, as shown by catches of 11 to 22 per net sweep in a few fields in Juneau, Monroe, and Waushara counties.

Potato leafhopper numbers were lower than what the WPB had expected following the period of hot and dry weather in mid-July.

Cutworm Count

Western bean cutworm (WBC) moth numbers continue to run much lower than a year ago.

Most of the moth catches this summer have been confined to a few counties in south central Wisconsin, topped by the 29 moths caught near Marcellon in Columbia during the reporting week ending on July 24.

The WPB was expecting the peak flight of WBC moths to occur this week. It pointed out that eggs have laid on both corn and soybeans by the moths which had arrived earlier.

Tolerance levels for the WBC at 90-95 percent of tassel emergence are infestations on up to five percent of field corn plants and four percent for sweet corn, the WPB indicated.

Late-pollinated corn is likely to be especially vulnerable to silk clipping and reduce kernel set because of a timing that is corresponding to the emergence of corn rootworm beetles during the first half of August, the WPB observed.

Infestations of five or more beetles per corn plant before pollination is complete should be countered with an insecticide treatment, the WPB recommended.

Very low numbers of second flight European corn borer moths were being caught in black light traps.

With much of the corn having biotech resistance to the corn borer, the WPB pointed out that pepper, potato, snap bean, and other vegetable plants are still vulnerable to the pest.

Vegetable Vagrants

Large imported cabbageworm larvae were found in Grant and La Crosse counties. The WPB indicated that the amount of effort to control the pest would depend on whether infested cabbage is headed for fresh market sale or to a processor.

Numerous reports of squash bug egg masses were also received by the WPB. It recommends a pyrethroid family treatment if there is more than one egg mass per vined plant.

The year's second bio-fix of the codling moth could be causing damage in apple orchards by early August, the WPB stated. It reminded orchard owners to rotate insecticides if they need to treat for the codling moth.

Iowa County was added to Wisconsin's list in 2013 for a catch of spotted wing drosophila flies.

Discoveries in three counties before the end of July should serve as early warning to growers of berries yet to be harvested this summer to set traps to identify the presence of this insect and apply insecticide if flies are detected, the WPB advised.

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