Field Notes 07-11-24

A rainy first half of the week over much of Wisconsin gave way to drier weather and developing heat by the end of the reporting period. Daytime high temperatures July 5-11 were near normal and ranged from the 70s to upper 80s statewide, while overnight lows were in the 50s and 60s. Historic rainfall (2.5-5.7 inches) on July 5 soaked parts of central and eastern Wisconsin, where downpours led to the breaching of a dam along the Little Wolf River near Manawa in Waupaca County. The National Weather Service estimates about 5.7 inches of rain fell in the city, following about 6.0 inches of rain in the preceding two weeks.

Meanwhile, the second cutting of alfalfa progressed at a slower-than-average pace in between rain showers, with growers reporting 68% of the crop in good to excellent condition. Ratings for both corn and soybeans were lower at 62% and 60% in the good to excellent categories, respectively, a 1% and 3% increase from the previous week. In addition to the surplus wetness and reports of destructive flooding early in the week, pressure from mid-season insect pests such as apple maggot flies, Japanese beetles, and potato leafhoppers continued to build in field crops and orchards.

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^{Western bean cutworm larva | Jocelyn Smith Univ of Guelph}

Emergence of western bean cutworm moths accelerated this week. DATCP’s network of pheromone traps captured a total of 485 moths for the period ending July 11, a sharp increase from 128 moths collected the previous week. The current state cumulative count is 724 moths in 67 traps.

Based on the UNL-UMN degree-day model for this pest, 25% emergence of this season’s moth population has occurred across much of the southern half of Wisconsin and 50% emergence is expected over the next two weeks. Egg laying on corn will increase as the moth flight peaks and scouting late-whorl and pre-tassel corn to estimate egg density is strongly recommended. In fields where egg masses and small larvae are found on 5% or more of the corn plants, a treatment applied at 90-95% tassel emergence will optimize caterpillar exposure to the insecticide. The eggs are laid on the upper surface of the top 3-4 leaves, often on the flag leaf, and the larvae can be found in developing tassels. Routine scouting for egg masses and small larvae is advised through the end of the month.

Japanese beetles have also become common at low levels in corn, soybeans, and fruit crops, and perimeter damage can be expected during the second half of July. For corn, the primary concern is to protect the silks from clipping since heavy beetle feeding on corn silks can impair pollination. Treatment may be justified for fields with three or more beetles per ear and silks that have been clipped to ½ inch when pollination is occurring (less than 50% complete).

Japanese beetles aggregate on plants in the edge rows, emphasizing the importance of obtaining a representative sample from several areas throughout the field before making control decisions. Border row spot treatments are sufficient if the beetles and damage are confined to the field edges.

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^{Corn earworm moth | Jim Moore MBP}

Early corn earworm migrants are arriving in Wisconsin, according to reports from DATCP’s corn earworm monitoring network. The pheromone traps located near Burlington, Fitchburg, Mayville, and Ripon registered counts of 19-69 moths this week. The total catch is 148 moths in nine traps as of July 11.

For fresh market sweet corn, the standard treatment threshold is 5-10 adult earworm moths per night for three consecutive nights (when green silks are present). The treatment threshold for tomatoes is seven moths per pheromone trap per week.

Moth counts for the state’s 15 pheromone trap monitoring sites will be published each Thursday through mid-September on the DATCP Corn Earworm page.

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^{Apple maggot fly | Thaddeus McCamant UMN}

Captures of apple maggot flies on orchard traps increased this week, with flies appearing on traps at eight of 26 cooperating locations in Brown, Columbia, Fond du Lac, Iowa, Kenosha, Oneida, Racine, Richland, and Walworth counties. The highest count for the week was a total of nine flies on an unbaited red sphere trap. Maintaining sticky traps will be important as emergence continues and egg laying on apples intensifies throughout the month.

The current recommendation for apple maggot monitoring is to place sticky red sphere traps in trees along the orchard perimeter (especially next to wooded edges) since the source of apple maggot flies is often nearby wild hosts. A perimeter trapping density of 6-12 unbaited red spheres per acre is suggested. The density can be reduced to one trap every 20 trees along the perimeter when the traps are baited. Orchards with a history of apple maggot problems should also place a few traps in the orchard interior in blocks with the earliest maturing apple varieties. The threshold is an average of one fly per trap if the traps are unbaited. When red spheres are baited with an ammonium enhancer, the threshold increases to an average of five flies per trap.

In addition to apple maggot flies, Japanese beetles are also appearing in higher numbers in Wisconsin apple orchards and vineyards. Damage to fruits, ornamentals and field crops is likely during the second half of the month. Insecticide options for this insect are limited and most chemical treatments offer only short-term repellency when applied before large aggregations of beetles begin to appear. For large populations, spot treating the blocks or perimeter areas with the highest beetle pressure with a “knock-down” or contact insecticide may offer temporary control. Strategies for reducing Japanese beetle damage without use of insecticides are discussed here: https://fruit.wisc.edu/2022/07/28/japanese-beetle-management-without-insecticides/.

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