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Looking Ahead
Volume 52 Number 15 Date 07/20/2007 ALERT Soybean aphid - Extremely high, potentially damaging populations of this pest have developed in the west central region. Surveys in La Crosse and Vernon counties found densities well above the economic threshold in 86% of the soybean fields sampled in the last week. Averages ranged from 251 to 1,160 aphids per plant with 100% of the plants infested. In the most heavily infested fields, soybean stems, petioles and leaves were saturated with up to 2,000 aphids per plant, and the lower leaves had become sticky with honeydew. In other areas of the state this insect is being found at moderate or low levels below 250 per plant, with a few exceptions. Soybean fields must be evaluated without delay if soybean aphid populations are to be optimally controlled. Foliar sprays are most effective in reducing aphid numbers and minimizing resurgences when applied during the R2 (full bloom) to R4 (full pod) stages of soybean growth. Spraying at or beyond R6 (full seed) has not been shown to prevent yield loss. To assess soybean aphid densities, examine 20 to 30 plants over 80% of the field. Closely inspect all parts of the plants for aphids, particularly the stem, newest trifoliates, and undersides of the lower leaves. Treatment is warranted when the economic threshold of 250 aphids per plant on 80% of the plants is exceeded. LOOKING AHEAD True armyworm - The larval offspring of moths monitored in black light traps during late June should soon be detectable in the margins of infested fields. Locally heavy populations of second generation larvae could develop in alfalfa, small grains, corn and peas before the month's end. Dry weather generally suppresses armyworm outbreaks, but under such conditions armyworm problems may be aggravated as larvae move to succulent corn plants when grasses dry up. The last time widespread armyworm problems occurred in Wisconsin, during the 2005 growing season, the earliest signs of an impending outbreak were noted around the third and fourth weeks of July. Corn rootworm - Peak adult emergence remains about two weeks away. Growers of multi-year corn should begin assessing beetle populations in early August and continue a scouting regimen at 7-10 day intervals through mid-September. Corn rootworm beetle sampling serves two purposes: to determine if beetle populations are high enough to interfere with pollination and to determine if a soil insecticide, commercial seed treatment, or corn rootworm transgenic corn hybrid should be used the following planting year. Western bean cutworm - The emergence of western bean cutworm moths continued to escalate this week at Wisconsin trap locations. The highest pheromone trap count of 122 moths was registered at Princeton in Green Lake County, while the highest black light catch recorded was 58 moths at Mazomanie in western Dane County. According to the degree day model for western bean cutworm, 25% emergence is expected around 1,319 GDD, 50% emergence is expected around 1,422 GDD, and 75% emergence is expected around 1,536 GDD (base 50F). Japanese beetle - DATCP surveys and reports from cooperators, consultants and residents indicate Japanese beetles are extremely numerous this season in soybeans, orchards, nurseries and backyards. Intensive trapping at 10 sites in Brown County resulted in the capture of 20 to 12,000 specimens per site (information from Vijai Pandian, Brown County UWEX). Of particular concern to Wisconsin apple growers is the attraction of Japanese beetles to young plantings of honeycrisp apples. In orchards where populations are high and feeding is noted on susceptible varieties or on immature trees, control measures may be necessary. Survey specialists observed moderate to high levels of defoliation (11-20%) attributed to this insect in Dodge County, and low levels of defoliation (<10%) in soybean fields in most southern and west central counties. Soybean growers are advised to be alert to lacy or skeletonized leaves in soybean fields; both are indicators of Japanese beetle activity. No more than 20% defoliation should be tolerated in soybeans between bloom and pod fill. Bean leaf beetle - Surveys in Rock, Green, Kenosha, Racine and Walworth counties revealed high numbers of bean leaf beetle adults on soybean foliage. Defoliation was estimated to be 5-10% on about 65% of the plants in some fields. Pod feeding and clipping by this generation of beetles is a distinct possibility in moisture stressed fields. Grasshoppers - Traditionally more of a problem during August than earlier in the season, various species of grasshopper are appearing in alfalfa fields, soybeans, and along roadsides in the central districts, particularly in the Central Sands. The populations of adults and nymphs present in many fields are not alarming, but are of concern since dry conditions are affecting growth of soybeans and third crop hay in these areas. Grasshoppers are a definite threat to dry forage crops and treatments can be applied to control potato leafhoppers simultaneously. Some fields should be cut immediately and treatments applied to regrowth. In soybeans, treatment should be considered if the economic threshold of 20% defoliation is exceeded. -- Krista Hamilton 
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