No. 14 Article 2/June 29, 2007

Protecting Pollination: It's Time to Begin Scouting for Corn Rootworm and Japanese Beetle Adults in Corn Fields

Corn rootworm and Japanese beetle adults are now common in corn fields throughout Illinois. As we have noted in recent issues of the Bulletin, emergence of western corn rootworm adults was very early this season, so rootworm adults are positioned to be a threat in many fields. As tassels begin to emerge, fields should be monitored carefully for both corn rootworm and Japanese beetle adults for potential silk clipping. Densities of at least five corn rootworm adults per plant typically are required to affect pollination in commercial corn fields. Treatment decisions need to be considered when three or more Japanese beetle adults per ear are present and pollination is not complete.

Excessive silk clipping by western and northern corn rootworm adults.

Japanese beetles on corn silks.

Obviously, a combination of both of these insect pests will be found in most fields, so a dose of common sense needs to be applied to the interpretation of the economic thresholds. Of most concern is the intensity of feeding to silks and the level of moisture stress in a field. Pay close attention to the amount of silk tissue protruding from the tips of ears. When ½ to 1 inch of fresh silk remains and soil moisture is abundant, successful pollination is likely occurring. Seed-production fields are likely to be at greater risk of economic losses caused by silk clipping. Many insecticides are labeled as rescue treatments for both corn rootworm and Japanese beetle adults to prevent excessive silk clipping. Please consult chapter 1 of the 2007 Illinois Agricultural Pest Management Handbook for a list of labeled products and rates.

There is considerable interest this season in tank-mixing fungicides and insecticides to prevent silk clipping and foliar disease. Before making a treatment decision, have you asked yourself any of these questions?

While these questions may seem academic, I raise them because I am concerned that many of the fundamental principles of IPM are increasingly being forgotten or ignored. A review of the pest management literature teaches us that there are often unpleasant consequences when pesticides are not used within an appropriate IPM framework.--Mike Gray

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