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INSECT 



SURVEY 



BULLETIN 





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itate 



Local Groups 



FOR IMMEDIATE RELEASE 



College of Agriculture 



University of Illinois 



and Natural History Survey, Urbana, Illinois 



U. S. Department of Agriculture Cooperating 



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June 50, 1967 



INSECT SURVEY BULLETIN NO. 12 ' ; ' '%$? 



This series of weekly bulletins provides a general Iook ar^the insect situation 

 (fruit insects excepted), along with suggested, abbreviated control measures. 

 Each individual should check his own fields to determine local conditions. 



Corn Insects 



European corn borer moth flight and egg -laying are almost complete in the southern 

 two-thirds of Illinois; egg hatch is more than 90 percent complete in this area. 

 Egg-laying is declining in the northern sections (north of Highway 6) . In the west- 

 southwest and west-central sections, some fields had 30 to 60 percent of the plants 

 showing recent whorl feeding; a few advanced fields had 100 -percent whorl feeding. 

 Corn heights in these fields ranged up to 78 inches, with tassel ratios of 22 to 53. 

 Many cornfields are over 50 inches in height in the western part of the state. 



Examine the rapidly-growing, most advanced fields (especially in bottomland areas) 

 for corn borer infestations. For optimum results, treatments in the southwest sec- 

 tion should be completed by now. Treatments, if needed, should be underway in the 

 west-central section. In the northern sections, treatments, if needed, should be- 

 gin about July 1. 



Many fields of advanced corn now have 30 to 60 percent of the plants showing whorl 

 feeding and a tassel ratio of 30 or over. These fields do not justify treatment 

 for control of first -generation corn borers. However, in these fields that have 

 an adequate tassel ratio for borer survival (35 or more), there is a high potential 

 for a second-generation brood to appear in late July. 



The combination of rains, delayed planting, and earlier-than-usual borer develop- 

 ment has thwarted the corn borer in most areas . The mortality rate of newly hatched 

 larvae has been high on corn with a tassel ratio of less than 35. Many plants with 

 whorl feeding 10 days ago are now free of it, due to the high mortality rate of the 

 larvae. 



To decide whether an insecticide can be profitably applied, measure the tassel 

 ratio of the field and determine the percentage of the plants with recent feeding 

 in the whorl leaves. To determine the tassel ratio, measure the height of the 

 plants with leaves extended; split the stalk open and measure from the tip of the 

 developing tassel to the base of the plant; divide the tassel height by the plant 

 height; and multiply by 100. That figure is the tassel ratio. If the tassel ratio 

 is at least 35 (preferably 40 to 45) and at least 75 percent of the plants show 

 whorl feeding, treatment is justified. Use 1 pound of actual diazinon in granular 



