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FOR IMMEDIATE RELEASE June 17, i960 



INSECT SURVEY BULLETIN NO. 9 



This is the ninth in a series of weekly bulletins on the general insect 

 situation in Illinois (fruit insects excepted), prepared by entomologists of the 

 Illinois Natural History Survey, University of Illinois College of Agriculture and 

 cooperating agencies • It is designed to forewarn people in Illinois of impending 

 changes in insect activity and to suggest abbreviated control measures. These re- 

 ports indicate only general trends. Each individual should check his own fields to 

 determine local conditions. 



Corn borer : The cool, wet weather this past week has retarded borer 

 development somewhat. Dead pupae were observed this week. In some cases pupal 

 mortality was as high as 20 percent. It is not known what is causing it or how 

 widespread it is. The last 10-15 percent of the moths that emerge account for most 

 of the damage. Reducing this segment of the first- generation borer population would 

 drastically reduce their importance. Weather during the next 10 days will be very 

 important to corn borer survival. 



At present it appears that the heaviest infestations will be in the area 

 north and west of a line from St. Louis to Kankakee. At the present rate of borer 

 development and plant growth, only early fields planted in late April or early May 

 are expected to be affected. Between 1 and 5 percent of the corn fields fall in 

 this category in the threatened area. 



Emergence is complete in the area south of Highway 36 and just north of 

 St. Louis. Egg counts in the most advanced fields this week ranged from 15 to 200 

 per 100 plants. About 20 percent of the eggs had hatched. On the most advanced 

 fields in this area, insecticides should not be applied until June 24 or later. 



Emergence is almost complete in central Illinois between Highways 36 and 

 6, and egg-laying is just starting. Where needed on field corn, insecticides 

 should probably be applied the last week of June and early July. 



Emergence in the area north of Highway 6 varies from 10 to 60 percent. 

 Peak moth flight will occur this week, and peak egg-laying will probably occur be- 

 tween June 24 and early July. Treatment probably should not be made in this area 

 until the last few days of June or the first week of July. 



To determine the need for treatment, examine 50 to 100 plants for signs 

 of borer feeding on the whorl leaves. If 75 percent or more of the plants show 

 recent whorl feeding, check the tassel ratio. Measure from the base to the tip of 

 the plant with leaves extended. Split the plant lengthwise, and measure from the 

 base of the plant to the tip of the developing tassel. Divide the tassel height 

 by the plant height, and multiply by 100. Take an average of five representative 

 plants per field. This is the tassel ratio. The corn will warrant treatment if 

 the tassel ratio is 30 or above. Treatment after borers have drilled into the 

 stalk proper will not be effective. 



DDT is still the insecticide most commonly used for corn borer. Apply 

 1 1/2 pounds per acre as a spray or 3/4 to 1 pound per acre as granules. When 

 spraying, arrange two or three nozzles over the row to cover the upper l/3 of the 

 plants, particularly the whorl zone. 



Other insecticides approved for use against corn borer include endrin 

 and toxaphene. Ensilage or stover of corn treated with DDT or toxaphene should 



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