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FOR TMEDlkTE RELEASE June 23, 1961 



INSECT SURVEY BULLETIN NO. 10 



This weekly bulletin on the general insect situation in Illinois (fruit 

 insects excepted) is prepared by entomologists of the Illinois Natural History Sur- 

 vey, 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 reports indicate only general trends, 

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



Corn borer moth activity has been retarded by the cold nights. They wiH 

 not lay eggs when temperatures are below 50° and egg laying is slow even between 

 50° and 60°,but with the first few warm, calm nights eggs should be deposited in 

 numbers. With cool nights moths will lay eggs in cornfields during cloudy calm 

 days. However, the longer the moths are delayed the more likely that wind and rain 

 storms will kill them before they lay all their eggs. 



In general, moth emergence is complete or nearly so south of highway 36. 

 From there north to highway 6, up to 20 percent of the moths have not yet emerged 

 from the pupal stage and in northern Illinois 20 to 30 percent are still to emerge. 

 With the moths yet to emerge and with the cool nights, egg laying may continue for 

 some time. 



Egg mass counts are quite variable from one area to another. The area 

 bounded by the Mississippi river and a line from St. Louis to Decatur to LaSalle to 

 Rock Island has the highest counts at present. Egg mass counts in field corn here 

 this week varied from to hko per 100 stalks in the most advanced fields. The 

 area north of a line from Danville to Decatur and that north of highway 6, had 

 counts ranging from to 60 per 100 plants in the most advanced fields. Egg lay- 

 ing may just be getting well underway in this general area. These counts are 

 about the same as they were in i960, lower than they were in 1955 and 1956, but higher 

 than in 1958 and 1959. Since corn is more advanced than in i960, the borer survi- 

 val may be higher. As usual corn borer development is earlier on the west side of 

 the state than on the east. 



General time to start treatments in the most advanced field corn in the 

 area south from Quincy to St. Louis is still sometime the week of June 26. From 

 Quincy north to Rock Island the time to start treating will be late the week of 

 June 26. Treatment where needed on the east side of the state will be a few days 

 later than this. Do not begin to treat field corn in the area north of highway 6 

 until the week of July 3. Delay treating those fields warranting it as long as 

 possible. The clearance height of the equipment may be a limiting factor in how 

 long you can wait to treat. 



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, but, if equipment clearance permits treatments 

 should not be applied until the ratio is at least 35 and preferably k-5. However, 

 treatment after borers have drilled into the stalk proper will not be effective. 

 Continued cool nights which retard egg laying may delay timing of treatments for 

 best control. 





