J-,V- I 



COLLEGE OF 

 AGRICULTURE 

 UNIVERSITY OF 

 ILLINOIS AT 

 URBANA-CHAMPAIGN 

 AND NATURAL 

 HISTORY SURVEY 

 URBANA. ILLINOIS 



INSECT. WEED & PLANT DISEASE SURVEY BULLETIN 



ITATE/COUNTY/LOCAL GROUPS/US DEPABTMEMT OF AGRICULTURE COOPERATING 



FOR IMMEDIATE RELEASE 



No. 13, June 17, 1969 



This series of weekly bulletins -provides a general look at the insect , weed, and plant 

 disease situation (fruit and commercial vegetables excepted) > along with suggested, 

 abbreviated control measures. Each individual should check his own fields to deter- 

 mine local conditions. 



INSECTS 



CORN INSECTS 



European corn borer egg-hatch in south-central Illinois below Route 16 reached its peak 

 with most of the corn too small for borers to survive. The majority of the corn there 

 is less than 40 inches tall, with about 20 percent over 40 inches and a tassel ratio of 

 15 or less in these taller fields. Late -planted fields in this area should be watched 

 carefully later on for second-generation borers. 



In central Illinois up to Route 9, moth -emergence is all but complete. Egg-laying is in 

 full swing with hatch just beginning. Very little whorl feeding is evident. Less than 

 10 percent of the fields have corn taller than 40 inches. Moths are beginning to con- 

 centrate on the earlier fields. The decision on whether treatment is needed in these 

 early fields can be made early next week. 



Between Routes 9 and 6, egg-laying has just begun, with 10 to 25 percent of the moths 

 yet to emerge. By late next week, egg -hatch will be occurring in the earliest fields, 

 and a decision can be made then about treatment. 



torth of Route 6, about 30 percent of the moths have yet to emerge and egg- laying is 

 starting. Most of the corn is about 10 inches tall. In a few fields, it is close to 

 30 inches high. Watch these early planted fields during the first week in July for 

 whorl feeding, and treat if necessary. 



In all areas of the state where the com borer is a potential problem, cool nights have 

 temporarily delayed egg- laying; also, moth -emergence. This delay can allow corn to reach 

 an optimum height for borer survival. 



To determine the need for treatment, measure the tassel ratio. Dig up a plant and mea- 

 sure from the bottom of the plant to the tip of the longest leaf. Split the plant and 

 find the developing tassel. Measure from the bottom of the plant to the tip of the 

 tassel. Divide the tassel height by the plant height and multiply by 100. If the tas- 

 sel ratio is 30 or over and if 75 percent or more of the plants have corn-borer feeding 

 on the whorl leaves, the field should be treated- -but not until the tassel ratio is at 

 least 35, preferably 40 to 50. If there is corn-borer feeding in the whorl, the per- 

 cent of plants required to justify treatment can be reduced with higher tassel ratios. 



