INSECT 



SURVEY 



BULLETIN 



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County 



Local Groups 



FOR IMMEDIATE RELEASE 



College of Agriculture 



University of Illinois 



and Natural History Survey, Urbana, Illinois 



U. S. Department of Agriculture Cooperating 



July 19, 1968 



ILLINOIS INSECT, DISEASE, AND WEED SURVEY BULLETIN NO. 19 



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

 ease, and weed situation (fruit and commercial vegetables excepted) , along with 

 suggested, abbreviated control measures. Each individual should check his own 

 fields to determine local conditions. 



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INSECTS 





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Corn Insects 



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Corn leaf aphids have increased rapidly in fields in many areas, especially across 

 the central part of the state. In many fields, populations have increased to the 

 point where more than half the plants in a field are infested with at least a 

 small colony; the infestation in some plants is moderate to heavy. A few fields 

 have an infestation as high as 100 percent. 



Corn leaf aphid infestations usually begin while the plant is 

 in the whorl stage probably, two to three weeks ahead of full 

 tassel, reaching a maximum during peak pollen shed. Infesta- 

 tions decline rapidly after that. The time to decide about 

 treatment is now . Fields where the plants are now in full 

 tassel will most likely escape damage; fields with more than 

 half the tassels showing may escape injury, while those with 

 a few or no tassels showing are most likely to suffer damage 

 and should be examined critically until after pollen shed. 



To examine a field for aphids, (1) pull 25 \\'horls in four places in a field, 

 (2) unroll the leaves, and (5) determine the percent of infested plants and degree 

 of damage. If the next two weeks are hot and dry, the population of aphids is 

 most likely to increase greatly, decreasing with cool, rainy weather. We have 

 observed very few predators. The black beetles with the four yellow spots, com- 

 monly seen in the whorl , are picnic beetles . They are not predators and do not 

 damage plants. 



Treatment is justified on late-whorl-stage corn (a few tassels showing) , if 

 50 percent or more of the plants have some aphids (a few heavy) and if the corn 

 is under stress (from low soil moisture, fertility, disease, etc.). Continue to 

 watch the infestation as long as growing conditions are good; if 15 percent or 

 more of the plants become heavily loaded with aphids, treat immediately. 



Seed fields should be treated if 50 percent or more of the plants have some 

 aphids (only a few heavy), even if the corn is not under stress. In general, 

 applications after all the silks have dried are disappointing. 



