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Fall Abunda nc c 
The Bureau of Entomology and Plant Quarantine cooperated with interested 
States again in the fall of 1940 in a surve* r to determine the relative abundance 
of the European corn borer in corn over a considerable portion of the area, in- 
fested by the insect. As a result, 3 >273 cornfields were examined in 25S 
counties of 19 infested States, l6S of the counties being surveyed by the Bureau 
and 90 by the Sta.tes. In each of 5 States, 2 small counties were combined and 
each pair treated as a, single county, and in 1 State 3 small counties were 
grouped in this way. The survey procedure adopted in 1939 was followed in 1940 
in all Sta.tes except Indiana and Maine. By 'this .method , 10 cornfields at. random 
were sampled within ea.ch county (except in Delaware where 20 fields per county 
were surveyed), the count of infestation being obtained by examining 25 consecu- 
tive corn plants taken at a given distance within the field from near the mid 
point of its most a.ccessible edge, and the number of borers per infested plant 
being;, determined by dissecting the first 2 plants found infested. In Indiana 
and Maine an average of 20 to 25 fields were surveyed in each county and the 
population figure for each field was based on an examination of 100 plants and 
the dissection of 5 or 10 infested plants. In either procedure the product of 
the percentage of plant infestation in a field and the average number of borers 
per infested plant provided a figure designated as the average number of borers 
per 100 plants. The population data derived in this way for the. individual 
fields were then grouped in the caJ.cula.tion of county averages, 
A summary by States of the data on corn borer abundance for a.ll counties 
surveyed in 1940 is presented in table 1, with comparisons of the figires for 
1939 end 1940 limited to the number of comparable counties included both years. 
In table 2 the average numbers of borers per 100 plants are given for each county 
surveyed in 1940 and all possible comparisons are made with similar data from 
1939. Both States and counties are arranged alphabet icaJly in the presentation 
of the data. In reading the tabulated data it should be noted that a zero 
recorded for any county- indicates a population so low that no infested plants 
occurred within the specified counts and does not mean the complete absence of 
the borer. In the accompanying map shaded areas indicate the relative abundance 
of the European corn borer over the part of the infested area in the United 
Sta.tes surveyed in 1940, and give the known distribution of the insect in 1940 
on a. county basis. Many of tho unshaded counties within the border of infesta- 
tion were surveyed in 1939 and, in general, found only lightly infested by the 
corn borer. h/ In the following paragraphs some of the outstanding results of 
the 1940 survey are discussed briefly. 
The principal centers of abundance of the European corn borer in the 
United Sta.tes in 1940 were as folloivsJ Southeastern Michigan, including most of 
the "thumb" section; the northwestern quarter of Ohio; nine counties near the 
eastern border of Indiana; four counties on tho southern edge of Lake Ontario, 
•^/insect Pest Survey Bui., Sup. to No. 9» v. 19* PP» 603-6IS. Dec. 15* 
1939 
