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JOURNAL OF ECONOMIC ENTOMOLOGY 
[Vol. 17 
year for a given locality. Their daily records also showed the wavering 
appearance of the flies and the egg-laying records of the fall brood. 
Predicting the Actual Fly-Free-Date 
On account of the extremely fluctuating emergence of the flies in the 
field and in the concentration boxes, together with the rate of egg-laying 
on the young wheat plants, the writers found the flax-seed 2 count to be 
the only constant and reliable factor to use in predicting the actual 
fly-free-date. This daily count determined the exact percentage of 
flies emerged, parasitized, as well as those in the larval and pupal stages. 
Moreover, when the count showed very few pupae and only a small per 
cent of larvae in contrast to a large number of empty flax-seeds, it was 
proof that the actual fly-free-date was at hand, and that the few flies 
yet to emerge would issue, lay their eggs and perish before wheat drilled 
at this time would come up. 
Iowa Observation Station Records 3 
Monona County station, Onawa, 1922:—A total of 19,225 eggs were 
laid on the 100 marked wheat plants during the 25-day emergence period 
of the fall brood from September 10 to October 4, an average of 769 
per day. From the two concentration cages 13,926 flies issued and 1,156 
were caught on the migration screen. The crest 'of the brood occurred 
September 29 (Fig. 12), on which day the actual fly-free-date was an¬ 
nounced. In 1923, conditions were radically different at this station. 
When the observations were started September 5, flax-seed counts 
showed that about 31 per cent of the puparia were viable, the rest 
having emerged as flies or parasites. This year, 1923, a total of 7,150 
eggs were laid on the 100 plants during a 19-day period, an average of 
375.7 per day; 477 flies issued in the concentration boxes and 182 were 
caught on the migration screen. The heaviest recorded egg deposition 
occurred between September 10 and 18, being interrupted by a rain and 
a four-day period of cool weather (Fig. 13). Seventeen and five tenths per 
cent of the puparia issued as parasites and 28 per cent of the holdover 
larvae were parasitized. The fly-free-date for this county, 1923, was 
announced on September 25, four days earlier than in the preceding 
year. 
2 The flax-seeds were taken from old wheat stubble dug up each day at random 
in the field. The puparia were classified as parasitized, emerged and viable. The 
latter were then opened to determine the per cent of larvae, pupae and unemerged 
parasites. 
3 For further details see la. Agr. Exp. Sta. Cir. 86, 1923. 
