84 
JOURNAL OF ECONOMIC ENTOMOLOGY 
[Vol. 11 
Reasons for Failure of Poisoned Bait in Field 
Judging from the experiments mentioned above, the failure of 
the poisoned bait in practice may be due to one of several causes, 
or combinations of them. It is certain that the flies do feed on 
on the bait with fatal results. However, other sources of sustenance 
may be more attractive in nature. On the other hand, flies have been 
observed to feed on the bait in the field. The results of the fly trap 
experiments rather indicate that large numbers do not feed after visit¬ 
ing the bait. The most important factor has not yet been mentioned, 
namely, climatic conditions. In Wisconsin, the critical period in the 
control of the onion fly is invariably rainy. Rains not only interfered 
with the regular application of the spray, but in many instances washed 
the bait away before the flies had time to feed on it. 
Investigations for Year 1916 
In 1916, therefore, the bait was given a trial in the same locality, but 
pie tins were used to hold the liquid, and the bait was modified by the 
addition of a sliced onion to each pan. Eight-inch pie tins were placed 
at the rate of 40 or more to the acre as soon as the onion appeared 
through the soil (PI. 3, fig. 2). Seven acres on one farm were thus 
treated, and almost as many acres on four neighboring farms. On 
portions of the seven acres 60 “Harper” fly traps were used, and 40 
extra large fly traps constructed for the purpose (PI. 4, fig. 1). 
The pans and traps were replenished as soon after showers as pos¬ 
sible, but the weather was extremely wet, even more so than in 1915. 
The experiment was started May 12 and discontinued June 25. The 
season was late, and when the writer was transferred to Madison on 
June 1, no eggs had appeared in the field or in the cages. The experi¬ 
ment was continued under the personal supervision of the grower, who 
attended to it very carefully. During June the maggots appeared in 
enormous numbers. 
On July 3 the writer visited the fields and found that many of them 
had been plowed up, the devastation was so complete. The few fields 
which remained intact showed injury conservatively estimated, after 
consultation with growers, ranging from 95 to 45 per cent. Plate 4, 
figure 2, photographed in August, shows one of the fields injured to 
the extent of 75 per cent. 
Conclusions 
For two seasons the poisoned bait for the onion fly has given de¬ 
cidedly negative results. 
Failure was due, to a great degree, at least, to adverse climatic con¬ 
ditions. These conditions are normal to this section of the country, 
however, and to other onion growing districts, I understand. 
