Seasonal Decline in the Effectiveness of Certain Insecticides Against Boll Weevil12

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Abstract
In field cage tests against Anthonomus grandis. toxaphene. benzene hexachloride, aldrin, and dieldrin were approx. 50% as effective in Oct. as in July, whereas undiluted C arsenate was approx. 67 % as effective. Effectiveness was much reduced in Sept. and Oct. as contrasted with that in June and July. Toxaphene was consistently less effective month by month from June to Sept., inclusive, for each of 3 years. Increased concns. and increased dosages up to 4 times the normal rate of application did not significantly affect the per cent kill of late -season weevils. It required 240 times as much toxaphene to kill the same percentage of boll weevils in Oct. as in July, and all the other insecticides tested required extremely high dosages to effect kill in Oct. comparable to that obtained in July. Aldrin and benzene hexachloride lost considerably more than 50% of their residual effectiveness within one day, whereas heptachlor, dieldrin, and toxaphene lost 39, 32, and 45%, respectively. Ca arsenate lost none of its effectiveness in one day and EPN had lost only 2%. After 5 days the chlorinated hydrocarbon insecticides were virtually ineffective.

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