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Monday - March 07, 2011

From: Boerne, TX
Region: Southwest
Topic: General Botany, Pests
Title: Half-life of the insecticide imidacloprid
Answered by: Guy Thompson


How long do systemic insecticides such as imidacloprid (Merit) remain active in nursery grown plants? Asclepias curassavica (tropical milkweed)is frequently grown with imidacloprid to prevent aphids but it also kills monarch caterpillars utilizing the milkweed. How long will the milkweed be toxic for monarch caterpillars? Thanks.


Imidacloprid is a chemical to avoid if at all possible.  It is a synthetic analog of nicotine and is slow to degrade in the environment.  According to the National Pesticide Telecommunications Network, it has a half-life in sandy loam soil of greater than one year.  In bright light it degrades faster, with a half life of 39 days. But even the degradation products that form inside plants are highly toxic to insects, and their half-life is not reported.

Imidacloprid has been implicated in the mysterious colony collapse disorder of honeybees. For this reason, certain countries, e.g., France, has outlawed its usage for many purposes.

It seems safe to assume that milkweeds treated with imidacloprid may be quite toxic to monarch caterpillars for an extended period.  Mr. Smarty Plants recommends the use of much less toxic sprays, such as Safer soap, which may be less effective on aphids but harmless to monarch caterpillars.


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