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Ask Mr. Smarty Plants is a free service provided by the staff and volunteers at the Lady Bird Johnson Wildflower Center.

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Sunday - July 28, 2013

From: Jacksonville, FL
Region: Southeast
Topic: Butterfly Gardens, Herbs/Forbs
Title: Effects or insecticide on Monarch butterflies
Answered by: Guy Thompson

QUESTION:

Thank you for fielding questions about plants!! Our nursery just informed us that their milkweed grower was using imidacloprid in their milkweed production. As a follow up to the question already in your database about imidacloprid on milkweed and its toxicity; Are NEW plants grown from 1) the stalks of denuded (by the poor caterpillars) plants and 2) grown from seeds from a imidacloprid treated plant - are those plants subject to the same half life time frame?

ANSWER:

The excerpt shown below must be the one you found in out database.

"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."

The half life of the chemical will be the same no matter where it resides.  The main question is the quantity that the caterpllars eat.  Imidacloprid is a systemic compound, meaning that it moves throughout the entire plant and not just the parts sprayed.  There might be enough on the denuded stalks to injure the caterpillars when they eat new leaves that have drawn the insecticide from the old stalks.  But the amount of imidacloprid that might have entered the seed would be diluted so much in new plants grown from it that there should be no danger. The caterpillars would not ingest a large enough amount of the chemical to damage them. So the conservative solution is to start new plants from seed.

 

 

 

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