Host an Event Volunteer Join Tickets

Support the plant database you love!

Q. Who is Mr. Smarty Plants?

A: There are those who suspect Wildflower Center volunteers are the culpable and capable culprits. Yet, others think staff members play some, albeit small, role. You can torture us with your plant questions, but we will never reveal the Green Guru's secret identity.

Help us grow by giving to the Plant Database Fund or by becoming a member

Did you know you can access the Native Plant Information Network with your web-enabled smartphone?

Share

Ask Mr. Smarty Plants

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

Search Smarty Plants
See a list of all Smarty Plants questions

Please forgive us, but Mr. Smarty Plants has been overwhelmed by a flood of mail and must take a break for awhile to catch up. We hope to be accepting new questions again soon. Thank you!

Need help with plant identification, visit the plant identification page.

 
rate this answer
1 rating

Monday - March 07, 2011

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

QUESTION:

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.

ANSWER:

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.

 

More General Botany Questions

What to do about grass dying under pin oaks in Iowa
December 10, 2008 - We have 2 pin oaks about 15 years old in our front yard. The grass has started dying out under and around them. What can we do?
view the full question and answer

Difference between Erigeron strigosus and E. annuus
July 07, 2011 - How can you tell the difference between Erigeron strigosus or Erigeron annuus. Does one have more flowers on it than the other? Thanks.
view the full question and answer

Possible reasons for yellow heads for Indian Blanket
December 13, 2005 - I had Indian Blanket flowers that had almost pure yellow heads. Will the seeds of these flowers produce plants that will have yellow flowers?
view the full question and answer

Will molasses harm beneficial organisms in my garden?
April 06, 2009 - If I use molasses in the garden, I am hoping this will NOT kill the beneficial nematodes and my earth worms, or other good bugs such as lady bugs? Thanks.
view the full question and answer

Native plants as accumulators of heavy metals in Texas
March 29, 2008 - I would like to know of any native plants that could be used as hyperaccumulaters of heavy metals in Texas.
view the full question and answer

Support the Wildflower Center by Donating Online or Becoming a Member today.