Different pesticides as insect killers


I recently came across a new study by a group of friends and colleagues that blew me away.  Of all the environmental pollutants and nasty things we use to kill pests, who knew that fungicides (chemicals used to kill fungus) would become a problem for bumblebee pollinators. This study:

McArt, S. H., C. M. Urbanowicz, S. McCoshum, R. E. Irwin and L. S. Adler. 2017. Landscape predictors of pathogen prevalence and range contractions in United States bumblebees. Proceedings of the Royal Society of London B 284: 20172181.

This study is part of a new generation of research looking a “non-target” effects and their mechanisms.  It reminded me of an awesome figure that artist Frances Fawcett did for Monarchs and Milkweeds on the different mechanisms that two other pesticides (the general term for chemicals used to kill pests) may harm monarch butterflies.  The figure ended up being one of those spare bricks that didn’t make the final cut. How do herbicides versus insecticides impact monarchs and milkweeds?  Well, the former kills the latter and the latter kills the former. In plain English, herbicides kill plants (usually weeds), in this case the milkweed host plant of monarchs, while insecticides kill monarchs directly.  It is likely that both are harmful to monarchs, but the mechanisms are decidedly different.  As far as we know, herbicides like roundup do not directly harm monarchs, but rather eliminate milkweed from agricultural fields.  How limiting milkweed is for monarch populations is currently being debated among scientists. Insecticides like the potent neonicotinoids often find their way into milkweeds along agricultural margins — only now is the data pouring in showing that even trace amounts have the potential to be harmful monarchs.

GMO plate
Two ways that pesticides (herbicides versus insecticides) may harm monarch butterflies.











In terms of agricultural yield, on average, weeds are the biggest pest f most crops, and diseases such as pathogenic fungi are the second biggest problem, followed by insects 3rd.  We have pesticides for each, and they are likely all harmful to our native butterflies. Another spare brick by photographer Ellen Woods below.

spare brick #2
spare brick #2, by Ellen Woods

2 Replies to “Different pesticides as insect killers”

  1. How long does it take for pesticides to be out of the plant? In the past, when first starting, I bought plants at a large chain store. I don’t remember if caterpillars died or not, but year after year now the plants return and I am having wonderful success. My question is, for myself and for new monarch enthusiasts, if you did buy a plant that unknowingly was treated with pesticides, could you cage the plant for a few weeks and the residue would leave the plant and it would be safe? What would the time frame be for this to happen?

    • I would guess at least a month … many insecticides are systemic, which means they are taken up by the plant, in which case it may be a couple of months… I think the following season would be safe for sure. Good luck on your garden!

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