WHEN do we know a host plant is safe (pesticide-free) to feed to caterpillars? 

Pesticide is a fearful thing when we’re raising butterflies and moths.


When we buy a plant or think our plants may have been treated with pesticides, we test the plant on one or two caterpillars before feeding it to all the caterpillars we are raising.  But the question is WHEN do we know a plant is safe to feed to caterpillars?  If the caterpillars are alive the next day, is the plant OK?


The answer is a simple and resounding NO.  Some pesticides will kill immediately.  Others take a day or two.  Some take three days (such as Bt).  Others will slowly kill them, by interfering with their mandibles and how much they can eat (lack of nutrition).  Yet others won’t affect them at all until they begin to pupate and won’t be able to finish the pupation process.


If caterpillars are fed some pesticides just before they pupate, they will emerge fine, inflate and dry their wings, and begin to fly before falling to the ground, quivering and dying with their wings folded downward over their legs.

The only fully correct answer is that a caterpillar should be fed the plant and it considered unsafe until it emerges and lives a day as an adult.  BUT …

In most cases, after a caterpillar that has been eating the plant has successfully and fully pupated, it is safe.  

That isn’t the answer we hoped for but it is the answer we want, the one that will help prevent more deaths.


14 thoughts on “WHEN do we know a host plant is safe (pesticide-free) to feed to caterpillars? 

  1. “Yet others won’t affect them at all until they begin to pupate and won’t be able to finish the pupation process.”
    I believe that a pesticide may be affecting my caterpillars though the plants are stated to be “pesticide free”. The caterpillars are beautiful, large at instar 5, OE (as best I can guess) free, given I have tested the monarchs which lay the eggs they come from and I have not read that OE affects formation of a chrysalis but may or may not effect the structure of the butterfly that ecloses. The chrysalis is approximately 1/2 formed and then all stops. This has happened three times now but does not happen on ALL plants from this grower.
    What types of pesticide might be responsible for this? systemic or non-systemic?

    It is tragic to see this happening to these incredible insects!

    Many thanks


    1. Rosemary, do you have pets with flea/tick medication, topical or oral? This sounds like a growth regulator pesticide, which is present in many flea/tick medications. The caterpillars are fine until they attempt to pupate. The growth regulator prevents them from going to the next stage. It is also in pesticides that on on plants, including organic pesticides. Were the plants neonicotinoid free or pesticide free?


      1. No, no dogs with topical flea or tick treatment and the grower makes a statement that the plants are pesticide free. Interestingly I noticed the plant had a strong smell which resembled a tobacco plant.

        I do believe that you know Brenda D. She helped us get our butterfly house going at the South Texas Botanical Gardens! Her daughter raises non-monarch butterflies and I raise some of the Monarchs. I love doing this and enjoy the testing and monitoring as well. It takes a large amount of time and I hate to see OE negative eggs NOT turn into butterflies due to chrysalis malformation which does not have to occur!

        I was working with a school group – the teacher wanted to have the children see the life cycle of the butterfly. Each child (N=20) had a plant inside of a habitat and observed the growth of their caterpillar through all instars until chrysalis formation. 9/20 had pupation with partial chrysalis (approximately 1/2) formation. These caterpillars had no skin contact without glove use for frass removal.
        The grower is the same.
        I was wondering about drift given the milkweed is with ornamentals and the like.
        I will do a bit of research about growth regulator pesticides to gather more information.

        I certainly do not wish to lay blame but was most curious about what was causing this given it has not happened before and it is now happening fairly consistently.

        Thank you so much for answering! If you know of a way I might get an answer to this problem, i.e. plant testing, etc. I would be MOST grateful to know.

        Rosemary Plank


      2. Drift happens, yes, and quite often. Sometimes a pre-pupa will partially pupate and for no known reason. If only one out of several fail to properly pupate, it is usually considered one of those that simply failed, for an unknown reason. When several fail, it is time to look for a reason. Nine out of twenty certainly is a reason to look for the cause.

        If you purchased the milkweed from a retail nursery that does not grow its own plants, the plants could have been treated at the wholesale nursery and the retail nursery not have a clue that they were treated.

        Over-spray happens all the time. Without a barrier, when one section of a nursery is sprayed, there will be some drift unless the day is 100% calm and the person spraying is consciously avoiding over-spray. Most nurseries keep different types of plants close together. To keep a good distance between different types of plants is a waste of valuable space.

        I’m sorry that I didn’t see your question earlier. If you want a plant tested, I suggest visiting your local county ag department and ask if they test or know of any place where you can have it tested.

        Some nurseries do use sprays or powders that are tobacco based to treat plants. I don’t think it interferes with pupation. I need to study more on the method in which tobacco works as an insecticide.

        I hope your next batch has a 100% successful emergence and release rate.


    2. Ask about fungicide use, some fungicides are very harmful to pollinators. It is very hard for a commercial grower to produce a crop without using fungicides.


      1. I start my plants in quality potting soil (not the cheap clumping stuff) and when I plant them, I am generous with Black Kow (composted cow manure).
        We also use just general inexpensive granular fertilizer.


  2. Yes, over-spray at the wholesale or retail nursery is a definite possibility. Tobacco is a pesticide and many plants are treated with tobacco as an organic pesticide. The percentage is much too high to be ‘one of those things’, a genetic or other type of glitch.

    Were the plants purchased from a retail nursery that buys from a wholesale nursery? Quite often, the wholesale nursery sprays and the retail nursery doesn’t, enabling them to say that they don’t use pesticides. Some simply don’t even think about the fact that the wholesale nursery could have sprayed the plants.


    1. It was a retail nursery. I had asked about pesticides at the retail nursery before given a ‘huntch’ that all things were not exactly right and received answer “We try hard but are not able to control for all things”.
      I will have to change to a wholesaler in the area that obtains plants from San Antonio.

      Thanks so much for your responses – will try to follow through with director at gardens. Might I share your comments with him?


      1. Yes, please feel free to share my comments. As a nursery, I fully understand. When we started our nursery, we used broad-spectrum pesticides. Only after we learned about butterflies did we change with what and how we treat our plants. Over-spray is easy so when we have to treat a non-butterfly host plant in one greenhouse, we don’t use host plants that are growing in that greenhouse for a few weeks.

        I’ve bought the exact same species of milkweed wholesale that looked totally different. One group was taller, a bit leggy, and the leaves weren’t near as beautiful. The other group was compact, multi-branched, dark beautiful leaves … the very appearance of the plants were our clue to ask about them. Yes, the wholesale nursery had just bought them from another wholesale nursery. Although the wholesale nursery we used hadn’t sprayed theirs (the leggy ones), they weren’t sure about the other nursery. We were able to use the leggy ones right away for food. The short beautiful ones were deadly (we tested with a couple of caterpillars) and it was weeks before we could use them. We learned then that even a wholesale nursery sometimes buys from other nurseries and don’t always know how those plants were treated.


  3. Thank you so much.
    I try to reuse the plants that I purchase – keeping them under a net outside to prevent use by butterflies that may have OE. I will put eggs on them from a non OE pair and do sterilize them to make sure.
    Is there something that may be used to nurture these plants that will not affect caterpillars, generally I use nothing but it would be nice to enrich the soil between uses rather than change the soil every other time the plant is used.

    Thanks so much, your website has been my set of instructions so to speak. I also find it fascinating to read the research by Hunter, de Roode, Agrawal, Altizer, etc.
    hope you don’t mind if I contact you again when I run into issues.


  4. Hi Ms. Smith – thank you so much for your column. I’m first year with raising monarch butterflies. Luckily I’ve had success with releasing 34 beautiful butterflies from eggs. It is now November in Jacksonville Florida. While trimming back my milkweed for the winter, I discovered 4 eggs. I decided to bring them in for raising before the freeze occurs. (There are many caterpillar predators here!) For the first time, 2 caterpillars died. When the remaining two went into chrysalis, one didn’t shed its skin fully. The chrysalis is deformed as a result. (I’ve taken 4 pictures of the chrysalis from each side.) The last caterpillar’s chrysalis looks to be “normal”. I’ve learned that my neighbor had the outside of their house sprayed because they saw a few insects inside, probably trying to come in from the cold. The milkweed that I use is throughout my backyard. The closest plants to their house are about 200 feet. With your knowledge, could this spraying of my neighbor’s house have impacted these caterpillars in this way?


    1. Yes, absolutely. Insect growth regulator type pesticides cause deformed or incomplete chrysalises. We have our house sprayed for termites, an hour south of you, and scheduled our yearly spraying for every November. The pesticide is always out of the soil by the time spring and butterflies arrive.


      1. Thank you for your reply Ms. Smith. With the fact that three of the four caterpillars didn’t make it to full chrysalis, may I ask for your suggested approach to this fourth chrysalis appearing to be “normal”? Should I allow it to emerge? I want to do the right thing and don’t want any suffering.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s