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interaction between the monarch caterpillars and the tachinid fly

just read the answer you gave in 2020 regarding the interaction  between the monarch caterpillars and the tachinid flies. We do  not raise monarchs  but have milkweed in an area of the garden, butterfiles deposited their eggs caterpillas ate, grew,  buterflies hatch and everone  was happy . Now all the sudden 2  caterpillars hang on a J and then went back to vericals and  3 chrysalis had the silky strings at the bottom 2 more chrysalis did not keep the jade green color one looks almost white, another brown.  Think that means they are infected.

Needless to say my heart is with the caterpillars not the fly. The comments on nthe Webb are addressed to people that raise moranchs  in enclosures , this is not our case

My aim is to at least control the infestation hence my writting to you. Want to be clear on the fly cycle, the caterpillars that unable to form the chrusalis clearly they are infected, will just dispose of it by freezing them.  Have the fly magots  already come out  ? the goal here is to make sure they do not become flies. Same for the infected chrysalis i saw somewhere that the egg is attached to  silk string. Hence those chrysalis should also be destroyed . Am I already too late and the eggs are lying on the ground.

In case we have them on the dirt what  to do ?, clean leaves and debris, read  like they dig themselves in the dirt  . Can we put something that will kill them with out killing the caterpillars that will walk around?

Any info would be greatly appreciated 

Thank you   

juliet, Malibu, CA - April 19, 2022

Curator Response

Hi Juliet,

In 2020, I wrote in response to the other inquiry that “I don't know of an effective way to kill, remove, or discourage tachinids and other parasitoids from attacking your Monarchs, other than bringing the whole plant/caterpillar setup indoors for the entire life cycle, which would be quite a commitment.” That is still the case. And in fact, bringing it all indoors is an even bigger commitment than it sounds like, since California law currently requires you to get a permit from the California Department of Fish & Wildlife in order to handle, collect, or rear Monarchs. Insect conservation organizations like the Xerces Society and Monarch Joint Venture have some good resources out there explaining why it’s for the best that we don’t pursue rearing Monarchs in captivity as a strategy to help the species, since captive rearing can have negative impacts on these particular animals. Check out this blog post by Monarch Joint Venture and this handy flyer by the Xerces Society

To rear butterflies for our exhibit, we have to follow a lot of regulations designed to protect the health of our animals, ecosystems, and local agriculture. For our Butterfly Pavilion staff, maintaining butterfly health is part of their work. They are trained in the safe use and disposal of chemicals and have a lot of experience with techniques to safely handle chrysalides without harming them. Conditions in our closed and sterile butterfly-rearing lab are totally different from those in a garden open to nature. So even though we raise a lot of butterflies each summer, we don’t intervene in the lives of wild butterflies. We encourage people to protect or create butterfly habitat, since protecting habitat is one of the best ways to help any species.

The forces of nature are powerful and highly evolved; in many cases resisting them can lead to more harm than good. Sometimes it’s better to work with nature than against it. As our Butterfly Pavilion Manager Kim Zsembik says, “Plant native plants often, then enjoy the show but don’t join the cast.” Remember that we're not just preserving butterflies, but the whole array of species and interactions that go along with them. If you want to up your milkweed game, check that you are growing native milkweeds, since the popular Tropical Milkweed can contribute the buildup of the protozoan parasite Ophryocystis elektroscirrha (also known as OE). You can read more about that in this flyer from Monarch Joint Venture.

If you’re interested in the general topic of how natural forces can aid gardeners, you might like to watch this video of a talk we recently hosted with the author of the book Garden Allies.

As a natural historian—not a pest-control exterminator—I retain the outlook I shared in that original inquiry, that the diverse and fascinating tachinid flies are just as much a part of nature as the Monarchs we find so beautiful.

Stay curious,

Schlinger Chair and Curator of Entomology Matthew L. Gimmel, Ph.D.

juliet response:

Thank you so much, good point. I was blidsided by my aim of helping the monarchs. As you know  many  environment  organizations keep telling us are in peril because uf the shortage of milkweed. Another point I may be missing is that since we have such benign climate here in th Ca coast we are promoting the so called over winter . I read somewhere not sure how well informed the source was that OE is more prevalent in the over winter season and it is not desirabel . Our goal here is not to  go inside  and get involved  in a larger commitment.. We don't plant the milkweed just keep it in small pots at a  small  area desiganted in the garden. Just started,  planted some  milkwed in Oct Nov and indeed the first butterflies had wing problems do to OE. We were so excited that now the butterfliees  hatched and could fly  so we though we were doing what we were set uo  to do . Another bias is the beauty of the monarchs, they are really magnificent. and now were so disapointed that some of the chrysalis were sick. We  learnt a lot. becaus of th initial  problem , thru the Webb we met a lady lipidopedist that runs a large operation in the adirondack mountains. We learn about OE and now about the flies  and another parasite chalcid wasps, thru her . But  from the pic  the problem here at the moment  most likely the flies. We will just dispose early of the chrysalis that look sick,  and/ or the catrpillars that are unable to hang  look infected and will and play the number game. Earlier we had many catrepillars maybe that attracted the flies. The goal is to keep milkweed avilable,  monitor the area. locate the chysalis monitor thier development, make sure the newly hatched has a place to hang and rejoice when it flies away. We now know that the time they stay in the chyssalis is weather dependent, thet hatched around 8 50 am hang and leave around  4pm if it is cold  or they hatched later they wait untill the next day. They are diurnal, the caterpillars protect themselves  under the leaves. So we need to learn how to help with out intervention. They come visit, we have a number of them  in the area everyday, looking for the nectar of other flowers  Hard to watch the well fed happy caterpillars not being able to become a healty buterfly. On the milkweed the nurseries have mainly the tropical variety,  also the thin leave one. The native  with the yellow flowers is hard to find.

Again Tu now that we know you have a set up will visit next time we are in Santa Barbara . Will read the blog and suggested recomendations to do our best to help        

Curator response:

You’re very welcome! If you want even more butterfly resources touching on these issues, we just shared a lot of information on our new Central Coast Butterfly FAQ page at