Will marigolds stop cabbage worms from attacking cabbage? This was a hot topic last week as the Facebook post shown below made the rounds in many gardening groups, everything from local Canadian groups, to Gardening in Western Australia. Through this one post, thousands, if not hundreds of thousands of gardeners are now convinced that marigolds stop cabbage worms.
That is too bad, since this may just be another myth that has no scientific basis.
In this post I will discuss why you should have been very skeptical when you first saw this post, and then I’ll look at the real science behind the claim.
The Start of This Marigold Companion Planting Myth
The above picture shows what I believe to be the initial posting of this claim and was posted by The Revolutionary’s Garden on Facebook. They have yet to explain where the image came from.
This posting did not start the myth which has been around for many years, but it certainly reignited interest and support for it.
Critical Examination of the Facts
It is becoming a necessity for everyone to question and critique information they see online. Here is my thought process as I reviewed the above post.
Who is the source? I had never heard of The Revolutionary’s Garden, so I went to their site and quickly flipped through some postings. It quickly became clear that they post fringe topics, many of which are controversial, such as concerns about irradiated milk, spiders that keep tiny frogs in their burrows, and ants that harvest gem stones. This should set off a red flag – the contents are suspect!
The description is not for the pictured garden. It confirms this in the first sentence, “We planted a tenth as many marigolds.” Why would the author not show the real test garden? The answer seems obvious to me. This picture gets much more attention than the real garden, and that is exactly what happened.
The description implies that marigolds prevent cabbage worms. It doesn’t actually say that, but it strongly implies it, and 99% of the readers who commented on the post, reached that conclusion.
What are cabbage worms? You will see below, that this term can mean a number of different things. The more vague the claim, the more people that get sucked into believing it.
There is no mention of a control. For those who are not familiar with the scientific method, have a look at The Scientific Method and Why Controls Are So Important. Without controls you can reach NO conclusions.
So what do we know? Someone grew some cabbage and had few cabbage worms, whatever they are, and they also grew some marigolds. Since the picture is not from their garden, we don’t know what their cabbage looked like, or how the marigolds were planted with respect to the cabbage. We don’t know the type of cabbage, nor the type of marigold. And without controls we don’t know if the marigolds had any effect on the cabbage crop.
In short, we have zero facts attached to a pretty picture, that implies support for the idea of companion planting.
What is a Cabbage Worm?
The term is very general and is used to describe a number of different pests, including the following.
- Pieris rapae (small white or cabbage butterfly) – a small, white, common butterfly.
- Pieris brassicae (large white or cabbage butterfly) – a larger white common butterfly.
- Pieris protodice (southern cabbage butterfly) – checkered white butterfly.
- Trichoplusia ni (cabbage looper) – a medium-sized moth.
- Hellula undalis (cabbage webworm) – brown mottled moth.
- Plutella xylostella (diamondback moth or cabbage moth) – small, grayish-brown moth.
- Mamestra brassicae (cabbage moth) – brown moth
One thing that is clear from research is that not all species of insect react the same way to a companion plant and this is true even for very similar looking species as in this list. Claiming pest control without actually identifying the pest is quite useless.
Marigolds as a Companion Plant
Marigolds are one of the most commonly recommended companion plants but many of the claims are myths. I have discussed marigolds in detail in Are Marigolds Good For Companion Planting? Here are some of the highlights.
- There are several species of marigold and just because one works in companion planting does not mean all work. For example, it is important to match the right species of marigold to the nematode you have, in order to control them.
- Calendula, the pot marigold, is commonly called a marigold and it may be better or worse than a real marigold.
- Lots of pests like marigolds, and so they can actually increase the number of pests in the garden.
Do Marigolds Stop Cabbage Worms?
What does the science say?
A study done in India looked at the number of army worms (Spodoptera exigua) and diamondback moths (Plutella xylostella) on cabbage using several different companion plants. The number of army worms (number of shot holes) was the same with or without marigolds (type and species not documented). The number of diamondback moths (larvae + pupae) was the same with or without marigolds. In both cases, the least amount of pests occurred when cabbage was grown with garlic.
When looking at pest damage it is also important to look at yield. The yield in the above study was the same with or without marigolds. Clearly, marigolds did not offer any advantage for these two pests, in the year the study was made.
A Polish study looked for various pests on cabbage grown with and without marigolds (T. patula nana) and with or without pot marigolds (C. officinalis). Marigolds resulted in less eggs of the white butterfly (P. rapae), the large white butterfly (P. brassicare) and the cabbage moth (Mamestra brassicae) and fewer larvae and pupae of the diamondback moth.
It should be noted that this study was done in each of 3 years and in some of those years there was no decrease in eggs, larvae or pupae. This shows the importance of multi-year studies in the agriculture field and in the home garden.
What was most surprising in this last study, is that the pot marigold was much more effective than the marigold.
Testing in the US with various herbs (not sure which ones) interplanted with collards showed that the herbs did not repel the white butterfly (P. rapae).
These are the only studies I found. If you find more on this topic, please add their link in the comments below.
What does science say?
It is unfortunate there aren’t more studies, but this disagreement as to the efficacy of using marigolds to control cabbage worms is not uncommon in this kind of research. Over time, with more work being done, a clearer picture will emerge.
For now, I think it is fair to say that there is no compelling evidence that marigolds work to keep cabbage worms off the cabbage. Even if they work for cabbage worms, they attract a lot of other pests (aphids, slugs) and pot marigold or garlic works better.
This is one of the studies about Tagetes sp. at least on pests. Lately biofumigation is becoming more promising implementation as well. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0233511
This study looked at marigold extracts – not companion planting. It tells us nothing about a benefit of using plants.
So I wish you would do a section on what actually works and why. Its peppered in your blog posts, but its so much debunking that its hard to get the nuggets of worth. Im not saying busting myths isn’t valuable, but worth is teaching and informing the ways to actually be good at gardening. I guess the point is entertainment though. Gardening in its true form might be kind of boring to read about.
I do that as I find them – but almost everything I check out – does not work.
https://www.gardenmyths.com/garlic-king-companion-planting/
Plant the flowers and herbs you like and plant them because they are pretty and/or useful! The cabbage/marigold picture is lovely! I have alyssum, calendula, and nasturtium scattered throughout my vegetable garden, and buckwheat cover crop, some of which I allow to bloom.
I found in Seattle that nasturiums and P. somniferum were both great trap crops for black aphids. It was hell on the nasturtiums, which needed to be pulled up and destroyed after a time, but on the P somniferum, the aphids confined themselves mostly to underside of the leaves, so the plants looked fine and the flowers, as beautiful as they are ephemeral. I used to joke that they aphids became addicted to the alkaloids that opium poppies are known for and couldn’t be bothered to find anything else to eat. Completely anecdotal. I have not had trouble with black aphids in Wisconsin, which is fine with me!
Did my PhD research on Tagetes. SOME of the species do have ecdyasis inhibitors and do help inhibit insect larvae from pupating into adults. Problem is – those are not the species commonly grown by gardeners.
Most of the Tagetes secondary compounds are more active against bacteria than insects. The completed dissertation is over 800 pages, with about 40 pages of citations. Much of my research lead to dead ends which are mentioned only in passing as dead ends.
Do like your blog – keep up the good work.
Dr. Jacqueline Soule, GardeningWithSoule.com
Thanks for the comment. Did you find cases where marigolds that gardeners use that would offer some value in companion planting? Maybe with other crops?
( Not being racist 🙂 ) I thought it was supposed to be confined to the FRENCH marigold that was/is efficacious, all others don’t work
As you can see other species have not been tested, so we don’t know if they work.
Robert,
For the first time ever I am sending a comment without having first read your post.
I will go back and read it, because I know it will contain a level of examination and analysis I would not have thought to consider.
But essentially, being short of time, I am confident of your probable conclusion — because I have bought into this practice as a young, beginning gardener. “Wow,” I thought, ” This is fantastic. All these years gardeners have been suckered into the industrial fraud of the need to use potent poisons on their food to control pests.”
And I still think that gardeners need to be conscientious regarding use of chemical pesticides, etc.
But I found that marigolds did not prevent infestation by anything. Did they slightly mitigate it? I don’t know. Did I plant the right kind of marigold, or in sufficient density? I don’t know. But if they were 50% effective, it wouldn’t be enough to substantially save my plants.
So, my conclusion is: use the least potent pesticide effective, use best possible observation so one can act effectively, hand pick/crush where possible, don’t think that companion planting is a cure all, though it can add a substantial aesthetic pleasure. And, above all, keep reading scientific analysts like Robert!