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Helping Monarchs

7 February 2023

There are many people in NZ who love monarchs and want to see the numbers increase. But there are some important points to remember.

Monarchs are part of an ecosystem; they are part of the food chain. Everything in Nature has pests, predators and parasites. A basic understanding of ecology is so important.


In NZ, our wētā are mostly vegetarian. If you found wētā eating your prize lichen, you might not think too highly of them. In turn, wētā get eaten by tuatara. Tuatara get eaten by kāhu (harriers) and ruru (moreporks). And so on – an example of a food web.

Monarch butterflies are in fact pests of milkweed, as are aphids!

Because we love monarchs so much it is only natural to see their numbers increase. This can be as simple as planting more milkweed, such as swan plant, and nectar sources. Or you might want to develop a more intensive relationship – and it is very important that this is done responsibly.

The milkweed community

Milkweed (swan plant) is not just for monarchs alone. In a milkweed community you will also find aphids, Aphidius colemani wasps, milkweed beetles, the swan plant seed bug, the swan plant flower moth, social wasps, tachinid flies and many other species interacting – all living in relative harmony in the wild. As soon as one species arrives, one of its foes arrives soon after. Only the healthiest survive. And that is the way Nature works: survival of the fittest.

All these species can have diseases. Infectious diseases are caused by pathogens (including bacteria, fungi, protozoa and viruses). With monarchs you may have heard of black death, NPV (nuclear polyhedrosis virus) or Oe (Ophryocystis elektroscirrha). These diseases will limit the number of monarchs on a milkweed so that the milkweed has a higher chance of survival. The diseases are all a part of the natural cycle and only build up when there is overcrowding.

When you are raising a large number of monarch butterflies it is much easier for an infection to spread.

Sometimes deformities are not due to a disease but physical circumstances. When a monarch ecloses without sufficient room for it to expand its wings, or it falls while eclosing and is unable to climb up to extend its wings down, the result can be a deformed butterfly.

Without scientific intervention (lab, microscope etc) you cannot be absolutely certain as to the cause of the deformity. For the sake of the other butterflies and caterpillars it is important that a deformed or sick monarch is isolated as you do not want to spread any infection. It is essential that they are not kept as pets.

Monarchs are wild animals, and you are doing the species a disservice by keeping them. Contagions are invisible – it is so easy for them to spread.

Say no to pumpkin!

Do not feed monarch caterpillars anything besides milkweed (e.g. swan plant). In the past we have suggested that other vegetables in the cucurbit family are fine as food for hungry monarch caterpillars. While some of the butterflies may eclose and look perfect, there is no evidence as to the long-term biological effects. Long term, it might make them more vulnerable to predators, for example. It is important that if you are going to be involved in raising more monarchs, that you do so responsibly and plan ahead. Yes, accidents do happen – ask any farmer!


The recommended way of euthanasing a monarch butterfly is to put it into a container and put the container into the freezer. Within a few minutes they are dead, and you have reduced the likelihood of other monarchs being affected.


If you are raising a large number of monarch butterflies, to reduce the likelihood of diseases you need to have a high standard of hygiene. Dispose of spent milkweed stems, tissues etc responsibly. Use a bleach solution to clean rearing containers regularly – follow the manufacturer’s directions. And rinse everything well.

Remember also that pesticides like plug-in pest controls and flea collars on pets are insecticides and may well affect your monarch caterpillars.

Outdoor Plants

If you suspect that a swan plant may be carrying the disease, i.e. that monarchs have left a disease on the leaves and as a result monarch caterpillars are picking up the disease, you can disinfect the plant.  Do this in a cooler time of the day:  Use a 20% bleach solution and spray on the plant. Also remember to spray the undersides of the leaves as well. Leave it on for 20 minutes and then wash off.

Saving Seed

If you want to save seed to share - or to help us with our fundraising - these instructions will be useful.

Instructions for seed collection


13 comments on “Helping Monarchs”

  1. I live in Carterton. Our butterfly garden is in planning stage so we had a large number of swan plants in the shade house which butterflies found and about 50 monarchs have hatched so far. Where would they winter? And is there a tree we can plant to give them a wintering place?

    1. Hello Jo

      Monarchs usually overwinter in very large (old) trees, especially those where they can leave behind a pheromone from previous years it seems. Quite often they are trees we call "macrocarpa" (Cupressus macrocarpa or Monterey cypress) and sometimes Eucalyptus (Eucalyptus globulus), pines, sheokes, pohutukawa. All of these trees have waxy leaves or needles.

      I suggest you put out a call in a couple of months around Carterton, for sightings in parks or on golf courses. You could try a local newspaper or a community Facebook group. We haven't heard of any major overwintering sites near you but would love to know if you're successful!

  2. I am still trying to find some caterpillars....has anyone else reported the dearth of butterflies in parts of Auckland since the storm. As previously mentioned, I release between 50-150 a year and there has not been a sighting since anniversary weekend. I find this alarming. I have been growing butterflies for twenty years.

    1. Hello Truda - there are still monarchs flying around parts of Auckland, but it does seem as there is less than usual. I posted on "Monarch Matchmaker" on Facebook as I could handle some caterpillars (an added attraction at an event I'm organising at the Blockhouse Bay Recreational Reserve next weekend) but no-one seems to have too many.

      I wonder if the cyclone HAS affected the numbers? However, in Hawkes Bay it would seem that there are more caterpillars than there are plants - and the cyclone was worse there than here in Auckland.

    2. I live in Hawkes Bay, I had 6 large trees of swan and had hundreds of butterfly last year and then the Storm, this year they have no returned. I have now seeded 200 plants, and I have about 100 plants ready to plant. Very sad they didn't come back but yes may have died in the Storm.

  3. This last weekend we ha hundreds of monach butterflies in and around our shelter belt and garden, never seen anything like it. Eating on bottle brush tree and covering shelter belt. We were in awe and watched them dance around. We don't have swan plants anywhere.

  4. We have plenty of caterpillars on our swan plants but the plants have only few leaves.
    What can we feed the caterpillars with? Any other plant to introduce?
    A colleague suggested pumpkin but reading your article I know it's not good for them.

    1. Check out the Monarch Matchmaker page on Facebook - add your own post with location and somebody might be able to help you. (If you don’t use, Facebook, one of your friends will probably be able to post on your behalf.)

  5. Hi there, has anyone else noticed monarch caterpillars being poisoned and sucked dry by a brown stink bug. I live in the King Country and despite covering my plants I've witnessed even very big caterpillars being killed in this way. It's not a normal green bug but I don't think it's the dangerous brown marmorated one either even though it does have some similarities.

  6. Hi. I am in Havelock North and have 12 monarch chrysalis hanging in my kitchen and am now wondering where I will let them go as it’s getting cold and wet. Can anyone help my with my next move.

    1. Hi Joanna. We think it’s cold, but Monarch do just fine in the cold and even rain so long as it’s not too bad, i.e. a snowstorm or a gale. I suggest that when they You put them outside in the sunniest And most sheltered position you can find. Out of any strong wins or heavy rain Don’t expect them to fly away immediately, it might take up to 24 hours They should be fine They do not need to feed in the first 24 hours or maybe even two days They are still settling down

      Hope that is helpful

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