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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.

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.

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.


7 February 2023

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