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Monarch Health

More than ever before monarchs need our help. In North America deforestation, pesticide use, GM crops and climate change are affecting numbers. Monarch populations are struggling here in NZ too: they are ravaged by social wasps, their major predator.


Commercial growers raise thousands of swan plants (a species of milkweed) and plants need to be in the best condition to be sold. Garden centres cannot sell plants covered with aphids, or bereft of leaves, so some growers use pesticides to protect their plants until they are sold.

Remember a female monarch can lay about 700 eggs (one was recorded as laying 1179!). Caterpillars grow almost 3000 times in size over two weeks… so one day there are lots of leaves and a day later the plant is just a stalk.

Some garden centres will not sell plants with pesticide on them and will also be able to advise on nectar sources and host plants for other butterflies and moths. The garden centres listed HERE go the extra mile to ensure their plants are caterpillar-friendly.

Approved By Butterflies Seal-FALook out for the 'approved by butterflies' sign on their door or merchandising material.

Always ask at your garden centre 'is this plant safe for caterpillars?' If that person doesn’t know, ask them to find out. Garden centre staff should be aware of how safe the plants are.

As well, here are some additional tips on how you can raise more magnificent monarch butterflies.

  • Grow your own plants from seed – that way you will know for sure whether the leaves are safe for caterpillars. Buy seed anywhere you see Yates products. Yates gives the MBNZT a donation for each packet sold. Plant seed now for next year.

  • Grow plants on from previous years so that they are in their second season and over 1 metre tall. If they are fertilised, watered and mulched, the growth in the spring and summer will almost keep up with the caterpillars.

  • When you buy plants, buy twice as many and protect some for next season’s monarchs. In the summer heat don’t plant the new plants – wait until the caterpillars have eaten the leaves, prune the dead stems, and THEN plant the the plant. The plants will be stressed enough being eaten without having to cope with being transplanted. Plant what’s left when the leaves have been eaten… but wait until it’s cooler (evenings) and keep it well-watered.

  • If you introduce new plants let the caterpillars move themselves to the new plant. Put the new plant next to the old one, or cut the stems with caterpillars on and lay the stems at the base of the new plant. New plant(s) may not immediately appeal to your caterpillars and it could be that the toxicity is at a different level to that in the plant it was eating. Water the new plant well to rehydrate it.

  • Seedlings that pop up in the wrong place can be left until you need food. Then cut the stem close to the roots and put it in a container of water, splitting the bottom of the stem before you do so it can better absorb water. Stand that next to the plant.

  • NEVER feed them alternative foods such as pumpkin, cucumber or other members of the cucurbit family. Monarch caterpillars are meant to eat milkweed (e.g. swan plant) and many will not eclose into healthy butterflies when fed the wrong food. As well, there is a risk that we are doing long-term damage to their internal systems.

  • Caterpillars shed their skin (moult) five times. Do not disturb them when they are moulting. They may choose to leave the plant as they sense that is where predators and parasites will find them. When they are moulting they will spin silk around themselves and disturbing them may kill them.   Observe the life cycle but minimise handling. Monarchs have been undergoing metamorphosis for hundreds of years without our help.

  • Pesticides such as fly sprays, plug-in insect controls and flea collars on pets can kill caterpillars. Sun-screen and cosmetics on hands can also affect them. New carpets and drapes can sometimes contain insect-proofing. Swan plants are NOT indoor plants. The humidity indoors could alter the toxicity level in the leaves and could poison your caterpillars.

  • When your plant is 15-20 cm tall pinch out the growing tip just above a node. The plant will now form two branches and bush out, creating twice as many leaves. A few weeks later pinch out the growing tip of the new branches. Also, a plant’s mission in life is to reproduce (set seed). Leaves feed caterpillars… flowers lead to seed. If you remove the flowers the plant continues to strive to fulfill its mission and as a result will feed more caterpillars.

  • If you are in a cooler area think about growing Asclepias incarnata or swamp milkweed. The plant will grow from a rhizome in the second and ongoing years, and will sprout soon after the last frost. It has a sweet but subtle vanilla scent, is very quick-growing and is a very popular nectar source as well. Seeds are on our ‘Shop’ page.

  • If you are in a tropical area, consider growing A. curassavica, the tropical milkweed. There are two varieties – one has scarlet/gold flowers and the other has yellow flowers. These add a bit of variety in your garden. Seeds are on our ‘Shop’ page.

  • If you have too many caterpillars or too many swan plants, consider putting a notice on our special Facebook Group: Monarch Matchmaker  or in a network such as ‘Neighbourly’. People will want to know what you have to offer, your neighbourhood and best way of contacting you. You might even be able to help out a local garden centre that has too many caterpillars on its swan plants!

  • Remember, swan plants and monarchs are poisonous so be careful when handling them. Monarchs store toxic steroids (known as cardenolides) from the swan plant and use them as a defence against some predators. The colour orange is a warning to say ‘I taste bad’.

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