Sydney NSW, Australia
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Question about monarch butterflies

Hello Pestnet members

In October 2018 I reared a small number of about 20 Monarch Butterflies.Approximately 5 caterpillars had migrated down the Swan plants and ended up pupating under the shelter of the ledge of a raised vegetable garden that I have commandeered to grow Swan plants in my backyard. The remainder pupated on branches and twigs of the plants that been their food source for the past few weeks. 

They all matured and produced another generation of small caterpillars which were happily feeding at Christmas time. When I returned home from holidays on the 13th January I was astounded to find that there was about 170 chrysalises hanging under the ledge, and there were many more caterpillars about to pupate in the same location. An interesting observation was that only 20 chrysalises hung in the branches of the swan plants. Note how denuded of foliage the plants have become after supporting such a large population of hungry caterpillars.Only one silly caterpillar had affixed itself to a leaf petiole to pupate and ended up falling to the ground because another caterpillar had chewed through the petiole. Can any entomologist illuminate me regarding Monarch Butterfly behaviour?. Why did such a disproportionate number crawl down under the ledge to pupate?
  1. Were they following a pheromone trail laid down by previous generations.
  2. Is there a primal instinct such as a salmon migratory genetic memory imprint that is passed on from one generation to another that has caused this behaviour?
  3. What causes avoidance of pupating under leaves when there is such a dense population of caterpillars. Previously I have noted that there was far more pupation on leaf sites when the population was less dense.
I’d appreciate the insight and wisdom of the Pestnet group on these questions. Many thanks 

Frank Visser

Posted on user's behalf



Hi Frank 

I can give you my understanding and experience of studying insect behaviour in the field for 40 years. Last instar caterpillars of many moth and butterfly species move into protected habitats for obvious reasons: mainly to move away from exposed and ‘at risk’ habitats to ‘shelter’ from environmentally harsh conditions (sun, rain, wind, cold, winter conditions), move away from a disappearing habitat (the host plant maturing and senescing, etc.), and also to move away from foliage/plant-searching predators and parasitoids. Species like Helicoverpa armigera pupate in the soil, while species like Pieris brassicae behave just like your monarch caterpillars. Evolution-wise this is probably a very smart move (to protect your pupal/changing life stage) and has therefore become ‘natural behaviour’. 

Hope helpful.


Graham Walker
IPM Scientist (Entomology)International Development & Aid
T: +64 9 925 3522
M: +64 272083077
The New Zealand Institute for Plant & Food Research Limited
Postal Address: Plant & Food Research
Private Bag 92169, Auckland Mail Centre, Auckland 1142, New Zealand
Physical Address: Plant & Food Research
120 Mt Albert Road, Sandringham, Auckland 1025, New Zealand


Frank Visser <>

Many thanks Graham

However this doesn’t explain why within one generation there should be such a dramatic change in pupation location. I’m thinking that there has to be some message imparted to the local population to make this distinct change in choice of location en mass.

Previously we have had a few caterpillars seeking the shelter of the garden ledge to pupate, but mostly within the shelter of the swan plants branches, however this year the overwhelming ratio of nearly 10:1 in favour of the garden ledge was spectacular.

Now they are all hatching out and we have up to 10 butterflies at a time sitting on the garden box ledge drying out before taking flight. Truly a beautiful sight. 
Kind regards

Frank Visser