Have you run out of milkweed (swan plant) and been tempted to feed them an alternative? Please don't do it.
Monarchs are milkweed butterflies, and their host plant is milkweed.
Most butterfly species are 'linked' to a single plant species, which is the food source for their larval form (caterpillar). These plants are referred to as host plants.
Milkweed / Swan Plant
When monarch caterpillars eat milkweed they absorb a variety of chemical compounds that make monarch caterpillars poisonous to potential predators. Milkweeds contain a cardiac poison (cardiac glycoside) that is poisonous to most vertebrates but does not harm monarch caterpillars. Various milkweeds have higher levels of these toxins than others.
In fact, cardiac glycosides are used in human medicine for treating heart failure and certain types of irregular heartbeats.
Have you watched a monarch laying eggs on your swan plant. Firstly, she tests the leaf for its suitability with her two forelegs called brushfeet. If it doesn't have the right balance of nutrients she moves on to another plant.
When we introduce hungry caterpillars to another food such as pumpkin or any other species in the cucurbit or gourd family, the caterpillars may eat it but they are not ingesting the food that they need. It's a bit like eating junk food ... our healthiest athletes don't eat junk food.
Asclepias and Gomphocarpus are a genus commonly known as milkweeds. They are in the Apocynaceae family. Pumpkins or squash are an entirely different genus (Cucurbita). Another plant which has been trialled as a host plant for monarchs in the past is Araujia sericifera, a fine which is a pest plant in NZ. It is closer to the milkweed genus, but not close enough to be a successful host plant. As well, A. sericifera has the common name of 'cruel plant' because it traps butterflies and moths by the proboscis, and they will be unable to free themselves.
Many of us have tried using alternative foods in the past. Monarchs fail to pupate or die in the latter larval stages. Butterflies emerge as weak specimens, with crippled wings, or unable to fly. And as well as those visible symptoms, we do not know what long-term damage we are doing to the species as a whole.
What to do?
This makes life difficult, doesn't it. You have too many caterpillars and not enough swan plant. There are some excellent tips on how to best utilise the swan plants you do have on our website here.
As well as those tips, NOW (when you are reading this) is the time to plan for swan plants for next year. Plant them now and keep them covered so you'll be able to see more magnificent monarchs next year.