As Mission Monarch coordinator, I’m often asked: So where are the monarch caterpillars? We had a bunch on our milkweed plants a few days ago – but there’s nothing there now.
There are a number of possible explanations. On the one hand, it happens that caterpillars leave plants for different reasons, whether to find new stems to munch on or simply a new place to make their chrysalis. But on the other, we tend to forget that over 90 percent of monarch eggs and caterpillars die before reaching the adult stage. Although different factors may be involved in the premature death of eggs or caterpillars (the weather, pollutants and interactions with the host plant, among others), monarchs are also not exempt from illnesses and predators.
Monarch caterpillar attacked by stink bug.
Crédit : Space for Life (André Payette)
Who are the monarch’s predators?
The monarch becomes toxic as it grows by storing milkweed cardenolides in its tissues. That toxicity provides it with a degree of protection against a range of predators. The example of a blue jay eating a monarch only to regurgitate it after ten or twelve minutes is well known! Nevertheless, new studies have shown that several species of arthropods (like insects and spiders) can attack monarchs in their natural environment. In 2019, researchers in fact reported 36 previously unreported monarch predators, including certain orthoptera (crickets), dermaptera (earwigs), hemiptera (bugs), spiders and even a lepidopteran (the milkweed tussock moth caterpillar).1 Even more surprising, some herbivorous arthropods, mostly grasshoppers, seem capable of feeding on small monarch caterpillars, sometimes accidentally (while eating milkweed leaves), but also intentionally. Also, with the help of surveillance video2, scientists have been able to lift the veil on the community of nocturnal arthropods that attack monarch eggs and caterpillars. Their assessment is that 74 percent of attacks directed against monarch eggs take place during the night.
The milkweed tussock moth caterpillar may inadvertently eat eggs or small monarch caterpillars found on leaves.
Space for Life (Sonya Charest)
Should we protect monarchs from these attacks?
This is a question that participants in the Mission Monarch program often ask. And it’s normal to wonder about it, the monarch population having been in decline for the last 25 years – but is breeding monarchs the right solution for preventing eggs and caterpillars from being exposed to predation?
Although it may sound counterintuitive, our opinion is that it’s better to leave them in their natural environment precisely so they can be exposed to these predation pressures. The reason behind our decision: natural selection. It’s through natural selection that the characteristics of the highest-performing individuals are maintained in the population and that a species can adapt to changes in its environment. If we prevent natural selection from doing its work, we’re possibly causing the monarch more harm than good.?
As human beings, let’s do our part to help the monarch butterfly by taking every step we can concerning the human-made factors that contribute to its decline (habitat loss, pesticide use, pollutant discharge, and so on) and by participating in various programs such as My Space for Life Garden and Mission Monarch.
In the case of predation, it has to be understood that this is a natural phenomenon the monarch has always been confronted with. So, let’s allow nature to do its work, as it has done so well for millions of years.
1 Hermann, S.L., Blackledge, C., Haan, N.L., Myers, A.T. & Landis, D.A. (2019) Predators of monarch butterfly eggs and neonate larvae are more diverse than previously recognised. Scientific Reports, 9, 14304.
2 Myers, A.T., Haan, N.L. & Landis, D.A. (2020) Video surveillance reveals a community of largely nocturnal Danaus plexippus (L.) egg predators. Journal of Insect Conservation, 1–7.