Monarchs – The Hazards of Life in the Wild

Debby Luquette
Adams County Master Gardener

(8/24) Everyone seems to be excited about the monarchs! Several people have told me they are seeing more of them this year. A few friends have even hand raised dozens. One friend confessed he raised them because of their high mortality rate in the wild - as high as 90%. ‘But why is that?’ he asked. That set me to looking and I am happy to share the answer. There is a lot of good research-based information about natural and human causes of monarch mortality on the internet.

Natural causes – predators, diseases, and even milkweed plants - eliminate large numbers of caterpillars. Monarch females can lay an average 300 – 400 eggs which hatch in about five days. Each larva emerges and then progresses through five instars (stages of development) which can last 9 – 14 days, depending on temperature, and they are exposed to danger all the time. We know that in a balanced ecosystem there will be predators looking for food. Mother monarch lays her eggs, a single egg per leaf, only to lose a few eggs to predators looking for a meal. Ants and lacewing larvae (yes, the ones we hope will eat our ‘bad bugs’) have been observed eating monarch eggs in natural settings.

When a monarch egg hatches, the hungry little larva starts munching away on its host plant, milkweed. We have heard that munching on milkweed protects monarchs from predators because it contains a chemical that makes the milkweed sap white, sticky and distasteful (a group of steroids called cardenolides). But the smallest and youngest larvae are vulnerable to predators such as mites, spiders, ants, wasps and true bugs until they have ingested and stored enough of the yucky stuff. The tiny larvae can also get trapped in the sticky sap and die! Older larvae can escape this fate by biting the main leaf vein at the point where it leaves the stem, allowing some of the sap to leak out. Eating the leaf from the edge inward also helps the larvae avoid too much sap.

The latex eaten by monarch caterpillars is stored in their outer skin. That should keep them safe, right? This defense works best against larger predators, mammals and birds, but there are smaller predators that can bypass the skin. Trachinid flies are parasitoids that lay their eggs inside the monarch caterpillar and their larvae ingest the monarch’s insides. Mites and other small insects, including some wasps, bugs and beetles, also get under the skin of the monarch larvae and eat the insides. And if this wasn’t enough, there are diseases and parasites that attack monarch caterpillars, as well.

Then we can talk about how we humans cause trouble for monarchs. Ideal monarch habitats are open fields with lots a flowering plants, including milkweed, among the grasses. Before North America was colonized by Europeans, this habitat was found in the Great Plains and in patches opened for farming by Native Americans in the forested East. Once the colonists started opening the eastern forests for pastures and crops, more monarch habitat became available, resulting in a larger monarch presence. In the last decades, increasing the amount of land needed for development has decreased the habitat of monarchs, though many gardeners helped ease the situation by adding high quality food in their yards. (Watch how you use pesticides, and be careful not to use seeds or transplants treated with systemic pesticides, including neonicotinoids.) Invasive plants have also taken a toll the native plant community by displacing the milkweed and nectar plants the monarchs need.

Agriculture presents another set of problems; chemical applications used to grow food, and lately genetically modified (GM) plants, adversely affect the monarch population. Fertilizers play a small role, decreasing some of the food value of milkweed for the caterpillars. Insecticides are generally indiscriminate in their effects; a chemical that is meant to eliminate ‘worms’ that eat crops, such as corn earworm, also kill monarch caterpillars. Interestingly, one of the first anticipated problems with GM plants involved corn carrying the gene to produce a natural larva killer, the BT protein. Remember BT corn? In the lab, milkweed leaves sprinkled with corn pollen from BT corn did kill monarch larvae, but in the field monarch mortality was not shown to be significant. However, plants modified to withstand herbicides are indirectly causing high monarch mortality. When herbicides that kill the ‘weeds’ but not the crop are applied to fields, the chemical drifts to the field edges where it kills the milkweed and wildflowers, causing a lack of food for larvae and adults.

One article I read discourages well-meaning folks from large-scale rearing of monarchs because of accidental outbreaks of diseases The authors do acknowledge that hand rearing monarchs is a valuable educational experience as well as being personally satisfying. Certainly, carefully raising a few by hand is fine; the problem is large scale rearing and commercial operations. Among the problems listed in the statement include monarchs possibly bringing diseases into the wild, some butterflies being less fit for life in the wild and passing those genes to the next generation, and interfering with ongoing research.

There isn’t much time left this year to enjoy watching these beautiful creatures that managed to escape so many dangers to reach adulthood. They are on their perilous journey to Mexico, where we hope they will find a hospitable forest to overwinter. In the meantime we can look forward to the return of the winter survivors, planning a garden with food available for both larvae and adults.