Thursday, November 1, 2012

Monarchs and their Enemies


Male monarch butterfly. © Beatriz Moisset

We are told that monarch butterflies are well protected against predators and indeed they are. Their caterpillars feed on milkweeds, rich on highly toxic substances known as cardenolides. Having developed resistance against these chemicals, the monarchs themselves are poisonous so predators avoid them. A young, inexperienced bird that takes a bite off of a caterpillar or a butterfly spits it out in disgust and may be sick afterwards. It learns its lesson after no more than one trial.

Are monarchs completely free from enemies, then? Not so, there are exceptions. A number of predators and parasites have also become tolerant or immune to the cardenolides. Thus, they can feed on monarchs with relative impunity. The list of predators includes birds, mice and insects. Monarchs can also become victims of certain pathogens.

Robin. © Beatriz Moisset
Brown thrashers, grackles, robins, cardinals, sparrows, scrub jays and pinyon jays are known to feed on monarchs. Some of these birds avoid the body parts with higher concentrations of cardenolides by eating only the abdomens or by eating this kind of food in moderation. When monarchs arrive in their wintering grounds in Mexico they are plump with stored fats that will keep them through the winter. A whole new set of predators is eagerly awaiting them. Mice feast mostly on dead and dying butterflies that have fallen to the ground. Several species of birds, especially black-backed orioles and black-headed grosbeaks take a heavy toll on the millions of wintering butterflies. A few months after their arrival in Mexico, the monarchs may have lost a fair amount of toxins, making them more appetizing. It is estimated that between 7 and 40% of them fall victims to predation in their roosting grounds.

Small milkweed bug. © Beatriz Moisset

Invertebrates may be worse than mice and birds at decimating the populations of monarchs. Eggs, caterpillars and pupae are all vulnerable. The small milkweed bug doesn't just eat milkweed. It becomes carnivorous in occasions and catches milkweed caterpillars.

Spined soldier bug. Podisus maculiventris. © Beatriz Moisset
The spined soldier bug is an indiscriminate predator of many species and is not averse to feeding on monarchs. It impales caterpillars larger than itself and feeds on the internal fluids. (View of a spined soldier bug feeding on a monarch caterpillar).

Polistes paper wasp. © Beatriz Moisset

Paper wasps, Polistes, sometimes attack monarch caterpillars or pupae to feed their young. Recent reports suggest that this form of predation is more common than originally thought. (View of several paper wasps collecting food from a monarch chrysalis).

Asian lady beetle or ladybug. © Beatriz Moisset

In addition to all these native predators, the introduced Asian lady beetle is becoming a serious enemy of monarch eggs and caterpillars.

Tachinid fly. © Beatriz Moisset

Several tachinid flies and parasitic wasps lay their eggs on the monarch caterpillars for their larvae to feed on. All these insect predators and parasites probably consume a substantial number of monarchs before they even reach maturity.


Parasitic wasp, Pteromalid. © Beatriz Moisset

You may feel sorry for the monarchs; but this is the way of nature. Everything is interconnected and monarchs are not the exception. Sad as it seems, these butterflies serve a purpose by being part of the food chain. They produce enough eggs to survive and prosper despite the attacks. They could go on for thousands of years as they have already done, feeding on milkweeds and providing food for other forms of wildlife. The biggest threat to them does not come from those that feed on them but from the changes we are introducing in the planet.

Another frequent enemy of monarch caterpillars is a predatory stink bug, Stiretrus anchorago (Anchor stink bug).

Parasites and Natural Enemies. Monarch Lab
Parasites Affecting Monarchs. University of Georgia


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© Beatriz Moisset. 2012

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