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| 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.
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| 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.
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.
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).
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).
In addition to all these native predators, the introduced Asian lady beetle is becoming a serious enemy of monarch eggs and caterpillars.
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.
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).
Further readings:
Parasites Affecting Monarchs. University of Georgia
Parasites and Natural Enemies. University of Minnesota
A Butterfly's Flashy Colors.
List of articles
© Beatriz Moisset. 2012
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| 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.
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| Spined soldier bug. Podisus maculiventris. © Beatriz Moisset |
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| 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).
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| 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.
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| 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.
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| 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).
Further readings:
Parasites Affecting Monarchs. University of Georgia
Parasites and Natural Enemies. University of Minnesota
A Butterfly's Flashy Colors.
List of articles
© Beatriz Moisset. 2012
yikes! time to plant more common milkweed!
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