Some trials and tribulations
by John Anderson – A Letter from Grandpa Nature
Monarch butterflies are distasteful to birds. They contain alkaloids called cardiac glycosides which make any bird consuming one retch, vomit, drool copiously and, in general, act very, very sick. The butterfly gets its chemical defenses while in the larval stage. As a caterpillar it eats milkweeds containing alkaloids and accumulates them in its body. When metamorphosis occurs the distasteful alkaloids end up in the butterfly. Therefore both monarch caterpillars and monarch butterflies are distasteful to birds. Any bird that eats one will get so sick that it will very likely never eat another.
Since we know about the monarch’s toxic defense it’s easy to imagine that they live rather carefree lives unbothered by predators. But that isn’t so. The alkaloids that make them distasteful to birds provide little or no protection against insect and arachnid predators and there are a surprising number of both. Mostly they target smaller, early instar monarch caterpillars.
I have seen pictures of both Crab Spiders and Jumping Spiders eating monarch caterpillars. Crab spiders are ambush hunters that wait for prey to blunder into their grasp. Jumping spiders are active hunters, leaping on prey from a distance. Like all spiders these species liquefy and ‘drink’ the internal parts of their victims.
There are also several of the true bugs that will feed on monarch caterpillars including Spined Soldier Bugs, Wheel Bugs, Ambush Bugs and Assassin Bugs. All are voracious predators that stab prey with their piercing mouthparts and drink their liquefied innards. I have seen spined soldier bugs feasting on caterpillars but all of these bugs are generalists. They will eat any caterpillar, not just those of the monarch.
Some wasp species are detrimental to monarch caterpillars. Paper wasps are hunters that kill and chew up Paper Wasp insects including monarch caterpillars to feed to their own wasp larvae. Ichneumon wasps are internal parasites. The female wasp injects her eggs into the Ichneumon wasp living caterpillar. The wasp grubs that hatch from those eggs eat their host from the inside, not killing it until they are finally old enough to pupate.
In the southern states that the monarchs have to colonize annually as populations move north, Fire Ants are a scourge, eating any and all insect life. Areas that have fire ants have almost no insects including monarchs.
Other monarch predators that were once native to Europe were intentionally introduced on this continent as biological controls for various destructive insect species. A beetle known as the Forest Caterpillar Hunter was released to control Gypsy Moth and Browntail Moth caterpillars. It is established in New England, south to Virginia and west to Pennsylvania. Another called the Golden Worm and Slug Hunter was also introduced to control gypsy moth caterpillars. It proved to be a generalist willing to feed on any caterpillar as well as on earthworms. It is mostly found in New England. Both have negatively impacted monarch populations.
Worse than either beetle is a fly which was also introduced to control gypsy moth caterpillars. This fly is known only by its scientific name, Compsilura concinnata. It is a most unremarkable creature, slightly smaller than a house fly and is an internal parasitoid on caterpillars. Unfortunately this fly produces four broods a year only one of which is synchronous with the emergence of gypsy moth caterpillars. The remaining three broods have to find other caterpillar hosts to feed on including monarchs.
All of these introductions occurred many years ago when the potential unintended impacts on other species including monarchs were not as well understood. But now the beetles and the fly are here to stay. Whatever impacts they may have are now integral parts of the world that we and the monarch caterpillars inhabit.
Monarch caterpillars are also susceptible to fungal attack, hosts to internal parasites and to non-lethal protozoan spores that may ultimately decrease their survival rates. But, the most lethal predator of monarch caterpillars is, oddly enough, milkweed. Milkweeds do not want herbivores such as monarch caterpillars eating their leaves. As a deterrent they produce the very alkaloids that make monarchs distasteful to birds and they produce extremely sticky latex sap. Monarch caterpillars have evolved to tolerate and even utilize the alkaloids but newly hatched monarch caterpillars are often engulfed by the sticky latex sap that gushes from a leaf when the tiny caterpillar starts to feed. One researcher estimated that 60% of newly hatched caterpillars were victims of milkweed sap during their very first attempt at feeding. They were killed by the milkweed’s defenses. And, they only eat milkweed. They have no choice but to eat it.
Even after all of the dangers that a monarch caterpillar must face it has been estimated that 10% will grow to full size, pupate and become butterflies.
Under normal circumstances, insects can stand heavy predation because they reproduce explosively. A female monarch may lay as many as 400 eggs. If 200 of those eggs produce females and each of them lays 400 eggs … well, you get the picture. Insects reproduce in great numbers and are eaten in great numbers. And, generally, enough survive to carry on the species.
However, researchers who estimate populations of overwintering monarchs in Mexico by counting the number of hectares of oyamel trees heavily covered with dormant butterflies believe that populations have been in decline for decades. There have been numerous theories put forth to explain their decline but no smoking gun has emerged. No single cause.
The monarch, Danaus plexippus, is probably one of the most studied species in the world. We know many ‘facts’ about it but we’re not even close to understanding its multigenerational continent spanning realities, and we may never be. And we’re not even close to understanding our impacts on the species. But the more I read the more I respect the monarch, Danaus plexippus, AKA The Wanderer. It’s a survivor. At least so far.