Monarch butterfly parasites

I have a problem, Houston! 

Dead monarch butterfly pupae - cause unknown

Recently I have had an entire cohort of chrysalises die. I don't know the cause of their demise, but perhaps it was tachinid flies, but at no time did I observe the larvae emerging on their sticky bungee cords. This is what the dead chrysalises look like. I found another three after taking this picture. Such a shame! 100% of the pupae have died. I posted the photos to Monarch Health's Facebook page and asked if they had any thoughts, to which they replied: "It could be one of a variety of pathogens/parasitoids, possibly Nosema bacteria, nuclear polyhedrosis virus, or parasitoid flies (tachinids) or wasps (chalcids)".

In this previous post, I wrote about predators that feed on monarch larvae and butterflies, specifically paper wasps and praying mantises. Today I bring to you the horrors of parasites and pathogens that use our favorite invertebrate as an unwilling host on which to feed, especially in the larval (caterpillar) and pupal (chrysalis) stage of its life-cycle.

Chalcid wasps and tachinid flies both lay eggs on or inject eggs into the monarch caterpillar. Once the eggs hatch, the larvae feed on the insides of the caterpillar. Tachinid flies time their own life cycle so that they pupate within the chrysalis and make their way through the chrysalis wall and drop to the ground from sticky threads. All sounds very yuky, doesn't it! 

Tachinid flies are typically identified by their red eyes. Here are some images of them from Flickr and Google. I actually saw one this morning on one of the milkweed plants near three caterpillars! And here are some images for chalcid wasps from Flickr and Google. Recognize any of these monsters from your own garden?

  

In the reading about parasites and pathogens that I have been doing today I have seen research that indicates that the greater and more concentrated the milkweed and caterpillar population, the greater the level of predation and concentration of parasites. I currently have a lot of milkweed plants in pots close together and I have also been bringing final-stage caterpillars from the milkweed at the front of the house to the back of the house to avoid them wandering into the street when they head off to pupate. From the density of milkweed and caterpillars there are plenty of signals to predators and parasites that host larvae are present. Perhaps I just made an easy target for the wasps or flies that might have parasitized the caterpillars, or if one had an infection, by placing them all on the same plant, the infection was spread. A bit like putting the guy with TB in the middle of a crowd of healthy people! I am learning more and more that it is best not to meddle. 

Asilidae (Robber fly or Assassin Bug)

I also caught this picture of a "robber fly" or "assassin bug" (Asilidae family) in the garden recently. These flies have a deadly proboscis that lances the caterpillar and injects fluids that immobilize the caterpillar and liquefies its insides for ingestion. More yuck! Aren't you glad you're at the top of the food chain?

Robber flies  (Asilidae) are very bristly. Their legs have long, sharp spines to hold onto the prey. Their faces have a dense coat of bristles, called the mystax, presumably to protect them from the legs and mandibles of struggling prey.

Check out more images of "Asilidae" at Flickr. 

I currently have another large batch of 5th instar caterpillars that are about to pupate. Let's hope that this will not be a repeat of the prior batch and that they will all emerge as healthy and beautiful butterflies.

Here's a good article on monarch butterfly predators and parasites from The Monarch Program: http://www.monarchprogram.org/common-monarch-predators-and-pathogens/.