Monarch – 4th Instar
This is the 4th instar stage Danaus plexippus larva (monarch caterpillar). It is 11 days post hatch and 26.2mm long.
Very few eggs laid actually make it to become pupae, let alone successfully pupate into healthy adults. The sources of mortality are many and come at different stages of the life cycle. Monarch eggs and larvae are routinely eaten by several insects and arachnids, and parasitized by flies and wasps.
One predator of D. plexipus is the non-native Asian lady beetle, Harmonia axyridis, which was intentionally introduced to the United States as a biological control as far back as 1916. H. axyridis preys primarily on aphids as many other lady bird beetles, but it also eats other soft bodies insects such as larval stages of Lepidoptera. H. axyridis has become a dominant generalist predator in many agricultural areas of the United States where common milkweed and D. plexipus are common as well.
In a 2003 paper published in the journal Biological Control, R.L. Koch et al. quantified the predation of H. axyridis on D. plexippus in both laboratory and controlled field experiments. They found that the 3rd instar lady bug larvae preyed on monarch eggs at a rate of up to 25 eggs per larvae per day. The rate of predation on 1st instar D. plexippus larvae was as high as 15 per H. axyridis per day. The adult H. axyridis also preyed on the eggs of D. plexippus at a rate that is a linear response to the prey density.
Though the field and lab experiments were conclusive that H. axyridis will eat significant numbers of eggs and 1st instar larvae of D. plexippus, these trials were conducted without aphids present. The availability of aphids may reduce predation rates on both eggs and 1st instar monarch larvae.
In our own garden we have seen a single D. plexippus egg hatch outside with a 1st instar larvae that was not seen after the first day. We “rescued” one viable egg and raised it indoors as an experiment for our son. We counted 14 other eggs which were positively ID’d and examined under dissecting microscope. All were found to have been penetrated with a slit cut into the wall of the egg and the egg insides eaten, often with a trail left on the outside of the egg.
In obtaining food for the 4th and 5th instar stages of the monarch, we cut a common milkweed plant at its base and brought it home. Each leave was examined for eggs, mites and fungus then washed and stored in a ziplock with dry papertowel in the refrigerator. That single plant had a further 16 egg remains on it. Each of which had a narrow slit through the case.
- Lepidoptera (Butterflies and Moths)
- Danaidae (Milkweed Butterflies
- Danaus plexippus
R Koch (2003). Susceptibility of immature monarch butterfly, Danaus plexippus (Lepidoptera: Nymphalidae: Danainae), to predation by Harmonia axyridis (Coleoptera: Coccinellidae) Biological Control, 28 (2), 265-270 DOI: 10.1016/S1049-9644(03)00102-6