Egg Mk. III, originally uploaded by eclectic echoes.
Egg Mk. I hatched outside, but the caterpillar disappeared when only 1-2 days old (presumed eaten)
Egg Mk. II I believe was the victim of a parasitic wasp or fly. Under the scope the yellow mass outside the empty, but non-eaten, egg looks to be the remains of a caterpillar that has exploded(?)
This egg is literally just a speck. This shot (a 1:1 pixel crop from the large size) was with a 100mm Macro, a 1.4x teleconverter and 48mm of extension tubes.
The egg itself is 0.5 mm (0.02″) in diameter and ~1.0 mm (0.04″) in height.
We watched the butterfly that laid an earlier egg (Egg Mk. I) on these same plants. She went from plant to plant sampling the nectar and exploring the leaves. She chose what appeared to be the healthiest of the four plants in the garden to lay her egg on. We judged it as the most healthy because when it was recently transplanted it had the largest most robust root system and it had recovered faster from the transplanting. When Egg Mk. I was laid all the plants, but one, were visually the same and appeared fully recovered from transplanting. The recent two eggs were similarly laid on the same plant. What made all three females choose the same plant?
Butterflies have complex chemoreceptors on their tarsi (roughly analogous to feet), which allow them to taste the flowers and leaves they land on. Several researchers have explored the chemical cues for oviposition (laying of eggs) in monarchs. They have found that certain flavinoid compounds in host plants play a role in stimulating oviposition in butterflies. When a female butterfly lands on a milkweed plant such as our butterfly weed (Asclepias tuberosa), using the tarsi on her forlegs and mid legs and using chemo receptors on the tips of her antennae, she can “taste” the species of plant, it’s age, physiological condition and key compound concentrations.
Monarch butterflies, like all Danaidae butterflies, lay their eggs on, and as larvae feed on, milkweed plants of the genus Asclepias. One characteristic of these plants is that they all contain cardenolide glycosides, a toxic steroid that can cause heart arrest in most vertebrate animals (notable exceptions include the black backed oriole which is a primary predator of monarch butterflies). Monarch larvae take up the cardenolides and sequester them in their body, so it is an important compound for the survival of the larvae and adults. In too high a concentration however, it can negatively impact larval development and even cause larval mortality, especially in the first instar stage. Too low a concentration and there is little or no actual chemical defense afforded to the butterfly, too high a concentration the butterfly risks increased mortality and deformity.
So our female butterfly was carefully selecting which plant to lay her egg on, to give the larvae the best chance of success.
- Lepidoptera (Butterflies and Moths)
- Danaidae (Milkweed Butterflies
- Danaus plexippus
Robert Baur, Meena Haribal, J. Alan A. Renwick, Erich Stadler (1998). Contact chemoreception related to host selection and oviposition behaviour in the monarch butterfly, Danaus plexippus Physiological Entomology, 23 (1), 7-19 DOI: 10.1046/j.1365-3032.1998.2310007.x
M. P. Zalucki, L. P. Brower, S. B. Malcolm (1990). Oviposition by Danaus plexippus in relation to cardenolide content of three Asclepias species in the southeastern U.S.A. Ecological Entomology, 15 (2), 231-240 DOI: 10.1111/j.1365-2311.1990.tb00804.x