3 years ago



Photo © Jeffrey E.

Photo © Jeffrey E. Belth Figure 3. Monarch caterpillar, fifth instar, chewing on common milkweed leaf. Photos © Jeffrey E. Belth Figure 4. Monarch chrysalis in the process of development. Monarchs lay their eggs only on plants in the Apocynaceae (dogbane family) in the milkweed subfamily Asclepiadoideae, genus Asclepias (L.) and related genera. Many milkweeds defend themselves from generalist herbivores by exuding sticky, bitter-tasting latex from cut leaves and other plant parts, and by producing compounds such as cardenolides that are toxic to many animals, including most vertebrates. Larvae of some milkweed butterflies are specialized to tolerate latex and accumulate cardenolides and/or other secondary compounds of the host plants into their bodies. They use the plant’s chemicals for their own defense against predators (Brower 1984), for pheromone production, and for other specific functions during their lifecycle (Brower et al. 2010, Agrawal et al. 2012). Monarch ESA Petition 20

After mating a female must soon find milkweed plants of a suitable species on which to lay her eggs. Some milkweed-family species have such high levels of toxins that even the larvae of milkweed-adapted species such as monarchs will not thrive (Zalucki et al. 2001a, b). Other milkweed species have such low cardenolide levels that larvae and subsequent adults may not be chemically protected from predation (Lynch and Martin 1993). Nutrient content of milkweeds varies with environment, and declines during the season (Oyeyele and Zalucki 1990, Agrawal et al. 2012), so a female needs to locate healthy plants young enough to support the full development of her offspring. Eggs are laid singly, on the underside of a young leaf or on a flower bud. The eggs are creamcolored or light green, ovate to conical in shape, and about 1.2 by 0.9 mm in size (Figure 2). The eggs weigh less than 0.5 mg each and have ridges running longitudinally from the pointed top to the truncated base. Eggs take three to eight days to develop and hatch into larvae (caterpillars). Larval monarchs take nine to 14 days to go through five instar stages before pupating. Instar stages can be distinguished by larval coloration and tentacle length, size of the head capsule, and other characteristics (Details of life history stages in this and following paragraphs, unless otherwise noted, are from the Monarch Larva Monitoring Project “Larval Field Guide,” available at:; and the Larval Monitoring Handbook, available at: The first instar larva, just out of the egg, is solid pale green and translucent, without banding coloration or tentacles. It eats the nutritious egg capsule first, and then uses a circular motion to eat milkweed leaf tissue without eliciting an overwhelming amount of latex that could entrap it. After the first molt, the second instar larva develops a characteristic pattern of white, yellow and black transverse bands. The opaque body is covered in short setae, and pairs of black tentacles start to grow, one pair on the thorax and another pair on the abdomen. The third instar larva has more distinct bands, particularly on the abdomen, and the two pairs of tentacles continue to elongate. Legs on the thorax differentiate into a smaller pair near the head and two larger pairs further back. These third-stage caterpillars begin to eat along leaf edges. The fourth instar is characterized by a new banding pattern on the thorax, and white spots on the prolegs near the back of the caterpillar. The fifth and last instar larva (Figure 3) has a more complex banding pattern and white dots on the prolegs, with front legs that are small and very close to the head. The fifth instar is large relative to the earlier instars; the body is 25 to 45 mm long and 5 to 8 mm wide, compared to the tiny first instar that is only 2 to 6 mm long and 0.5 to 1.5 mm wide. The body mass of fifth stage caterpillars has increased about 2000-fold from first stage instars. Fifth stage instar larvae often cut the petiole or midrib of milkweed leaves to restrict the latex flow so that they can eat more leaf tissue to support the last growth period before pupation. Larvae must eat constantly to ingest enough milkweed to increase in mass so dramatically within a few weeks. Larvae in the final stages of development stop feeding to search for a location to form a pupa, or chrysalis, the last stage of development before the emergence of the adult butterfly (Figure 4). The fifth stage larva attaches itself securely to a chosen leaf or branch with a silk pad, latching on with its hind legs and hanging down. The larva then molts to reveal an opaque, blue-green chrysalis adorned with gold dots. At normal summer temperatures, adult morphology develops Monarch ESA Petition 21

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