Frequently Asked Questions

Egg/ Larva

Does the disappearance of a 5th instar caterpillar mean it was eaten?: While it is possible that a 5th instar caterpillar could have been eaten, the most likely reason for the disappearance of the caterpillar is that it has left the milkweed to pupate. After a caterpillar pupates, the chrysalis can be very difficult to find, making it appear as if the monarch is no longer at the site. Click here or here to see related questions.
Why do I see caterpillars crawling on the ground instead of on milkweed?: Monarchs are known to crawl from plant to plant for a number of reasons. This can happen when the original milkweed plant does not have many leaves, or if there is a lot of competition for leaves on a single plant, they will also leave in search of a plant they can have to themselves. Additionally, caterpillars will leave to escape the heat; sometimes they seek out shade on the ground during the hottest parts of the day. Late stage (5th instar) caterpillars will also often leave the milkweed plant to find a sheltered place to pupate, and will travel as far as 10 meters away!
How many eggs does a female monarch butterfly lay?: A female butterfly only lays eggs over a 2-5 week period. During this time, she probably lays an average of 300-400 eggs in the wild. Numbers in captivity are higher - about 500-700 depending on things like temperature and the conditions under which the female is kept. In captivity, monarchs have been recorded laying as many as 1100 eggs! Of course, most monarchs in the wild don't survive the egg and larva stage, but are eaten by predators and are an important part of their ecosystem. It is not necessary to bring monarchs in from the wild to raise and protect them, the most important thing we can to is create habitat! Read our handout Raising Monarchs: Why or Why Not? or watch our March 2019 webinar Monarchs in a Web of Life - Predators, Parasites, and Disease for more information.
Why does an egg turn black/dark before it hatches?: Just before a monarch egg hatches, the dark pigmentation of the monarch's head capsule develops. This is visible through the translucent egg shell and gives the egg its characteristic grey dot!
How can I tell if late eggs/larvae are part of the reproductive or migratory generation?: In short, the fall migratory generation isn't cleanly divided from the later summer breeding population. If a butterfly is reproductive, it won't successfully migrate and overwinter because reproduction requires a huge investment of their body/lipid reserves, so they don't live as long once reproductive. Reproductive diapause is when monarch's reproductive organs are delayed in maturing - they wait until the next spring at their overwintering grounds. Once their reproduction organs mature, they'll continue to lay eggs on milkweed until they die. This doesn't bode well for a window of late-season eggs/larvae with limited success in fully developing and completing the migration. That said, the 'pre-migration' phenomenon does show that reproductive monarchs are moving south and continuing to lay eggs. This doesn't mean they'll make it to Mexico and survive the winter. Because the last of the reproductive butterflies from July/August continue to lay eggs until their death, and the migratory butterflies are emerging at the same time, there is no exact time that all butterflies you see are either one or the other (reproductive or migratory).
What are monarch caterpillar’s tentacles and antennae for?: The caterpillar’s tentacles are sensory organs. Caterpillar’s eyesight is poor, and tentacles are tactile. They aid in navigation on the front. They may also play a role in defense/predator confusion on the rear, leading a potential predator to think that the monarch’s rear is its head. Caterpillar antennae (located near their mandibles) aid in smelling and are used to find food.
When a monarch lays eggs, are they all the same sex? Does the temperature or other environmental cues influence the sex?: As is the case in most animals, butterfly sex is determined when the egg is fertilized. Interestingly, in butterflies, it is the female gamete (the egg) that determines the sex of the offspring. In mammals, females have 2 "X" chromosomes and males have an "X" and a "Y" chromosome. If an egg is fertilized by a sperm carrying an X chromosome, it becomes a female (XX). If it is fertilized by a sperm carrying a Y chromosome, it becomes a male (XY). In butterflies (and birds), males are ZZ and females are ZW, so some eggs carry a W chromosome, and some a Z chromosome. There is no important difference between Y and W (and X and Z) chromosomes; scientists just use different letters to help differentiate species in which males and females determine the sex. Temperature and other environmental cues do not influence sex, and individual females produce both male and female offspring. There are some animals for which environmental cues determine sex (some reptiles, for example), and some that actually change sex during their development (some fish). But butterflies (including monarchs) and mammals are not in either of these groups.
How does a caterpillar choose a place to pupate?: When monarchs are in their chrysalis, they are vulnerable to predation by wasps and flies. It's important for caterpillars to find a spot that they feel secure from predators, as well as sheltered from wind and rain. Caterpillars do not usually pupate on their host milkweed plants. Instead, they move as far as 10 meters from their initial plant to a tree, another plant, or even the side of a house!
My monarchs are eating dill or parsley! What's going on?: What you are looking at is not a monarch, since monarchs only eat milkweed (genus Asclepias). We get this observation and question quite often, because the caterpillars of the black swallowtail butterfly resemble a monarch caterpillar. Black swallowtail larvae feed on a variety of different host plants, but cultivated garden plants like dill, parsley, celery, and fennel are a few on the list. If you come across a caterpillar that you don't know, try identifying the host plant that it is on. This may help you in determining what Lepidoptera (butterfly or moth) species you are dealing with!