Monarch caterpillars eat milkweed and only milkweed. The genus Asclepias is the primary host across North America, though monarchs that have colonized other parts of the world also use closely related plants in the milkweed family. This obligate relationship goes beyond simple nutrition: the toxic compounds in milkweed leaves are both a hazard the caterpillar must manage and a chemical arsenal it steals for its own defense. Which milkweed species a caterpillar feeds on turns out to shape everything from how fast it grows to how well it resists parasites.
Milkweed Species That Monarchs Use in North America
More than 70 species of Asclepias grow across North America, and monarchs will feed on many of them, but not all species are equally good hosts. A study that raised caterpillars on nine milkweed species common in the Midwest found that survival from hatching to adulthood ranged from about 30% to 70% depending on the species, with an average around 58%. The best performers included poke milkweed (A. exaltata) and butterfly weed (A. tuberosa), while tall green milkweed (A. hirtella) and prairie milkweed (A. sullivantii) produced significantly lower survival rates.1PubMed Central. Milkweed Matters: Monarch Butterfly (Lepidoptera: Nymphalidae) Survival and Development on Nine Midwestern Milkweed Species Common milkweed (A. syriaca), the species most people picture when they think of monarchs, falls somewhere in the middle and is by far the most widespread host across the eastern half of the continent.
In the western United States, the lineup shifts. California monarchs lay eggs on narrow-leaved milkweed (A. fascicularis), showy milkweed (A. speciosa), and desert milkweed (A. erosa), among others. Lab tests with California desert milkweeds showed that newly hatched caterpillars were least attracted to A. fascicularis but showed no strong preference among the other three species tested.2Journal of Insect Behavior. Larval Host Choice of the Monarch Butterfly (Danaus plexippus L.) on Four Native California Desert Milkweed Species That said, A. fascicularis is still commonly used in the wild because it is so abundant, which illustrates a point that recurs throughout monarch ecology: what caterpillars eat in practice often comes down to what their mothers can find.
Why Only Milkweed
Milkweeds produce a group of toxic compounds called cardenolides, which are found across all Asclepias species in varying concentrations and structural diversity. These toxins interfere with a critical enzyme in animal cells, one that maintains the sodium-potassium balance needed for muscles and nerves to function.3PubMed. Toxicity of Milkweed Leaves and Latex: Chromatographic Quantification Versus Biological Activity of Cardenolides in 16 Asclepias Species For most insects, eating milkweed would be fatal. Monarchs carry specific genetic mutations that make their own version of this enzyme resistant to cardenolides, allowing them to eat the leaves without poisoning themselves.4Biological Journal of the Linnean Society. Comparative genetics of Na+/K+-ATPase in monarch butterfly populations with varying host plant toxicity
But monarchs don’t just tolerate cardenolides. They absorb them from the leaves, transport them across the gut wall, and store them in their body tissues, concentrating the toxins above the levels found in the plant itself.5PubMed. Spatial metabolomics reveal divergent cardenolide processing in the monarch (Danaus plexippus) and the common crow butterfly (Euploea core) This chemical theft is what makes monarchs distasteful to birds and other predators. The bright orange-and-black coloring of the adult butterfly is an honest warning signal backed by real toxicity, and the whole system depends on the caterpillar eating the right plants. Research suggests that the genetic resistance to cardenolides evolved not primarily to survive eating toxic food, but to enable the sequestration itself, which is the truly adaptive trick.6PubMed Central. Milkweed butterfly resistance to plant toxins is linked to sequestration, not coping with a toxic diet
How Caterpillars Handle Milkweed’s Physical Defenses
Milkweed doesn’t make it easy for caterpillars, even ones adapted to its chemistry. The plants produce a sticky white latex that flows through pressurized channels in the leaves and stems. When a tiny first-instar caterpillar bites into a leaf, that latex can glue its mouthparts shut or coat its body. Milkweed leaves also carry trichomes, fine hairs on the leaf surface that slow down small caterpillars and make feeding physically difficult. These two defenses work together: in experiments, latex and trichomes both increased caterpillar mortality and reduced the growth of survivors, and the effects were additive rather than overlapping.7Ecological Entomology. Complementary effects of two plant defence traits on behaviour and performance of monarch butterfly caterpillars
Older caterpillars, especially in their later instars, have developed a remarkable workaround. They bite through the midrib or petiole of a leaf before feeding on it, draining the latex pressure so they can eat without getting glued. This “sabotage” behavior was long assumed to be purely defensive, a way to disarm the plant. But a 2024 study flipped that interpretation. Researchers found that monarch caterpillars actually drink the latex that flows from the cuts they make. After severing a petiole, caterpillars had significantly higher concentrations of cardenolides in their gut contents compared to before. The authors concluded that monarchs have transformed what was once a strategy to avoid latex into a strategy to consume it, deliberately seeking out the toxin-rich sap to boost their chemical defenses.8PubMed Central. Late-instar monarch caterpillars sabotage milkweed to acquire toxins, not to disarm plant defence
Not All Cardenolides Are Created Equal
The picture gets more complicated at the molecular level. Milkweeds produce up to 200 structurally different cardenolides, and monarch caterpillars process them very differently depending on their chemical structure. When researchers fed caterpillars purified individual cardenolides, they found that simpler compounds like frugoside and calactin were stored intact, with more than 90% of the toxins in the caterpillar’s body matching what it ate. But two nitrogen-and-sulfur-containing cardenolides, uscharin and voruscharin, were not stored at all. Instead, the caterpillars converted them into other compounds, and the conversion was inefficient: caterpillars on these two toxins sequestered the fewest total cardenolides and excreted the least, meaning much of the compound simply disappeared, presumably broken down internally.9eLife. Cardenolide toxin diversity impacts monarch butterfly growth and sequestration
This matters because voruscharin is the most abundant cardenolide in tropical milkweed (A. curassavica), one of the most commonly used host plants worldwide. It accounts for roughly 40% of the leaf cardenolides in that species, and it actually slows caterpillar growth. So while tropical milkweed supports monarchs and is widely planted in gardens, its dominant toxin is one the caterpillars can’t effectively use for their own defense and that imposes a metabolic cost to process.
The Tropical Milkweed Problem
Tropical milkweed (A. curassavica) deserves its own discussion because it is planted so widely and because it creates real complications for monarchs, especially in the southern United States. Unlike native milkweeds, which die back in winter and force monarchs to migrate, tropical milkweed stays green year-round in warm climates. This disrupts the normal migration cycle. Monarchs exposed to tropical milkweed as caterpillars were more likely to become reproductively active in the fall, when they should be entering a non-reproductive migratory state. Females developed mature eggs, and males exhibited mating behavior, at a time of year when migrants from native milkweed were shutting down reproduction.10PubMed Central. Exposure to Non-Native Tropical Milkweed Promotes Reproductive Development in Migratory Monarch Butterflies
The year-round availability of tropical milkweed also fuels the buildup of a protozoan parasite called Ophryocystis elektroscirrha (OE). In migratory populations, the annual die-off of milkweed essentially resets the parasite load: spores on old plants are eliminated when the plants go dormant. Where tropical milkweed persists through winter, breeding continues uninterrupted and parasite spores accumulate on the leaves. Field surveys and citizen-science data have confirmed that infection rates are substantially higher in sedentary monarch populations that breed year-round on exotic milkweed compared to migratory populations.11PubMed Central. Loss of migratory behaviour increases infection risk for a butterfly host For gardeners in the Gulf states and similar warm regions, the standard recommendation is to cut tropical milkweed to the ground in late fall to mimic the natural die-back of native species, or better yet, plant native milkweeds instead.
How Plant Quality Shapes Caterpillar Growth
Beyond toxin chemistry, the basic nutritional quality of a milkweed plant affects how well a caterpillar develops. Nitrogen content, which roughly correlates with protein availability, is a key variable. In experiments where monarchs were raised on milkweed leaves with either high or low nitrogen content, caterpillars on lower-quality leaves simply ate more. They consumed a greater volume of leaf tissue and ended up weighing just as much as caterpillars on high-nitrogen leaves, without taking significantly longer to complete their larval development.12Environmental Entomology. Compensatory Feeding in Danaus plexippus (Lepidoptera: Nymphalidae) in Response to Variation in Host Plant Quality This compensatory feeding is a useful adaptation, but it only works when there is enough plant material to eat through. In a habitat where milkweed is sparse, a caterpillar on a nutrient-poor plant may not have the luxury of simply eating twice as much.
Climate change introduces another wrinkle. Milkweed grown under elevated carbon dioxide levels tends to have altered chemistry, including lower cardenolide concentrations in the leaves. When caterpillars were fed foliage from milkweed raised in high-COâ‚‚ conditions, they compensated by increasing their rate of cardenolide sequestration, managing to maintain similar toxin levels in their wings as adults. But the high-COâ‚‚ conditions also affected wing shape, potentially altering flight performance.13Functional Ecology. Phytochemical changes in milkweed induced by elevated CO2 alter wing morphology but not toxin sequestration in monarch butterflies The full consequences of this are still playing out, but it is one example of how a plant-insect relationship this tight can be rattled by changes that seem distant from it.
Where Mothers Lay Eggs Versus Where Caterpillars Thrive
You might assume that female monarchs lay their eggs on whichever milkweed species gives their caterpillars the best chance of survival. That assumption turns out to be only partially correct. When researchers compared the milkweed species that female monarchs preferred for egg-laying against the species where caterpillars grew best, the rankings didn’t line up neatly. Mothers sometimes preferred milkweed species that produced average larval outcomes, while species that supported strong caterpillar growth went underused.14Entomologia Experimentalis et Applicata. Oviposition preference and larval performance of North American monarch butterflies on four Asclepias species
Several factors could explain the mismatch. A female monarch’s egg-laying decisions may be influenced by plant traits that matter to her but not to the caterpillar, such as leaf surface texture, plant height (which affects visibility to predators), or even the local density of other monarchs. She may also be hedging her bets across multiple plant species rather than concentrating eggs on the single best one. Whatever the reason, this means that a backyard planting of a less-preferred but highly suitable milkweed species can still attract monarchs if the female happens to encounter it.
Milkweed as Medicine
One of the more surprising findings in monarch research is that the host plant can function like a pharmacy. The same cardenolides that deter bird predators also appear to suppress the OE parasite mentioned earlier. When infected monarchs were raised on a high-cardenolide milkweed species, their lifespans were reduced by about 30% compared to uninfected individuals. But when infected monarchs were raised on a low-cardenolide species, the lifespan reduction was roughly 55%. In other words, eating a more toxic plant cut the parasite’s impact nearly in half. Uninfected monarchs lived equally long on both plant species, so the benefit was specific to combating infection, not a general health boost.
This raises the possibility that female monarchs may sometimes choose milkweed species not only for their nutritional quality but for their medicinal properties, a form of what ecologists call therapeutic self-medication. Some lab evidence supports this idea, though in the wild, the decision-making is likely messier, constrained by which milkweed species are growing nearby.
What Monarchs Eat Outside North America
Monarchs have established breeding populations across the Pacific, including Hawaii, Australia, New Zealand, and parts of Southeast Asia. In these regions, native Asclepias species don’t exist, so the butterflies rely on other plants in the milkweed family that were introduced from Africa and Asia. In Australia, the primary hosts are balloon plant (Gomphocarpus physocarpus) and swan plant (Gomphocarpus fruticosus). In Hawaii, monarchs use G. physocarpus along with crown flower (Calotropis gigantea), a large shrub native to India.15Evolution. Host plant adaptation during contemporary range expansion in the monarch butterfly Tropical milkweed (A. curassavica) also shows up in many of these regions, since it has been widely distributed by humans as an ornamental.
These non-Asclepias hosts still produce cardenolides, which is why monarchs can use them at all. But the specific cardenolide profiles differ from what North American milkweeds produce, meaning the caterpillars in these populations are processing a chemically distinct diet. Whether this has begun to drive genetic or physiological divergence in these populations is an active area of research. What is clear is that the monarch’s dietary requirement isn’t locked to the genus Asclepias specifically. It is locked to the cardenolide-producing branch of the milkweed family, which gives the butterfly a somewhat wider menu than the common “monarchs eat milkweed” shorthand suggests.
Practical Advice for Planting Milkweed
If you want to support monarchs, the single most useful thing you can plant is a milkweed native to your region. In the eastern and central United States, common milkweed (A. syriaca), swamp milkweed (A. incarnata), and butterfly weed (A. tuberosa) are widely available and well-supported by research. In the western states, showy milkweed (A. speciosa) and narrow-leaved milkweed (A. fascicularis) are good choices. Planting multiple species is better than planting one, because it gives females options and caterpillars a range of nutritional and chemical profiles to develop on.
Avoid planting tropical milkweed (A. curassavica) in regions where it won’t die back naturally in winter, which generally means anywhere in USDA hardiness zones 9 and above. If you already have it, cutting it to the ground between October and February forces a reset that mimics native plant dormancy and helps break the parasite cycle. In cooler zones where it freezes out annually, it poses less of a problem, though native alternatives are still preferred for the ecosystem-level benefits they provide to pollinators and other wildlife beyond monarchs.
One thing worth knowing: a milkweed plant stripped bare by caterpillars is not a failed plant. Healthy milkweed typically regrows after defoliation, sometimes within a few weeks. The caterpillars eating your garden down to the stems is exactly the point.