Why Are Monarch Butterfly Numbers Still Dropping

The Numbers Are Still Bad and Researchers Are Puzzled

Monarch butterfly conservation has gotten complicated with all the conflicting headlines and half-measures flying around. The western population hit a new low in 2023 — just 30,000 individuals wintering in California’s coastal forests. That’s a 96% collapse from 1980s estimates. The eastern population, larger but equally fragile, hovered around 140 million at last count. Both figures fall short of the 100 million target conservation groups set nearly a decade ago.

As someone who has spent months reading through recent population surveys, I learned everything there is to know about how deep this frustration runs inside the research community. Money has flowed. Habitat has been restored. Nonprofits coordinated across state lines and international borders. Yet the population curve hasn’t reversed. It’s flatlined — or dipped further.

Today, I will share it all with you.

The central mystery is this: recovery efforts treat monarchs as if the problem were simple resource scarcity. Plant more milkweed. Protect the forest. The interventions are sound in isolation. But when researchers map actual population data against restoration effort, the relationship breaks down. Something else is wrong. Or several things are, simultaneously — compounding faster than any single fix can address.

Milkweed Is Coming Back but It Is the Wrong Kind

But what is the milkweed problem, exactly? In essence, it’s a case of well-intentioned planting creating the wrong conditions. But it’s much more than that.

Here’s the first actual bottleneck, and it’s counterintuitive enough that most people get it completely wrong. Milkweed is returning to the landscape. Just not the right species in the right places.

Frustrated by declining habitat, native plant gardeners across the Midwest and South launched a well-intentioned wave of monarch-friendly planting starting in the early 2000s using whatever seeds and starts they could find at local nurseries. Monarch organizations, state environmental agencies, and native plant societies all pushed homeowners and farms to get involved. The problem: many programs defaulted to tropical milkweed — specifically Asclepias curassavica. It’s showy. Grows fast. Commercially available and cheap, usually $8 to $15 per plant at Home Depot or Lowe’s.

Tropical milkweed stays green year-round in warm climates. Native milkweed dies back in fall. That difference matters catastrophically for monarchs. The perennial foliage creates two serious problems. First, it breaks the environmental cue that triggers fall migration — monarchs evolved to read dying milkweed as a signal to leave before frost arrives. When milkweed never dies, that signal fails. Second, tropical milkweed harbors higher loads of Ophryocystis elektroscirrha, a protozoan parasite that weakens monarchs and spreads on contact. A monarch reared on evergreen milkweed carries more parasites and stands far less chance of surviving a 3,000-mile migration.

Probably should have opened with this section, honestly. It’s the fastest-moving variable researchers can actually point to and say: “This we created.”

In Texas, Florida, and California — wherever tropical milkweed has taken root — monarchs are arriving at overwintering sites more weakened than their predecessors. Some conservation groups have since pivoted to removing tropical milkweed entirely and replanting native species. But the damage compounds across years. A weakened cohort in 2020 produces fewer eggs in 2021. That population produces fewer migrants in 2022. Don’t make my mistake of underestimating how long that setback actually persists.

Climate Shifts Are Breaking the Migration Timing

Monarchs don’t have a GPS. They navigate using temperature and day length. For millions of years, those cues aligned perfectly with milkweed availability along the migration corridor. Spring temperatures rise, day length increases, milkweed sprouts, monarchs fly north. Eat. Breed. The next generation continues the relay.

That alignment is fragmenting now.

Across the Midwest, spring arrives earlier. Milkweed emerges one to three weeks sooner than it did in the 1990s. But monarchs are still using ancestral temperature and daylight cues to trigger departure from Mexico — cues that haven’t shifted to match. The result is a widening gap. Monarchs reach historical breeding grounds only to find milkweed already mature or declining. They’ve missed the optimal egg-laying window. The cohort produced is smaller. Eggs laid on tough, senescing milkweed simply don’t develop as well.

That’s what makes the Texas bottleneck so alarming to us who follow this closely. Every spring, the entire northbound monarch population must pass through Texas — essentially a narrow geographic funnel. Spring conditions in south and central Texas during March and April carry outsized impact on the entire eastern population’s breeding season. In 2024, an extended drought coincided with late March freezes that killed early milkweed growth. The population cohort moving through Texas that year was notably depleted by summer. One bad month. One funnel. The whole year affected.

Climate scientists can model this. Researchers can document it. Reversing it requires either shifting the migration window — biologically implausible on short timescales — or ensuring milkweed availability across a broader seasonal window. That’s expensive and demands coordination across states sharing no common regulatory framework.

The Overwintering Sites in Mexico Are Still Under Pressure

Assuming a population survives the migration, it faces Mexico’s Oyamel fir forests in Michoacán. Twelve specific forest reserves totaling roughly 140,000 acres, where up to 500 million monarchs historically clustered. That was the destination their ancestors used for thousands of years.

The forests face three simultaneous stressors right now. Illegal logging has slowed — enforcement genuinely improved in the early 2010s — but it hasn’t stopped. Local actors continue felling trees for timber and fuelwood. Storm damage has increased; a single winter storm in 2002 killed an estimated 500 million monarchs in the oyamel zone alone. Climate warming is also shrinking viable roosting altitude. Monarchs need cool, humid conditions. As higher elevations warm, the suitable zone creeps upslope — toward trees more exposed to wind and catastrophic weather events.

I’m apparently someone who obsesses over carrying capacity numbers, and the math here never gets more comforting. Even if a monarch population survives the journey north, breeds successfully, and returns south, it arrives at a destination that’s smaller, hotter, and more vulnerable than the one its ancestors knew. A smaller carrying capacity means a lower ceiling on population size — permanently, until something changes on the ground in Michoacán.

What Would Actually Move the Needle for Monarchs

Recovery is possible. Not guaranteed on current trajectories — but possible. So, without further ado, let’s dive in to what the data actually supports.

First, you should prioritize large-scale native milkweed restoration — at least if you want interventions that match the actual scale of the problem. Not suburban gardens. Agricultural margins. Highway right-of-way. Strategic placement along the Texas corridor and through Kansas, Oklahoma, and Missouri, filling gaps during critical spring migration weeks. This requires sustained funding and coordination, but it’s the most direct lever researchers consistently identify.

Policy changes on herbicide use might be the best second option, as monarch recovery requires milkweed in agricultural landscapes — and that milkweed has been essentially eliminated. That is because glyphosate-tolerant crops now dominate the Corn Belt, and herbicide application has intensified decade over decade. Limiting application near field margins, rewarding farmers — say, $50 to $150 per acre — for maintaining milkweed strips would reduce seasonal availability loss. Politically difficult. Economically contentious. But measurable, and the data supports it.

Third, international coordination on the Mexican forest reserves. No single U.S. action secures those forests. It requires sustained funding, enforcement, and real partnership with Mexican conservation groups and local communities in Michoacán — communities that currently bear the economic burden of conservation without proportional support. Without that piece, the migration endpoint remains compromised regardless of what happens north of the border.

This new framework of coordinated, bottleneck-specific intervention took shape over several years of failed broad-stroke approaches and eventually evolved into the strategy conservationists know and cautiously advocate today. It’s not simple. It’s not cheap. But it’s specific, testable, and grounded in what the population data actually shows — which is more than can be said for most of what came before it.