Honeybees face a combination of parasites, pesticides, poor nutrition, and environmental disruption that together drove an estimated 55.6% loss of managed colonies in the United States between April 2024 and April 2025. No single factor is responsible. The threats interact, weakening bees on multiple fronts at once, which is why colony losses have remained stubbornly high for over a decade.
Varroa Mites: The Top Killer
The USDA ranked Varroa mites as the number one stressor for honeybee operations during every quarter of 2024. These tiny parasites, each about the size of a pinhead, attach to adult bees and developing larvae inside the hive. For decades, scientists assumed the mites were drinking bee blood. A 2019 study in the Proceedings of the National Academy of Sciences overturned that idea: Varroa mites actually puncture the bee’s body and digest its fat body tissue, a critical organ that functions like a combined liver and immune system.
The mites use specialized mouthparts to break through the bee’s exoskeleton, then inject salivary fluid that dissolves fat body cells from the inside out. This is called extraoral digestion, essentially liquefying tissue before consuming it. Losing fat body tissue cripples a bee’s ability to store protein, detoxify pesticides, and fight off infections. It also shortens the bee’s lifespan dramatically, which matters most in fall when colonies need long-lived winter bees to survive until spring.
On top of the direct damage, Varroa mites are confirmed carriers of at least 5 debilitating viruses and potentially 13 others. Deformed wing virus is the most well-known. When a mite feeds on one bee and moves to the next, it injects viral particles directly into the wound, bypassing the bee’s outer defenses entirely. A colony can tolerate a low mite population, but once numbers cross a threshold in late summer, the combination of fat body destruction and viral spread can collapse a hive within weeks.
Pesticides and Chemical Exposure
Neonicotinoids, the most widely used class of insecticides worldwide, are specifically designed to target the nervous system of insects. They bind to receptors in the bee’s brain that normally handle nerve signaling, locking those receptors in an “on” position. This causes overstimulation, then dysfunction. The result isn’t always immediate death. At low concentrations, the damage is subtler and arguably worse for colony survival.
Imidacloprid, one of the most common neonicotinoids, impairs honeybee learning, navigation, and foraging behavior at concentrations as low as 5 to 10 parts per billion. Bees exposed at those levels eat less, forage less efficiently, and bring back fewer resources. Queens exposed to just 10 parts per billion over an extended period laid 30% fewer eggs than unexposed queens. Fewer eggs means fewer worker bees, which means less food coming in, which means the colony slowly starves even when flowers are available.
Neonicotinoids also suppress the bee’s immune system and increase oxidative stress, making bees more vulnerable to every other threat on this list. But they’re not the only chemical problem. Fungicides, often considered “bee-safe” because they target fungi rather than insects, can dramatically amplify the toxicity of other pesticides. One study found that the fungicide propiconazole made a common pyrethroid insecticide 16 times more lethal to honeybees. The insecticide alone had a hazard ratio of 110; mixed with propiconazole, that ratio jumped to 1,786. Farmers routinely tank-mix fungicides and insecticides for a single spray application, exposing bees to combinations that are far more dangerous than either chemical alone.
A Gut Parasite That Starves Bees From the Inside
Nosema ceranae is a single-celled fungal parasite that infects the lining of a honeybee’s midgut. Once inside, it hijacks gut cells to reproduce, destroying the intestinal lining in the process. The parasite also suppresses genes responsible for gut tissue renewal, so the damage can’t be repaired. An enzyme critical for nutrient absorption and reducing gut inflammation drops significantly in infected bees.
The practical effect is that infected bees can’t properly absorb the food they eat. They become malnourished even with adequate nectar and pollen available. In laboratory experiments, cages of bees infected with Nosema ceranae reached 100% mortality by day 14, while control groups remained largely alive. In real-world hives, the infection is rarely that uniform, but it chronically weakens individual bees and shortens their productive lives. Bees fed supplemental vitamins, minerals, and amino acids showed significantly reduced Nosema spore counts, suggesting that good nutrition provides some defense, but bees in nutritionally stressed environments get hit hardest.
Poor Nutrition From Shrinking Habitat
Honeybees need pollen from a diverse mix of flowering plants to get the full range of amino acids, vitamins, and minerals their bodies require. Nine amino acids are essential for worker bee development, including lysine, tryptophan, and valine. No single plant species provides all of them in the right proportions. When colonies sit next to thousands of acres of a single crop, they get a nutritionally incomplete diet even if pollen appears abundant.
Colonies near low-diversity agricultural landscapes show reduced brood rearing and shorter lifespans, and they become more susceptible to parasites and diseases. This creates a vicious cycle: poor nutrition weakens immunity, which lets Varroa and Nosema do more damage, which further reduces the colony’s ability to forage effectively.
Habitat loss also forces bees to fly farther for food. A study tracking foraging distances found that small-scale construction covering just 1% of a colony’s foraging range nearly doubled the average flight distance, from 0.69 to 1.28 kilometers. Energy expenditure jumped from 7 to 13 joules per trip. That may sound small, but multiplied across thousands of foraging trips per day across the entire colony, it represents a serious drain on the hive’s energy budget.
Climate Change and Timing Mismatches
Warming temperatures are shifting when flowers bloom and when bees become active, but not at the same rate. For every 4°C increase in long-term average temperature, flowering advanced by about 14 days across plant species studied. Generalist bees advanced their activity by about 17 days for a similar temperature shift, but specialist bees shifted by only 8 days. When bees emerge before flowers bloom, or flowers finish blooming before bee populations peak, the result is a gap where pollinators can’t access the food they need.
These mismatches are most pronounced at northern latitudes, where temperature changes have been largest. Even a few days of disconnect between peak bloom and peak bee activity can reduce the total food a colony collects during a critical buildup period, leaving it weaker heading into summer and fall.
Migratory Beekeeping Adds Stress
More than 1 million hives are trucked to California each year just for almond pollination, and many of those same colonies travel thousands of additional miles for other crops throughout the season. This isn’t free. Bees from migratory colonies had significantly shorter lifespans than bees from stationary colonies. In one controlled comparison, stationary bees lived an average of 19.5 days while migratory bees lived 18.0 days, a meaningful gap at the individual level that compounds across an entire colony.
Researchers found elevated oxidative stress in migratory bees, though food scarcity had an even larger impact on stress levels than transportation alone. The effects showed up after just one or two moves, suggesting that colonies trucked repeatedly across the country experience cumulative damage that standard studies likely underestimate.
Colony Collapse Disorder: Still Happening
Colony Collapse Disorder, the phenomenon where adult bees vanish from a hive leaving behind a queen, food, and capped brood, has not disappeared. The USDA still tracks it as a distinct category. In the first quarter of 2025, 148,410 colonies were lost with CCD symptoms in operations with five or more hives. CCD is defined by four criteria: few or no dead bees in or around the hive, rapid loss of the adult population despite a queen and brood being present, food reserves that remain unrobbed by neighboring colonies, and losses not explained by Varroa or Nosema alone.
CCD grabbed headlines in the mid-2000s, and public attention has faded, but the losses continue. Most researchers now view CCD not as a single disease but as the end result of multiple stressors converging. A colony weakened by mites, exposed to pesticides, and nutritionally stressed may reach a tipping point where foragers simply fail to return. The bees don’t die in the hive. They die in the field, disoriented or too exhausted to make it back.
Why Combined Threats Matter Most
The reason honeybee losses remain so high is that these threats rarely act alone. A bee carrying Varroa-transmitted viruses is less able to detoxify pesticide exposure. A colony on a monoculture diet has weaker immune responses to Nosema. A hive stressed by long-distance transportation enters almond groves already depleted, then gets exposed to fungicide-insecticide tank mixes during bloom. Each stressor lowers the threshold at which the next one becomes lethal.
This is why pinpointing a single cause of honeybee death has proven so difficult. The answer is almost always “several things at once,” with Varroa mites and pesticide exposure sitting at the center of most colony failures, and nutrition, pathogens, climate shifts, and management practices shaping how fast and how hard those central threats hit.