Spring Buildup: 10,000 Bees to 50,000 by June
The signal isn't temperature. It's light.
A honey bee colony's annual cycle is triggered primarily by photoperiod - the length of the day. As day length increases past roughly 10 hours in late January through February (depending on latitude), photoreceptors in the bee's compound eyes detect the change, and the information propagates through the colony via behavioral and pheromonal signals. Temperature matters too - bees don't fly below about 50 degrees Fahrenheit (10 degrees Celsius), and brood rearing requires a sustained brood nest temperature of 95 degrees Fahrenheit - but the initial trigger for spring buildup is the lengthening day.
The queen responds first. Through winter, she may have been laying nothing (in northern climates with extended broodless periods) or a small patch of brood (in milder climates where the cluster maintains a small brood area through winter). As days lengthen, her egg-laying rate increases. Slowly at first - perhaps 50 to 100 eggs per day in February. Then faster: 500 per day by mid-March, 1,000 per day by early April, and 1,500 to 2,000 per day by May.
Two thousand eggs per day. Each egg weighs about 0.1 milligram. The queen weighs roughly 200 milligrams. She's laying her body weight in eggs every day, sustained for weeks, fueled by royal jelly provided by nurse bees who are themselves consuming vast quantities of pollen and honey.
The Pollen Trigger
The critical input for spring buildup isn't honey. Colonies typically have enough stored honey to fuel the initial expansion (a well-prepared colony enters winter with 60 to 90 pounds of honey, and uses roughly 40 to 60 pounds over winter, leaving a reserve). The critical input is pollen.
Pollen provides the protein - roughly 20 to 30 percent protein by weight, depending on the plant source - that nurse bees need to produce brood food (royal jelly and worker jelly) for developing larvae. Without fresh pollen, nurse bees can produce brood food only from their own body reserves - their fat body tissue and stored vitellogenin - which depletes them rapidly and limits the amount of brood the colony can raise.
The first pollen of spring is the trigger that shifts the colony from survival mode to growth mode. In much of the eastern and midwestern United States, the first significant pollen sources are:
Red maple (Acer rubrum). Blooms in February to March, often the first tree pollen available. Produces abundant pollen but minimal nectar.
Elm (Ulmus spp.). Blooms in February to March in the mid-Atlantic and Midwest.
Pussy willow (Salix spp.). Blooms in March to April, producing both pollen and nectar. Willows are among the most important early spring forage plants for bees.
Dandelion (Taraxacum officinale). Blooms in March to May depending on latitude. The most reliable early pollen and nectar source across most of the US. Much maligned by lawn care enthusiasts, dandelions are one of the most important bee plants in North America.
Fruit trees. Apple, cherry, plum, pear - bloom from April through May, providing abundant nectar and pollen during the peak buildup period. These are the same blooms that commercial pollination contracts pay for.
The timing of these blooms relative to the colony's buildup determines how fast the colony can grow. A colony that has access to pollen in February starts growing earlier and reaches peak population sooner than a colony in a location where pollen isn't available until April. The 4 to 6-week head start can mean the difference between a colony that's at full strength for the main nectar flow and one that's still building up when the flowers start.
The Exponential Math
Colony population growth during spring buildup is approximately exponential, with a doubling time of roughly 3 weeks.
The math: a queen laying 1,500 eggs per day produces roughly 10,500 eggs per week. Worker development takes 21 days from egg to emerging adult bee. At steady state, with 1,500 eggs per day and 21 days of development, the colony has approximately 31,500 immature bees (eggs, larvae, pupae) in the brood nest at any time, with about 1,500 new adults emerging daily.
But it takes 3 weeks for the first eggs laid in the spring ramp-up to produce adult bees. During those 3 weeks, the existing winter bee population is declining as older bees die. The colony goes through a brief population dip - the "spring dwindle" - before the new adults begin emerging and the population curve inflects upward.
The spring dwindle is normal and predictable, but it's the period when the colony is most vulnerable. The population is at its annual minimum. The brood nest is expanding (consuming energy to heat a growing area of comb). The incoming resources (pollen and nectar) may be inconsistent due to spring weather variability. A cold snap, a late frost, or a week of rain during the spring dwindle can stall the buildup by killing the early blooms and forcing the colony to consume stored honey to survive.
By early to mid-April in temperate zones, the first spring brood is emerging. The population inflects upward. The colony enters the exponential growth phase: more bees mean more foragers, more foragers bring in more resources, more resources allow the queen to lay more eggs, more eggs produce more bees. The positive feedback loop runs until the colony reaches its maximum population (typically 50,000 to 60,000 bees for a strong colony in a standard Langstroth hive) or until the colony swarms, which is the natural brake on exponential growth.
The Brood Nest
The brood nest during spring buildup is a precisely organized structure. The queen works on the central frames of the hive, laying in concentric rings that expand outward and onto adjacent frames as the available brood area increases.
The organization follows a consistent pattern: eggs in the center, surrounded by larvae (young larvae nearer the center, older larvae farther out), surrounded by capped brood (pupae developing under wax caps), surrounded by a band of pollen (stored within easy reach of the nurse bees), surrounded by honey (the energy reserve). This concentric arrangement - sometimes called the "brood nest sphere" - is maintained through the interactions of thousands of individual bees, each responding to local cues: the queen walks to an empty cell near existing brood and lays; nurse bees cluster on brood-temperature comb and feed larvae; foragers deposit pollen near the brood nest edge where nurse bees can reach it.
During spring buildup, the brood nest expands from perhaps 2 to 3 frames (the overwintered brood area) to 8 to 10 frames over the course of 6 to 8 weeks. The expansion is limited by the queen's laying rate, the availability of pollen (which limits brood food production), the colony's ability to thermoregulate the expanding brood area, and the physical space available in the hive.
A colony that runs out of space during the buildup - when every frame is filled with brood, pollen, and honey, with no empty cells for the queen to lay in - enters swarm preparation. The congestion of bees, the reduced circulation of queen pheromone in the crowded hive, and the lack of laying space trigger swarm cell construction. The colony begins building queen cells on the frame bottoms and preparing to divide.
The Disease Window
Spring buildup is also the period when brood diseases gain traction. The expanding brood nest provides the substrate that brood pathogens need: thousands of developing larvae in warm, humid conditions.
American foulbrood spores, which may have been dormant in the comb over winter, can infect spring larvae. Nosema infection - particularly Nosema ceranae - often peaks in spring, debilitating adult bees during the critical buildup period. Chalkbrood, a fungal disease of larvae, is most common during the cool, damp conditions of early spring when the colony's thermoregulatory capacity is strained by the expanding brood area.
Varroa mites begin their own exponential growth alongside the colony's brood expansion. Mites that overwintered on adult bees enter the first brood cells of spring and begin reproducing. If the fall treatment was effective, the spring mite population starts low and takes months to build to damaging levels. If the fall treatment was inadequate - or if the colony acquired mites through late-fall robbing - the mite population may already be elevated, and the spring brood expansion amplifies it rapidly.
The colony's growth rate has to outpace its disease and parasite load. A healthy colony expanding at 2 to 3 percent per day can outgrow moderate disease pressure - the new bees emerging faster than the diseases can debilitate them. A weak colony growing at 1 percent per day with a high mite load may never reach critical mass. The spring buildup is a race between the colony and its pathogens, and the colony that starts with more bees, more stores, and fewer mites wins the race.
The Starvation Risk
The cruelest irony of spring beekeeping: colonies that survived the entire winter can starve in March.
The mechanism: the colony emerges from winter with reduced honey stores (most of the original 60 to 90 pounds has been consumed). Spring buildup is in progress - brood rearing demands energy (honey) and protein (pollen). But the spring blooms haven't started yet, or they've been delayed by cold weather. The colony is spending faster than it's earning. If the remaining honey runs out before the income resumes, the colony starves.
This is most common during extended cold snaps in March or April - periods of 5 to 10 days when temperatures remain below flying temperature and the colony is confined to the hive, burning through stores without the ability to forage. The brood nest is now too large to abandon (the brood would die), so the energy expenditure for heating continues at the spring rate even though no income is flowing.
Spring starvation kills more colonies in some years than winter cold does. A colony that is heavy with honey in December and light in March has consumed its reserves. The risk is highest in northern climates with long springs and unpredictable weather - the transition zone between winter and summer foraging that can last 6 to 8 weeks.
The Reward
A colony that navigates the spring buildup - that survives the dwindle, finds early pollen, outgrows its parasites, doesn't starve in March, and doesn't swarm in May - arrives at the main nectar flow in peak condition: 50,000 to 60,000 bees, a queen laying at maximum rate, a brood nest packed with healthy larvae, and a foraging force of 15,000 to 20,000 experienced field bees ready to exploit the bloom.
The numbers are staggering when the buildup works. In a strong nectar flow (basswood, clover, alfalfa, citrus - depending on region), a full-strength colony can bring in 10 to 20 pounds of nectar per day. The house bees process the nectar - adding enzymes, evaporating water, filling and capping cells of honey. A colony that enters the main flow at peak population and peak health can produce 60 to 100 pounds of surplus honey in a 4 to 6-week flow.
That surplus - the honey that exceeds the colony's own needs and becomes the beekeeper's harvest - represents the compound return on everything that went right during the buildup. The early pollen that fueled brood food production. The nurse bees that fed the larvae. The larvae that became foragers. The foragers that found the nectar flow. Each step in the chain had to succeed for the surplus to exist.
The colony that comes through the spring buildup strong is the colony that pays the rent. The colony that stumbles - late start, disease pressure, starvation, early swarming - arrives at the flow understaffed, and the honey that might have been stays in the flowers.
Spring isn't the beginning of the season. It's the exam for the previous year's preparation. The fall feeding, the mite treatment, the winter configuration - all of it was for this. Ten thousand bees in March becoming fifty thousand by June, powered by pollen and honey and 2,000 eggs a day from a queen who has been waiting all winter for the light to change.
The light changes. The buildup begins. And the beekeeper, standing next to a hive that hums with the sound of exponential growth, watches the population curve bend upward and knows: this is either the year that works, or the year that doesn't. The bees will tell you which. They're telling you right now.