Reviewed by Thomas B. · Last Updated September 24, 2026 · How we review
Bees that tremble, cannot fly, and crawl out of the hive are most often showing chronic bee paralysis, a disease of adult bees caused by chronic bee paralysis virus (CBPV). The virus attacks nerve cells. Sick bees shake, then become paralyzed, and usually die within a week. Many turn black, hairless, and shiny as nestmates chew at them. The result is a pile of dead and dying bees at the entrance.
Trembling alone does not prove a virus. Pesticides, nosema, other paralysis viruses, and some normal behaviors look similar at first. Below you’ll find what CBPV looks like, how to rule out lookalikes, how to get it confirmed in North Dakota and Minnesota, and what to do next.
Key Takeaways
- CBPV has two forms that often overlap. In one, bees tremble, can’t fly, and have bloated abdomens and splayed wings. In the other, bees turn black and hairless and guards push them out of the hive.
- It spreads by contact and shared food, mostly in crowded colonies. The UK National Bee Unit says it is not thought to be spread by Varroa mites.
- Pesticide kills leave dead foragers across the whole apiary, often in every hive at once. CBPV builds up as piles at one or a few entrances.
- No medicine treats CBPV. Thymol and antibiotics do nothing against a virus.
- Instead, give the colony room and pollen, check the queen, and move dead bees away from the entrance.
Table of Contents
- What Chronic Bee Paralysis Looks Like
- How It Spreads: Why Mites Are Not the Main Route
- Trembling Bees That Are Not CBPV
- Field Checks to Sort It Out
- Getting It Confirmed in North Dakota and Minnesota
- Managing an Affected Colony
- Frequently Asked Questions
What Chronic Bee Paralysis Looks Like
The clearest guide for beekeepers is the UK National Bee Unit’s information note on chronic bee paralysis, written with Newcastle University. It describes the disease moving from repeated movements to partial paralysis, full paralysis, and death within a week.
The trembling form
Bees shake their bodies and wings and cannot fly. They crawl on the landing board, drop into the grass, and climb plant stems. Their abdomens often look swollen and their wings sit at odd angles. The USDA’s Diagnosis of Honey Bee Diseases handbook notes that inside the hive they are usually on the top bars. The National Bee Unit adds that they ignore smoke.
The black, hairless form
Healthy bees nibble at sick ones while trying to remove them, which strips their hair. The sick bees look black, shiny, and almost greasy, and guards drive them away from the entrance. With all the fighting and the dark, worn-looking bees, this form is easy to mistake for robbing.
What happens to the colony
The sign at colony level is a growing pile of dead and dying bees at the entrance and on the bottom board. The National Bee Unit reports that colonies are lost in 40 to 50% of cases. The disease is seen most often in early summer. In the Red River Valley that means June and early July, when colonies are most crowded and nearby canola, sunflower, and soybean fields are being sprayed. That overlap is why careful diagnosis matters here.
CBPV is also becoming more common. Newcastle University reports it was recorded only in Lincolnshire in 2007, but in 39 of 47 English counties by 2017 (Budge and colleagues, 2020, Nature Communications). In the US, the national disease survey first found it in 2010 in under 1% of samples. By 2014 it was in 16%.
How It Spreads: Why Mites Are Not the Main Route
A lot of advice online, including the earlier version of this page, gets this wrong. Deformed wing virus and the acute paralysis viruses are closely tied to Varroa. CBPV is not.
The virus has several ways that it can move between honey bees, but it is not thought to be transmitted by Varroa mites.
Adult bees pick up the virus from infected food or from contact with sick and dead bees. Crowding speeds this up. Dead bees stay infectious for months, so a pile of them at the entrance keeps infecting live bees. Budge’s study found the disease was nearly twice as likely in apiaries whose owners had imported bees. Keep treating for Varroa, because other viruses still matter. Just don’t expect a mite treatment to stop CBPV. See our seasonal Varroa treatment calendar.
Trembling Bees That Are Not CBPV
The USDA handbook warns that flightlessness and unhooked wings are general symptoms of many disorders. It also notes that toxic chemicals can cause paralysis-like symptoms.
| Cause | What you see | Pattern | Best field clue |
|---|---|---|---|
| Chronic bee paralysis virus | Trembling, flightless, bloated or black hairless bees | One or a few hives; piles build over days; early summer | Shiny, hairless bees plus a growing pile at one entrance |
| Pesticide poisoning | Twitching foragers; some spin on their backs | Often every hive at once, right after a spray | A sudden carpet of dead bees across the yard |
| Acute, Israeli acute, or Kashmir paralysis viruses | Trembling and paralysis like CBPV | Colonies with heavy Varroa, late season | High mite count; only a lab can separate them |
| Deformed wing virus | Crumpled, stubby wings on young bees | Heavy Varroa | Wing damage, not shaking |
| Nosema apis | Crawling bees near the entrance | Spring, after confinement | Spores under a microscope |
| Normal behavior | Tremble dances, washboarding | Healthy, busy colonies | Bees fly normally and don’t pile up dead |
The National Bee Unit gives the most useful rule. Pesticides leave carpets of dead foragers across the apiary. CBPV kills bees inside the colony and leaves piles at the entrance. Bees spinning on their backs point to pesticides.
Not all shaking is sickness. Foragers do a “tremble dance” when they can’t find a bee to unload their nectar, as Thomas Seeley showed in a 1992 paper in Behavioral Ecology and Sociobiology. Washboarding is harmless too. Also rule out zombie bees, which are parasitized by a phorid fly and wander around at night. For crumpled wings, see our guide to deformed wing virus signs.
Field Checks to Sort It Out
Step 1: Count the affected hives
If every hive has dead foragers on the same morning, suspect pesticides. If one or two hives have a growing pile and their neighbors look fine, suspect CBPV.
Step 2: Look at the bees, not just the pile
Look for shiny, hairless bees being mauled by guards and for bees trembling on the top bars. Crumpled wings point instead to deformed wing virus.
Step 3: Take a mite count
High Varroa makes the acute paralysis viruses more likely. A low count does not rule out CBPV.
Step 4: Freeze samples
Bag fresh dead and dying bees from the entrance and freeze them. Also take dated photos and a short video. Write down any recent spraying nearby, and check BeeCheck for applicator notices.
Getting It Confirmed in North Dakota and Minnesota
You cannot confirm CBPV by eye. The USDA handbook says lab methods are needed. The BeeMD identification tool notes that acute and chronic paralysis are nearly impossible to tell apart without testing. The free USDA Beltsville service doesn’t list viruses among its tests. The North Dakota Department of Agriculture’s beekeeper page points to the National Agricultural Genotyping Center in Fargo. Its qPCR honey bee pathogen panel tests for CBPV, the acute paralysis viruses, deformed wing virus, and both nosema species. The lab won’t give treatment advice, so go over the results with your state apiary program. Our guide to laboratory testing for honey bee diseases covers how to handle samples.
If you suspect pesticides, report the kill quickly. In North Dakota, call the Department of Agriculture’s Pesticide and Fertilizer Division. In Minnesota, the Department of Agriculture investigates bee kills after a written complaint. It also compensates beekeepers for confirmed acute poisonings if they are registered on BeeCheck. Anywhere in the US, you can also report it to the EPA.
Managing an Affected Colony
There is no cure. The old advice to feed thymol or spray antibiotics doesn’t work, because neither acts on a virus, and bee antibiotics in the US need a veterinarian. The National Bee Unit recommends these steps instead:
- Add space. An extra brood box or super reduces the crowding that spreads the virus.
- Make sure there’s pollen. A pollen shortage makes symptoms worse. After canola finishes, that may mean a pollen patty.
- Check for a laying queen. Colonies with CBPV often lose their queen.
- Clear out the dead. Widen the entrance and move piles of dead bees away from the hive. Don’t remove the floor, because that invites robbing.
- Reduce drifting. Point hive entrances in different directions. See our guide to stopping drifting.
There is no evidence that changing the queen will help manage the disease.
Requeening is still widely sold as a cure, which is why this matters. Replace a queen that has died, but don’t expect a new one to end the outbreak. Colonies that survive are often weak, so feed or combine them before winter. For more, see preventing chronic bee paralysis virus and CBPV vs pesticide poisoning.
Frequently Asked Questions
Why are my bees shaking and unable to fly?
The most common disease cause is chronic bee paralysis virus, especially if the bees also look black and hairless and pile up at one entrance. Pesticides, acute paralysis viruses, and nosema can look similar.
Is chronic bee paralysis spread by Varroa mites?
The National Bee Unit says it is not thought to be. It spreads through contact and contaminated food, and crowding makes that worse.
How can I tell pesticide poisoning from bee paralysis virus?
Pesticides usually hit every colony at once, right after a spray, and leave dead bees across the yard. CBPV builds up over days in one or a few hives.
Can a colony survive chronic bee paralysis?
Yes, often. The National Bee Unit puts colony losses at 40 to 50% of cases, so about half survive, though many come out weak.
Will requeening cure a colony with trembling bees?
No. There is no evidence that a new queen helps. Replace her only if she has died or failed.
Where can I get bees tested for paralysis viruses in North Dakota?
Try the National Agricultural Genotyping Center in Fargo. It runs a qPCR panel that includes CBPV. Check its submission guidelines before you ship.
The Bottom Line
Trembling, black, hairless bees piling up at one entrance in early summer are the classic sign of chronic bee paralysis. Before you settle on that, rule out a spray event and a mite-driven virus, and freeze a sample. Then give the colony space and pollen, keep it queenright, and clear the dead.
