September Varroa Treatment: Your Last Chance Before Winter Damage Sets In
A colony treated on September 15 instead of August 15 will have 30% fewer high-quality winter bees to form the cluster. That 30% reduction reflects one additional month of winter bee production under high mite pressure. The September treatment still works; the September-treated colony still has a fighting chance. But it starts winter with less margin than the August-treated colony.
If you missed the August window, September is your backup. Here's how to make it count.
TL;DR
- September treatment decisions should be based on a current mite count, not calendar date alone
- Temperature constraints in September may limit which treatments are effective in your climate zone
- PHI timing for September treatments affects when honey supers can be added or must be removed
- Log a mite count before starting any September treatment to calculate efficacy post-treatment
- VarroaVault's treatment reminders for September account for regional temperature and flow calendars
- Recording September treatment dates creates the audit trail needed for state inspection compliance
Why September Is a Diminished Window, Not a Closed One
The winter bee biology argument for August treatment is about protecting brood laid in August from high mite levels. September treatment is still valuable because:
- Queens are still laying in early September in most US climates, and September brood produces additional winter bees
- Treatment in early September (September 1-15) protects mid-to-late September brood
- Even partial protection of the winter bee cohort is better than no protection
- A colony left untreated through September enters October with potentially 4-6% mite loads, which is a colony in serious decline
The calculus changes after October 1 in northern climates: brood rearing slows and brood cells become less available for mite reproduction, reducing the reproductive cycle advantage. But treating in October is still better than not treating at all for colonies still above threshold.
The September Urgency Score
VarroaVault's September urgency score shows the countdown to the point where winter bee quality can no longer be protected. The score accounts for your location's:
- Average queen brood cessation date
- Average first frost date
- Treatment cycle length for your selected product
A colony in Minnesota needs faster action in September than one in Virginia, because the Minnesota season ends earlier.
The urgency score increases every day from September 1. By September 15, most operations are in high-urgency territory. By October 1, you're doing damage control, not prevention.
Fastest September Treatment Options
Speed matters in September in a way it doesn't in August. With a narrowing window, you want treatment cycle completion as fast as possible.
Fastest option: OA vaporization extended protocol
Three applications over 14-21 days. Protocol can start September 1 and complete by September 15-21. 90-95% efficacy. Works regardless of temperature. No honey super concerns.
This is the recommended first choice for September treatment urgency because you can start immediately without a temperature concern.
Good option (if temperatures cooperate): MAQS
7-day treatment. If your temperatures will stay between 50-85°F for the next 7 days, MAQS gives you the fastest single-cycle completion of the formic acid options. Post-treatment count scheduled for September 14-21 depending on start date. Brood penetration is a bonus.
Good option (moderate temperature flexibility): Formic Pro
14-day treatment. Still faster than Apivar. Effective, brood-penetrating, usable with supers on if needed.
Caution with Apivar in late September: A 42-day Apivar treatment started on September 15 doesn't complete until late October or early November. Strip removal in November may conflict with pack-down timing. If you're going to use Apivar in September, start no later than September 10-15 depending on your climate and pack-down schedule. Calculate: treatment start date plus 42 days must fall before your planned pack-down date.
What If You're Starting September at High Mite Loads?
If you're starting September with a count above 3% and haven't treated yet, this is an emergency response situation:
- Start treatment within 48 hours of confirming the count.
- Choose the fastest effective option (OA vaporization extended protocol or MAQS if temperatures allow).
- Reduce entrances immediately on this colony and all neighboring colonies to limit robbing transmission.
- Plan for a second treatment cycle if post-treatment count is above threshold.
- Log everything in VarroaVault with the urgency context noted.
A colony at 5% in September is a colony in active decline. Treatment may save the colony or it may be too late, depending on how rapidly the population is declining. Document what you find at treatment time to inform your program analysis in winter.
The September-to-October Sequence
If you treat in September and achieve good efficacy (post-treatment count below 1%), you can still do the winter broodless OA dribble in October. That two-step fall sequence (September treatment + October broodless dribble) still achieves good winter preparation, just with less winter bee protection than August treatment would have provided.
The dribble in October is still essential: it catches remaining mites that survived the September treatment cycle and eliminates them before winter cluster formation.
Logging September Urgency in VarroaVault
VarroaVault's September urgency score appears as a dashboard element for all active hives that haven't received fall treatment. The score counts down from 100 (September 1) toward 0 as the window closes.
When you log a September treatment, the urgency counter resets and the dashboard shows treatment active status. The post-treatment count reminder schedules automatically for the appropriate window.
See also: Fall treatment window and August varroa treatment.
Frequently Asked Questions
Is September too late to treat for varroa?
No, but it's late. September treatment still protects September brood (which includes some winter bees still being raised in early September) and brings mite levels down before colony population decline accelerates. A colony treated in September has 30% fewer high-quality winter bees than one treated in August, reflecting the additional month of winter bee production under high mite pressure. September is your backup window, not a closed window: don't skip treatment because you missed August.
Which treatments work fastest for September urgency?
OA vaporization extended protocol (3 applications over 14-21 days) is the fastest option with no temperature restrictions. MAQS (7-day treatment) is fastest if temperatures will stay 50-85°F throughout the treatment period. Formic Pro (14 days) is moderately fast. Apivar (42-56 days) should be started no later than September 10-15 to complete before pack-down; after mid-September, faster-cycle treatments are preferred for most operations.
Does VarroaVault show the urgency of late-season treatment?
Yes. VarroaVault's September urgency score counts down from September 1 based on your location's average season end date and treatment cycle lengths. The score appears on your dashboard for any hive that hasn't received fall treatment. As the score drops toward 0, it indicates the narrowing window for protecting winter bee quality. The score resets to "treatment active" status when you log a fall treatment.
How do I know if my varroa treatment is working?
Run a mite count 2-4 weeks after the treatment ends and compare it to your pre-treatment count. The efficacy formula is: ((pre-count - post-count) / pre-count) x 100. A result above 90% indicates effective treatment. Results below 80% should trigger investigation for possible resistance, application error, or reinfestation. Log both counts in VarroaVault to track efficacy trends across treatment cycles.
How often should I check mite levels in my hives?
At minimum, once per month (every 3-4 weeks) during the active season. Increase to every 2 weeks when counts are near threshold or after a treatment to verify it worked. In fall, monitoring frequency matters most because the window to treat before winter bees are raised is narrow. VarroaVault's monitoring reminders can be set to your preferred interval for each apiary.
What records should I keep for varroa management?
Each record should include: date of count or treatment, hive identifier, monitoring method used, number of bees sampled, mites counted, infestation percentage, treatment product name and EPA registration number, dose applied, treatment start and end dates, and PHI end date. State apiarists typically expect this level of detail during inspections. VarroaVault captures all of these fields in a single log entry.
Sources
- American Beekeeping Federation (ABF)
- USDA ARS Bee Research Laboratory
- Honey Bee Health Coalition
- Penn State Extension Apiculture Program
- Project Apis m.
Get Started with VarroaVault
The information in this guide is most useful when you have your own mite count data to apply it to. VarroaVault stores every count, flags threshold crossings automatically, and builds the treatment history you need for state inspections and effective management decisions. Start your free trial at varroavault.com.