Varroa and Robbing Season: Why September Mite Levels Spike

By VarroaVault Editorial Team|Updated Mar 2026

Robbing season can triple the mite load of a healthy colony within 2 weeks if a nearby colony is collapsing. This is the mechanism behind the September shock many beekeepers experience: a colony that was at 1.5% in July, treated correctly, and came back to 0.5% in August is somehow at 3% in September. The treatment worked. Robbing undid it.

Understanding robbing season is essential for fall varroa planning.

TL;DR

This guide covers key aspects of varroa and robbing season: why september mite levels spike
Mite monitoring should happen at minimum every 3-4 weeks during active season
The 2% threshold in spring/summer and 1% in fall are standard action points based on HBHC guidelines
Always run a pre-treatment and post-treatment mite count to calculate efficacy
Treatment records including product name, EPA number, dates, and counts are required for state inspection compliance
VarroaVault stores all monitoring and treatment data with automatic threshold comparison and state export formatting

Why Robbing Transmits Varroa

When a colony collapses from mite infestation, it weakens over weeks. Declining populations, reduced guard capacity, and abundant stores (mite-killed colonies often die with full honey stores) make them irresistible to robbers. Honey bees from strong, well-managed neighboring colonies will attack and rob the weakening hive.

Those robbing bees enter a colony with potentially 15-20% mite infestation on adult bees. They forage inside the hive, pick up mites on their bodies, and carry them home. A single robbing event can deliver dozens to hundreds of mites to a previously clean hive. Repeated robbing over days can overwhelm the treated mite load entirely.

This is why September counts can look like treatment failure even when the treatment worked perfectly.

When Robbing Season Peaks

August 15 through September 30 is the peak robbing period in most US climates. This corresponds to the late summer and early fall nectar dearth: when major nectar sources are finished (goldenrod hasn't started yet, spring and summer flows are over), foraging bees shift to robbing behavior.

The risk is highest when:

Multiple colonies are present in a small area
Some colonies in the area are weakening from any cause (mites, disease, dearth)
Entrances aren't reduced on weaker colonies
There's been a recent treatment that killed many bees in a colony, temporarily weakening defense

Protecting Your Hives During Robbing Season

Reduce entrances on all but the strongest colonies. A one-bee-width entrance is all a full-strength colony needs for summer ventilation. Weaker colonies should have even smaller entrances. This reduces the number of attackers that can enter simultaneously and lets the guard bees defend more effectively.

Remove or destroy collapsing colonies promptly. A colony that's clearly failing from mite infestation should be treated immediately and aggressively, or if beyond recovery, removed and the equipment frozen or treated to kill remaining mites. A collapsing hive left in place becomes a mite dispersal point for every colony within 1-2 miles.

Avoid opening hives during dearth conditions. Opening hives when nectar is scarce can trigger robbing attempts. Keep inspections brief, close hives promptly, and avoid leaving frames exposed.

Count more frequently. During robbing season, your monthly counting schedule isn't enough. Count every 2-3 weeks from August 15 through October 1. Mite loads from robbing can double or triple in 2 weeks. A count that looked fine on August 15 may need emergency treatment by September 1.

If Your September Count Shocks You

If your post-treatment count showed good efficacy in early August and your September count is significantly higher, robbing is the most likely explanation. This is not necessarily a resistance event.

To distinguish robbing reinfestation from resistance:

Resistance: Post-treatment efficacy was below 80% on the August count
Robbing reinfestation: Post-treatment efficacy was 90%+ in August, then counts spiked in September

If it's robbing reinfestation, your treatment worked. You need to treat again (choose a different product class to maintain your rotation), reduce entrances aggressively, and investigate what the mite source in your area is. Is a neighboring operation's colony collapsing? Is one of your own colonies failing?

VarroaVault Robbing Season Alerts

VarroaVault activates a robbing season alert mode between August 15 and September 30. During this window, the count overdue reminder shortens from 30 days to 21 days, prompting more frequent monitoring. Your dashboard shows a robbing season flag with a note recommending increased testing frequency.

If you log a count above 2% after a previous count that showed good treatment efficacy, VarroaVault's trend analysis notes the pattern as consistent with reinfestation rather than treatment failure, helping you distinguish the two scenarios.

Frequently Asked Questions

Why do mite counts spike in September?

Late summer and early fall nectar dearth triggers heavy robbing behavior. Healthy colonies rob weakening or collapsing colonies that often carry extremely high mite loads (15-20% infestation). Robbing bees pick up phoretic mites and carry them home, rapidly increasing mite loads in previously treated colonies. This is the mechanism behind September count spikes in well-managed apiaries: the treatment worked, but robbing reinfestation from a nearby mite source undid the progress.

How do I protect my hives from robbing season mite transmission?

Reduce entrances on all but the strongest colonies to limit robber access. Remove or treat collapsing colonies promptly rather than leaving them as mite dispersal points. Avoid opening hives unnecessarily during dearth conditions. Increase count frequency to every 2-3 weeks during August 15 through October 1 so you catch reinfestation early enough to treat before it becomes severe.

Does VarroaVault warn me about robbing season?

Yes. VarroaVault activates a robbing season alert mode between August 15 and September 30, shortening the count overdue reminder from 30 to 21 days to prompt more frequent monitoring. The dashboard displays a robbing season flag. If your counts show a post-treatment spike consistent with reinfestation rather than resistance, VarroaVault's trend analysis notes this pattern to help you distinguish the two scenarios.

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.

What is Varroa and Robbing Season: Why September Mite Levels Spike?

Varroa and robbing season is a phenomenon where mite levels spike in September due to collapsing colonies attracting robber bees. When a mite-infested hive weakens, neighboring colonies raid its honey stores and carry varroa mites back with them. A colony that tested at 0.5% in August can reach 3% by September through no fault of its own management. Understanding this mechanism is critical for accurate fall varroa planning and treatment timing.

How much does Varroa and Robbing Season: Why September Mite Levels Spike cost?

This article is free educational content published on VarroaVault. VarroaVault itself offers free mite monitoring tools and paid plans for beekeepers who want automated threshold alerts, treatment record storage, and state inspection compliance exports. There is no cost to read the guide, and basic monitoring tracking is available without a subscription.

How does Varroa and Robbing Season: Why September Mite Levels Spike work?

Robbing season works by creating a chain reaction of mite transmission. A collapsing colony weakens its guard bees, exposing abundant honey stores. Robber bees from healthy hives enter freely, collect honey, and pick up hitchhiking varroa mites in the process. Those mites are then introduced into previously healthy colonies. This can triple a colony's mite load within two weeks, overwhelming treatment gains made earlier in the season.

What are the benefits of Varroa and Robbing Season: Why September Mite Levels Spike?

Understanding September mite spikes helps beekeepers avoid misdiagnosing their own management. A treatment that appeared successful in August may look like a failure by September due to robbing, not poor efficacy. Recognizing this protects against unnecessary repeat treatments, guides timing of entrance reducers and robbing screens, and supports better fall colony preparation for winter survival with lower mite burdens.

Who needs Varroa and Robbing Season: Why September Mite Levels Spike?

Any beekeeper managing colonies through late summer and fall needs this information, especially those in areas with high apiary density. Hobbyists with one or two hives near other apiaries are particularly vulnerable since they cannot control neighboring colony health. Commercial beekeepers moving hives for pollination contracts also face elevated robbing risk. Anyone who has experienced unexplained September mite spikes despite correct summer treatment will benefit from this guide.

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.