Primefact Varroa mite management options in NSW

Integrated Pest Management (IPM) and Varroa control

Integrated pest management (IPM) uses a variety of techniques to keep Varroa numbers below a level where they cause economic damage to the colony and beekeeper. IPM involves:

• Understanding the Varroa life cycle (PDF, 410.15 KB)
• Rigorously monitoring Varroa levels (at least 4 times per year, including before and after treatment) and evaluating results
• Applying integrated pest management controls appropriate to your beekeeping operation, environment and colonies’ developmental phase if Varroa levels are too high
• Repeat monitoring and management as much as necessary, rotating different types of chemical controls to prevent Varroa from developing chemical resistance.

1.Understanding the Varroa life cycle

Varroa’s life cycle and development follows the seasonal development and breeding of its honey bee host. Honey bees and varroa cycle through four phases over time. The length of each phase can vary widely between climate types.

For example, control measures required in cold temperate versus tropical beekeeping conditions may be different. In some locations, there is one cycle of the four phases per season while, in other locations, more than one cycle may occur or a cycle may be skipped altogether.

Four life cycle phases of honey bees and Varroa (Figure 1):

• Dormant (occurs only if there is a broodless period – typically Winter or dearth)
• Population Increase (typically Spring)
• Population Peak (typically Summer)
• Population Decrease (typically Autumn)

Varroa mite populations increase and decrease with the seasonal changes in colony population. Mite populations reach their highest levels soon after the brood and adult honey bee populations reach their peak. When the bee population and the amount of bee brood decline, mite numbers drastically increase on adult bees.

Where Varroa has been established in a state or country for several years, seasonally Varroa numbers eventually decrease, along with the adult bee population. The size of the mite population at the start of the bee ‘Population Decrease’ phase is critical because the colony needs to be healthy enough to rear enough bees to survive the ‘Dormant’ phase.

During broodless periods, all mites are carried on adult bees. However, many locations in Australia will not have the ‘Dormant’ phase. When the ‘Dormant’ phase is skipped or absent and there is reduced, continual brood rearing, mite reproduction may be continuous, which is likely the case in most mainland Australian conditions.

Understanding the seasonal changes your colonies’ go through, particularly around brood development and broodless periods, will inform the best management control tools for your situation.

2.Rigorously monitoring Varroa levels and evaluating results

Routinely monitoring Varroa levels in a portion of your colonies in each apiary is critical to making good Varroa control decisions.

Monitoring at least 4 times per year is recommended and can be done by alcohol wash, soapy water wash, or sugar shake of 300 bees per colony using methods shown on this video at https://bit.ly/46s6cqi.

How many hives should I monitor?

Beekeepers with only a few hives are best to monitor all their hives regularly. Beekeepers with many hives should regularly monitor a proportion of all their hives. Where beekeepers have greater than 10 hives per apiary and are only monitoring a portion of their hives 4 times per year, it’s recommended to sample colonies both from the centre and outer edges of the apiary.

• Small apiary
  • Fewer than 10 colonies at an apiary = all colonies should be monitored for Varroa
• Medium to large apiary, multiple apiaries
  • Greater than 10 colonies per apiary = Sample 10 or more colonies in each apiary. If possible, include colonies from the centre as well as the outer edges of the apiary.

Varroa infestations will vary between colonies, even in the same apiary, so the more colonies that can be monitored, the greater accuracy you’ll have evaluating your mite levels. In the early years of Varroa establishment, where populations have not stabilised and re-infestation is common, sampling more than 10 colonies per apiary and rotating which colonies are monitored is highly recommended.

Evaluating mite numbers

The number of Varroa you find will depend on your location, how long Varroa has been established in the surrounding environment, the time of year, your previous Varroa IPM, and other situation specific considerations. When your colony needs chemical treatment depends on the brood rearing phase your colony is in and the number of Varroa you find in the results of your monitoring (i.e. alcohol wash, soapy water wash, sugar shake).

EXAMPLE: During spring, a NSW beekeeper alcohol washes 300 worker bees (1/2 cup or 125ml volume) from a brood frame of one hive and finds a total of 10 Varroa mites.

To calculate the Varroa %: divide the # Varroa by ÷ # bees sampled and multiply X by 100.
10 Varroa ÷ 300 bees = .03 X 100 = 3% (3 mites per 100 bees).

Because it’s spring and the beekeeper’s colony is in a ‘Population Increase’ phase, a 3% Varroa infestation means immediate Varroa control is needed. If Varroa control is not taken, colony loss is expected.

The following Varroa treatment thresholds (Table 2) were developed by the Honey Bee Health Coalition, a group of North American beekeepers, bee researchers, government agencies and other honey bee experts. Over time, Australian bee experts will develop specific Varroa treatment thresholds for Australia’s unique conditions, which may change the current recommendations.

Table 2. Treatment Thresholds by Colony Phase: Varroa % = # Varroa/100 adult bees (adapted from Honey Bee Health Coalition 2022 by E. Frost). In brackets (# mites found) is the total mites found in an alcohol wash, soapy water wash, or sugar shake of 300 worker bees.

Quick mite infestation evaluation in the apiary

To simply evaluate Varroa infestation levels in the field, the beekeeper can count the number of mites detected per 300 worker bee sample and reference Table 2 control guidelines.

3.Applying integrated pest management (IPM) controls

IPM involves many different control methods for a beekeeper to use throughout the season. IPM encourages the use of non-chemical controls first, known as cultural and mechanical controls, in order to minimise chemical use and residue risk, and the risk of Varroa developing chemical resistance. When a Varroa threshold is met, chemical control is required to ensure colony survival. The type of control tools (Table 3) you use will be specific to your beekeeping scale and style, apiary environments and colonies’ developmental phase (Table 4). IPM tools include the following controls.

Table 3. Effectiveness of common Varroa controls (adapted from Jack and Ellis 2021 by Nadine Chapman, Emily Noordyke, E. Frost). * Effective where Varroa populations have not developed resistance.

The IPM cultural, mechanical and chemical controls described above (Table 3) will vary in effectiveness in Australia depending on local climates and seasonal specifics. Seasonal control recommendations for Australian conditions are outlined below (Table 4).

Table 4. Varroa control options by seasonal phase (adapted from Honey Bee Health Coalition 2022 by E. Frost)

4. Repeat monitoring and management as much as necessary, rotating different types of chemical controls to prevent Varroa from developing chemical resistance.

Varroa progress rapidly through their life cycle. Rotation of different chemical controls (such as alternating synthetic and non-synthetic treatments) is critical to ensure Varroa doesn’t develop resistance. When Varroa is repeatedly challenged with a specific active ingredient, it’s likely to develop resistance. Increasing dosage or use of more frequent applications may result in the speedy development of Varroa chemical resistance. This reduces the number of options for effective mite control. Rotating products with different active ingredients (e.g. IPM) during the season or between different seasons, will help slow development of Varroa mite resistance to critical chemical control tools.

Acknowledgements

This PrimeFact has undergone critical internal and external peer-review, including by Dr. Nadine Chapman, Dr. Ben Oldroyd and Dr. Doug Somerville.

References

Cameron, J.J., Ellis, J.D., Integrated Pest Management Control of Varroa destructor (Acari: Varroidae), the Most Damaging Pest of (Apis mellifera L. (Hymenoptera: Apidae)) Colonies, Journal of Insect Science, Volume 21, Issue 5, September 2021, 6, doi.org/10.1093/jisesa/ieab058

Goodwin, M., Taylor, M. (2007). Control of Varroa: A Guide for New Zealand Beekeepers. New Zealand Ministry of Agriculture and Forestry.

The Keystone Policy Center on behalf of The Honey Bee Health Coalition (2022). Tools for Varroa management: A guide to effective Varroa sampling and control. honeybeehealthcoalition.org/wp-content/uploads/2022/08/HBHC-Guide_Varroa-Mgmt_8thEd-082422.pdf