Effects of Pesticides

Pesticide Toxicity

Pesticide toxicity is a complex issue, with new debates emerging regularly. While lists of pesticides toxic to honey bees may help beekeepers and growers of pollination reliant crops to identify and manage the risks of pesticide application and honey bee poisoning, the following information is also critical to consider.


Hives of bees pollinating an almond orchard. Bryn Jones

Spray Drift

One of the most common causes of bee poisoning is spray drift. This is where a pesticide has been applied to a crop that is not in flower but has drifted onto flowers of other plant species. This problem can often be eliminated by mowing underneath the canopy of horticultural crops before spraying. It is much more difficult with flowering weeds growing amongst a crop. In this case, only non-bee-toxic chemicals should be used.

Other tips to minimise off target spray drift include:

  • Always read and follow the product label directions, including any restrictions. Many labels now detail weather conditions, droplet size, and equipment and spray drift restraints and/or mandatory no-spray zones to help users manage drift.
  • Before you start, check that the weather conditions are suitable for spraying (ie wind speeds between 3–15 km/h, blowing away from sensitive crops/areas, Delta T between two and eight, temperature below 28oC, no inversion layer present). If the weather is unstable or unpredictable, don’t spray. Continue to monitor weather conditions while spraying and stop spraying if it turns unfavourable.
  • Discuss your spray plans with neighbouring properties, particularly if you plan to spray near a sensitive crop or area. This provides neighbours with the opportunity to implement protective measures on their property and can help avoid complaints later on.
  • Check for susceptible plants, animals and areas (eg streams, bee hives) close to the target area and put strategies in place to protect them from drift (eg use a buffer zone or leave an unsprayed buffer next to a susceptible crop).
  • Ensure your equipment is set up and calibrated correctly.
  • Use a nozzle or sprayer setting that produces the largest possible droplet size (coarsest spray quality) to reduce the risk of drift without compromising the efficacy of the chemical.
  • Choose a chemical formulation that is less likely to drift off-target. For example, use amine formulations of 2,4-D instead of 2,4-D high volatile esters which are more prone to drift as vapour during or after application.


It had been assumed that most fungicides are not toxic to adult honey bees. Although this is probably correct, a recent study has demonstrated that the pollen collected by bees from flowers sprayed with fungicides becomes contaminated with fungicides. Some of these fungicides are toxic to honey bee larvae when fed at the same concentration. The effect of fungicides on honey bee larvae has not been extensively researched, so that a complete list of fungicides that are likely to be toxic to larvae is not reported. Also, fungicide warning labels do not normally include warnings about honey bee larvae.

It is therefore good practice not to apply fungicides to flowers unless it is essential for disease control. If the safety of the fungicide to honey bee larvae is not known, the issue should be discussed with the beekeeper before it is applied to flowers. Some fungicides can also affect pollination by affecting the ability of the pollen grain to germinate on the stigma, which may adversely affect pollination. Unfortunately, there have been few investigations on how frequently this will be a problem. Where possible, avoid spraying fungicides onto flowers.

Hives of bees pollinating macadamia trees

Hives of bees pollinating macadamia trees. Geoff Manning


Surfactants are used to improve the effectiveness of chemicals, to decrease the droplet size or to increase the penetration of chemicals on a plant or insect. Some surfactants used with insecticide and fungicide sprays are toxic to bees. When water is sprayed onto bees it usually forms discrete droplets on the surface and does not damage the bees. However, surfactants allow the water to penetrate the body hairs, which will kill bees. Unfortunately, the toxicity of most surfactants has not been tested and they do not carry bee warning labels. Unless it has been demonstrated that a surfactant will not kill bees, it should not be applied to bees even if the pesticide with which it is mixed is non-toxic to bees. Don’t spray surfactants onto honey bees unless they have been shown to be bee-safe.

Water itself can be a problem if it is cold or if it is applied at high pressure or high volume. The effect is similar to when surfactants are used. To avoid killing bees and adversely affecting pollination, it is good practice to avoid applying even bee-safe chemicals while bees are visiting the crop. Care should also be taken to avoid applying sprays directly onto beehives.

Do not apply sprays or water directly on bees or beehives.

Synergism – “Chemical Cocktails”

Another risk for beekeepers and farmers to be aware of, is the risk associated with combining two or more pesticides together, which although “bee-safe” in their own right, may be harmful to bees when mixed with other chemicals. Applying one or more chemical products together is a common practice in some horticultural situations (eg applying a fungicide with an insecticide in orchards).

Honey bees use an enzyme called P450 to detoxify chemicals, and if this enzyme is being used to detoxify one chemical, it may not be available to detoxify the other, resulting in poisoning.

It is difficult for beekeepers and farmers to ascertain which products will pose a threat when applied together. The simple “jar test” practiced by many farmers, only provides an indication of whether or not the combination of chemicals can be applied without solidifying and damaging equipment. It does not provide any indication of the effect that the mixture will have in terms of its efficacy or its impact on bees. In the absence of expert advice, the most responsible approach is for beekeepers and farmers to assume that chemical combinations are toxic, and to avoid this practice in situations where honeybees may be affected. Alternatively, beekeepers should consider relocating their hives to avoid the risk.

Managing the Risk of Honey Bee Poisoning

To reduce the risk of honey bee poisoning events occurring, it is important to understand how honey bee poisoning may occur.

Honey bee poisoning events may occur in some of the following ways:

  • When a chemical is applied directly to a flowering crop while bees are foraging.
  • When a chemical is applied to a crop that is flowering and bees subsequently forage on contaminated nectar, pollen or water or alight on a contaminated plant part.
  • When a chemical is applied to a crop not in flower, but is also applied to non-target plants that are flowering (eg weeds), at the same time.
  • When pesticide drifts onto bees, flowering plants, hives or the bees’ water source.
  • When a worker bee carries contaminated nectar, pollen or water back to the hive, contaminating the colony.
  • When an areas within the bees’ flight path is sprayed.
Hives of bees pollinating

Hives of bees pollinating a sunflower field. Kathy Keatley Garvey

Management Practices for Farmers

There are a variety of management practices that both farmers and beekeepers can do to reduce the risk of a honey bee poisoning event. For farmers, this includes:

  • Reduce the need for chemical use by adopting integrated pest management (IPM) principles where appropriate.
  • Contact the owners of any hives in the area well before spraying, so that they have an opportunity to relocate or protect their hives. Also consider owners of hives on adjacent properties, bearing in mind that bees commonly forage within a 5 km radius, sometimes further.
  • If using spray contractors, inform them of the location of any hives that may be affected and ensure that they understand the importance of reducing the risk of honeybee poisoning.
  • Read and always comply with the chemical product label.
  • Choose appropriate spraying conditions so as to avoid the chance of spray drift affecting non-target flowering crops, hives, and water.
  • Ensure that bees are not foraging in the target area. This is a condition of some product labels and is therefore a legal requirement. This may include bees from hives located on adjacent properties, and feral bees as well. Remember, poisoning bees reduces future pollination potential.
  • Mow flowering weeds inside the target area to reduce the damage to bees that may otherwise forage there.
  • Talk to your agronomist and apiarist to help select chemicals that pose a low risk to bees, whilst still achieving the required outcome for your business.

Management Practices for Beekeepers

Beekeepers can reduce the risk of a honey bee poisoning event by following the below practices:

  • Before placing the hives on site, work with the farmer to plan a chemical application program that works for both parties. If the farmer needs to apply chemicals, discuss when this will occur and how the risk can be managed. For example, the hives may be able to be placed on site after chemical application, or removed prior to chemical application.
  • Ensure that the farmer and neighbouring landowners have your full contact details so that you can be contacted quickly.
  • Notify aerial operators operating in the area.
  • Advise other nearby land managers, such as local councils.
  • Ensure that at least some of your hives are marked, or your apiary is signposted with your contact details (including your mobile phone number) so that other people can contact you urgently if required. Ensure that the lettering is large enough to be read from a distance, as people will not want to move close to hives.
  • Select sheltered areas that offer some protection against spray drift. Consider the prevailing wind and seek advice from locals.
  • Ensure that bees have access to clean water. Bees require a lot of fresh water, and if it isn’t readily available, they may search further afield and drink chemically contaminated water from flowers and other sources.
  • Identify an appropriate area nearby (but at least 3 km away) that hives can be temporarily relocated to if needed.
  • Inspect hives regularly so symptoms of bee poisoning can be identified early.
  • Relocate hives before bees are forced to forage across a wider area than was originally intended. Bees have been known to forage up to a 12 km away from hives when necessary.

Responding to a Poisoning Event

An increase in the number of managed hives available for crop pollination is crucial to the continued prosperity of the agricultural industry. Further development of the managed pollination sector will also provide important opportunities for the honey bee industry. However, a significant barrier in this regard has been the risk that beekeepers face in relation to honey bee pesticide poisoning.

Although it is important to put in place mechanisms to ensure that these instances do not occur, it is also important to understand how to respond to a honey bee pesticide poisoning event if they do occur. The information contained below summarises some of the symptoms that may be associated with the hives, how to best manage hives in affected areas, as well as information about who to report the matter to for investigation.

A honey bee poisoning event.

A honey bee poisoning event. Notice all the dead bees at the front of the hive. Bryn Jones

Identifying the Symptoms of Poisoning

  • Significant numbers of dead bees are found outside the hive entrance.
  • In severe cases, dead adult bees will be found inside the hives as well and brood will die from starvation, overheating or chilling (due to inability of adult bees to feed brood and regulate hive temperature).
  • Adult bees all die within a few days of each other.
  • Most or all hives in an apiary may be affected.
  • Dead adult bees often have their wings unhooked and at odd angles to their body, their proboscis fully extended, and their hind pair of legs outstretched behind them.
  • A lack of foraging bees can be observed leaving the hive.
  • Live adults may look sick (eg move slowly, show signs of paralysis, jerky motion).
  • Remaining bees may behave aggressively.
  • Queen failure may occur within 30 days.

Managing Affected Hives

  • Move hives to a safe area.
  • Remove excess supers so that colonies can stay warm.
  • Consider removing pollen and honey which may be contaminated, and dispose of appropriately if necessary.
  • Feed colonies inside the hive with a 1:1 sugar and water solution until recovery. This helps compensate for the lack of fresh nectar resulting from reduced bee numbers. It may also be necessary to feed either pollen or pollen substitute.
  • Add sealed brood and adult bees from healthy hives if needed. Ensure that young bees are added as well, to assist with the feeding of unsealed brood in the weakened hive.
  • Observe hives for signs of queen failure or supersedure, which may occur a number of weeks after the poisoning event.

Investigation and Reporting

The action that a beekeeper takes will most likely be determined by the severity of the poisoning event and the beekeeper’s relationship with the farmer concerned, or other farmers in the area. Always remember that bees can fly over 5 km, so the poisoning event may not have occurred with the farmer that the beekeeper is working with, it could be a neighbour for instance who does not know that there are bees in the area. Regardless however, it is a good idea to collect samples that can be sent to a laboratory for analysis if required. Proper investigation is important to ensure that farmers, who have done the right thing, are not unfairly blamed. There are three types of samples that can be taken:

  • Dead bees from outside the hive.
  • Dead bees and comb from inside the hive.
  • A swab sample from the outside of the hive.Providing a swab sample from the outside of the hive is appropriate if you suspect that a chemical has come directly into contact with the hives (eg spray drift or aerial application), whereas the other two sample types should be taken for all poisoning events. The following things should be considered when collecting samples:
  • Bees should be picked up with sterile tweezers or gloves, and comb should be cut with a sterile knife.
  • A swab can be taken from the outside of the hive using a clean tissue or cotton wool ball.
  • All samples should be contained in sterile specimen jars, such as the type commonly used for sending honey samples for laboratory testing. If such a jar is unavailable, a clean, sealable glass or plastic container can be used (eg snap-lock bags).
  • If unable to be dispatched immediately on ice, samples should be frozen. This slows the breakdown of both dead bees and chemical residues.
  • Samples should be properly labelled (eg ‘bees from outside the hive’ or ’20 cm x 20 cm swab sample from the outside of a hive’). When submitting a swab sample, it is important to advise the laboratory of the size of the area that was swabbed.
  • 25–40 bees, or at least 20 g, should be collected per sample.
  • For swab samples, area of at least 20 cm x 20 cm should be swabbed.
  • For comb samples, a piece of comb at least 10 cm x 10 cm should be sampled.

Sources: www.beeaware.org.au

Funded by the Erasmus+ Program of the European Union. However, European Commission and Turkish National Agency cannot be held responsible for any use which may be made of the information contained therein.

Designed by Omer Mentes