Pesticides and bees – an overview

Janet Lowore, Programme Manager for Africa, Bees for Development

The FAO Code of Conduct on the Distribution and Use of Pesticides defines as pesticides, ‘any substance or mixture of substances of chemical or biological ingredients, intended for repelling, destroying or controlling any pest or regulating plant growth’. Pesticides are used to kill fungi, weeds and animal pests, especially insects and mites. Pesticides contain more than 1,000 active ingredients. These ingredients make pesticides effective killers of different living organisms. Many pesticides are designed to kill insects, and this means that in addition to killing the target insect species, it is inevitable that other insects including bees are also harmed.

Our modern era of pesticide use stems from the 1940s. Concern over the unintended negative consequences of pesticide use was flagged in 1962 with the publication of the famous book Silent Spring by Rachel Carson. Carson wrote about how a single application of DDT could continue killing for months and years. This led to an era of greater control and regulation in some countries, but the problem is still with us. The companies that produce pesticides try to downplay the negative impact and spend considerable effort in ‘proving’ their safety. This is not surprising given the amount of money at stake in the industry.

There are different broad groups of pesticides. The older-type pesticides, such as DDT were widely used until the full extent of their harmful consequences caused them to be banned or heavily controlled, along with many other Persistent Organic Pollutants (POPs). A new class of pesticides, neonicotinoids came into widespread use in the 2000s. Following increasing concern about the impact of neonicotinoids on bees, the EU implemented a partial ban in 2013, prohibiting use of some neonicotinoids on crops attractive to bees.

Types of pesticides

Organochlorines

Example: DDT

DDT was widely used in the 1940s to 1970s. It caused widespread and catastrophic harm to wildlife because it does not biodegrade and instead accumulates through the food chain. DDT is now banned for most uses in most parts of the world but is used to kill malaria-spreading mosquitos in some countries.

Organophosphates

Exampled: Malathion

Highly toxic and widely used insecticides used in agriculture, gardens, homes and veterinary practices. Highly toxic to all insects, including bees. Interferes with insects’ nervous systems preventing them from moving or breathing. Malathion is applied in dusts and sprays and is harmful to people if inhaled or comes into contact with their skin.

Carbamates

Example: Sevin

Carbamates are heavily used in agriculture as fungicides, herbicides and insecticides. Carbamate insecticides vary in their spectrum of activity, mammalian toxicity and persistence. Used as either dusts or sprays. Carbamate pesticides kill insects in a similar fashion as organophosphate insecticides but some carbamates are less toxic and degrade faster than organophosphates. Sevin (one example) is moderately toxic to mammals, but highly toxic to honey bees.

Pyrethroids

Pyrethroids are a group of man-made pesticides similar to the natural pesticide pyrethrum. Pyrethroids are a broad-spectrum insecticide and widely used to control insects in agriculture. Considered less toxic to mammals than organophosphates and relatively cheap. Sub-lethally toxic to honey bees and harmful to aquatic wildlife.

Neonicotinoids

Example: Imidacloprid

First used commercially in the 1990s, ‘neonics’ are now among the most popular insecticides in the world. They are coated onto crop seeds and – being water soluble – are taken up and dispersed throughout the plant. This mode of action is called systemic and explains how the toxin enters the nectar and pollen. Sometimes they are sprayed onto foliage. They are especially effective against sucking pests (such as aphids). Increasing evidence that neonics are sub-lethally toxic to honey bees led to a partial ban in the EU in 2013.

Spraying cow pea with pesticide

Photo © IITA

How pesticides harm bees

1. Direct contact via crop-spraying. Bees are at high risk when spraying is carried out when a crop is in flower, or neighbouring plants are in flower, as this raises the likelihood of direct contact between bees and the spray.

2. Ingestion of nectar and pollen from crops which have been seed-treated with neonicotinoids. Neonicotinoids are systemic which means that the toxin is taken up by every part of the plant, including nectar and pollen.

3. Direct contact with dust used to coat seeds. When dust-coated seeds are planted, the dust can become dispersed and accidentally come into contact with foraging bees.

4. Contamination. Residues from treated seeds can be taken up by non-targeted plants and enter water courses. Consequently the residues can be indirectly ingested by bees through this wider environmental contamination.

Understanding the impact on bees

Bees can be killed outright after coming into contact with a toxin. This is called ‘acute lethal toxicity’. Sometimes beekeepers report seeing a mass of dead bees at the hive entrance. This is a sign of ‘acute lethal toxicity’ and is perhaps the most visible evidence of bee poisoning. This can be a consequence of direct contact via crop-spraying.

Bees can also suffer from ‘sub-lethal toxicity’, or in other words the bees do not die, but they become sick or impaired. Bees may or may not recover from these sub-lethal effects. The consequences are much more difficult to detect because the ill-effects may be less easy to see (for example, a bee becomes a less efficient forager) or the ill-effect may occur some months after the application of the pesticide. In the case of ‘sub-lethal toxicity’ linking cause and effect is difficult by direct observation. Sub-lethal effects on bees can include:

• Disorientation and difficulty in finding their way back to the hive

• Reduced foraging ability

• Impaired memory ie cannot remember where the forage is located

• Failure to communicate ie cannot communicate information about forage to other foragers

• Delayed development of bee larvae

• Weakened immune system.

Measuring the cause and effect

Research into pesticide use and impact is difficult. Incidents of acute lethal toxicity can be evidenced by mass deaths near the hive and are easily observed by beekeepers. Bee deaths in the field are hard to notice and might be detected only by a colony dwindling in strength. But poisoning does not always result in instant death. Nerve agents can cause bees to become disorientated or weak and to lose their ability to forage well. Laboratory studies are one way to substantiate cause and effect, but these too are problematic. It is often argued that laboratory tests are unrealistic because it is hard to know the actual dosage ingested by bees ‘in the field’. Pesticides companies have exploited some of these difficulties. For example, if a laboratory study shows that bees die when coming into contact with a particular insecticide, pesticide companies may still defend the insecticide by saying that ‘in real life’ bees receive a lower dose than that tested in the lab. For this reason, it is useful also to analyse the traces of insecticides in the bodies of bees and larvae. Beekeeper experience and observation is also very powerful. Their perceptions are unlikely to convince pesticide companies to withdraw certain chemicals from use, but they can sometimes convince public opinion, local-decision makers and farmers, if they present their arguments clearly.

Spraying pesticide in Ethiopia

Photo © BfD Ethiopia

Bibliography

CARSON,R. (1962) Silent Spring. Penguin Classics; New edition (28 Sept. 2000)

MCAFEE,A. (2017) A brief history of pesticides. American Bee Journal. July 2017.

Ontario Ministry of Agriculture, Food and Rural Affairs. Pollination and bee poisoning prevention. Available from: http://www.omafra.gov.on.ca/english/food/inspection/ bees/pollination.htm#table1 [accessed 20/11/2018

Ontario Ministry of Agriculture, Food and Rural Affairs. Best Management Practices for Pollination in Ontario Crops: Types of pesticides. Available from: http://www.pollinator.ca/bestpractices/images/PDF_ Pesticides%20-%20Types%20of%20Pesticides.pdf [accessed 20/11/2018]

Oregon State University. National Pesticide Information Centre. General factsheet on Imidacloprid. Available from: http://npic.orst.edu/factsheets/imidagen.html [accessed 20/11/2018]

Oregon State University. History of pesticide use. Available from: http://people.oregonstate.edu/~muirp/ pesthist.htm [accessed 20/11/2018]

Pesticide Action Network. Bees and other pollinator factsheets Nos. 1 to 10. Available from: http://www. pan-uk.org/resources/#bees_and_other_pollinators [accessed 20/11/2018]

UNEP Stockholm Convention. The 12 initial POPs under the Stockholm Convention. Available from: http://chm. pops.int/TheConvention/ThePOPs/The12InitialPOPs/ tabid/296/Default.aspx

University of Hertfordshire. Pesticide Properties Database. Properties of Malathion. Available from: https://sitem.herts.ac.uk/aeru/ppdb/en/Reports/421. htm [accessed 20/11/2018]