Review Article
Behavioral Changes Due to Sub-lethal Doses of Pesticides in Bees
Institute of Agriculture and Animal Science, Tribhuvan University, Nepal
LiveDNA: 977.33050
Bees are indispensable components for both natural as well as agricultural ecosystems1,2. About 75% of crops are animal pollinated1,3 and bees significantly contribute to the pollination4,5. Bee pollination improves both the quality and quantity of crops production6-8. Similarly, insect-pollinated crops are a vital source of nutrition throughout the world9 and among the insects, bees are important groups of pollinators4.
However, the decline in the bee population is a huge concern rising all over the world10-12. Among the different reasons, the widespread use of pesticides is majorly responsible for the decline of bees13-16. Pesticides exposure to bees causes several negative impacts such as altered gut microbiota17, susceptible to pest and diseases18,19, brood development and function20, behavioral changes21-24 and bee colony health25,15. Pesticides also impair on worker performance as foraging efficiency and navigation ability26-30. Further, it is reported that pesticide residue in brood comb decreased adult life span, increased brood mortality and they became more susceptible to pathogens which resulted in reduced honeybee colony affecting their health31. These effects disturbed normal colony cycle32 which led either in bee decline or colony collapse.
Honeybees are social insect and all behaviors are essential to run normally in a healthy colony. For example, foraging behavior to collect pollen, nectar, water and resin33, hygienic behavior for disease and parasite resistance34,35, communication to inform the location of food source to other foragers36, reviewed by I'Anson et al.37. Similarly, in a normal healthy honeybee colony, nurse workers clean the cells, care the broods and support to store the food whereas middle-aged bee maintains the hive, receives and processes nectar. Besides, they guard the nest entrance and after 21 days go for forage38-41. Although there may not be immediate mortality after exposure to sub-lethal doses of pesticides, it does have chronic effects on bee health. Multiple review papers discussed the negative impact of pesticides on bees e.g.38,42. In addition to this recent review on the effects of sub-lethal dose of pesticides on bees is hardly found. Although, Thompson43 and Desneux et al.44 reviewed on effects of sub-lethal doses of pesticides after those multiple new findings have been explored. So, this is essential to keep in account that doing the review is the utmost need for the proper direction in further studies. Hence, this paper mainly reviewed previously published results and behavioral impairment due to different types of pesticides on bee species.
FORAGING BEHAVIOR
Foraging behavior is important to collect the nectar, pollen, water and propolis to the bees and to pollinate the crop6,45,33. Several cultivated crops and wild plants depend on the bee for effective and proper pollination so efficient bee foraging is essential factor4,45,46. However, several previous studies showed that sub-lethal doses of pesticides impair the foraging behavior of bee24,47,48. Nectar or pollen detection in bees is conducted by sophisticated nervous activity which was found disrupted by neurotoxic pesticides49. The sub-lethal dose of imidacloprid and clothianidin impaired the homing and foraging activities of worker honeybees as they failed or delayed to return in the hives or their feeding sites23,24,47,48. For example, imidacloprid treated orally with sugar water at the lowest concentration of 50 μg L1 and higher than 1,200 μg L1 resulted in delayed normal foraging interval and abnormalities in revisiting the feeding sites24. Similarly, another experiment reported that Cry1Ab protoxin, deltamethrin and imidacloprid has been affecting the foraging activity50. According to them, the mean number of foraging honeybees visited was 43.4, 30.7 and 23.7 before, during and after treatment by protoxin respectively. Likewise, in the case of the deltamethrin test, the mean numbers were 1.7, 0.7 and 1.0 before, during and after the treatment respectively. Finally, in imidacloprid mean visited numbers initially was 23.7, during the treatment 4.8 and 20.4 after the treatment.
Other activities in foraging behavior were also found to be affected by the sub-lethal dose of imidacloprid like less nectar or pollen collection23, delay in revisiting for foraging51. Also, due to the reduction of olfactory capacity worker bees became incapable to locate the food source49. In the case of bumblebees, exposure to imidacloprid reduced foraging motivation. They visited the less robotic flowers, slowed down to commence for foraging and did not visit all three provided flower colors compared to the control52.
Including these effects, precocious foraging was reported by multiple studies53,54. For example, imidacloprid treatment with a concentration of 5 μg kg1, ppb induced precocious foraging, fewer orientation flights and reduced lifetime foraging flights by 28%52. Due to sensitivity towards the stressors, less effective foraging and precocious foraging remained seriously responsible for the colony collapse in honeybee25,55,56. Similarly, the application of Diazinon also found inducing precocious foraging in honeybees53.
COMMUNICATION
The application of pesticides also affected communication in honeybees. Treatment with imidacloprid was found to affect the waggle dance perform by foragers57. Similarly, it is reported that communication in honeybee was impaired which resulted in the decline of social behavior after 30-60 min with single-dose and ad libitum administration of 500 and 100 ppb imidacloprid respectively58. Likewise, contamination with 20 ppb imidacloprid decreased the frequency of waggle dances of forager bees59. Also, treatment with parathion with 0.3 μg/bee caused the incorrect direction (angle of dance) on a vertical surface and incorrect distance on a horizontal surface60. Forager bees must remember the location, direction and amount of the food source to inform the colony with dance36. But after the application of the sub-lethal dose of pesticide, the memory of the forager bees got to be impaired61,62 and they often forgot the location or direction of the food source. Also, the application of pesticides affected the development and function of the mushroom body21,63,64 which resulted to affect communication in the bee colony. Also, nAChRs is responsible for memory and learning65,66 and social behavior which are present in mushroom bodies67,68. However, nAChRs were found impaired by the application of pesticides69. Moreover, according to Schmuck57 and Kirchner 70 probably due to impairment by the insecticide on motor neurons signal transmission communication in honeybees gets affected.
HYGIENIC BEHAVIOR
Hygienic behavior is a behavioral response of worker bees that exhibit removing dead, diseased or pesticides infected individuals from the colony71. Disease and parasite resistance of the bee colony depends on the hygienic behavior of workers34,35. There are very few studies on the effect of sub-lethal doses of pesticides on hygienic behavior in the bee. However, it is reported that the hygienic behavior of the colony was significantly decreased when exposed to the pesticides72. For example, honeybee colonies treated with 50 ppb and 100 ppb imidacloprid significantly reduced the hygienic behavior along with the removal of freeze-killed brood by 63.3 and 73.7%, respectively compared to 97.4% in control72.
OTHER BEHAVIOR
All usual activities must go normally in a normal colony but it has been found that the pesticides change such activities. Neonicotinoids (imidacloprid, thiamethoxam, clothianidin, dinotefuran) affects in the motor function and behavioral changes of adult workers of honeybees (Apis mellifera)73. According to Migdał et al.74 aggression, walking, grooming, reversal, mobility, rebuild of comb behavior of honeybee colonies were found affected by the application of different pesticides. Moreover, imidacloprid treated with 100 and 500 ppb a.i., bees were less active, communication capacity got affected and this also results to impair in social behavior53. Sub-lethal doses of pesticides caused more time in self-cleaning, trembling dance, abdomen trucking in comparison to the control bees75. Pesticides cause direct intoxication as trembling, tumbling, abdomen tucking and cleaning, rubbing the hind legs together76.
LEARNING AND MEMORY
As presented the summary of sub-lethal doses of pesticides on bees in Table 1, multiple previous studies reported that learning and memory in bees were found impaired by the application of pesticides10,49,62. For example, exposure of 25.6 μg L1 (20.8 ppb) imidacloprid concentration for four days significantly impaired short-term learning by 87% and memory retention by 85% as compared with control bees62. In another separate study, testing a sub-lethal dose of imidacloprid affected learning performance in honeybee and increased the cytochrome oxidase labeling in the calyces of the mushroom bodies21. Similarly, both neonicotinoid and non-neonicotinoid pesticide groups found causing a significant negative impact on memory and learning in Apis species and Bombus under both chronic and acute application22. Testing of sub-lethal toxicity of different nine pesticides from 2.2-940 μg L1 pesticides concentrations in sucrose solution in Apis mellifera fipronil, deltamethrin, endosulfan and prochloraz, reduced the learning performance whereas bees found to be lacking learning abilities in k-cyhalothrin, cypermethrin, s-fluvalinate, triazamate and dimethoate77. It is also reported that exposure to sub-lethal doses of pesticides (imidacloprid, coumaphos or their combination) significantly reduced the olfactory learning in bees61,73.
There are both chronic and sub-lethal doses of pesticides that harm the brain as well as mushroom body64,78,79 which lowers down the learning ability and memory80-82. In addition, nicotinic acetylcholine receptors (nAChRs) respond to cholinergic neural signaling play a vital role in honeybee learning and memory65 was found affected by the application of pesticides70. As a result, it harms the olfactory learning and memory in honeybees21,47 because of the change in the way that neurons in the honeybee’s mushroom bodies function83. Moreover, the application of the neonicotinoid, imidacloprid down-regulates nAChRs in their brains70, causes brain cell death84, motor function reduced85,73, decreased the hive entrance activity47, predator avoidance ability23, harms foraging48, impaired visual learning86, impaired navigation to the nest29,87 and deficient rewarded olfactory learning61,88,89. Finally, different biological processes like ribosomes, the oxidative phosphorylation pathway, tyrosine metabolism pathway, pentose and glucuronate interconversions and drug metabolism were also found to be affected by the sub-lethal dose of pesticides90, which also might impair the learning and memory of bees.
Table 1: | Summary of the effect of a sublethal dose of pesticide on bee behavior |
Therefore, this clearly shows that even a small amount of pesticides causes severe negative effects on bees learning and memory. As a result, different behavioral changes occurred as they were unable to learn or memorize the activities.
MUSHROOM BODY
The behavior described above primarily depends on the mushroom body in social insects91,92. It plays a vital role in olfactory learning, memory and other behavior in honeybees e.g.93-95. But the application of pesticides found impaired growth, development and function of the mushroom body21,63,64 which resulted in different behavioral changes in bees.
The calyces are important sensory input regions in the mushroom bodies, while sub-lethal doses of imidacloprid affected the neural development of the honey bee brain by immune-labeling synaptic units in the calyces of mushroom bodies. The density of the synaptic units in the region of the calyces, responsible for olfactory and visual functions, found to be decreased after being exposed to a sub-lethal dose of imidacloprid64. Similarly, it was found that sub-lethal doses of neonicotinoid insecticides decreased the synaptic density of the mushroom body calyx of honey bees and the bees exposed to 0.04 μg imidacloprid per bee larvae in the larval stage exhibit an impaired olfactory associative behavior in the adult stage89. Also, the growth and development of the mushroom body of Stingless Bee (Melipona quadrifasciata anthidioides) were found negatively affected by the application of imidacloprid96. They found that untreated bees mushroom body volume was normally increased, whereas, treated with 56 μg a.i./bee with imidacloprid was reduced by 36% by volume at eight days of emergence. Similarly, the mushroom body calyces, relatively volumes were significantly small in worker bees exposed to pesticides as compared to control97. Nicotinic acetylcholine receptors (nAChRs) are responsible for cholinergic synaptic transmission and play vital roles in cognitive and behavioral processes65 which are located in the mushroom bodies67,68. They were also found impaired by the application of pesticides70. Also, it is reported that treatment with 8100 ppb imidacloprid induced mushroom bodies cell death after 1 day of treatment98.
It is concluded that pesticides, especially insecticides, impaired the different behavioral activities in honeybees. Not only pesticides contaminated forager bees but the collected nectar or pollen by them contained pesticides even in trace amounts which caused problems in the colony. Bee behavior depends on the higher functionality of the sensory and integrative nervous system which is affected by exposure to even a small amount of pesticides both at larval and adult stages.
This review paper will be the basis to the researchers to explore further the effect of sub-lethal doses of pesticides in the bee. This review explored that even trace amounts of pesticides impaired bees’ behaviors. Hence, not only exposure to lethal doses causes multiple negative impacts on the bee but the sub-lethal doses are also harmful to the bees.