Under laboratory observation, the neonicotinoid imidacloprid was found to affect the mobility of honeybees at low doses. This effect was dose-dependent and changed with time (Suchail et al, 2001; Lambin et al, 2001), revealing that the time of observation could be crucial in detecting some of the more subtle effects of insecticides. In another laboratory experiment, sub-lethal doses of imidacloprid caused significant reductions in mobility. Bees were less active than untreated bees, although this effect was transitory. Bees also showed a loss of ability to communicate, and this could have profound effects upon social behaviour (Medrzycki et al, 2003). Navigation and orientation For some pollinators, visual learning of landmarks is important for spatial orientation. For example, honeybees use visual landmarks to navigate to a food source, as well as to communicate accurately to the rest of the colony about distance and direction to it. Pesticides might affect both the learning of visual patterns during foraging trips and the communication of this information back in the hive. The pyrethroid deltamethrin has been shown to alter the homing trips of foragers treated topically with sub-lethal doses, decreasing the number of flights back to the hive in treated foragers (Vandame et al, 1995). A recent highly sophisticated study performed under semi-natural conditions with honeybees showed that bees eating pollen or nectar contaminated with the neonicotinoid thiamethoxam, even at very low doses, can get lost on their way back home. As a result, they are twice as likely to die within a day, making the colony weaker and putting it at greater risk of collapse (Henry et al, 2012).
“In the case of honey bees, impaired feeding behaviour can induce a drastic decline in hive population. Most of the large-scale farming areas, when food resources are reduced to cultivated plants, the repellent effect of pesticides may reduce pollen and nectar uptake, potentially leading to a demographic decrease of the colony.” – Desneux et al, 2007 Pyrethroids are probably the best-known case of pollinator-repellent insecticides, and this avoidance behaviour was assumed in many cases to be an adaptation for reducing the risk of exposure (Desneux et al, 2007). However, it was subsequently shown that pyrethroid applications during peak foraging activity (in broad daylight) result in high exposure levels (see discussion in Desneux et al, 2007). “Therefore, a repellent effect must not be misconstrued as providing any protection against exposure to pesticides.” – Desneux et al, 2007 Pesticide exposure can also reduce the capacity of bees to detect food sources. For example, fipronil applied topically at low concentrations to honeybees decreased their capacity to sense low-sucrose concentrations by about 40% relative to the capacity of untreated bees (El Hassani et al, 2005). Imidacloprid repels some pollinators (pollinating flies and beetles), so their exposure might be reduced, but as a result pollinators could starve if the only feed available is from imidacloprid-treated crops in agricultural regions. In addition, if insects do avoid visiting the flowers of treated crops, this could impact adversely on crop yields, depending upon the strength of the response and how abundant the pollinators are (Easton and Goulson, 2013).
The neonicotinoid imidacloprid has also been shown to impact honeybee foraging trips at low concentrations, causing delays in feeding trips and increased losses when bees are fed sub-lethal doses of the pesticide (Yang et al, 2008). Foraging trips of honeybees were reduced by between 20% and 60% when exposed to either the neonicotinoid imidacloprid or the pyrethroid deltamethrin. Deltamethrin also induced changes in learning capabilities (RamirezRomero et al, 2005).
Bees in Decline Greenpeace Research Laboratories Technical Report (Review) 01/2013 31