Some people like a perfectly green carpet of grass; I welcome the biodiversity of clover, dandelions and other weeds (although I could do without the painful thistles). Of course, I also notice many pollinating insects including bumblebees seem to enjoy visiting the flowers that bloom in the lawn. However, concerns have arisen over the use of insecticides and the health of our pollinators. There has been extensive talk about the collapse of honeybee colonies and declining populations of native pollinators like bumblebees. One area of research is the use of insecticides, especially neonicotinoids, and how they affect bees but most studies were done in lab environments, not in a lawn where the bees would collect nectar from flowering plants. Larson, Redmond and Potter decided to study both clothianidin, a neonicotinoid insecticide, and chlorantraniliprole, an anthranilic diamide, and examine how the insecticides affected bumblebees that foraged on treated lawns.
Neonicotinoids are systemic insecticides that are potent selective agonists of nicotinic acetylcholine receptors in insects, and are applied as sprays or granules during the spring for control of root-feeding grubs and other pests. Anthranilic diamides are a new class of insecticides that activate insect ryanodine (calcium channel) receptors and cause lethal paralysis in sensitive species. Chlorantraniliprole shows low acute bee toxicity, a promising development, but this anthranilic diamide has not been tested in the field.
The experiments described in the PLOS ONE article imported the bumblebee Bombus impatiens and confined the bees to ~9m2 of turf with an estimated 30% white clover that had been treated with the neonicotinoid clothianidin, chlorantraniliprole or remained untreated. There were 10 separate colonies, each with a fertilized queen and 20 workers, for the three treatment conditions, and the bees were allowed to forage for six days. Hives were visually inspected on days 5 and 6, the number of foragers in the plot counted and the hive weighed on day 6 before being moved to a new location where the bees foraged for six weeks on a 700 hectare horse farm without insecticide treatment nor had any of the surrounding farms been treated. The hives were then closed and brought to the lab for assessment of hive weight, live versus dead bees, live versus dead pupae, queen presence, size of honey pot and weights of adult, living bees. The results from this experiment showed that the six-day exposure to clothianidin resulted in greater bee mortality and reduced foraging compared to the untreated or the chlorantraniliprole-exposed bees. Furthermore, the clothianidin-exposed bees gained weight more slowly even after being moved to the farm with no insecticide present and produced no new queens, an indicator that the colony was not thriving. However, the chlorantraniliprole-exposed bees showed a similar profile to untreated control bees in all areas measured, including producing new queens.
A second experiment using the same set up (clover turf with colonies of 20 workers and a fertilized queen) with five replicates of each treatment or control plot, with and without irrigation, and allowed bees to forage on the plot for two weeks before the hives were brought to the lab. The turf was mowed, the clover was allowed to bloom again and a second batch of bumblebees of similar size and age as the first set were introduced to the same plots and allowed for forage for two weeks. The researchers saw no difference between irrigated and unirrigated plots so the data were combined for the same treatment or control conditions. The first set of bumblebees showed the same effect as the previous experiment: more dead worker and pupae and reduced honey pot for the clothianidin-treated plots while the bees exposed to chlorantraniliprole was more similar to bees exposed to untreated plots. However, bees exposed to treated plots after mowing and forming new clover blossoms all looked equivalent whether the plot had been treated with insecticide or not. Interestingly, the authors noted that the bees on the mown chlorantraniliprole-treated plot actually had fewer dead worker bees compared to the bees from the mown untreated plot.
Larson, Redmond and Potter also examined whether bees avoided turf treated with clothianidin or chlorantraniliprole compared to untreated areas. These areas were not confined with imported bees but rather involved daily counts of foraging honey bees (Apis mellifera) and bumble bees (Bombus spp.) with each plot twice in less than an hour over the course of a week. The numbers of bee visitors indicated there was no avoidance of any insecticide-treated areas. All three conditions (untreated, sprayed with clothianidin or sprayed with chlorantraniliprole) had equivalent numbers of bees visiting the clover in the surveyed area.
These experiments demonstrated that exposure to neonicotinoid insecticide did have long-term consequences for bumblebees when they had only been exposed to a treated area for six days, even if the bees had access to untreated plots for six weeks more. The authors acknowledged that the bees exposed to clothianidin could have shown recovery if the experiment had run longer but a delay in producing new queens also means the queens are less likely to survive. However, once clothianidin-treated clover blossoms were removed, bees were not negatively affected by the insecticide. Using alternatives to neonicotinoids like chlorantraniliprole may benefit bee health while allowing humans to keep their green carpet of turf grasses. More studies over the longer term would ensure that alternative insecticides would not impair the health of foraging bees so we can enjoy their buzzing, bumbling and pollinating for many years to come.
Larson, J.L., Redmond, C.T. and Potter, D.A. (2013). Assessing insecticide hazard to bumble bees foraging on flowering weeds in treated lawns. PLOS ONE, PMID: 23776667