By Raymond A. Cloyd
Professor and Extension Specialist in Horticultural Entomology/Plant Protection
Kansas State University
Department of Entomology
Recently, there have been concerns associated with the potential direct and indirect effects of neonicotinoid systemic insecticides on bees. The neonicotinoid systemic insecticides include imidacloprid, thiamethoxam, dinotefuran, clothianidin, and acetamiprid. These active ingredients are present in many products available to both professionals and homeowners. The concern is affiliated with exposure from foliar applications and exposure to pollen and nectar that may be contaminated via applications to the soil or growing medium. These insecticides have a higher selectivity for insects compared to mammals than other insecticides in the chemical classes, organophosphate and carbamate. The mode of action of the neonicotinoids is as agonists at the nicotinic acetylcholine receptors of insects. The specific proposed benefits of any systemic insecticide (not just the neonicotinoids) includes 1) plants are generally protected throughout most of the growing season without the need to make repeat applications, 2) minimal issues regarding drift (when applied as a drench or granule) compared to foliar applications of insecticides, and 3) less direct impact on natural enemies and bees.
Despite these benefits, there have been many sound scientific studies conducted, although primarily under laboratory conditions, that have demonstrated both lethal (direct mortality) and sub-lethal (affecting reproduction and/or survival) effects associated with neonicotinoid systemic insecticides on honey bees and bumble bees. However, this entire issue regarding the concern of how neonicotinoid systemic insecticides may directly and indirectly affect bees is related to two factors: one is the Oregon incident that occurred in June 2013 in which a landscaper sprayed 55 blooming European linden trees with dinotefuran (Safari) for control of aphids…so the use of the insecticide was mainly as a contact…and ended-up killing approximately 55,000 bumble bees in a Target parking lot in Wilsonville, OR. However, the label states specifically “This product is highly toxic to bees exposed to direct treatment or residues on blooming crops or weeds. Do not apply this product or allow it to drift to blooming crops or weeds if bees are visiting the treatment area.” What should have been a great extension/outreach opportunity to educate people on the importance of reading the label was “twisted” into a means to promote the banning of neonicotinoid systemic insecticides. It is difficult to understand why the Oregon Department of Agriculture did not stress the point regarding the off-label use more. In fact, the landscaper was fined a modest $555.00 for killing 55,000 bumble bees and not reading the insecticide label…what is wrong here?
The second instance was related to a 2013 publication [Gardeners Beware: Bee-Toxic Pesticides Found In “Bee-Friendly” Plants Sold At Garden Centers Nationwide] by the Friends of the Earth based on an extremely poorly constructed preliminary study regarding the sampling of nursery plants treated with neonicotinoid systemic insecticides from three locations (CA, Washington, D.C., and MN). Overall, this study demonstrated ‘nothing’ especially since the study failed to quantify the concentration of active ingredient in the pollen and nectar (they simply combined leaves, stems, and flowers). Furthermore, only seven out of thirteen (54 percent) of the plants sampled (tomato, squash, saliva, gaillardia, pumpkin, zinnia, and aster) tested positive for one or more neonicotinoid insecticide. Therefore, no general inferences can be justifiably made on the impact of neonicotinoid systemic insecticides on bees. However, this information was accepted as demonstrating that ornamental plants treated with neonicotinoid systemic insecticides are toxic to bees. So, this represents another instance of misinformation or lack of substantial reliable information.
It is important to remember that the impact of systemic insecticides (non-neonicotinoids) on bees and other pollinators is not new phenomenon. Below are three publications from studies that demonstrated the direct impact of certain systemic insecticides on bees.
1) Glynee-Jones, G. D., and W. D. E. Thomas. 1953. Experiments on the possible contamination of honey with schradan. Ann. Appl. Biol. 40: 546-555.
2) Jaycox, E. R. 1964. Effect on honey bees of nectar from systemic insecticide-treated plants. J. Econ. Entomol. 57: 31-35.
3) Lord, K. A., M. A. May, and J. H. Stevenson. 1968. The secretion of the systemic insecticide dimethoate and phorate into nectar. Ann. Appl. Biol. 61: 19-27.
One question that needs to be addressed is—will the banning of neonicotinoid systemic insecticides in actuality preserve bees? In all likelihood producers and homeowners are going to use contact insecticides such as carbaryl (Sevin) and pyrethroid-based insecticides as sprays on a frequent basis, which in the long-term will be more detrimental to bees than systemic insecticides. In addition, there could be problems associated with pesticide drift, direct and indirect effects on natural enemies (e.g., parasitoids and predators), issues affiliated with residues on leaves and flowers, and the potential for insecticide resistance due to the selection pressure placed on insect and mite pest populations.
Presently, the emphasis has been on the neonicotinoid systemic insecticides; however, what about other systemic insecticides such as acephate (Orthene), disulfoton (Di-Syton), and dimethoate (Cygon) that are still commercially available to homeowners although both disulfoton and dimethoate have or are being phased-out. Then what about chlorantraniliprole (Acelepyrn) and spirotetramat (Kontos)? Will these be the next targets?
What professionals and homeowners can do in regards to utilizing pesticides without harming bees is to use selective products (e.g., Dipel) with short residual activity, time applications accordingly when bees are less active such as the early morning or evening, and use plants in landscapes and gardens that are less susceptible to pests.
Below are a number of general comments and questions regarding neonicotinoid systemic insecticides that need to be taken into consideration:
1. Can neonicotinoid systemic insecticides (NSI) be absorbed into plants, and become present in pollen and nectar thus making floral resources toxic to bees?
2. Are NSI present in pollen and nectar at concentrations that cause lethal or sub-lethal effects?
3. Will exposure via pollen and nectar result in lethal, sub-lethal effects, or no effects?
4. Can NSI contaminate or accumulate in weeds and/or wildflowers?
5. Exposure to contaminated pollen and nectar may increase honey bee susceptibility to parasites and pathogens by compromising the immune system.
6. What about interactions and multiple factors? For example, what about the effect of combination products and interactions with fungicides?
7. What about timing of application? In general, and based on scientific research, residues of the active ingredient may occur at higher levels in pollen and nectar when applications are made before or during bloom.
8. What about the effects of the metabolites, which tend to be more toxic to insects, associated with the NSI?
Some of the points mentioned above have been demonstrated based on scientific research. Furthermore, there are numerous factors that may influence variation in residue levels in pollen and nectar including timing of application, application method, application rate, number of applications (carry-over effect), formulation, water solubility, plant type and flower morphology, plant age and size, soil type and organic matter content, environmental conditions (e.g, light intensity), and bee age and size. This clearly highlights the complexity of the issue.
Also, it should be noted that honey bees can travel four miles from a hive and they typically gather nectar and pollen from a wide-range of flowers (in fact, the primary food source of the European honey bee is clover and alfalfa) during the season thus possibly diluting “contaminated” pollen and nectar by collecting from different flowers.
In conclusion, we need to be aware of the direct and indirect impact of all pesticides (e.g., insecticides, miticides, and fungicides) on bees. Furthermore, it is critical to read the label of any pesticide to determine if there are any effects on bees. We also have to understand that there is no clearly defined “smoking gun” because many factors may be contributing to bee decline globally including parasites such as the varroa mite (Varroa destructor), pathogens (e.g., Nosema cerane), loss of habitat, nutritional deficiencies, habitat fragmentation, intense management strategies (“bee feedlots”), poor beekeeping, and pesticides.