Neonicotinoid Turf Insecticides and Pollinators
A disturbing trend observed the last several years is the rapid and dramatic decline of honeybee populations. While most enjoy honey and are familiar with beekeeping as a hobby, it may be less well known that healthy honeybees along with various native bee species are an essential part of agricultural production. Bees are responsible for the pollination of a wide range of agricultural crops each growing season; crops valued in the billions of dollars that are critical to the stability of our food system.
Public and private interests are currently scrambling to identify the mechanics of this pressing issue, commonly referred to as honeybee decline or colony collapse disorder (CCD). It is important to note that commercial honeybee colonies are affected along with wild, native pollinator populations. Current theories on the honeybee decline phenomenon point to a factor or combination of factors among parasites, disease, low genetic diversity, poor nutrition, loss of habitat, management stress and pesticide use. The state of honeybee health has been detailed in a joint comprehensive report released in 2013 by the USDA and EPA: https://www.usda.gov/sites/default/files/documents/ReportHoneyBeeHealth.pdf
Concerning the role of pesticides, neonicotionoid insecticides such as imidacloprid, clothianidin and thiamethoxam have been implicated as a potential contributing factor to honeybee decline. Neonicotinoids at present are used widely throughout the world in both agricultural and horticultural settings because they are effective, convenient, and much less toxic to vertebrates than older materials. Neonicotinoids are labeled for control of a range of insects, but the most common use pattern in turf care is as preventive materials for the control of white grubs (applications targeting grubs are typically made between mid-June and late July in the Northeast).
Regardless of any possible role of neonicotinoids in honeybee decline, every applicator has a continual obligation to apply all pesticides in a responsible, informed, and careful manner. The most fundamental aspects of Integrated Pest Management (IPM) involve the reduction of pesticide use to the lowest possible level, and minimizing the impact of applied pesticides to non-target organisms and the greater environment. Best management practices for reducing neonicotinoid contact with bees essentially parallel the guidelines for responsible IPM. Preventive white grub control is on the minds of many turf managers beginning in late spring. When considering a decision to use preventive control for white grubs, ask yourself the following questions:
Do I need preventive control for grubs? Other than employing good fundamentals to grow healthy and stress-resistant turf, there are no specific cultural practices that reliably lessen injury from white grubs. Therefore, preventive grub control insecticides (applied before damage occurs) are commonly used in situations where user expectations will not permit turf damage from white grub feeding or associated damage from foraging animals or birds. A preventive approach is usually preferable to the most effective curative (applied after damage occurs) insecticide, trichlorfon, which is a more mobile material with higher toxicity to vertebrates. Trichlorfon also cannot be applied on school grounds in Massachusetts, thus preventive applications are the only option for turf managers in such areas.
Best practices, however, dictate that preventive grub control should be used only if certain conditions are met. Preventive control only makes sense when a certain tolerance threshold for grub damage exists, and there is 'probable cause' to believe that grub populations may exceed the threshold within the current season. Such 'probable cause' normally consists of a documented history of white grub damage based on previous scouting at the site. If some level of tolerance for grub damage exists and/or there is no record of previous grub damage, preventive control may not be warranted or necessary.
Do I need 'blanket' applications? Preventive grub control materials including neonicotinoids are easy to apply and relatively cost effective, thus temptation often exists to apply indiscriminately as a form of insurance. Preventive materials, however, should only be applied to specific areas that have a documented history of damaging populations. For example, if a residence has a history of grub damage to the back yard but no record of damage in the front, then the preventive material should only be applied to the back yard. This saves budget, time and labor resources while at the same time reducing unnecessary application and possible adverse impact to non-target organisms and the environment.
Am I using the correct approach? Timing is a critical consideration when using a preventive approach for white grub control. Neonicotinoid insecticides have little or no effect against the large, mature grubs that are present in the spring and in the fall. The appropriate preventive use pattern is to time the application so that the material is in place when the grubs are in the most susceptible stage of development, which is immediately after egg hatch. The life cycles of the various grub species found in New England are relatively similar: adults fly in early summer and lay eggs in late June to late July; larvae feed on turf roots from early July through mid-autumn and again in the spring; pupae are present in the soil for a week in mid-June to mid-July; and adults emerge early the following summer to complete the cycle. Neonicotinoids for grub control, therefore, must only be applied within the period from approximately mid-June to early August in most years. Applications at other times will be markedly less effective against grubs, while still carrying the potential to affect non-target insects like bees.
Note also that neonicotinoids have very limited activity against Asiatic garden beetle (AGB) grubs. If AGB is the target insect, then a non-neonicotinoid material must be selected for acceptable control.
How can I minimize potential bee impact when applying? Best practices for application involve techniques to minimize exposure of foraging bees to direct application or application residue. Do not apply neonicotinoids to turf when commonly associated forage plants, such as clover, are in bloom. This might mean applying either earlier or later in the preventive window for grubs, depending upon prevailing conditions. Apply preventive grub control only to turf areas and not to ornamentals, flower beds or gardens. Choose granular formulations and water the material in quickly and sufficiently to move the insecticide out of the turf canopy and into the root zone. If using liquid formulations, spray under calm conditions to reduce the potential for drift. Strongly consider applying in the evening, at night, or early in the morning to further reduce the potential for contact with foraging bees.
Are there alternatives to neonicotinoids for preventive grub control? Chlorantraniliprole (AceleprynTM for commercial applicators, GrubExTM for homeowners) is a non-neonicotinoid insecticide that is extremely effective against all white grub species (as well as caterpillars) and has no activity against bees, ants, or wasps. The product also has a very low level of vertebrate toxicity, so much so that a signal word is not required on the label, not even 'Caution'. An important aspect of using chlorantraniliprole is to account for the fact that it takes 60 to 90 days to fully dissipate in the soil for optimum effectiveness against grubs. Therefore, the preventive window is earlier than for the neonicotinoids, usually mid-April to early June in most years.
Paenibacillus popilliae ('milky spore') and various strains of entomopathogenic nematodes are labeled for white grub control and have no known impact against bees. Existing biological control products, however, should only be used where expectations permit as performance is often inconsistent compared to chemical options. Current biologicals also vary considerably in terms of their availability, cost and effectiveness against specific white grub species.