Pesticides & Pollinators

By Dr. Chrissie A. Segars, Dr. Eric Reasor & Dr. Chris Williamson
From the February 2024 Issue

Well maintained grass lawns are integral to landscapes. They contribute to quality of life, aesthetics, recreation, and environmental health. However, there is a complex and increasingly scrutinized relationship between man- aged turfgrass, pesticide use, and the well-being of pollinators.

Pollinators are a vital resource and can potentially be impacted by the application of pesticides in managed turfgrass. Yet such use is common to prevent damage and negative impact caused by weeds, diseases, or other insects. So how do we find a balance?

These interactions emphasize the need for solid research and a balanced approach to maintaining healthy turfgrass while also safeguarding pollinator populations. Pollinators provide an essential ecosystem service, necessary for reproducing over 80% of plant species and contributing to 35% of global crop production, according to Entomologist Dr. Timothy D. Schowalter, author of Insect Ecology. If you were to put a number on it, this “service” is valued at over $18 billion annually in the U.S. alone, where honeybees and wild pollinators aid in pollinating more than 100 crops, according to the USDA.

Pollinators are a vital resource and can potentially be impacted by the application of pesticides in managed turfgrass. Yet such use is common to prevent damage and negative impact caused by weeds, diseases, or other insects. So how do we find a balance?

Published research studies have aimed to understand the interaction of pesticides and pollinators in managed turfgrass, which is vital to offering solutions to mitigate potential impacts. Based on those studies, this article summarizes simple and practical steps that can be taken to reduce the impact of pesticides on pollinators.

(Photo: PBI-Gordon)

Mowing Practices

Mowing is the most common practice in managed turfgrass. Flowering weeds are common in turfgrass and can be attractive to pollinating insects, while equally being the bane of property managers. To prevent pollinator exposure to direct contact with pesticides, it’s recommended to mow off the flower heads of common broadleaf weeds (i.e., white clover) before treatment. In cases where the insecticide label specifies not to mow before treatment, managers can perform the application first, then follow it with mowing to remove treated flowers.

Research on nectar from broadleaf weed flowers treated with systemic insecticides suggests that if new flowers grow in treated areas, the residues transported from the roots of the weeds to the flowering portion are below levels considered hazardous to pollinators.

Formulations

The formulation of an insecticide is a crucial factor in determining its potential hazard to pollinators. Formulation research is limited to insecticides and further exploration into other pesticides will be beneficial.

While the inherent toxicity of the active ingredient is important, how the insecticide is delivered or applied and how it interacts with the environment can significantly affect its impact on pollinators.

Turfgrass insecticides come in various formulations, such as liquid sprays, granules, or seed dressings or coatings. They are designed to be taken up (absorbed) and distributed (translocated) within the plant after application. While the inherent toxicity of the active ingredient is important, how the insecticide is delivered or applied and how it interacts with the environment can significantly affect its impact on pollinators. The key distinction lies in a formulations potential to directly be found in flowering parts of plants (e.g., nectar and pollen) that pollinators feed and rely on.

A study comparing systemic insecticide liquid sprays to their granular counterparts found that granular applications pose a reduced risk to pollinating insects when applied to flowering weeds in turf. Granular products are applied to the soil and then typically watered or irrigated. This process helps the granules move into the soil where plant roots can absorb them. As a result, insecticide residues are absorbed and transported throughout the plant to protect against pests. The systemic movement of the insecticide within the plant focuses on preventing damage from below-ground pests or pests that feed on various plant parts, such as leaves.

Because granular applications are less likely to come into direct contact with the above-ground, flowering portions of plants, they pose a reduced risk of being found in nectar and pollen. This can be an important consideration for protecting pollinators when flowering weeds or ornamental plants are present in the treated area.

Liquid formulations can potentially pose more of a risk because they typically make direct contact with the weed flowers and/or any visiting pollinators. However, this risk can be mitigated through irrigation or the removal of flowering weeds through mowing, as discussed.

Therefore, the choice of formulation, along with tactful or careful application timing and methods can play a critical role in minimizing the potential exposure and impact of insecticides on pollinators while still effectively managing pest issues in turfgrass.

Herbicides & Pollinators

As a manufacturer of primarily herbicides, PBI-Gordon has facilitated and partnered research with professors and researchers across the U.S. on evaluating the effect of commonly used herbicides on pollinators in managed turfgrass areas. These partnerships include Purdue University, Auburn University, Davey Tree, and Virginia Tech. Research by Virginia Tech was recently published in a peer-reviewed journal.

Three field studies were conducted at Virginia Tech between 2021 and 2022 in white clover–infested turf-type tall fescue. The objectives of this research, by Navdeep Godara, were to assess the influence of various herbicides on white clover floral quality, bloom density, and pollinator foraging in weedy turfgrass. Treatments included: a non- treated control, 2,4-D; Dicamba; MCPP; Trimec Classic™; SpeedZone®; and a non-active ingredient formulation blank.

Herbicide treatments were applied to the tall fescue/white clover stand and then pollinator foraging and white clover flowers were assessed every morning for eight days following the application. Bees—including honeybees, solitary bees, and bumble bees—represented the majority (79%) of insect visitors to white clover flowers. Honeybees were the most abundant, accounting for 80% of total bee visits. Hoverflies, butterflies, and wasps made up smaller proportions.

The Landscape As Ecosystem

The study found bee visitation was reduced by ≥55% per day in all herbicide treatments. Honeybee visitation, specifically, was reduced by ≥60% daily regardless of herbicide applied. In all cases, insect visits to herbicide-treated plots reduced to zero in less than two days.

White clover flower density was an influential factor in pollinator foraging frequency. Following herbicide application, white clover flower density was reduced by ≥21% per day. White clover flower quality was also affected by herbicide treatments. The herbicide 2,4-D discolored white clover flowers by 16% per day, while other herbicides discolored flowers by more than 20% per day.

However, flower degradation was slower than the decline of insect visits. In other words, pollinators ceased to use the white clover as a food source more quickly than the herbicides had a visual effect on flowers. These findings indicate that while herbicide treatments did affect the appearance and quality of white clover flowers, the decline in flower quality was not the primary factor driving the rapid departure of pollinators from the treated areas. Other factors, such as the presence of the herbicide, might have influenced the behavior of pollinators more than the visual quality of the flowers. The study suggests that reduced insect visitation may be associated with nectar depletion, which happens rapidly after herbicide treatment.

The study results indicate that honeybees and other insect foragers frequently leave herbicide-treated areas in less than two days following a treatment. Therefore, the potential interaction of herbicides to pollinators is low. This rapid decline in insect visits doesn’t correspond with loss of floral density and flower quality. To protect pollinators from insecticide exposure, this study suggests a solution: treating weedy flowers with herbicides two days before applying insecticides. But more research is needed to support this.

Tips At A Glance

As you schedule Spring applications, keep these tips in mind:

Mowing prior to insecticide application removes flowers which reduces chances of pollinators interacting with pesticides. If the label recommends not mowing prior to insecticide application, mowing can be done after application to remove contaminated flowers.
The choice of formulation is critical for reducing risk to pollinators. Granular products are highlighted as a reduced risk option.

Insect visits to herbicide treated areas were zero within two days, according to a recent study. Reduced insect visitation may be associated with nectar depletion, which happens rapidly after herbicide treatment, thereby also reducing pollinator risk.
There is potential to lessen pollinator exposure to insecticides by treating weedy flowers with synthetic auxin herbicides several days prior to an insecticide application; but more research is needed.

These are practical, research-based options for those striving to mitigate potential adverse impacts and safeguard pollinators. For more info, visit the references at right.

Dr. Segars, Dr. Reasor, and Dr. Williamson are research scientists with PBI-Gordon Corporation. Segars is the Western Research Scientist located in Bend, OR. Reasor is the Southeast Research Scientist based in Dallas, TX. Williamson is the Midwest Research Scientist located in Defiance, OH. 

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