Introduction

This publication includes information blueberry growers should consider when selecting fungicides and insecticides to apply during bloom and application strategies to minimize harm to pollinators. Insect pollinators, particularly wild and managed bees, are necessary to achieve adequate fruit set and marketable berries in southern highbush blueberry production. Bees facilitate both self-pollination, including pollination within an individual flower or bush, and cross-pollination, or that between bushes of different cultivars. In the absence of insect pollinators, berries may form, but they will be significantly smaller and misshapen and will take longer to ripen than bee-pollinated berries (Danka et al. 1993; Campbell et al. 2018; Mallinger et al. 2021). For these reasons, insect pollinators are essential to the production of marketable and profitable southern highbush blueberries.

Blueberry growers in Florida typically stock their fields with honey bees (Apis mellifera) as well as managed bumble bees (Bombus impatiens) (Mallinger et al. 2021). Wild insect pollinators, including the native southeastern blueberry bee (Habropoda laboriosa), native carpenter bees (Xylocopa spp.), and native butterflies and wasps, also contribute to pollination (Campbell et al. 2018; Mallinger et al. 2021; Rogers et al. 2014). Both managed and wild pollinators are susceptible to pesticide applications, especially when those applications occur during the bloom period when pollinators are actively foraging in blueberry fields. Growers must balance disease and pest protection and adequate insect pollination.

How are insect pollinators exposed to pesticides?

Pollinators can be exposed to pesticides in several ways (Figure 1), including those listed below:

• Direct contact with an aerial spray: most likely to happen if the pesticide is sprayed during the day and in favorable weather for pollinator activity (warm and relatively sunny).
• Contact with the chemical while it is still active on the crop plant during the period of residual activity: most likely to happen when pesticides are applied in the daytime and during weather favorable for pollinator activity. This also includes contact with pesticide residues on flowering weeds within the crop field or in the vicinity of the crop field.
• Drinking contaminated water: this is particularly an issue when pesticides are applied via irrigation. If there are leaks in the drip irrigation system, or if water pools in low areas of the field, bees may drink the contaminated water. Pesticide residues from other modes of application may also be present in surface or groundwater.
• Consuming contaminated nectar and/or pollen: this is particularly an issue for systemic products. If the pesticide is systemic (i.e., taken up by the plant and expressed throughout the plant tissues), it may be present in the nectar and pollen of the crop plant even well after application. Though the concentration of the pesticide within nectar or pollen is often relatively low, consuming contaminated pollen or nectar can have sublethal effects on adult bees or lethal effects on the bee brood (Yang et al. 2008; Whitehorn et al. 2012; Stoner and Eitzer 2012).
• Via bee nesting materials, including soil, mud, leaves, and other natural materials. Wild bees use a variety of natural materials to create their nests. Contaminated soil, leaves, or other materials can harm wild bee larvae living and growing in these nests. Systemic insecticides have proven to be highly mobile, which increases the likelihood that they will contaminate nesting materials (Goulson 2013; Main et al. 2014; Long and Krupke 2016).

How do pesticides affect pollinators?

The effects of pesticides on pollinators can broadly be classified into lethal and sublethal effects. Lethal effects occur when the pesticide directly kills either adult foraging bees or developing brood. Lethal effects are typically measured by exposing adult honey bees to different concentrations of the pesticide both in contact exposure and oral exposure and determining the lethal dose or lethal concentration that kills 50% of individuals (i.e., the LD/LC 50). The lethal toxicity of a pesticide is sometimes examined with developing brood or with other pollinator species (e.g., bumble bees) and can vary significantly across life stages and species (Mussen et al. 2004; Wade et al. 2019).

In addition to lethal effects, pesticides can have a variety of sub-lethal effects on bees that include impaired learning and memory in adult foragers, weakened immune systems in adults and brood, and a reduction in reproduction, including fewer new queens or fewer total offspring. Pesticide exposure can also affect behaviors directly relevant for pollination, such as the overall foraging activity of individual bees or the attraction of bees to the crop plants (Morandin et al. 2005; Mommaerts et al. 2010; Wu et al. 2011; Gill et al. 2012; Tschoeke et al. 2019).

Pesticides can also interact with one another, especially when they are applied in tank mixes. Some pesticides are known to have synergistic effects, i.e., they enhance the toxicity of other pesticides applied simultaneously. This is especially true for fungicides; while many fungicides are not highly toxic by themselves, they have been found to increase the toxicity of insecticides when both are applied together (Pilling and Jepson 1993; Manning et al. 2017; Wade et al. 2019; Bigante et al. 2021). Fungicides have also been found to have significant sublethal effects on bees, including weakening bee immune systems. For these reasons, while many fungicides are not considered highly toxic to bees, care should be taken when applying them during bloom, especially when applying them simultaneously with insecticides.

Selecting Pesticides During Blueberry Bloom

Insecticides and fungicides that are commonly applied during blueberry bloom in Florida are listed below in alphabetical order of the active ingredient (note that this may not be an exhaustive list) along with their general toxicity to bees and aspects of residual activity or persistence in the environment (Tables 1 and 2). The general toxicity rating is based on the LC/LD 50 to honey bees measured through contact and/or oral exposure with practically non-toxic > 50 ug/bee; low toxicity < 50 and > 11 ug/bee; moderate toxicity < 11 and > 2 ug/bee; and high toxicity < 2 ug/bee (1 ug = 1/1,000,000 g and 1 g ~ 1/30 oz). Note that, counterintuitively, higher-toxicity products have a lower LC/LD 50, indicating that less active ingredient is needed to result in 50% mortality. A high LD/LC 50 conversely means that a large amount of active ingredient is needed to result in 50% mortality, and thus the product is less toxic. When selecting a product to apply during bloom, it is important to look not only at its toxicity but at whether it is systemic, whether it will persist in the environment (i.e. have persistent residual activity), and whether it may produce synergisms (interactions with other chemicals that may increase toxicity of one or more of the chemicals).

Tips for Limiting Pesticide Effects on Bees and Other Pollinators

The following tips can help with decision making and reducing pollinator exposure to pesticides during the blueberry bloom period (Figure 2):

• Implement IPM strategies and other control methods to limit chemical sprays during bloom.
• Use existing action thresholds when possible.
• Follow label instructions. Many products with a high toxicity to bees specify that the product should not be applied in any mode of application during bloom and/or when bees are in the crop field.
• When possible, and especially during bloom, select a non-systemic insecticide with a short residual activity and no to low toxicity to bees.
• When possible, avoid using tank mixes of insecticides and fungicides during the bloom period to reduce synergistic effects.
• Apply pesticides, including fungicides and insecticides, in the evening to allow for the longest period of time to pass before bees forage. Cool temps and/or wet conditions may prolong residual activity.
• If using a pesticide with a moderate to high toxicity to bees during bloom, be sure to follow label instructions and also contact your beekeeper in advance so that they can consider moving or covering hives during application and for a period of time after application.

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