A recent study has provided valuable insights into the optimal timing for spraying insecticides on alfalfa seed fields to minimize harm to pollinators. Kelsey Graham of the USDA-ARS Pollinating Insects Research Unit in Logan, Utah, and Kimberly Hageman of Utah State University sought to balance effective pest control with the protection of these crucial pollinators, which are vital to both crop production and biodiversity.

It is generally recommended to apply pesticides in the evening or early morning when pollinators are less active. By avoiding peak pollinator activity during midday, farmers can reduce the risk of pesticide exposure to pollinators while still managing pest populations, offering a more sustainable approach to crop protection. This study tested if and when an evening application would be more favorable than an early morning application.

“We wondered if applying pesticides at night was effective at reducing residue exposure for bees the next day since most pesticides are likely to undergo slow dissipation at night,” Graham said. “Since each type of pesticide behaves differently in the environment, we chose to investigate the behavior of three insecticides used by alfalfa seed growers with the aim of providing tailored recommendations.”

Hageman added, “Not much is known about the timing of adult lygus bug activity or their preferred positions within the alfalfa canopy. This was an important data gap because pesticides are most effective against pest insects that are in the upper plant canopy during application so that droplets land directly on them, or they are actively crawling on recently sprayed leaves.”

The study found that evening applications generally led to higher pesticide residue levels over a longer period compared to morning applications. Therefore, the recommendations for application of insecticides when applied during mainly warm and sunny conditions are:

Prebloom “clean-up” applications:

• Bloom is less than one week away: a morning application to ensure faster pesticide dissipation and safety of pollinators when bloom occurs.
• Bloom is more than one week away: an evening application to reduce speed of pesticide dissipation and boost efficacy against target pests.

• Use pesticides less toxic to bees, such as flonicamid (Beleaf). Consider evening applications to reduce dissipation rates and hopefully promote efficacy against the target pest.
• If sulfoxaflor (Transform) is needed, wait until close to the end of bloom to avoid bee exposures and conduct an evening application to avoid direct contact with pollinators.

“We hope that growers use this research to help make decisions about insecticide applications to keep bees safe, increase pollination rates, and increase efficacy against target pests,” Graham said. “The key take-home message is that the timing of insecticide applications is important for both bee safety and efficacy against lygus. Carefully consider if a shorter or longer dissipation is needed based on the status of the crop and what is being applied.”

Further research is planned to better understand insecticide exposure to bees through various materials, such as alfalfa leaves and pollen, and how these exposures impact bee health and their ability to pollinate. “We are excited to fill in the gaps in knowledge about how pesticide exposures differ,” Hageman said. A full copy of the final report can be found at: https://alfalfa.org.

This article appeared in the February 2025 issue of Hay & Forage Grower on pages 12-13.

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