Location: Pollinating Insect-Biology, Management, Systematics Research
Project Number: 2080-21000-019-103-S
Project Type: Non-Assistance Cooperative Agreement
Start Date: Jun 1, 2024
End Date: May 30, 2026
Objective:
This project is designed to address the need for managing alfalfa pests while simultaneously ensuring the safety and health of alfalfa leafcutting bees (ALCBs) (Megachile rotundata) used for pollination of alfalfa seed crops. Insecticide management strategies that satisfy both requirements are essential. Pest pressure in alfalfa fields often demands insecticide use, while protecting pollinators from harmful levels of pesticide exposure is needed to ensure high pollination yield for optimal seed production and efficient ALCB propagation with minimal pollen ball production. The use of insecticides to control pests while not significantly harming pollinators is a delicate undertaking, with the balance depending on the interplay between bee introduction timing, insecticide residue concentrations, insecticide exposure and accumulation in bees, and toxicity thresholds. The Cooperator has previously observed certain pesticides persisting on plant leaves for three weeks after an application. This may be beneficial for pest control but means the pesticide exposure time for pollinators is lengthy. The Pesticide Dissipation in Agricultural Land (PeDAL) model, developed by the Cooperator, predicts changing residue concentrations in alfalfa leaves following application, providing a quantitative foundation to the pesticide management equation needed. The Cooperator has also demonstrated that for some insecticides, transfer from sprayed alfalfa leaves into ALCB provisions can be a significant pesticide exposure route for ALCB larvae. Nonetheless, several unknowns in the equation still need untangling. In particular, there is much to learn about how bee introduction timing affects exposure and insecticide accumulation in ALCB adults and their nests. By focusing on the following four objectives, the aim is to fill several knowledge gaps that currently hinder solid guideline development for the safe release of pollinators in alfalfa seed crops without compromising pest control.
Objectives
1. Determine how bee introduction wait time affects insecticide accumulation in adult female ALCBs following insecticide application.
2. Determine how bee introduction wait time affects insecticide accumulation in nest materials and the prevalence of pollen balls following insecticide application.
3. Determine how accumulation varies with insecticide formulation and how the insecticide dissipation rate effect exposure to, and accumulation of insecticides in, adult female ALCBs and their nests.
4. Develop an extension to the PeDAL model for use in estimating insecticide exposure and accumulation in adult female ALCBs and their nests under specified application scenarios.
Approach:
Four insecticides commonly used to control insect pests in alfalfa seed crops in the western United States were selected for this study. Chlorpyrifos and lambda-cyhalothrin are considered older generation insecticides (Group A insecticides). Sulfoxaflor and afidopyropen are newer generation insecticides (Group B insecticides). Thirteen 10 ft x 10 ft mesh cages will be assembled in an established alfalfa seed field. Each insecticide will be applied to alfalfa in cages on three dates (3 dates for each of 4 insecticides = 12 cages). Alfalfa in Cage 13 will be used for the control experiment and will not be sprayed with insecticides. Alfalfa leaf samples will be collected at regular intervals following application; these samples will be used to monitor insecticide dissipation loss rates and exposure to bees during nest construction. Bee boards and ALCBs will be introduced to all cages, including the control cage, following various wait times after insecticide application. Bees will be captured and nests will be collected for insecticide quantification. The influence of wait time will be investigated for each of the four insecticides and used to establish the PeDAL model extension. ALCB Health will be monitored by comparing the number of nests and cells produced in each cage.
Field experiments will be conducted at the alfalfa seed field managed by the USDA-ARS Pollinating Insect Research Unit (PIRU) at USU’s Greenville Research Farm in North Logan, UT. Pesticides will be applied at maximum allowed concentrations with a backpack sprayer. Group A insecticides will be applied 21, 14, and 7 days before bees are released into the cages (Day 0). This spray schedule was selected because Group A insecticides are known to be toxic to pollinators and have spray restrictions during bloom. Group B insecticides will be applied on Days 21, 7, and 1 before bee introduction. A minimum of 48 female ALCBs will be released into each cage.
Alfalfa leaves (10 g per sample) will be hand-picked at three randomly selected areas in each cage. Leaf samples will be stored at -20 ¿ in the lab until insecticide analysis. For cages sprayed on Day -21 of the experiment, leaf samples will be collected immediately before application and then at regular intervals for three weeks; insecticide concentrations in these samples will be used to measure insecticide dissipation rates, which will be used in developing the exposure prediction model. On Day 5 of the experiment, approximately 50% of the introduced bees will be captured and 10- 15 ALCB nests removed for insecticide analysis. The remaining nests and bees will remain in cages until Day 14 when nest building should be complete; these nests will be analyzed for pollen ball prevalence using x-ray imaging. Insecticides will be extracted from alfalfa leaves, bees, provisions, and nest materials using Energized Dispersive Guided Extraction (EDGE). Chlorpyrifos, lambda-cyhalothrin, and sulfoxaflor will be quantified using gas-chromatography tandem mass spectrometry (GC-MS/MS); afidopyropen will be quantified using liquid chromatography mass spectrometry (LC-MS). Data will be used to develop an extension of the predictive model.
