Project Overview
Alfalfa is an economically important crop in the Intermountain Western United States. Grown primarily as a nutritious forage crop for livestock or as a nitrogen-fixing cover crop, the economic value of alfalfa has increased substantially in recent years.
Managing for insect pests, including the alfalfa weevil (Hypera postica), is a significant challenge in alfalfa production systems. Alfalfa weevil larvae can significantly reduce crop yield and quality by feeding on alfalfa leaves. In the western United States, pyrethroid – a synthetic insecticide – has been the predominant method of managing alfalfa weevil populations; however, given recent environmental and health concerns regarding the widespread use of synthetic insecticides, as well as concerns over the development of insecticide resistance in alfalfa weevils, there is a need to find alternative ways of managing this high-risk pest.
One such alternative for managing alfalfa weevil populations is to shift alfalfa harvests earlier, thereby killing alfalfa weevils during their high-risk larval stage and preventing the majority of larval-induced crop damage. This study assessed the impacts of alfalfa harvest timing on alfalfa weevil survival and crop damage across a wide geographic range in the Intermountain Western United States.
Farmer Takeaways
- Alfalfa producers must consider the trade-offs between alfalfa yield, quality, and insect pest risk; waiting longer to harvest might result in higher yields, but only if insect pest risk is low. If insect pest risk is high, waiting longer to harvest might result in yield and quality reductions by exposing the crop to alfalfa weevils during their highly-destructive larval life stage.
- Windrows may serve as a refuge for adult alfalfa weevils and facilitate alfalfa weevil survival; as such, growers should remove windrows from the fields quickly in order to minimize weevil risk to subsequent crops.
Project Objectives and Approach
Determine the impact of alfalfa harvest date on alfalfa weevil populations in the Intermountain West region of the United States
- Alfalfa production fields were sampled for three consecutive summer growing seasons in Wyoming, Colorado, and Montana. Criteria for field selection included: irrigation, lack of insecticide use within the year, and spatial independence/separation from other sample locations.
- Sampling occurred twice during each summer: once pre-harvest (~1 week prior to alfalfa harvest) and once post-harvest (~2 weeks after alfalfa was cut).
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- Pre-harvest data was collected on alfalfa stem density, height, plant stage, and damage score, along with alfalfa weevil density, life stage, and egg count within several quadrats per field.
- Post-harvest alfalfa weevil count data was collected using a vacuum sampler under and adjacent to windrows, the gathered cut plant material left to dry on the ground prior to baling.
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Determine the impact of alfalfa harvest date on natural alfalfa weevil parasitoids, including the parasitic wasp Bathyplectes curculionis
- Alfalfa weevil parasitism rate was measured by randomly selecting 50 late-stage (third and fourth instar) alfalfa weevil larvae from the sampled populations and dissecting/examining them under a microscope for evidence of parasitoid (B. curculionis) larvae.
Key Findings
Harvest date, alfalfa plant stage, and sampling state (WY, MT, CO) were significant predictors of the proportion of pre-harvest alfalfa weevils that were in early (first or second instar) life stages
- On average, the proportion of pre-harvest alfalfa weevils in their early larval life stages (first or second instar) decreased as harvest date increased. In other words, a higher proportion of alfalfa weevil larvae were observed at early harvest dates, and a higher proportion of adult alfalfa weevils were observed at later harvest dates, suggesting that an earlier alfalfa harvest may be beneficial for minimizing larvae-related crop damage.
- Each state (WY, MT, CO) had significantly different alfalfa weevil life stage proportions, with Montana having the highest proportion of early instar weevils and Wyoming having the lowest proportion. This indicates that specific geographic and climatic conditions may influence alfalfa weevil development.
Maintaining windrows in the field may facilitate adult alfalfa weevil survival
- On average, fields where windrows had been removed had approximately 0.5 fewer alfalfa weevils per square foot than fields where windrows were still present, providing evidence that windrows may provide a refuge for adult alfalfa weevils and facilitate weevil survival post-harvest.
Alfalfa weevil parasitism rates by B. curculionis were not associated with alfalfa harvest date, pre-harvest weevil density, nor the proportion of early instar alfalfa weevil larvae
- There were no clear associations between alfalfa harvest date and the parasitism rate of alfalfa weevil by the parasitic wasp B. curculionis. Longer-term studies are warranted to understand how harvest date-related changes to alfalfa weevil population density may impact natural parasitoids.
Resources
Herreid, J. S., Rand, T. A., Cockrell, D. M., Peairs, F. B., & Jabbour, R. (2024). On-farm harvest timing effects on alfalfa weevil across the Intermountain West region of the United States. Frontiers in Insect Science, 4.
Read MoreLocation
Colorado, Montana, WyomingCollaborators
Tatyana Rand, United States Department of Agriculture
Darren Cockrell, Colorado State University
Frank Peairs, Colorado State University
Randa Jabbour, University of Wyoming
Region
Northwest, Plains
Topic
Insect/Pest Management, Cropping Systems
Category
Grain and Field Crops, Hay and Pasture
Year Published
2024
