Alfalfa weevil (Hypera postica) is the most problematic pest in Wyoming alfalfa fields. One of the reasons this pest is so tricky for producers to manage, is because infestations can be highly variable and difficult to predict. They can be very problematic in some years and fields, and not at all in others. As a result, producers have the difficult task of deciding if, how, and when to manage for weevil.
One of our research objectives was therefore to better understand what factors make an alfalfa field more likely to have higher numbers of alfalfa weevil. To investigate this we looked at characteristics of alfalfa fields themselves, such as size, vegetation height, and the amount of weeds. We also zoomed out and looked at some characteristics of the surrounding landscape. Previous work has found that crop fields located in landscapes that contain more non-crop habitats (think shrublands, wetlands, forests etc…) are more likely to have a greater number of natural enemies (predatory insects and parasitoids), who have the potential to suppress pest populations. With this in mind we looked at the amount of non-crop habitat, alfalfa crop, and other crops in the landscape surrounding alfalfa fields, to see if any landscape characteristics were associated with numbers of alfalfa weevil.
We looked at these in-field and landscape factors across 20 alfalfa fields in southeastern Wyoming and found that the amount of alfalfa crops in the landscape and the amount of non-crop habitat in the landscape were both factors associated with alfalfa weevil densities.
When looking at the more immediate landscape (the landscape in the surrounding 500 meters) we found that alfalfa weevil densities were positively associated with the amount of alfalfa crops. This mean the more alfalfa in the immediate landscape, the more likely that a crop field will have greater numbers of alfalfa weevil.
Interestingly when we zoomed out and looked at the broader landscape (the landscape in the surrounding 3000 meters), we found that weevil densities were positively associated with the amount of non-crop habitat. While we’re not entirely sure why weevil densities are higher in crop fields with landscapes containing more non-crop habitat, we hypothesize that these non-crop habitats might contain alternative host plants for alfalfa weevil. For example, yellow sweetclover (Melilotus officinalis) is commonly found along roadsides and in pastures and is another plant that alfalfa weevil will feed on. It’s therefore possible that in southeastern WY, non-crop habitats may still be providing habitat for alfalfa weevil.
Given these findings, if producers have options about where to plant a new alfalfa field, it may be important to consider the proximity of nearby alfalfa fields and the amount of non-crop habitat surrounding them.
Chaplin-Kramer, R., O’Rourke, M.E., Blitzer, E.J., Kremen, C., 2011. A meta-analysis of crop pest and natural enemy response to landscape complexity. Ecol. Lett. 14, 922–932.