Conservation biological control of alfalfa weevil in Wyoming

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Commodities

Practices

Alfalfa weevil is a pest of concern throughout the Western United States. Biological control of the weevil has been attempted through extensive releases of over 17 million parasitoid wasps of the alfalfa weevil in the 1950s and again in the 1980s.  These parasitoid populations still persist in Wyoming, but alfalfa weevil outbreaks continue to be variable and severe. Conservation biological control, managing habitats to promote already existing natural enemies, is a promising avenue for improving control of alfalfa weevil. The overall goal of this project was to determine if alternative habitats at the landscape and local scale have the ability to enhance biological control of alfalfa weevil. At the local scale, a field study was conducted to compare visitation to annual and perennial flowering strips by parasitoids, other natural enemies, and pests. At the landscape scale, parasitism rates and in-field characteristics in producer fields across southeastern WY were measured and compared with landscape cover types. On-farm we found that annual flowering habitats supported both predators and a large number of herbivores, while perennial habitats contained increased predators but did not have increases in herbivores. Looking at the landscape scale, we found that the proportion of crops in the landscape was positively associated with pests and natural enemies at varying spatial scales. In particular, alfalfa weevil densities were positively associated with alfalfa at the 500m scale, and positively associated with natural and semi-natural habitats at the 2000m and 3000m scale. This information may be useful to producers interested in conserving natural enemies to reduce alfalfa pests.

Introduction

Intensification of cropland has lowered habitat diversity in agricultural landscapes leading to fewer alternative resources for natural enemies of pests. Alternative habitats near agricultural fields can provide overwintering habitat, refuge from disturbances, and additional food sources important for many natural enemies (1). Parasitoids are one type of natural enemy that kill their host in the process of parasitism, making them important biological control agents. Non-native parasitoid wasp species have been introduced for biological control of agricultural pests, including the alfalfa weevil (Hypera postica). Although there have been a number of studies that contribute to our knowledge of alfalfa weevil parasitism, it is still difficult to predict this parasitoid’s effectiveness for a given year or field. However, a still unexplored driver of parasitism is the effect of alternative habitats. Furthermore the effect of these alternative habitats can be assessed from the landscape perspective, where producers have no control over the habitat features surrounding their farm, or from an on-farm perspective, where producers can actively manage these habitats. The overall aim for identifying and understanding drivers of parasitism of alfalfa weevil is to improve biological control and reduce the severity of pest outbreaks.

The complexity of landscapes surrounding agricultural fields has been shown to influence the activity and abundance of natural enemies and pests in those fields (2). In particular, more complex landscapes (those with more natural and semi-natural habitat) have been shown to increase the activity of natural enemies in crop fields. In this study I aim to use landscape complexity and in-field characteristics to better predict the density of alfalfa pests and their natural enemies.

On the on-farm scale, incorporating flowering resources into agricultural landscapes can provide parasitoids access to alternative food sources (3) which promote parasitism through 1) increased parasitoid longevity and fecundity and 2) more time searching for hosts. Access to a carbohydrate food source has been shown to increase the longevity and eggs laid by the adult female of the most prevalent parasitoid in WY, Bathyplectes curculionis (4). We therefore hypothesize that providing parasitoids with floral resources near alfalfa crop fields can increase parasitism rates of alfalfa weevil. If this theory can be applied to pest control in intermountain west alfalfa fields, it would give producers another management tool for controlling a chronic and potentially devastating pest. 

References:

1. Landis, D. A., Wratten, S. D., & Gurr, G. M. “Habitat management to conserve natural enemies of Arthropod pests in agriculture.” Annual Review of Entomology, 45 (2000): 175–201.
2. 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.
3. The Xerces Society. Farming with Native Beneficial Insects: Ecological Pest Control Solutions. N.p.: Storey, 2014.
4. Jacob, Helens S., & Edward W. Evans. “Influence of Carbohydrate Foods and Mating on Longevity of the Parasitoid Bathyplectes curculionis (Hymenoptera : Ichneumonidae).” 29.5 (2000): 1088–1095.

The aim of this study was to examine drivers of parasitism of alfalfa weevil by B. curculionis at a landscape and local scale.  At the landscape scale we focus on comparing parasitism rates between multiple field sites with different landscape characteristics and addressing these objectives:

Objective 1: Determine if landscape composition (percentage of crop and non-crop habitat) affects parasitism rates, and densities of pests and natural enemies in producer fields.

Objective 2: Evaluate whether flowering weeds and/or aphid populations in alfalfa fields could be providing parasitoids with in-field resources.

At the local scale we focus on how parasitoids and natural enemies visit flowering alternative habitats, addressing the following objective:

Objective 3: Assess how abundance and community composition of arthropod functional groups (herbivores, predators, parasitoids, and pollinators) differ between different habitat types (annual flowers, perennial native flowers, alfalfa, control of fescue grass).