Evaluation of Plant Composition and Strip Size on the Effectiveness of Native Plant Conservation Strips for Sustainable Enhancement of Beneficial Insect Communities

View the project final report

• 2010 annual report
• 2011 annual report

Commodities

• Fruits: apples, berries (blueberries), cherries

Practices

• Education and Training: extension
• Natural Resources/Environment: biodiversity, habitat enhancement
• Pest Management: biological control

Beneficial insects and their provision of ecosystem services are at risk of decline in agroecosystems, and an important approach for conserving these insects in resource-limited habitats is to meet their ecological requirements. Previous research has shown that providing areas of flowering plants that bloom throughout the season supply beneficial insects with the resources they need, but little is
known about the importance of wildflower plot size for supporting wild bees, insect natural enemies, or the ecosystem services these insects provide. At the Trevor Nichols Research Center in Fennville, MI, we established wildflowers of equal proportions of flowers from a group of 12 native, perennial species in plots with sizes ranging from 1-100 m2. In these plots we found native bee density responds positively to increasing wildflower plot size, which corresponded to an increase in seed set of three wildflower species within the plots. We also found that natural enemy density, group richness, and diversity of natural enemy groups increased with plot size. The density of insect herbivores was lower in all flower plots than in the control samples, whereas the diversity of herbivore groups was significantly higher in flower plots. Comparing population growth of sentinel soybean aphids among plot sizes, aphid colonies were smaller as plot size increased, resulting in higher biocontrol service index values. Providing beneficial insects with flowering resources resulted in significantly greater densities of native bees and natural enemies as well as increased pollination and pest control than in smaller flower plots or mown grass areas. Our results indicate that the beneficial insects are sensitive to the area of floral resources in the environment, even at relatively small scales.
Therefore, larger wildflower plots are more suitable for the conservation of beneficial insects and their provision of ecosystem services.

Introduction:

Populations of beneficial insects and their provision of ecosystem services, including pollination and biological control, are at risk of decline, particularly in agroecosystems (Biesmeijer et al. 2006; Landis et al. 2008). These declines are particularly due to the scarcity of native and flowering plants, insecticide use, and loss or fragmentation of habitat (Landis, Wratten & Gurr 2000; Goverde et al. 2002b; Carvell et al. 2006). Consequently there has been growing interest in developing approaches to conserve beneficial insects and these strategies often involve integrating floral resource patches into farmland (Bianchi, Booij & Tscharntke 2006; Kremen & Chaplin-Kramer 2007; Isaacs et al. 2008; Letourneau & Bothwell 2008).

Flowering plants can provide necessary resources for beneficial insects in agricultural landscapes including nectar, pollen, and nesting sites. Vegetation structure and flower abundance have been shown to be key factors for insect species richness, abundance, and species composition (Zurbrügg & Frank 2006), so manipulation of the habitat in field margins or around cropped areas by establishing flowering plants and grasses can increase beneficial insect populations in agricultural systems (Long et al. 1998; Kells, Holland & Goulson 2001; Rebek, Sadof & Hanks 2005). A well-designed flowering border adjacent to a field will provide necessary resources for pollinators and natural enemies of crop pests during periods when pest numbers and crop flowers are low, thus maintaining high populations of predators, parasites, and bees supported by provision of nutrients throughout the season (Landis, Wratten & Gurr 2000; Ahern & Brewer 2002; Büchi 2002; Sanchez, Gillespir & McGregor 2003; Wanner, Gu &; Dorn 2006; Wanner et al. 2006). Research has shown that using flowering plant strips adjacent to fields helps support beneficial insects in agricultural landscapes, but much of this work has been done with non-native, annual or biennial flowering plants (Baggen & Gurr 1998; Dufour 2000; Carreck & Williams 2002). Although annual flowers often bloom within one growing season, they commonly require annual sowing, making it costly to properly maintain a well-established flowering strip of annual flower plants (Fiedler & Landis 2008; Tuell et al. 2008). Hence there have also been projects in the North Central Region to help support beneficial insects using native perennial flowering plants that are adapted to the local environment and are less likely to become invasive within adjacent crop fields (Fiedler & Landis 2007a; Fiedler & Landis 2007b; Tuell et al. 2008).

When farmers have been approached about integrating flowering plant strips adjacent to their cropland a common question is asked: “how big do the flowering plant strips need to be?” It is expected that insect population density will be greater in larger habitat patches that have more resources to support populations (Slobodkin, 1980), but few studies have examined this pattern in relation to beneficial insects. Beneficial insects respond positively to the addition of flowering resources in farmland, but different insect taxa respond to these manipulations in varying ways (Fraser et al., 2008; Osborne et al., 2008; Tscharntke et al., 2008), and may also respond to habitat at different scales. Meyer et al. (2007) and Olson and Andow (2008)found that larger grassland habitat areas resulted in greater insect abundance and diversity. In a study by Heard et al. (2007), landscape composition influenced bee abundance, but flower patch size had no effect. Conversely, Meyer et al. (2007) observed that increasing flowering strip size increased abundance and diversity of pollinators. In terms of insect natural enemies, there has been a documented positive response to the size of host patch size (Bach, 1988b; Olson & Andow, 2008), but there is little information on the response of natural enemies or herbivorous insects as a function of wildflower patch size. For many farmers, cropland equates to money, so it is likely not an easy decision for farmers to dedicate potential cropland to flowering plant strips. Better understanding of how beneficial insects and their services respond to the size of wildflower plots will assist farmers’ decision-making for beneficial insect conservation. This project is designed to determine the impact of wildflower plot size on attracting beneficial insects and the subsequent response of pollination and biological control, so that these strips are justifiably cost-effective for farmers.

This research project has three main objectives: 1) Determine the response of beneficial insects to the size of wildflower plots, 2) determine the effect of wildflower plot size on pollination and biocontrol within the plots, and 3) use our results to increase producer and public awareness of using native flowering plants to support beneficial insects in farmland.

Note: The objectives have changed slightly from the initial proposal. We had issues with the establishment of native annual plants and we had to replace the original objective of determining whether native flowering plots composed of a mixture of annual and perennial wildflowers will support beneficial insects earlier and remain established longer than those strips composed of only perennial or annual wildflowers.