My overarching research goal is improve biocontrol by spiders in agroecosystems by determining the value of drainage ditches to their conservation and community assemblage. This goal can be divided into two research objectives:
1. To assess changes to the spider communities living in drainage ditches throughout the year
2. To determine the extent of spider movement from agricultural drainage ditches to adjacent agricultural fields
The purpose of our project is to ascertain the value of agricultural drainage ditches for conservation biocontrol in croplands, focusing specifically on the role of spiders as natural enemies. Spiders are the most abundant generalist predators in agroecosystems, consuming 4-8 million tons of prey items annually worldwide. Due to the generalist feeding strategy of spiders and their abundance in agroecosystems, spiders have recently become an interest of pest managers in sustainable agriculture. Drainage ditches are common features on farms in the eastern United States that present an undisturbed habitat on farms that may influence spider diversity and abundance in croplands. Spider diversity is important for sustainable agriculture as it can affect pest suppression in croplands, as more diverse spider communities are thought to provide enhanced pest suppression when compared to less diverse communities. Drainage ditches have previously been examined to determine their benefits to beneficial arthropods, but the potential benefits of ditches to spiders have not been determined. This information is important for sustainable agriculture as it tends to rely on natural enemies to suppress pest insect populations. Identifying sources of natural enemies, such as spiders, near croplands is important for sustainable farmers in order to preserve and bolster pest predation in their fields. Previous research has suggested that spiders may migrate to croplands from the areas surrounding the field as prey items (such as agricultural pests) become more abundant. Yet, the level of overlap between spider communities outside of croplands and those within croplands remains unknown. Determining if spider communities migrate from drainage ditches to croplands throughout the growing season is important for sustainable agriculture, as this information will guide drainage ditch management practices to enhance conservation biocontrol.
The methodology that will be used in data collection for this project remains the same as that proposed in the original proposal for this grant. The majority of the data collection for this project will be conducted in summer 2019. We have included a copy of the original proposed methodology that will be implemented this summer for reference. We plan to begin sampling in early 2019 for Objective 1, and along the growth cycle of a soybean field planted at Cutfresh Organics in mid-May for Objective 2.
Objective 1: Assess changes to the spider communities living in drainage ditches throughout the year
Assessing changes in spider communities within drainage ditches throughout the year is essential for understanding how effective drainage ditches are at fostering spider diversity. To complete this objective sampling is necessary along multiple drainage ditches to determine if drainage ditches at different farms possess different spider communities. All sampling described in for this objective will take place within two drainage ditches on the properties of Cutfresh Organics and Homestead Farms Inc., two organic farms on Maryland’s Eastern Shore. One ditch from each farm will be sampled, which were selected due to their distance from confounding sources of variation, such as forests. In order to gain a representative sample of the spider communities at the ditches of these farms, foliar sweeps and pitfall traps will be utilized. Foliar sweeps will be conducted monthly for 12 months along 30m of the drainage ditch. These samples will be processed by hand and all spiders collected will be preserved in 100% EtOH. 3 pitfall traps will be deployed monthly at each drainage ditch. Propylene glycol will be used to capture ground-dwelling spiders in the pitfall traps. These traps will be collected 5 days after deployment. Spiders collected during this time will be hand-picked from the pitfall traps and stored in 100% EtOH until identified to genus. Both of these sampling methods are necessary to collect an accurate and representative sample of the spider community in each ditch. Spider community diversity will then be calculated using Hill numbers to determine the typical number of spider genera present in each ditch in each month. Bray-Curtis dissimilarity will then be used to analyze the communities to determine how similar the ditch communities are as the year progresses and to each other. The significance of these differences will be assessed using one-way ANOVA and multiple means comparisons.
Objective 2: Assess the movement of spiders from agricultural drainage ditches to their associated agricultural fields
The sampling descried here will also take place on the property of Cutfresh Organics, along the same drainage ditch and crop field described above. To access spider movement from drainage ditch to agricultural field, spiders must be collected in the crop field, drainage ditches, and the area between the crop field and drainage ditches. In addition to the sampling sites created to assess the spider communities, 4 sampling sites will be created along a parallel beginning in the drainage ditch and ending in the crop field. One sampling site will be created in the drainage ditch and another will be created in the crop field, away from field margins. Additionally, two sites will be created at the field margin, between the drainage ditch and crop field, along the parallel the first two sampling sites were created upon. All 4 of these sites will be sampled at 4 distinct stages of the soybean growth cycle: after the field is seeded, at growth stage R1, growth stage R4, and just after harvest. These sampling dates encompass the entire crop growth cycle and as a result, changes in the spider communities inhabiting the cropland and drainage ditches can be examined as the season progresses. Stages R1 and R4 were selected to determine the spider and pest communities present with and without soybean pods present. Foliar samples composed of 100 sweeps will be conducted along within a 30m square within each sample site on each sampling day. Sweeping this area will provide an accurate representation of the spider community present on the soybean plants at the time of sampling. Additionally, one pitfall trap per sampling site will be deployed at each sampling date and retrieved 5 days later to collect ground running and wandering spiders. The use of pitfall traps is necessary to collect a representative sample of the spider community on the ground at the sampling site. The use of these 2 sampling methods should provide an accurate representation of the spider communities present at within the drainage ditch and in the field. Only using either sampling method would create a sampling bias towards spiders of specific feeding guilds and habitats. All sample sites and sampling will be replicated 3 times in the drainage ditch and crop field. Foliar sweep samples will be processed by hand and stored in 100% EtOH. Pitfall trap samples will also be handpicked to remove spiders and soybean pests, which will also be stored in 100% EtOH. Additionally, any economic soybean pests will also be collected and identified from the pitfall and sweep sampling methods and curated in a similar fashion. Knowing how abundant soybean pests are at the time of sampling will allow us to see how spider abundance changes in response to changing prey population numbers as the growing season progresses. Ethanol will used to curate our collection instead of pinning due to the fragility of spiders when they are pinned. All spiders collected throughout the study will be identified to genus, while common soybean pests will be identified to the lowest taxonomy. Effective spider taxa will be calculated through the use of Hill numbers, and community differences will be analyzed using Bray-Curtis dissimilarity. These metrics will provide both the number of typical spiders present in these communities and a measure of how similar the communities are. Significance community differences between the field and drainage ditch will be analyzed through ANOVA and multiple mean comparisons.