Enhancing Hedgerow Systems in Fruit Tree Production to Improve Beneficial Insect Diversity and Abundance

Objective 1: To increase diversity and complexity of existing hedgerow systems and measure their impact on pollinator and natural enemy density and diversity

Three locations have been selected for this project: Central Florida (citrus), North Florida (citrus satsuma mandarin), and Georgia (peaches). In each of these locations, the experimental setup will be adapted to the local orchard. Because we are using existing windbreak systems, we need to adapt our treatments for each specific location in collaboration with the local farmer.

Field trial in Central Florida (Citrus): cooperative farmer James Shinn

The citrus farm selected for this experiment is located in Lake Alfred, FL. Our cooperator James Shinn is the owner of half a dozen citrus groves of large size situated within a 10 mile radius. Two citrus groves with uninterrupted 0.5 miles windbreaks will be selected with one field acting as the treatment (enhanced hedgerows) and the other as a control (unenhanced hedgerows). Each grove has windbreaks that are currently made up of a single row of mature eucalyptus. All have room on each side of the windbreak to allow for the planting of flowers and vines on one side, and shrubs on the other side. In the treated field, there will be 6 treatments installed adjacent to the grove, each will be 5 grove tree rows length (row middles are 18-20 ft.), separated by two buffer rows and with treatments replicated 4 times. In this location, the following treatments will be applied: 1) hedgerow alone; 2) hedgerow + shrub; 3) hedgerow + wildflower; 4) hedgerow + shrub + wildflower; 5) hedgerow + shrub + vines; 6) hedgerow + shrub + wildflower + vines.

An additional field situated 1 mile away from the experimental groves will be used as a control to compare a grove with flower enhancement and a grove without flower enhancement.

Field trial in North Florida (Satsuma): Cooperative farmer Kim Jones.

The Satsuma grove selected for this experiment is located in Monticello FL, is 75 tree rows in total length, with each row approximately 24 trees. Along the North edge, a windbreak consisting of two rows of cypress has been erected. The current distance between the windbreak and the citrus grove is about 40 feet. According to Kim Jones, our cooperating farmer, they need 35 feet to operate tractors between the citrus grove and the windbreak, giving us 5 feet to implement our treatments. Because the cypress trees are still short in size, we will only implement vines and flower stripes in this grove. There will be 4 treatments, each 5 rows length, separated by one buffer row, and with treatments replicated 3 times. In this location, the following treatments will be applied:1) windbreak alone, 2) windbreak + vines, 3) windbreak +flower strip, 4) windbreak + vines + flower strips. An additional field situated 3 miles away from the experimental grove will be used as a control to compare a grove with flower enhancement and a grove without flower enhancement.

Field trial in Central Georgia (Peaches): Lee Dickey

Our experimental field will be located in Byron, GA. This field consists of 3-year old peaches with high population of San Jose scale. The windbreak length is situated on the northern side of the grove, reaches approximately 5 m height, and is constituted of a single row of cypress and oaks.

There will be 6 treatments, each 5 rows length, separated by one buffer row, and with treatments replicated 4 times. In this location, the following treatments will be applied: 1) hedgerow alone; 2) hedgerow + shrub; 3) hedgerow + wildflower; 4) hedgerow + shrub + wildflower; 5) hedgerow + shrub + vines; 6) hedgerow + shrub + wildflower + vines.

An additional field situated 2 miles away from the experimental grove will be used as a control to compare a grove with flower enhancement and a grove without flower enhancement.

Plant selection

Plant selection has been based on the following criteria: 1) the plants have to be native to avoid invasiveness in the landscape; 2) the plants can be grown from central Florida to central Georgia; 3) plants provide season-long resources; 4) plants are inexpensive and require low maintenance to favor adoption by growers.

Vines will be Coral Honeysuckle Lonicera sempervirens. This vine has been selected because it is adapted for Central Florida and Georgia conditions, is native to Florida, and can grow in part shade (Brown & Knox, 2007). In addition, these vines have been reported as highly attractive to pollinators and natural enemies (Harris et al., 2016), and require minimal maintenance (http://gardeningsolutions.ifas.ufl.edu/plants/ornamentals/coral-honeysuckle.html). Vines will be placed along the windbreak and supported by an inexpensive system of poles and wires or on existing fence structures. Irrigation systems will be implemented to ensure fast growing and good flowering of the vines.

Flower strips will be constituted of Coreopsis (Coreopsis auriculata). Coreopsis are native to the southeastern United States, fast growing flowers requiring low maintenance, and highly attractive to natural enemies and pollinators (Harris et al., 2016). They can be grown in both Florida and Georgia. In addition, Coreopsis produces flowers with extrafloral nectaries, which provide easily accessible nectar to natural enemies (Wackers, 2004). 

Shrubs will be buttonbush (Cephalanthus occidentalis). Buttonbush are native to both Georgia and Florida, medium growing reaching 20 feet tall when mature, require low maintenance, and have no known serious insect or disease problems. Buttonbush produce white flower heads that are very attractive to bees and butterflies (Madhumita et al., 2003). . Irrigation will be provided to all new plantings to increase immediate growth.

Predatory insect density assessments

A high-power vacuum insect sampler (D-Vac Vacuum Insect Net – Model 24, Rincon-Vitova Insectaries, Ventura, California) will be used to assess the density of mobile predatory insects. This sampling device have an airflow capacity of 21.23 m³/min at the collection head with a 3.75 H.P engine. For each sample, the collection cone will be applied for 30 s onto approximately 2 m² of the canopy of the targeted plant. For each treatment and control plot, the windbreak (that include shrub, flowers, vines, litter) and 1^st and 3^rd rows of the adjacent citrus/peach grove will be sampled in separate bags. The insects will be collected into mesh bags that will be replaced for each sample. Mesh bags will be brought back to the laboratory and placed in a freezer (−4 °C) for 24 h. Insect predators including ladybeetles, syrphidae and lacewings collected within each bag will be counted and identified to the lowest taxonomic unit possible under a stereomicroscope. In addition, yellow sticky traps will be deployed on the border of the windbreak, and at the 1^st and 3^rd rows of the citrus grove (3 sticky traps per plot). Yellow sticky traps will be changed every 2 weeks from April to November. Predatory insects will be counted and identified to the lowest taxonomic unit possible.

Pollinator insect density assessments

Bloom period duration and flower density will be measured weekly throughout the year by counting the total number of inflorescences per plot. Nectar quantity and sugar concentration from wildflowers, vines and bushes will be measured three times during the bloom period for each flowering plant in the treatment plots in order to determine each plant’s relative value to bees and other pollinators. On the day prior to sampling, in all plots in bloom, 10 inflorescences per plot will be randomly chosen and bagged to exclude insect visitors for 24 hours. The following day, bags will be removed and nectar volume per inflorescence will be measured using microcapillary tubes. Additionally, nectar from all inflorescences within a plot will be pooled and tested for sugar concentration using a low-volume handheld refractometer. This will be repeated three times for each species of flowering vine, wildflower, and shrub at each site.

To determine relative plant species attractiveness to pollinators, visitation rates will be recorded monthly to each flowering species within each plot. Each plot in bloom will be monitored for 5-mins during which time a trained observer will walk around the perimeter of the plot and record each individual pollinator visit to a flowering vine, shrub, or wildflower. Bees will be identified to groups including honeybees (Apis mellifera L.), bumble bees (Bombus spp.), and other bees (i.e. Melissodes and Lasioglossum spp.). Visitation rates by other pollinators will be recorded to group including butterfly, fly, wasp, and crawling insect (e.g. beetles). The plant species on which the pollinator was found will be recorded. Observations will be made when temperatures are at least 10 °C and with no precipitation.

Additionally, to investigate density of pollinators and syrphidae at a larger scale, experimental orchards enhanced with flowers and vines will be compared with control orchards without flower enhancement. To do so, white, blue, and yellow “bee bowls” with a 1% detergent solution will be placed on a platform at 1 m height for a period of 48 hrs deployed once per month over the year. Four sets of 3 bowls (one per color of blue, white, and yellow) will be placed within the hedgerow enhanced plots and also three rows into the crop field adjacent to the hedgerow plots. Insects from these bowls will be collected, pinned, and identified to species.

Measure of abiotic factors.

A series of measurement stations will be established on the leeward side at 1H (1H = a distance of one windbreak height), 3H, 6H, if possible 12H along a transect perpendicular to the windbreak. At least two separate transects will be established on the leeward side of each windbreak at each site. Automated weather stations will be placed at each measurement stations to measure and record windspeed, temperature and relative humidity. Windspeed will be measured using HOBO windspeed smart sensors and temperature/relative humidity will be measured using HOBO temperature/relative humidity sensor. Both the sensors will be mounted on stands at a height of 2 m from the ground. Automatic measurements will be taken every 10 seconds and hourly averages will be recorded in data logger. A reference station will be established at each site. Existing weather stations will be used as reference stations where available. (Tamang et al., 2010)

Farmer involvement:

Each treatment has been presented, discussed and approved by farmer cooperators. As mentioned earlier, each experimental design has been adapted to the individual farm operation so that the farmer can continue their operation without any disturbance. All farmer cooperators agreed to help plant the vines, shrubs, and flowers, and maintain them for the duration of the study. The PI and co-PIs visited each farm and had a meeting with each farmer to better assess the dimensions, the potential locations and the logistics associated with this project.

Statistical analyses.

As we expect that most of the data will not be normally distributed, the numbers of key natural enemies including ladybeetles and lacewings as well as pollinators will be analyzed with general linear mixed models with Poisson distribution. Fixed variables will be treatments, field, and date after treatment implementation. Blocks nested within fields will be included in the models as random variables.

Objective 2: To assess the effects of improved hedgerows on pest abundance and rates of herbivory.

A comment on our pre-proposal pointed out that adding companion plants may also attract pests. We agree that a wrong selection of companion plants can exacerbate certain pest problems. As our main objective is to increase natural enemy populations to obtain a better control of some major fruit tree pests, measurements of the density of insect pests directly in the experimental groves will be performed. We will also monitor for pests found in treatment groves (with enhanced hedgerows) and associated with the companion plants that are not found in our control groves, suggesting that they are infesting the orchard via the hedgerows.

In citrus orchards, the density of adult Asian citrus psyllid (ACP) will be measured bi-weekly from the vacuum sampling and sticky trap assessment from objective 1 (Martini et al., 2015). In addition, the density ACP nymphs will be measured bi-weekly by visually inspecting 10 developing flush per plot for psyllids.

In satsuma, the density of rust mites will be assessed bi-weekly on 1cm² area of fruit surface taken on an individual fruit. Magnifying lenses (10X) fitted with 1-cm² grids will be held to count mites. All motile rust mite stages present under the grid will be counted. Similarly, the number of Citrus red scale will be assessed by counting the number of female scales on the surface of the fruit. For rust mites and scales, 10 samples on the 1^st, 3^rd and 6^th rows from each hedgerow plot (for a total of 30 orchard samples per adjacent hedgerow plot) will be sampled weekly during the fruiting season. We are opting to use nondestructive sampling methods; therefore, participating growers will not lose yield due to the experiment.

In peach orchards, San Jose scale (SJS) crawlers (the immature, mobile stage) will be sampled bi-weekly on single trees at 3 distances along two transects for each of the 6 hedgerow treatment plots. Thus, we will sample 6 trees per treatment plot for a total of 36 sampling sites (trees) per orchard. We will sample trees on the 1^st, 3^rd and 6^th rows from the hedgerow along two transects that are perpendicular to the border (hedgerow). Based on SJS development, we will begin to monitor crawler activity at 500 degree-days (base 51^oF and maximum 90^oF starting February 1^st) using a piece of double-sided sticky tape positioned over a strip of black electrical tape and wrapped around an infested branch. Three scale monitoring tapes will be deployed per sampling tree and will be removed and replaced on a weekly basis. We will monitor scale crawlers for 3 weeks during three main periods of activity starting at 500 degree-days, 2,000 degree-days, and 3,500 degree-days. The number of SJS crawlers on a 5 cm section of tape will be counted using a stereomicroscope and seasonal means will be compared across the 6 treatments.

Objective 3: To investigate the effect of hedgerow connectivity on beneficial insect abundance and diversity.

In order to assess how the structure and connectivity of hedgerows affects their relative benefit to beneficial insects, 30 to 50 sites between Georgia and Central Florida will be selected along a gradient of hedgerow connectivity to other hedgerows or woodland patches. Some sites will be on farms owned by our participating farmers (but will not include the experimental fields used for objectives 1 and 2). All the sites will consist of fruit tree orchards (citrus or peaches) with hedgerows. For each location, the density of ladybeetles, syrphidae and pollinators will be assessed by vacuum and bee bowl sampling as described previously. There will be 10 vacuum samples and 6 bowl samples conducted per site and repeated three times a year. Any elongated tree planting with a length of more than 20 m and a width of less than 20 m will be considered as a hedgerow. For each site, landscape information such as hedgerow length, width and height, hedgerow orientation, and percentage of urban, agricultural and natural habitats within a 250 m, 1 km, and 2 km circle radius from the sampling point will be determined. Hedgerow connectivity will be assessed by measuring the total area of uninterrupted tree patch including the hedgerow and the distance between the hedgerow and the next closest forest patch using remotely-sensed land-cover maps (Tischendorf & Fahrig, 2000; Campagne et al., 2006; Cranmer et al., 2012).

To examine the effect of local grove characteristics on ladybeetle and pollinator abundance, total grove area and hedgerow length will be calculated using Arcview®, with aerial photography. Digitized polygons will be applied manually to the sampled groves to calculate grove area and hedgerow length. For each location, the abundance and diversity of natural enemies and pollinators as measured by diversity indices will be correlated to both hedgerow connectivity metrics and to local grove characteristics.    Graph theory will also be used to quantify the physical connectivity of hedgerow networks. In this method, graphs are constituted of nodes and edges. A graph is connected if a path between each pair of nodes exists. Each node is given an order depending of the number of edges which are connected to it. In our case, hedgerows will be assimilated to graph edges and intersections between these hedgerows to nodes. It is possible to distinguish four configurations of hedgerow intersections: only one edge is connected to the node, or 2, 3 or 4 edges are connected to the node. We hypothesize that an area with over 3 edges will have better biological connectivity than areas where node order is lower.

Objective 4. Educate growers to the need of hedgerow conservation, with workshop and field days.

Three field days will be organized during this project, one in central Florida, (Lake Alfred), North Florida (Quincy), and Georgia (Athens). Growers, extension agents, and industry representatives will be encouraged to attend each event. Field days will either be at the cooperative grower’s site or at a local research/extension center and include visits to orchards with improved hedgerows, presentation of the data collected from the project, and demonstration of the ecological services brought by hedgerow conservation. At the end of the workshops a survey will be conducted to assess the proportion of growers that currently have hedgerows and understand what barriers exist to enhanced hedgerow implementation. 

During orchard visits, both researchers and growers will share information on the planting, maintenance, challenges, and benefits of the enhanced hedgerows. For each field day, attendees will be provided with a notebook containing handouts of presentations and copies of educational documents created to provide guidance to growers wanting to implement enhanced hedgerows (EDIS documents for FL field days, UGA Extension Publications for GA).