Progress report for GS21-236
Pecan orchards are often attacked by arthropod pests and sap-sucking aphids are among the group that concerns growers due to their potential to impair tree productivity potential. Aphid management practices in pecan orchards are currently reliant on the use of synthetic insecticides, which are known to negatively impact the environment, non-target organisms (e.g., natural enemies and pollinators) as well as humans. Hence, the development of alternative and more sustainable aphid management in pecan systems should not be left aside. Improving biological control using predatory natural enemies is a plausible option because pecan orchards are known to harbor diverse predators of aphids (e.g. ladybeetles, lacewings, syrphid flies, and minute pirate bugs); however, the specific roles of these biocontrol agents play in pecan fields has received relatively little attention. To advance the augmentative and/or conservational biological control programs, it is crucial to understand the trophic interactions among these common natural enemies. Thus, our objectives are to 1) examine the positive services (i.e. predation) and disservices (i.e. intraguild predation) of predatory arthropods in pecan systems, and 2) determine if services depend on different locations within pecan trees (i.e. in the upper and lower canopies) and vary during the season (early and late season) through molecular gut content analysis. This study will contribute to a strong ecological basis for advancing sustainable pest management in southern US pecan orchards. In 2021, sampling was completed successfully. Samples were taken from the pecan trees during the months of June, July, August, and September coinciding with important pecan phenological stages of the trees. Samples have been processed, sorted, and prepared for near future DNA extraction. We have also learned some important lessons during the field season regarding how predator-prey coexist within the pecan trees in the different strata and the potential interactions we will reveal during the molecular analysis phase. We expect to initiate the molecular phase of this project once sampling and initial sorting have been completed for both seasons.
Our main goal with this project is to understand the specific roles that predatory natural enemies play within the southern US pecan agroecosystems and contribute to a strong ecological basis for advancing sustainable management through:
- Identifying the community of pecan-resident predators that contribute to the biological control of each aphid species (e. black pecan aphids, yellow pecan aphids, and blackmargined aphids).
- Identifying potential intraguild predation events among predators of aphids.
- Understanding how tree strata (upper versus lower canopy) or seasonal differences (early versus late season) play a role in the interactions among predators and with their prey.
Additionally, we will use the information acquired in 1, 2, and 3 to pursue our fourth goal with this project, which is:
- To produce a visual, durable, extension field guide to natural enemies and their role as biocontrol agents of pecan aphids for Georgia pecan growers.
This study will be performed in two experimental stages: field collection (in 2021 and 2022) and laboratory analysis of samples via molecular techniques, as described below:
Site characteristics and experimental design:
The research has been conducted in an experimental field located in Byron, Georgia at the United States Department of Agriculture Southeastern Fruit and Tree Nut Research Laboratory (32.665550, -83.729287). The experimental orchard consists of approximately 110 mature pecan trees var Pawnee. The trees are about 40 years old, standing between 16 to 17 meters tall. The study is a randomized complete block design with two factors [tree strata (spatial) and time (temporal)]. Six trees (blocks) were randomly chosen within the orchard and samples were taken from three locations (treatments) within the tree (i.e., the upper, middle, and lower canopies) and at four points of the season (time series).
In the 2021 season, the pecan orchard was assessed in four critical points of the season that correlates with pecan physiological events; post pollination (June) rapid nut development (July), kernel filling (August), and before shuck split (September). Two samples were taken from three locations within the trees (i.e., the upper, middle, and lower canopies). The two samples were taken from opposite sides of the trees (one on the east and one on the west side) with the aid of a telescopic crawler boom lift (JLG 660SJC). Insects were collected using a reverse blower (i.e., a suction sampler) that ran for 30 seconds and moved gently sideways around pecan branches that are within the reach of the operator standing on the lift basket. Immediately after collection, the samples were placed in gallon-size zip lock bags and stored inside ice coolers to stop or slow down biological processes (more specifically genetic material -DNA- degradation) and other ecological interaction events (e.g., predation)) from occurring among the collected individuals. Samples were brought to the laboratory and stored in a -20 freezer. All predators within the samples are being identified and transferred to individual vials containing 95% ethanol and stored in the -20 freezer until DNA extraction to avoid contamination and degradation. This process will be repeated in 2022.
After samples have been processed we will perform DNA extraction, and then use a multiplex polymerase chain reaction (PCR) approach to screen predators for evidence of feeding on target insect pests (especially aphids) and natural enemies (including other predators and parasitoid wasps). We will use the available aphid, predator, and parasitoid-specific primers in the optimized primer mixes that allow screening for up to 9 target DNAs in one PCR reaction. Predators that are rough to identify because they are damaged or because they are in their immature stages (which can complicate identification) will be sent for DNA sequencing. It is worth mentioning that during these processes, each predator count as one sample. In this way, we will be able to gain not only qualitative but also quantitative information on the frequency each predator eats specific prey or engage in intraguild predation events. Additionally, to have an overview of what the predators use as prey and non-prey food within the pecan system, we intend to select part of the data to create pool predator samples (divided by family or genus) and screen them using the metabarcoding approach that amplifies all insect and/or plant DNA within the polled samples and subsequently send these samples to sequencing..
The results from the molecular analysis will initially be used for a descriptive work of the roles of naturally occurring predators in aphid control as well as their interactions with other natural enemies. That is, we will identify the presence of prey in the predators’ gut as well as the frequency they eat it. Then, identify key “beneficial” species, those with primarily pest feeding and “negative” or neutral species, that tend not to feed on pests of interest or often prey upon key natural enemies. This information will be set up as a comparison between levels of pest control services and disservices. Finally, we will analyze if there are effects of tree strata as well as seasonality, in the structure of the predator community and the aphid management services they provide. To achieve that, we will compare the richness and abundance of natural enemies as well as how the strength and frequency of their pest control versus intraguild predation events may be altered with strata and season period.
In 2021, sampling was completed successfully. Samples were taken from the pecan trees during the months of June, July, August, and September coinciding with important pecan phenological stages of the trees. Samples have been processed, sorted, and prepared for near future DNA extraction. We have also learned some important lessons during the field season regarding how predator-prey coexist within the pecan trees in the different strata and the potential interactions we will reveal during the molecular analysis phase. We expect to initiate the molecular phase of this project once sampling and initial sorting have been completed for both seasons. A more complete report of the first year of this project including the abundance and spatial distribution of biological control agents within the pecan canopy during the 4-month period will be provided in the near future along with other updates.
Educational & Outreach Activities
In the near future, a preliminary analysis of the data collected until the moment will be presented in a scientific meeting (ESA Annual Meeting). And after the sampling period has been completed and data analyzed, a visual, durable, extension field guide to natural enemies and their role as biocontrol agents of pecan aphids will be produced for Georgia pecan growers.