Project : USDA ARS

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Research Project: Basic and Applied Approaches for Pest Management in Vegetable Crops

2023 Annual Report

Objectives
1. Develop and enhance germplasm for host plant resistance of sweetpotato and watermelon that are resistant or tolerant to economically important pests, including whiteflies and soil dwelling pests. 1.A. Develop and characterize watermelon germplasm with resistance to whiteflies and incorporate the resistance into advanced breeding lines. 1.B. Develop sweetpotato germplasm clones that are resistant to soil dwelling pests and have desirable horticultural traits. 2. Assess whitefly-virus-host plant interactions and effects of biotic and abiotic factors on vegetable pests and their biological control agents. 2.A. Determine the effect of biotic and abiotic factors on populations of whiteflies and biological control agents of whiteflies in vegetable production systems. 2.B. Assess the impact of biotic and abiotic factors on whitefly:host-plant:virus interactions and whitefly endosymbionts. 3. Develop new or improved methods for the management of insect pests (including whiteflies and soil dwelling pests) and whitefly-transmitted viruses in vegetable crop production systems. 3.A. Identify and characterize genomics factors and develop novel genomics-based biotechnologies that would impede virus acquisition and transmission from whiteflies to plants. 3.B. Characterize genetic diversity and population structure of the sweetpotato weevil within the U.S. 3.C. Characterize infochemicals and plant-based chemicals affecting vegetable pests (e.g., click beetles, sweetpotato weevil and whiteflies) for use in detection, monitoring, and biologically-based management. 3.D. Identify and characterize sources of pickleworm resistance in cucumbers. 4. Develop sweetpotato germplasm lines adapted to low input, sustainable production systems, especially lines that are productive under weed competition. 4.A. Identify and characterize sweetpotato germplasm that is tolerant/competitive with weed pressure within sustainable production systems.

Approach
Conduct laboratory, greenhouse, and field experiments to identify sources of resistance and evaluate genetic populations to determine resistance against the sweetpotato whitefly in watermelon, against soil insect pests, weeds and whitefly-transmitted viruses in sweetpotato, and resistance against pickleworms in cucurbits. Assay chemical and physical mechanisms of resistance to pests using tools including gas chromatography-mass spectrometry (GC-MS), and Y-tube olfactometers. Use PCR-markers and other genomic technologies, such as genotype by sequencing, to identify sequences linked to the studied characters and to locate controlling genes on linkage maps. Cross appropriate germplasm to facilitate the incorporation of resistance into advanced breeding lines or new cultivars. Assess the competitive advantage against weeds of sweetpotato genotypes with more vigorous growth habits in comparison to less competitive conventional cultivars, identify competitive genotypes with good horticultural quality, and evaluate them as a component in integrated management systems for conventional and organic growers. Use a recurrent mass selection breeding approach to generate sweetpotato clones with high levels of resistance and good horticultural characteristics. Continue ongoing searches for new resistances or tolerances among watermelon and sweetpotato accessions from the U.S. Plant Introduction System and other collections. Investigate the influence of climate and biotic factors on insect populations and secondary endosymbionts and virus transmission by using field and controlled environments. Study the epidemiology of whitefly-transmitted viruses using biological assays and molecular techniques. Infochemicals used by vegetable pests in mate- and host-finding will be assessed using chemical, electrophysiological and behavioral studies for pests such as click beetles. Make improved plant germplasm available for use by the vegetable industry.

Progress Report
Research addressing Objective 1 was conducted on host plant resistance. Research on the transmission of cucurbit leaf crumple virus (CuLCrV) and sweetpotato leaf curl virus (SPLCV) to watermelon and sweetpotato, respectively, by the sweetpotato whitefly was continued with laboratory and field tests. Watermelon cultivars determined to be susceptible or resistant to CuLCrV using agroinoculation techniques were tested for virus transmission by whiteflies. A field study was conducted to determine the rates of SPLCV dissemination from infected sweetpotato in adjacent fields and in the natural environment to healthy sweetpotato. New quantitative techniques for CuLCrV detection using digital PCR were developed for measuring virus titer in whiteflies and plant material. We are conducting multiple bioassays on selected pepper lines for their resistance to whiteflies. The pepper lines were selected based on their response to the guava root knot nematode. In collaborative research, over 25,000 seeds from breeding nurseries were harvested by ARS researchers and 3,000 seeds were sent to North Carolina State University per collaborative agreements. Sweetpotato germplasm (~ 800 samples) was genotyped with a 3K SNP (single nucleotide polymorphisms) array developed in collaboration with Breeding Insight at Cornell University. Selection of improved sweetpotato germplasm was continued with over 5,000 1st year seedlings and over 150 advanced clones evaluated in replicated field plots. A single breeding nursery was established to develop new germplasm with insect and nematode resistance. Sweetpotato weevil samples were obtained from locations in Florida, Louisiana, South Carolina, and Texas. Genome sequencing data from 40 sweetpotato weevils from Georgia, Hawaii, South Carolina, and Texas was used to create KASP markers for genetic characterization and these markers are being validated on samples collected from Georgia, Hawaii, and South Carolina. In collaborative research between Fort Valley State University and ARS, the population dynamics of insect pests and beneficial insects (pollinators and natural enemies) on 24 snap bean cultivars was assessed. Cultivar ‘Jade’ had high whitefly resistance, and cultivars ‘Greencrop’ and ‘PV-857’ harbored the lowest number of adult potato leafhoppers and tarnished plant bugs. Temperature and relative humidity were related to the population levels for whiteflies, Mexican bean beetles, bees, and predator ladybug beetles. Research addressing Objective 2 was conducted on the effect of biotic and abiotic factors on insects and virus transmission. After the establishment of isoline colonies and subsequent transfer and maintenance of subsets of the isolines at different temperatures, collections of representative whiteflies from each temperature treatment have continued. The isoline colony was determined to harbor two secondary endosymbionts along with the primary endosymbiont Portiera aleyrodidae. Digital PCR is being used to quantify the baseline endosymbiont compositions and evaluate changes in whitefly endosymbionts. New sensitive and specific primers were designed for detection of the sweetpotato leaf curl virus from whiteflies and plant materials. Evaluations of shifts in endosymbiont communities in response to temperature are underway. DNA was extracted from individual males and females and groups of male or female whiteflies for 14 temperature-time point collections, and digital PCR evaluations of endosymbionts are ongoing. Colonies of whiteflies from the same parent have been established on selected vegetable crops, and cultures of tomato yellow leaf curl virus and sweetpotato leaf curl virus are being maintained in ongoing transmission research. The effects of whitefly maternally inherited facultative endosymbionts on virus transmission were studied. Evaluations of the transmission efficiency of CuLCrV sweetpotato whitefly populations harboring different secondary endosymbionts were performed using CuLCrV infectious clones. Studies to determine the effects of endosymbionts on whitefly transmission and host plant virus titers are ongoing. Additional whitefly samples were collected from collards maintained at a range of temperatures and from overwintered populations in Tifton, Georgia, and Charleston, South Carolina, to evaluate shifts in endosymbiont communities in response to different temperatures and overwinter survival. Collaborative work was conducted on whiteflies in vegetables by ARS in Charleston, South Carolina, with researchers at Oklahoma State University and São Paulo State University. A new technique for discriminating B. tabaci cryptic species was developed and endosymbionts present in Oklahoma populations of B. tabaci were detected. ARS researchers in Charleston, South Carolina, described a new species of Encarsia parasitizing a whitefly (Aleurocybotus species) in Florida; this whitefly is a pest on ornamental Muhly grass and was described based on morphological and molecular sequence data and formally named. Morphological and molecular studies of Eretmocerus, an economically important group of parasitoids imported and released for control of B. tabaci in the U.S., is in progress. Sequence data will help to clarify species delimitations among Eretmocerus that are closely related and difficult to discriminate morphologically. Accurate species identification is needed for successful biological control programs. Research addressing Objective 3 was conducted in field and laboratory settings. Cucumber germplasm (67 plant introductions and 9 commercial pickling cultivars) was screened for resistance to natural infestations of pickleworm in replicated field trials. Field studies were conducted in Virginia, North Carolina, and South Carolina on the reproductive biology of click beetles. One study used a sex pheromone previously discovered by ARS researchers in Charleston, South Carolina, to evaluate a multi-modal beetle trap, i.e., one combining sex pheromone and light cues. In related studies, pheromone traps were used for a 2nd year to characterize the region-wide phenology of click beetles, to determine if pheromone trap captures can be used to time insecticide applications for click beetles in sweetpotato, and to determine the relationship between beetle captures in pheromone traps and wireworm densities in nearby soil samples. A field study was conducted to evaluate use of a sex pheromone for mating disruption of a click beetle pest of crops. Concerning pheromone discovery and evaluation, sex attractants for several species of click beetles were identified. A field study evaluated use of sex pheromone lures of multiple species together in the same trap to improve cost efficiency. Although click beetles are a significant pest of root vegetables, they are difficult, if not impossible, to identify to species morphologically in their larval wireworm state. Research pairing morphological study with DNA sequencing for species identification is underway. Over 300 specimens have been collected for morphology and DNA analyses. DNA has been extracted from 240 specimens. The mitochondrial genomes of another major pest of sweetpotatoes, Cylas formicarius (the sweetpotato weevil) have been assembled for 40 individuals collected from South Carolina, Georgia, Texas, and Hawaii, and a phylogenetic analysis was conducted. These first mitochondrial genomes assembled and made publicly available for sweetpotato weevils collected in the United States will be useful for analysis of intraspecific divergence, phylogenetic relationships, and population discrimination. The effects of virus acquisition on whitefly endosymbiont communities are being determined. Studies on the endosymbiont compositions of whiteflies feeding on infected versus healthy plants are in progress. Research was conducted to assess how virus acquisition affects whitefly gene expression. Feeding on infected versus healthy plants is known to affect gene expression in whiteflies. We focused on the effect of virus acquisition on whitefly gene expression. The sweetpotato whitefly was fed on tomato infected with tomato yellow leaf curl virus, and RNA was extracted from the whiteflies and sequenced. Gene expression analyses are in progress. Collaborative research between the University of Georgia and ARS in Charleston, South Carolina, was conducted on a study to assess the role that a protein called DNA methyltranferase 1 (Dnmt1) has on the biological processes of whiteflies. We found that the Dnmt1 gene serves as an essential role in the reproduction of female whiteflies; it affects whitefly egg production and the ability of the eggs to hatch. Collaborative research between the University of Georgia and ARS in Charleston, South Carolina is ongoing on the evaluation of the role of insecticides and potential insecticide resistance in the management of whiteflies and whitefly-transmitted viruses. Although trials evaluating the use of insecticides for suppression of whitefly-transmitted viruses continued, the data continue to show weak control regardless of pest and virus pressure. Research addressing Objective 4 was conducted on sweetpotato lines to minimize the threat from weeds. A total of 55 first year seedlings (FYS) that had combined suitable storage root traits and upright vigorous plant habit were selected from 5,000 FYS. Twenty-six advanced selections are being evaluated in replicated field trials in 2023, and the remaining FYS will be evaluated in 2024. An open pollinated breeding nursery was established to create new germplasm resistant to soil dwelling pests with modified plant architecture to be competitive with weed pressure.

Accomplishments
1. Sweetpotato clones with improved insect resistance and weed tolerance. Management of weeds and insect pests are of concern to sweetpotato growers due to limited options. Sweetpotato yield can be severely reduced under weedy conditions. Crop tolerance/resistance to weeds and insect pests offer an effective sustainable solution. ARS researchers in Charleston, South Carolina, along with Clemson University collaborators, investigated the effects of weed-free interval and sweetpotato clone on weed counts for naturally occurring weed species, storage root yield, and insect resistance to the major pests of sweetpotato. Our results demonstrate that breeding for cultivars and/or germplasm that are competitive with weed interference and resistant to the major insect pests of sweetpotato offers benefits. We identified two sweetpotato clones (USDA-17-037 and USDA-17-077) that had low weed counts, broad insect resistance, and the highest yielding among those tested. This research demonstrates that developing sweetpotato cultivars that are competitive with weed interference through novel plant architecture (erect growth habit) and resistant to insects is useful for developing an effective pest management strategy with particular benefit for organic and sustainable growers.

2. Genomic technology for whitefly management. The sweetpotato whitefly is a globally important crop insect pest that is difficult to manage, even with insecticides. The use of RNA interference (RNAi) technology is a strategy for pest control that scientists are studying for several pests. Collaborative research between the University of Georgia and ARS researchers in Charleston, South Carolina, found that the Dnmt1 gene (a protein called DNA methyltranferase1) serves an essential role in the reproduction of female whiteflies. The researchers found that it affects whitefly egg production and the ability of the eggs to hatch. These findings will aid scientists in the development of a RNAi-based pest management strategy for whiteflies and other pests.

3. Tolerance of a ladybug predator to several insecticides. Insecticides are commonly used as a pest management strategy against pests in Brassica crops such as collard and kale. Collaborative research between researchers in Brazil and ARS researchers in Charleston, South Carolina, assessed 10 commonly used insecticides for their compatibility to a ladybug predator (Eriopis connexa) in Brassica crops. Although feeding behavior by the beetle was affected in some cases, several of the evaluated insecticides (Bacillus thuringiensis, cyantraniliprole, chlorantraniliprole, deltamethrin, chlorfenapyr, spinosad, azadiracthin, and spiromesifen) were compatible with the predator ladybug in Brassica crops regarding its survival and egg laying. The results about this natural enemy demonstrates that it may have a positive impact on helping to manage pests in the Brassica cropping system when several selected insecticides are used by growers.

4. Enhancing trapping to detect and monitor adult wireworms. The corn wireworm is an economically important pest that feeds on root and tuber crops in the United States. A recent discovery by ARS, Charleston, South Carolina, researchers and collaborators of the corn wireworm sex attractant provides a new method to detect and monitor this pest during its adult stage. A collaborative study among researchers at North Carolina State University, University of California, Virginia Polytechnic Institution & State University, University of Florida, and ARS researchers in Charleston, South Carolina, was conducted to optimize trap placement and type of trap for corn wireworm capture for monitoring as compared with the currently used attractant-based pitfall trap. The researchers found that the population of the corn wireworm varies in the southeast, and that traps placed at waist-high and with lures of up to 2-week-old resulted in the most capture of beetles. Findings from this research will help researchers to develop practical pest management options for wireworms.

U.S. DEPARTMENT OF AGRICULTURE

Vegetable Research: Charleston, SC