Location: Biological Control of Pests Research
2019 Annual Report
Objectives
Objective 1: Discover new efficacious natural products, including fire ant-specific pathogens, for Integrated Pest Management (IPM) of invasive ant species.
Sub-objective 1A: Evaluate naturally occurring toxins for invasive ant control.
Sub-objective 1B: Evaluate entomopathogenic agents for invasive ant control.
Objective 2: Determine gene function and utilize existing genomic resources to develop gene disruption approaches for mitigating the impact of invasive ants.
Sub-objective 2A: Perform database comparisons to identify candidate genes for disruption.
Sub-objective 2B: Functionally characterize specific candidate genes targeted for disruption.
Objective 3: Improve existing and design new biopesticide delivery systems as part of Integrated Pest Management (IPM) programs for invasive ant species.
Sub-objective 3A: Enhance the efficacy of biopesticides by improving adjuvants and synergists.
Approach
Current practice for controlling invasive pest ants depends heavily on synthetic insecticides, which have hazardous impacts beyond their intended uses. New safer and more sustainable technologies are needed to improve pest ant management. This project plan describes research to develop new products for managing invasive ants, particularly the imported fire ant, Solenopsis invicta, and the tawny crazy ant, Nylanderia fulva. This research will focus on 1) identifying new biopesticides, including naturally occurring toxins and biological control agents; 2) identifying and characterizing targets and methods for gene disruption; and 3) developing and improving delivery systems to maximize the efficacy of newly developed and existing biopesticides. This research will produce innovative products and methods for managing invasive pest ants.
Progress Report
Sex pheromones are critical in the reproduction of insects and they can be used to monitor the presence, abundance, and spread of insects and to develop environmentally friendly methods for insect control (Sub-objective 1A, Evaluate naturally occurring toxins for invasive ant control). It was believed that sex pheromone(s) play a critical role in mate success of fire ants. However, so far, no fire ant sex pheromones have been determined due to the fact that fire ants never mate in the laboratory. Elucidation of sex-specific compounds will greatly help scientists determine the sex pheromone. Using an innovative method of chemical analysis, a series of male and female specific compounds have been determined. Among them are two oxo fatty acids that were only found in males. Five compounds were found only in female alates, including mellein, ß-springene, trans-ß-ocimene, cis-ß-ocimene, and 6-pentyl-2H-pyran-2-one. A pyridine alkaloid, 2-methyl-6-undecanylpyridine, was found to be exceptionally abundant in fire ant female alates. All these compounds are available either by chemical synthesis or donation from our collaborators, which builds a solid foundation for a better understanding of the fire ant sex pheromone.
Like all other social insects, fire ants depend on sophisticated pheromonal communication for maintaining colony cohesiveness, sociality, and defense. Fire ants can detect pheromones by using various morphologically distinct olfactory sensilla on the antennae. For the first time, olfactory sensilla, basiconic sensilla, of fire ants were functionally characterized for their responses to about 90 individual compounds, which including 30 ant cuticular hydrocarbons (CHCs), and ant extracts. Single sensillum recording (SSR) was used to detect olfactory neuron responses by female alate and worker specific olfactory sensilla basiconica. These sensilla contain multiple olfactory receptor neurons which displayed a broad spectrum of sensitivity to the tested compounds. Moreover, these sensilla collectively detect every general odorant and CHCs, and allow them to be classified into three broad groups. It was the first time to record the sensilla response of fire ants to CHCs. Whether fire ants respond to the CHCs had been a very controversial issue for many years. Since behavioral responses to CHCs were also demonstrated in fire ants, this controversy was solved. These studies also provided a comprehensive overview of olfactory discrimination in fire ants, which not only advanced our knowledge on the fire ant olfactory system, but ultimately may aid in fostering the design and development of olfactory-based strategies for controlling fire ants (Sub-objective 1A, Evaluate naturally occurring toxins for invasive ant control).
Since bait is a very important component in fire ant manage, knowledge on how fire ants search for food (foraging behavior) is important in developing new fire ant bait formulations. It was demonstrated that fire ants can use tools during forage, and they modulate tool use strategies in response to perceived risks. Surfactants in the sucrose solution can significantly increase the risk of drowning and fire ants. Fire ants can utilize several tools including sand, pine needle, and sponge around the nest to reduce mortality and facilitate feeding. More pine needles and sponges are moved to liquid baits under the increasing surfactant concentration. Sand use strategy is very unique. Workers moved less sand into liquid baits, but more sand around the baits under higher foraging risk. To reduce the drowning risk under high surfactant concentration, workers were able to build a syphon bridge which can help in collecting sucrose and reducing the risk. Workers have higher foraging activity, fewer drowning risk and more sucrose intake amounts when tools are available. Fire ant’s tool use strategy was novel in animal kingdom, which may be potentially exploited in developing fire ant new baits (Sub-objective 1A, Evaluate naturally occurring toxins for invasive ant control).
A formulation based on hexyl benzoate was evaluated for controlling fire ants in potting soil. This formulation was effective in controlling fire ants in potting soil at the application rate of 1,000 ml liquid (0.1% concentration) per gallon of pot volume. No ants were found in the pots 60 days after treatment. It was also found that a low concentration of this formulation can induce significant aggression among fire ant nestmates. Such aggression is lethal to the larvae and pupae. This discovery opens a new revenue to study the mechanism of fire ant nestmate recognition and potential use in the development of novel management strategies (Sub-objective 1A, Evaluate naturally occurring toxins for invasive ant control. Sub-objective 3A: Enhance the efficacy of biopesticides by improving adjuvants and synergists).
Described discovery of two known ant viruses and three novel viruses in fire ant juveniles in the Mississippi Delta. First genome, SINV2-MSD, was published in a Microbial Research Announcement manuscript. The second of these is the first report of a virus only known from crazy ants and now identified in a fire ant. Performed multiple reference-based assemblies of 6 sequenced (paired Illumina reads) samples of Solenopsis invicta to identify RNA viruses. Analyzed sequences and discovered three viruses, all of which were unique, to varying degrees. Transcriptome sequencing of immature ants reveals complete genome sequence of a new isolate of Solenopsis invicta virus 2 from the Mississippi Delta; accepted and published. Log# 354155, not yet resubmitted: Complete Genome Sequence of a New Isolate of Nylanderia fulva virus 1 from Solenopsis invicta. Specific primers for PCR-based virus detection have been designed. Virus research has been incorporated into the new Project Plan. A manuscript describing discovery of known & novel viruses has been prepared and sent to internal reviewers (Objective 2. Determine gene function and utilize existing genomic resources to develop gene disruption approaches for mitigating the impact of invasive ants).
Synergist can be useful in improving the efficacy of fire ant bait products. Synergistic effect of toxins from camelina plants on fire ant bait formulated using entomopathogenic fungus Beauveria bassiana (GHA and NI8) was investigated. Glucosinolates derived from Camelina (C.) sativa such as sinapic acid, ellagic acid, and chlorogenic acid were tested at laboratory conditions for their effect on observed mortality to fire ant immature and reproductive castes. In treatment, oil-extracted C. sativa seed meal was substituted by a mix of defatted soy flour (78%), corn oil (21%) and Sinapic acid 0.22%, ellagic acid 0.11%, and chlorogenic acid (0.07%) (the same concentrations as being reported in C. Sativa oil), defatted soy flour and corn oil were used because they have similar nutritional content as C. sativa meal. In control, the same mixture devoid of the glucosinolates was used. There were significant differences in the number of dead workers and dead reproductive castes between control and treatment. The control showed significantly more dead workers and reproductive castes than the treatment. However, the control also had a significantly higher ending live biomass than the treatment. Analyzing the data with General Linear Model and including ending biomass weight as an independent variable did not show significant differences between control and treatment in the number of dead workers. Differences in the number of dead workers seemed to be better explained by differences in colony size between control and treatment (Sub-objective 1B: Evaluate entomopathogenic agents for invasive ant control. Sub-objective 3A: Enhance the efficacy of biopesticides by improving adjuvants and synergists).
Accomplishments
1. Formulations for controlling fire ants in potting soil. The long distance movement of fire ants often occurs because of human activity, particularly through the transport of infested potted plants. To slow or prevent their spread, federal quarantines are currently in place. The potted plants must be treated with a significant amount of synthetic insecticides before they can be moved from quarantined areas. A major concern for such treatment is the potential detrimental effect of insecticides to the environment. Environmentally friendly products are desired. A natural formulation using hexyl benzoate as an active ingredient was evaluated for controlling fire ants in potting soil by ARS scientists in Stoneville, Mississippi. This formulation was effective in controlling fire ants in potting soil. No ants were found in the pot 60 days after treatment. This formulation may be used as an alternative to synthetic insecticides in quarantine treatment. It was also found that this formulation at low concentrations can induce significant aggression among fire ant nestmates. Such aggression is lethal to the larvae and pupae. This discovery may also open a new revenue to study the mechanism fire ant nestmate recognition.
2. Identification of fire ant attractants. Bait technology is commonly used in fire ant management. To reduce the repeated use of bait products, there is a need to improve the performance of fire ant bait products. Fire ant attractants can be used to improve the efficacy of bait products. Multiple fire ant attractants have been identified from different sources. Three compounds from Ylang-ylang oil, an essential oil extracted from the fragrant yellow flowers of the tree Cananga odorata, are identified as fire ant attractants. Pyridine alkaloids were identified the first time in fire ants by ARS scientists in Stoneville, Mississippi. In collaboration with chemists at the University of Mississippi and Zhejiang University, five fire ant pyridine alkaloids were synthesized and their biological activities were evaluated. One pyridine alkaloid was found to be an attractant to fire ants. These compounds may be potentially used to improve fire ant bait efficacy by facilitating ants to find and accept the baits.
3. Discovery of fire ants virus. In order to reduce our dependence on synthetic insecticides in managing fire ants, new safer and more sustainable technologies are needed. One approach is biological control using fire ant pathogens, including fire ant viruses. Two known ant viruses and three novel viruses were discovered in fire ant juveniles in the Mississippi Delta by ARS scientists in Stoneville, Mississippi. Multiple reference-based assemblies of 6 sequenced (paired Illumina reads) samples of Solenopsis invicta were performed to identify these RNA viruses. These viruses can potentially be directly exploited or engineered for the biological control of fire ants.
Review Publications
Allen, M.L., Rhoades, J.H., Sparks, M., Grodowitz, M.J. 2018. Differential gene expression in Red Imported Fire Ant (Solenopsis invicta) (Hymenoptera: Formicidae) larval and pupal stages. Insects. 9(4):185.
Allen, M.L. 2019. Transcriptome sequencing of immature ants reveals complete genome sequence of a new isolate of Solenopsis invicta virus 2 from the Mississippi Delta. Microbiology Resource Announcements. 8:(18)1-3.
Zhao, R., Lu, L., Chen, J., He, Y. 2018. Volatile terpenes and terpenoids from workers and queens of Monomorium chinense (Hymenoptera: Formicidae). Molecules. 23:1-14.
Lee, B., Park, C., Park, M., Riddick, E.W., Roh, G., Chen, J., Cha, D.H. 2019. Ethyl formate fumigation for the disinfestation of red imported fire ants Solenopsis invicta Buren. Journal of Asia-Pacific Entomology. 22(3):838-840.
Chen, J., Rashid, T., Feng, G., Feng, Y., Zhang, A., Grodowitz, M.J. 2019. Insecticidal activity and structure-activity relationship of methyl benzoates analogs against red imported fire ants, Solenopsis invicta Buren (Hymenoptera: Formicidae). Journal of Economic Entomology. 112(2):691-698.
