Location: Chemistry Research2013 Annual Report
1a. Objectives (from AD-416):
1. Determination of physiological and biochemical mechanisms, elicitors, and regulators involved in the interactions of plants with insects and their natural enemies. 1a. Isolation and identification of elicitors or biotic agents of induced plant volatile emission and other inducible plant defenses. 1b. Elucidation of the signaling interactions that mediate insect and insect elicitor induced plant defenses and volatile emission. 2. Identification and evaluation of chemicals that regulate or influence behaviors, including foraging, mating and oviposition, of important pest insects. 2a. Isolate and identify plant volatiles and insect produced pheromones that in combination attract male and female pepper and cranberry weevils. 2b. Identification of oviposition deterring pheromone from pepper weevil. 2c. Interaction between Insect and Host Volatiles on Reproductive Development of Anastrepha spp. (Dipera:Tephrididae). 2d. Identification of oviposition deterring pheromones for Tephritid Fruit Flies. 2e. Influence of Fruit Volatiles on attraction of Small Hive Beetle. 3. Improve the efficacy of alternatives to methyl bromide fumigation for management of stored-product insects by developing new attractants and behavior-altering chemicals that affect Sitophilous spp. (rice and maize weevils) and Tribolim castaneum (red flour beetle) for the purpose of understanding the chemical ecology of these insects and developing practical methods for detection and control.
1b. Approach (from AD-416):
Identify and evaluate chemicals that regulate or influence behaviors, including foraging, mating and oviposition, of important pest insects. Determine the physiological and biochemical mechanisms, elicitors, and regulators involved in the interactions of plants with insects, insect natural enemies and other organisms. This research will utilize numerous interactive bioassays with insects, plants, purified biochemicals and other organisms. Isolation and identification of new bioactive chemicals that mediate insect behaviors and plant-insect interactions will be achieved using a combination of approaches including preparative GC, HPLC, preparative flash chromatography, GC/MS, FT-IR, NMR, micro-degradation and synthesis where applicable. Major target insects for this research will include pest Lepidoptera, Coleoptera and Diptera that attack fruit and vegetables and coleopteran pests of Honey Bees. Other target insects may be selected as needed during progression of the project.
3. Progress Report:
Studies were conducted to isolate and identify acidic terpenoid phytoalexin defenses in maize resulting from insect and microbial attack. Two novel sesquiterpenoids were structurally determined using nuclear magnetic resonance spectroscopy techniques. One of these new compounds, termed zealexin A4, exhibits significant antifungal activity and occurs at high levels in the kernels of specific maize lines that resist aflatoxin accumulation. Characterization of stable homozygous maize mutants in the gene AntherEar2 demonstrated that this encoded enzyme is the first committed step in kauralexin diterpenoid biosynthesis. In the process we also identified an unusual oxylipin that accumulates to high levels in tissues following wounding. Early bioassay results are consistent with the oxylipin having negative effects on pest growth. Studies were conducted to gain mechanistic insight into how European corn borer may alter defense responses in maize tissues. Analysis of European corn borer oral secretions following ingestion of tryptophan demonstrated increased production rates of the plant phytohormone indole-3-acetic acid. Application of indole-3-acetic acid to maize stem tissues resulted in complete metabolism and removal within 12 hours and did not alter stem protein levels. Studies are on-going to supply continuous levels of indole-3-acetic acid as are present from larval excretions into the feeding tunnel. The Pepper weevil oviposition deterring pheromone has been identified as a combination of two compounds with different functions. Research on oviposition deterrent pheromones of Tephritid fruit flies of led to determination that oviposition deterrents deposited after oviposition have interspecific activity. Collection and separation of the deterrents by HPLC for two species has resulted in isolation of biologically active fractions which are currently being further purified. Studies on molecular regulation of reproduction in the Varroa mite led to the complete sequencing of 5 genes responsible for reproductive competence. Physiological studies documented the effects of these genes and their interaction with honeybee development. The studies pave the way for development of molecular approaches to control the mite.
1. Plant defenses recovered by altering a pest evasion strategy. Following attack by pests, plants activate a wide range of protective biochemical defenses. Insect herbivores are often recognized by plants through the presence of elicitor molecules in larval oral secretions that greatly amplify the wound-induced plant responses. The existence of negative insect-derived effectors that suppress plant defenses is also predictable yet very few specific biochemicals with inhibitory activity have been characterized. By examining the interactions of generalist (Fall armyworm) and specialist (Velvetbean caterpillar) pests on cowpea, scientists at the Center for Medical, Agricultural and Veterinary Entomology in Gainesville, FL discovered that Velvetbean caterpillars convert a potent plant elicitor into a competitive inhibitor of plant recognition. Biochemical screens identified a simple alteration of the plant-derived elicitor precursor that was resistant to further Velvetbean caterpillar modification. When Velvetbean caterpillars ingested trace amounts of a modified plant protein sequence they elicited significantly greater plant defenses in cowpea herbivory assays. The importance of the discoveries is the demonstration that some crop pests evolved methods to overcome plant defenses where as other pests do not.
2. Understanding the reproductive physiology of the Varroa mite. The varroa mite is the major pest of the honey bee in the world and few, if any effective methods to control the mite are available because the pest rapidly develops resistance. Despite considerable study essentially nothing is known about the reproductive physiology of the pest. Scientists at the USDA ARS, Center for Medical, Agricultural and Veterinary Entomology identified the genes for vitellogenin proteins that are critically important for reproduction by Varroa mite. Identification of these genes provides a significant target for disruption using molecular pesticide techniques based on RNA interference to control or disrupt the physiology of the pest Varroa mites.
3. Understanding dietary physiology of Varroa mites. Despite being the most serious pest of Honey bees in the world nothing is known about how the mite processes food it gets from parasitizing Honey bees. Scientists at the USDA ARS, Center for Medical, Agricultural and Veterinary Entomology identified the gene for the most abundant protein in the Varroa mite and found that it is a carrier protein responsible for lipid transport in the mite. The levels of Vg transcript was found to be decreased during the reproductive period of the female mite when the female is not feeding on adult bees. This carrier protein provides a significant target for disruption using molecular techniques to control or disrupt the physiology of the pest Varroa mites because lipids are critical compounds required for survival of the mite.
Choe, A., Chuman, T., Von Reuss, S.H., Doseey, A.T., Yim, J., Ajredini, R., Kolawa, A.A., Kaplan, F., Alborn, H.T., Teal, P.E., Schroeder, F.C., Sternberg, P.W., Edison, A.S. 2012. Sex-specific mating pheromones in the nematode Panagrellus redivivus. Proceedings of the National Academy of Sciences. 109(51):20949-20954.