Submitted to: Annals of the Entomological Society of America
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/31/2013
Publication Date: 7/15/2013
Citation: Backus, E.A., Rangasamy, M., Stamm, M., McAuslane, H., Cherry, R. 2013. Waveform library for chinch bugs (Hemiptera: Heteroptera: Blissidae): Characterization of Electrical Penetration Graph (EPG) waveforms at multiple input impedances. Annals of the Entomological Society of America. 106:524-539. Interpretive Summary: This study used a newly developed electrical penetration graph (EPG) monitor, the four-channel, Backus and Bennett AC-DC monitor, to identify waveforms recorded during feeding by adults of two economically important chinch bug species: southern chinch bug, Blissus insularis, feeding on St. Augustinegrass, and western chinch bug, B. occiduus, feeding on buffalograss. This is only the third time any heteropteran (“true bug”) species has been recorded using EPG, and the first heteropteran recording with this monitor. Waveforms of western and southern chinch bugs were similar, and both showed long periods of likely stylet-searching and ingestion phases. The flexibility of using multiple amplifier sensitivities, possible only with the AC-DC monitor, was valuable for determining whether the waveforms were caused by biological voltages inherent in the plant cells, or electrical resistance due to valves or pumps opening and closing inside the insect’s head during feeding. Such knowledge allowed researchers to propose that salivation, ingestion, or other biological processes might be occurring during the waveforms, without having to resort to experiments using microscopy and other time-consuming techniques. Understanding the biological meanings of EPG waveforms is essential for the application of EPG monitoring to host plant resistance studies, to aid in developing crop plants that will resist the feeding of pest insects without requiring insecticides.
Technical Abstract: Electrical penetration graph (EPG) monitoring has been used extensively to elucidate mechanisms of resistance in plants to insect herbivores with piercing-sucking mouthparts, or stylets. Characterization of waveforms produced by insects during stylet probing is essential to the application of this technology. In studies described herein, a four-channel, Backus and Bennett AC-DC monitor was used to characterize EPG waveforms produced by adults of two economically important chinch bug species: southern chinch bug, Blissus insularis, feeding on St. Augustinegrass, and western chinch bug, B. occiduus, feeding on buffalograss. This is only the third time a heteropteran species has been recorded using EPG, and the first recording of Blissidae. Chinch bug probing was recorded with either AC or DC applied voltage, no applied voltage, or voltage switched between AC and DC mid-recording, at input impedances ranging from 106 to 1010 Ohms, plus 1013 Ohms, to develop a waveform library. Waveforms exhibited by western and southern chinch bugs were similar, and both showed long periods of putative pathway and ingestion phases (typical of salivary sheath-feeders) interspersed with shorter phases, termed transitional J wave (suspected to be an X wave) and interruption. The flexibility of using multiple input impedances with the AC-DC monitor was valuable for determining the electrical origin (R versus emf components) of the chinch bug waveforms. It was concluded that an input impedance of 107 Ohms combined with either DC or AC applied substrate voltage is optimal to detect all R- and emf-component waveforms produced during chinch bug probing. Knowledge of electrical origins allowed biological meanings of the waveforms to be hypothesized, prior to time-intensive, future correlation experiments using histology, microscopy, and other techniques.