|GODFREY, L - University Of California|
|AKBAR, W - Monsanto Corporation|
|CLARK, T.L. - Monsanto Corporation|
|Rojas, Maria - Guadalupe|
Submitted to: Journal of Insect Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/23/2017
Publication Date: 12/30/2017
Citation: Backus, E.A., Cervantes, F.A., Godfrey, L., Akbar, W., Clark, T., Rojas, M.G. 2017. Certain applied electrical signals during EPG cause negative effects on stylet probing behaviors by adult Lygus lineolaris (Hemiptera: Miridae). Journal of Insect Physiology. 105:64-75.
Interpretive Summary: Cotton production in the mid-southern United States is severely affected by direct damage to cotton squares during feeding by the tarnished plant bug, Lygus lineolaris. In addition, close relatives of L. lineolaris, such as numerous stink bug species, have recently become important new pests in many crops, worldwide. Development of lygus- and stink bug-resistant varieties of crops depends upon better understanding of the means by which feeding causes plant damage. One of the most important tools for such research is electropenetrography (EPG), in which an electrical signal is applied to a plant on which a gold wire-tethered insect is feeding. Insertion of insect mouth parts into the plant completes an electrical circuit; feeding behaviors alter current resistance that is recorded as waveform output. In over 50 years of EPG science, it has been assumed that there would be no (or very tiny) negative effects on the insect from applied electricity during EPG. This assumption has been true for small-bodied insects like aphids, thrips, and psyllids, whose recordings represent most EPG studies to date. Recently however, EPG studies of large-bodied lygus bugs and stink bugs have been published. The wider mouth parts of such large insects suggest that more electrical current could enter the insect’s body. The present study compared EPG feeding waveforms from Lygus lineolaris on pin-head cotton squares using AC or DC applied signals of varying voltage levels, as well as different amplifier sensitivies. Results showed that differences in feeding behaviors were indeed caused by different monitor settings. Results strongly support the need to tailor instrument settings to the size of each insect species recorded. These findings will improve the use of EPG for L. lineolaris and other large pest insects, making possible better studies of host plant resistance or insecticidal assays, ultimately leading to better control of such pests in cotton and other crops.
Technical Abstract: This study is the first to fully evaluate whether electrical signals applied to insects during electropenetrography (EPG; also called electrical penetration graph) affect insect behavior. During EPG, electrical signals are applied to plants, and thus to the gold-wire-tethered insects feeding on electrified plants. The insect completes an electrical circuit; changes in voltage reflect insect stylet probing/penetration behaviors that are recorded as waveform output. In over 50 years of EPG science, it has been assumed that there would be no (or negligible) negative effects on the insect from applied electricity during EPG. This assumption has been true for small-bodied hemipterans, representing the majority of EPG studies to date. Recently however, EPG studies of large-bodied hemipterans such as heteropterans and sharpshooter leafhoppers have been published. The wider stylet diameters of such large insects suggest that large insects would have lower inherent resistance, thus allowing more electrical current to enter the insect’s body. In addition, high input resistor (Ri) settings on the electropenetrograph (monitor) could cause high voltages to develop across the insect-plant interface. The present study compared feeding by Lygus lineolaris on pin-head cotton squares using an AC-DC electropenetrograph. Effects of AC or DC applied signals were examined in two separate, factorial studies, each comparing four Ri levels (10^6, 10^7, 10^8 and 10^9 Ohms) against four voltage levels (2, 50, 150 and 250 mV). Results showed that differences in both probing and non-probing behaviors were indeed caused by different signal types, Ri levels, or applied voltages. For both signal types, negative effects were greatest at 10^8 and 10^9 Ohms Ri and/or 150 and 250 mV. Behavioral aberrations appeared to be greater for DC than AC applied signals, probably due to muscular tetany from DC. Results strongly support the need for flexible but standardizable Ri and voltage levels, to tailor instrument settings to the size of each insect subject. These findings will facilitate further EPG studies of Lygus spp., such as host plant resistance or insecticidal assays/bioassays. This study will also inform EPG studies of similar, large heteropterans in the future.