Symbiont infection and psyllid haplotype influence phenotypic plasticity during host switching events

Many herbivorous insect species exhibit phenotypic plasticity when using multiple hosts, which facilitates survival in heterogeneous host environments. Physiological host acclimation is an important part of it, yet the effects of host acclimation on insect feeding behavior are not well studied, particularly for insect vectors of plant pathogens. We studied the combined effects of host acclimation and infection with a plant pathogenic symbiont on feeding behavior of Bactericera cockerelli, an oligophagous psyllid widespread in both crop and natural habitats that feeds primarily on Solanaceae and transmits an economically important plant pathogen, Candidatus Liberibacter solanacearum (CLso). We used a factorial design and the electrical penetration graphing technique to disentangle the effects of host acclimation, CLso infection, and psyllid haplotype on the within-plant feeding behavior of B. cockerelli during conspecific and heterospecific host switches. This approach allows to connect phenotypic plasticity with the role of B. cockerelli as a vector by quantifying the frequency and duration of behaviors involved in CLso transmission. We found significant reductions in multiple metrics of B. cockerelli feeding efficiency, exacerbated by infection with CLso, which could lead to reduced transmission of this pathogen. Psyllid genotype was also important; the Central haplotype exhibited less dramatic changes in feeding efficiency than the Western haplotype during heterospecific host switches. Our study shows that host acclimation and heterospecific host switching directly alter feeding behaviors underlying pathogen transmission, and that the magnitude of feeding efficiency reductions depends on both host genotype and infection status.

Methods
Plants were taken from the greenhouse and placed in two Faraday cages hosted on opposite benches in the same room, one with a Giga-8 DC-EPG amplifier and the other one with a Giga-4 DC-EPG amplifier (EPG Systems, Wageningen, The Netherlands). The plastic saucers beneath the pots were filled with water to ensure soil electrical conductivity during recordings. We randomly selected a leaf and gently turned it upside down to expose the abaxial surface, then secured it to a small piece of Styrofoam using a glass slide laid across the surface and affixed by a rubber band (if necessary, thin wooden sticks were also used to position this arrangement for easy leaf access). We collected fourth instar nymphs from select colonies in the morning before setting up EPG recordings. To simulate stress associated with foraging for a new host, nymphs were allowed to sit without plant access for 4 hours in a 10x1.5 cm petri dish (Fisher Scientific, Pittsburgh, PA, USA) sealed with parafilm. After this period, each nymph was taken with a paint brush and placed on a 10-µl pipette tip connected to a vacuum pump (NestEcho, Chaozhou, China) tube to prevent the insects from moving.

To create electrical circuits that included a plant and psyllid, we tethered each nymph by attaching a 12.5 μm thick, 2.5 cm long gold wire (Sigmund Cohn Corp., Mt. Vernon, New York, USA) to the pronotum using a small drop of conductive water-based silver glue (EPG Systems) under a Bausch & Lomb StereoZoom 4 Microscope configured on a boom stand (Cambridge Instruments, now Leica, Deerfield, IL, USA) and illuminated by LED-6W dual gooseneck lights (AmScope, Irvine, CA, USA). The opposite end of the wire was glued with solvent-based silver glue (Dag 503 62% silver coating, Ladd Research, Essex Junction, VT, USA) to a copper wire electrode welded to a brass nail. Tethered nymphs were connected by the brass nail to the EPG probes wired to the DC-EPG amplifiers and placed on the previously prepared leaves, hanging a few millimeters away from the leaf, and a second electrode was inserted into the soil of each potted plant, close to the main stem to ensure contact with the roots, to close the electrical circuit.  

Once all nymphs were in place, the Stylet+ d Windows software (EPG systems) was started, and nymphs immediately lowered to touch the leaves. During the initial probes, the output voltage of each channel was set to about 2V on the DC-EPG amplifier. We recorded the feeding behavior of psyllids from 12 channels simultaneously using the two DC-EPG amplifiers over a 14-hour period, during which LED lights on top of the Faraday cages were kept on by a timer to maintain a 16:8 L:D photoperiod in the room, with the goal of getting at least 20 recordings per psyllid/plant combination for each experiment. Each week three to four sets of recordings were carried out on a new set of plants, which were assigned randomly to EPG channels, and each day a different leaf was selected on each plant, and new nymphs were used.