In this study, we quantified the flight capacity of B. cockerelli for the first time by using a flight mill to determine how flight performance is affected by CLso infection. We found that CLso-infected B. cockerelli were significantly less likely to engage in long distance flights when compared to CLso-free insects. Accordingly, our data suggest that CLso is detrimental for B. cockerelli propensity to disperse. This agrees with previous research on other aspects of B. cockerelli biology. For example, CLso has a negative effect on B. cockerelli fitness (oviposition and nymphal survival), with CLso haplotype B being more pathogenic than haplotype A (Nachappa et al. 2012; Yao et al. 2016). A difference in flight propensity but not in flight capacity may indicate that the effect of CLso on B. cockerelli is stronger at the behavioral than at the physiological level. In fact, this is consistent with our observation that psyllid weight was not affected by CLso status. It is unknown how CLso causes the observed shift in flight behavior, but previous work has shown that CLso infection is linked to changes in B. cockerelli feeding behavior, as CLso-infected psyllids probed plants more quickly and fed for longer periods (Valenzuela et al, 2020). Pathogens such as viruses can also affect the behavior of its vectors. For example, a study performed with dengue virus suggests that virus infection increase the movement of its vector Aedes aegypti by affecting neural activity (Lima-Camara et al. 2011). We believe a similar mechanism may apply for the effect of CLso on vector behavior if the bacterium interferes with insect organs responsible for processing environmental cues used to decide to engage in flight. However, this hypothesis needs to be studied further, ideally with functional genomic studies.
On the flight mill, B. cockerelli performed two types of flights, short flights where insects moved a few meters, and long flights where insects traveled between 12 meters to one kilometer. However, flight duration was typically short. This result is congruent with the flight behavior reported for other psyllid species (Kobori et al. 2011; Martini et al. 2014; Antolínez et al. 2021) and is also consistent with the clumped distribution of B. cockerelli and the cluster distribution pattern of zebra chip reported in potato fields (Butler and Trumble 2012; Henne et al. 2012; Thinakaran et al. 2015; Dahan et al. 2017; Henne and Thinakaran 2020). Generally for flying insects, long distance flights are less common than short flights but long distance dispersal is vital to drive the colonization of new habitats and thus, population expansion (Clark et al. 2001; Petrovskii et al. 2011). According to our data, maximum long-distance dispersal in the absence of wind for B. cockerelli can go up to 980 m, but because wind may assist psyllid dispersal under field conditions (Henne et al. 2010; Cameron et al. 2013; Antolínez et al. 2022), actual dispersal distance may be higher. Among insects that engage in long flights, average distance dispersed ranged between 122 and 185 m depending on pathogen status, with exposed/infected individuals being more prevalent in the 0 to 150 m range from a psyllid source. Flights longer than 200 m were only performed by 12% or less of the tested insects (CLso-free = 4/32 (12%), CLso infected = 1/37 (2%)) (Table 2), which is consistent with values calculated by Cameron et al. (2013) to estimate the dispersion of potato psyllids in field conditions. Accordingly, we believe that natural population expansion in B. cockerelli is more likely the result of distance flown over repeated flights in the range of meters, rather than mass long-distance migration events.
Finally, it is important to acknowledge the consequences that pathogens and their interactions may have on vector physiology and behavior, which, in some cases, can increase pathogen transmission (Ingwell et al. 2012; Mauck et al. 2012, 2018; Gandon 2018). This has been extensively described for persistently transmitted viruses and their vectors (Stafford et al. 2011; McMenemy et al. 2012; Liu et al. 2013; Moreno-Delafuente et al. 2013; Vogels et al. 2017; Safari et al. 2019; Wang et al. 2020; Lee et al. 2022). However, few studies have explored the effects of plant pathogenic bacteria on vector dispersal behavior. For Candidatus Liberibacter species, only one study has explored pathogen effects on the flight capacity of its psyllid vector (Martini et al. 2015). In this case, the infection with Candidatus Liberibacter asiaticus, the causal agent of citrus huanglongbing disease, increased the occurrence of long-distance flight of the Asian citrus psyllid (Diaphorina citri), which could increase its own spread. However, our study shows the opposite for B. cockerelli and CLso, suggesting that psyllid and Liberibacter interactions are likely highly specific to each host-pathogen circumstance.
Overall, our study demonstrates that CLso haplotype B negatively affects flight performance of the B. cockerelli western haplotype. The pathogen appears to reduce insect participation in long-distance flights. Extrapolating flight mill results to dispersion in natural conditions should be done with caution, since tethered flight may under or overestimate dispersal (Riley et al. 1997; Taylor et al. 2010; Minter et al. 2018). However, the flight mill is a valuable tool to assess flight performance under controlled conditions. By refining our understanding of both flight propensity and flight duration, our study provides a starting point for the development of recommendations for defining CLso risk zones and survey points around new B. cockerelli infestations. This is especially important in South America, the center of Solanaceae biodiversity, where the current expansion of B. cockerelli and its associated pathogen (CLso) represent a significant threat to food security and may have negative impacts on endemic collections of germplasm that serve as a source of genetic diversity for breeding. Future work should investigate whether this effect depends on the CLso and vector haplotypes involved and whether it is the result of direct pathogen effects, indirect plant-mediated effects, or a combination of both.