Endosymbionts are present within a wide diversity of hosts, and more than one endosymbiont may occur within a host. An endosymbiont may be obligate or facultative for host survival and/or reproduction. Despite the prevalence of endosymbionts throughout many insect orders, little is known about the role and maintenance of endosymbionts within insect hosts. Two endosymbionts, an obligate symbiont (primary symbiont) and facultative symbiont (secondary symbiont) are harbored within the invasive red gum (eucalyptus) lerp psyllid, Glycaspis brimblecombei, in California. An extensive survey of diversity and frequency of G. brimblecombei’s secondary symbiont was conducted in multiple populations throughout the state of California using PCR detection, restriction enzymes, cloning, and sequencing. A total of 380 G. brimblecombei individuals were screened for secondary symbionts among 19 populations. Based on molecular screening results, only one type of secondary symbiont was present among G. brimblecombei populations in California. Interestingly, secondary symbiont infection frequencies among G. brimblecombei populations varied dramatically from 0-75%, and were significantly related to parasitism pressure by Psyllaphaegus bliteus, a solitary endoparasitoid. Predictions, from a general infection frequency model, suggest that fidelity of vertical transmission and differential host susceptibility to a parasitoid and fecundity of infected relative to un-infected psyllids may play a major role in maintaining observed variation in infection frequencies.