Culex quinquefasciatus: status as a threat to island avifauna and options for genetic control

Abstract

The avifauna endemic to islands is particularly susceptible to population declines and extinctions resulting from the introduction of non-native pathogens. Three pathogens of concern are the avian malaria parasites, the avian poxviruses, and West Nile virus—each of which can be transmitted by Culex quinquefasciatus, a highly adaptive and invasive mosquito. Culex quinquefasciatus has dramatically expanded its range in recent centuries and is now established throughout much of the tropics and sub-tropics, including on many islands that are remote from mainland landmasses and where this geographic separation historically protected island species from mosquito-borne diseases. The potential for ecological disruption by Cx. quinquefasciatus has been particularly striking in the Hawaiian Islands, where the introduction and transmission of avian malaria and avian poxvirus led to the extinction of several endemic bird species, with many more at risk. With Cx. quinquefasciatus now present in many insular communities and global trade and tourism increasing links between these areas, both to each other and to mainlands, there is growing concern that patterns of avian decline in Hawai‘i may be played out in other insular ecosystems. The implementation of traditional methods for Cx. quinquefasciatus control, including larval source management, is often impractical at large scale and when breeding sites are numerous and difficult to locate—typical issues associated with invasive species removal. One alternative approach would be the utilisation of genetic control methods, several of which have been successfully developed in other mosquitos such as Aedes aegypti and the malaria vector Anopheles gambiae. However, the development of similar tools for Cx. quinquefasciatus has been comparatively limited. Here we review the threat that Cx. quinquefasciatus poses as a vector of avian pathogens to island avifauna and discuss specific examples of at-risk bird populations on the islands of Hawai‘i, New Zealand and Galápagos. We also review the major options for the deployment of genetic control tools against Cx. quinquefasciatus, and discuss the current state of the field with a focus on radiation-based sterilisation, transgenic methods, and transinfections using the bacterial endosymbiont Wolbachia.

Perceived Barriers to the Use of Assisted Colonization for Climate Sensitive Species in the Hawaiian Islands

“…Managers may delay consideration of assisted colonization while waiting for other tools to be developed if those tools are perceived as providing a higher chance of recovery. For example, as avian malaria spreads to higher elevations on Kauaʻi, managers may delay assisted colonization due to the ongoing effort to develop new mosquito control techniques (Harvey-Samuel et al 2021 ).…”

Management of avian malaria in populations of high conservation concern

“…The native ranges of Plasmodium parasites are distributed worldwide, in diverse habitats (e.g. Nearctic, Palearctic, Oriental, Neotropical and Australian ecozones) [ 2 ], and co-vary with similarly wide-ranging ornithophilic mosquito vectors such as Culex quinquefasciatus [ 6 ]. In addition to this wide historic distribution, Plasmodium parasites have been introduced to new regions where highly virulent species have led to substantial population declines and extirpations of endemic birds [ 7 ].…”

“…Because many Hawaiian passerines are already at high extinction risks due to habitat loss and the introduction of non-native predators, the additional pressure of avian malaria in this and similar systems (e.g. New Zealand and Galápagos Islands) is of high conservation concern [ 6 ]. Understanding how to predict the effects of avian malaria and how to limit new vector pathways into additional ecosystems is critical to ongoing conservation action to prevent extinction in these high-risk avian species [ 14 ].…”

Double-tap gene drive uses iterative genome targeting to help overcome resistance alleles

“…For example, mosquitos can be modified with beneficial genes to prevent carried pathogens 4 – 6 or with detrimental gene alterations to suppress the vector population 7 – 9 . Gene drives also offer promising solutions in crop pest population control 10 and invasive species suppression 11 , 12 , such as for the rodents 13 , 14 currently impacting island conservation efforts 15 . Briefly, CRISPR gene drives operate by biasing their own inheritance from Mendelian (~50%) toward super-Mendelian (>50%) by converting heterozygous germline cells to homozygosity.…”