Five things to know about genetically modified (GM) insects for vector control.

Vector-borne diseases cause immense suffering and economicdamage. Vector control remains a key element of mitigation andcontrol strategies, particularly for pathogens such as dengueviruses for which there are no specific drugs or vaccines. Yetexisting vector control tools are limited; toxic chemicals are themainstay but difficult to deliver due to vector behaviour, emergingresistance, and/or environmental concerns. Genetically modifiedvectors—presently only mosquitoes—offer complementary newapproaches to integrate with the best existing methods. Modifiedmosquitoes will actively seek out wild mosquitoes as mates, withhigh species specificity and minimal off-target effects.Within this overall scheme, many different genetic modifica-tions have been proposed, all delivered via this mating-basedmechanism (‘‘vertical transmission’’). These may be classifiedaccording to the persistence of the modification: ‘‘self-sustain-ing’’ genetic systems are intendedto persist or spread invasivelyin the wild population after an initial release period, while ‘‘self-limiting’’ systems will disappear relatively rapidly unlessmaintained by more releases. Another classification is byintended effect: ‘‘population suppression’’ strategies aim, likemost current vector control programmes, to reduce the numberof vector mosquitoes in the target area, while ‘‘populationreplacement’’ strategies aim to reduce the ability of affectedmosquitoes to transmit specified pathogens, with any reductionin total number of mosquitoes being incidental. In either case,the intended result is fewer competent vectors, thereby reducingthe force of infection. In computer simulations, several suchstrategies are capable of eliminating transmission in theprogramme area.These approaches are not entirely new. Some proposals [1] aresimply applications of modern genetics to improve on the classicalSterile Insect Technique (SIT) [2], in which radiation-sterilisedinsects are released to mate with wild counterparts and therebyreduce the reproductive potential of the target pest population,leading to suppression or even local elimination. SIT has beenused successfully on large and small scales against some majoragricultural pests. This close relationship to an existing methodmeans that the rollout, use, strengths, and weaknesses of such self-limiting population suppression strategies are fairly predictableand well understood. For self-sustaining strategies, looser analogiesmay be drawn with classical biological control, in which an exoticpredator or parasite is introduced with the intention that it shouldestablish permanently and thereby help control the pest. Thisanalogy highlights both key strengths of self-sustaining systems—potential long-term benefit without further human action—andweaknesses—relative lack of control post-release—relative to self-limiting ones. Simulation modelling is a vital tool to inform straindevelopment and risk assessment and mitigation, especially of themore invasive self-sustaining systems in which release is essentiallyirreversible.