A Culicoides midge

Anders L., Flickr.

A potential weapon in our war against insect disease vectors are nanoparticles – silver nanoparticles in particular seem to have insecticidal properties. This element silver is known to have antimicrobial properties, and is exploited by the pharmaceutical industry in things like wound dressings and to sterilise surfaces. It is hypothesised that silver nanoparticles (counterintuitively) work by damaging tissues, which triggers cytotoxicity by producing antimicrobial reactive oxygen species.

Using silver nanoparticles to combat disease vectors isn’t new. For example, researchers showed silver nanoparticle-covered grain particles can kill the larvae of Culex quinquefasciatus, a mosquito that transmits viruses like Zika, West Nile and Eastern equine encephalitis. Now, researchers from Kansas State University, University of Arkansas, and the USDA are trying to find out if silver microparticles are toxic to other insect vectors. Enter the tiny blood-sucking midge.Culicoides sonorensis is a biting midge that transmits economically important and often lethal pathogens of wildlife and livestock, such as the bluetongue virus, epizootic haemorrhagic disease and vesicular stomatitis virus in the United States. Bluetongue is of particular concern in the UK as it is a notifiable disease that was detected again within the country in November 2023. One method to control midge populations uses chemical insecticides on aquatic larval sites, which can harm non-target species. Silver nanoparticles could be a good alternative, as research suggests that while they were toxic to various species of mosquito larvae, it took a much higher concentration to harm non-target organisms like the mosquitofish Gambusia affinis, giant water bugs, and backswimmers.

Larval Mortality

In their study, Osborne and colleagues first played games with third-instar C. sonorensis larvae reared in cell culture plates. For one week, the team daily scored larval mortality after treatment with:

1. Silver nanoparticles
2. Sorghum polymer particles (synthesised from black sorghum, a type of great millet producing flavonoids with larvicidal properties),
3. Silver nanoparticles bound to sorghum polymer particles, or
4. Bathed in pure water as a control.

The final results showed that while sorghum polymer particles alone had no effect on larval mortality, both the silver nanoparticles and silver-sorghum polymer cocktails did. As predicted, different concentrations of silver nanoparticles produced varying levels of mortality. Silver nanoparticles were insecticidal at concentrations exceeding 50 mg/L, and by the end of the seven days, there was 90% mortality in larvae treated with 200 mg/L. Impressively, when treated with concentrations of 200 mg/L or above, more than 75% of the larvae were pining for the fjords in less than a day. Furthermore, silver nanoparticles bound to the sorghum polymers were also larvicidal; 75% of the larval groups treated with 200 mg/L or above died within a day post-treatment.

Antimicrobial Effects

Armed with the knowledge of which treatments were larvicidal, they performed a second test to know whether it was the antimicrobial effects of the nanoparticles causing death, or a direct effect on the larvae. To do this, they plated samples of the different larval-treatment solutions onto agar plates. Culicoides larvae have a broad diet and happily eat different microorganisms, including bacteria. If silver nanoparticles heavily reduce levels of bacteria -basically the larvae’s dinner- starvation would ensue. This or the silver nanoparticles were disrupting the larval gut microbiome to trigger death.

While sorghum polymer alone didn’t impact bacterial growth, the silver nanoparticles did, but only at a concentration of 400 mg/L. Larval bacteria were only culturable from concentrations of 200 mg/L or less. Because the larvae still rapidly died at this concentration (with viable bacteria), it was probably not the collateral loss of bacteria causing the larvae’s death, but rather the silver nanoparticles bullets themselves. The control group further supported this, as the larvae bathed in pure water devoid of bacterial growth survived until the end of the experiment.

Summary

The authors showed that silver nanoparticles aren’t just toxic to mosquitoes; other disease vectors are susceptible to these deathly silver bullets. In addition, while the black sorghum microparticles were not toxic themselves, larvae ate them with the voracity one would enjoy tapioca pearls in bubble tea, so they hold promise as a vehicle to deliver other insecticidal microparticles. Not only that, but sorghum is relatively easy to produce from grain (a renewable resource unlike silver), and naturally degrades in the environment.

Although the authors provide promising results for the silver nanoparticles as larvicides, further research must look at the safety of adding them to aquatic ecosystems. If these nanoparticles have the capacity to negatively impact other bacteria-devouring creatures, would this be an ethical or sustainable solution? We won’t even tackle the expense of silver nanoparticle manufacturing and how such nanoparticles in waterways would survive exposure to acid rain.

In closing, one final food for thought and a haiku remains – as mosquitoes and midges are slain by these silver nanoparticle bullets, should they be reclassified as dipteran werewolves?

Annoying midges
But how can I bite you back?
With silver dentures