The Old Reliable for generations of entomologists, the light trap remains perhaps the best catchall collecting device to sample large numbers and species of insects, but it also may mask infestations of some insect-borne illnesses and even expose humans to disease vectors. So suggests a paper published today in the Journal of Medical Entomology, catchily titled “The Dark Side of Light Traps.”
For more than a century, light traps, in recent times supplemented by suction or odor baits, have been the device of choice for collecting insects active by twilight and night. Insects most often sampled for purposes of human and animal health include disease vectors such as mosquitoes, biting midges, and sand flies.
Old Reliable, however, can have a troublesome flip side. Over-reliance on light traps can be counter-productive at best and potentially dangerous at worst, say Emily McDermott, Ph.D., postdoctoral fellow at the Walter Reed Army Institute of Research, and Bradley Mullens, Ph.D., professor of entomology at the University of California, Riverside.
“Perhaps the most critical overall limitation of light traps is their potential to inaccurately reflect the species composition of host-biting insect communities in an area,” they write. Among insects drawn by light traps, moreover, are species carrying diseases transmittable to humans and livestock, which could be exposed to infection in the vicinity of the devices.
Still, it is no secret why scientists like light traps. Modern light traps are cheap, inexpensive and portable, often resembling a small tube feeder for backyard birds. Size, in this case, matters not only for easy portability but for concealment from vandals. All in all, no other traps catch so many kinds of insects so easily, which makes light traps ideal for initial general surveys. Be that as it may, interpretation of data based on light traps may be biased by their limitations, often due to behavioral quirks of insects and background conditions, especially when it comes to epidemiology, the study suggests.
“If used alone, light traps may fail to collect important or infected vectors, and light traps are inefficient or ineffective when competing ambient light is present,” McDermott and Mullens argue. Street lights or a full moon—not to mention global light pollution—may significantly reduce a light trap’s haul, even though insects are swarming. And, while ambient light may trigger high insect activity, it competes with the trap, limiting its attractiveness. Consequently, public health officials could underestimate the number of disease vectors, and the risk of infection, in the locale sampled. “These concerns are especially troubling when light-trap data are used to inform policy decisions meant to protect human and animal health,” say the authors. Moreover, the emergence of diseases such as dengue fever, chikungunya, and Zika, spread by diurnal mosquitoes of the genus Aedes, has increased the need for alternative traps since those baited with light are relatively useless by day.
Rarely, say the authors, “the use of light traps may put people and animals at risk” by drawing large numbers of insect vectors that are strongly attracted to light near human habitations. “We believe that any attractive trap poses a risk of drawing in infected vectors closer to hosts, but it is an area in which more research is needed,” McDermott says. Studies in South America support her opinion. Research showed that triatomine, or “kissing,” bugs (which spread Chagas disease, a scourge in Latin America) are drawn by artificial lighting to homes where human contact usually occurs. “In rural areas, researchers may want to consider whether to place light traps away from livestock enclosures and habitations,” say the authors.
Originally, light traps were not particularly efficient because they used incandescent bulbs, emitting mostly light mostly as infrared, invisible to most nocturnal insects, which sense wavelengths in the green-to-ultraviolet range. UV blubs and LEDs, however, have remedied that flaw and work far better. Whatever the light source, the wavelength it produces can be critical. In Egypt, traps baited with blue or green LEDs collected only about one-fifth the number of sand flies (Phlebotomus and Sergentomyia spp.) as those baited with red.
Yet, the fact remains that scientists still have much to learn about how many insects respond to light. Sole reliance on light traps to survey Culicoides biting midges, say the authors, led scientists to believe that they were active from dawn through dusk and not in winter. It turns out some are diurnal while other nocturnal species fly by day in summer. “Light traps on their own could quite possibly miss such species entirely,” according to the paper.
Beyond that, the same insects may respond unevenly or differently to light. One type of trap drew Culicoides from a range of two to four meters in one location yet up to 30 meters in another. Even sexes of the same species may respond differently to light. Female Lutzomyia whitmani sand flies find light much more attractive than do males. The very presence of a pathogen in some insects may even change their response to light. Once infected, Culicoides sonorensis midges carrying the BTV virus, which causes the blue tongue livestock disease, are repelled by light.
Far from disrespecting light traps, though, the authors say that the traps “will continue to have a place in medical and veterinary entomology, and in some cases may be the most appropriate choice.” They caution, however, that “broad and critical discussion about the validity of the epidemiological conclusions from these collections is needed in order for entomologists and epidemiologists to avoid the dark side of light traps.”