Everyone knows that certain flying insects are attracted to artificial lights in the evening. This can be seen at countless outdoor fixtures every night as swarms of bugs fly around the glow of the lamp. Aside from being a potential nuisance, this could have consequences in terms of human health when insects that carry human diseases are included in that swarm.
Insects that help spread and transmit diseases are called vectors. The types of insect vectors and the diseases they spread vary greatly by geographic region, season, and local conditions. Many are bloodsucking and spread infection between animal and human hosts through blood-borne transmission, biting an infected individual and then transmitting the disease through subsequent bites. Some vector-borne diseases are spread by mechanical transmission (where the disease is transported by the insect but does not undergo development on or in the insect host), while others require their insect host as an essential step in the disease organism's life cycle. Some examples of flying vectors and the diseases they spread are:
Mosquito species (Figure 1) represent the greatest insect health hazard to large populations of people. Malaria and Dengue fever, both spread by mosquitos, affect hundreds of millions of people every year. Most species of mosquito are crepuscular, meaning that they are most active in the low light environment during dusk and dawn. Three species of mosquitos are responsible for the following diseases:
Aedes – Chikungunya, Dengue fever, Rift Valley fever, Yellow fever, Zika
Anopheles – Malaria, Lymphatic filaiasis
Culex – Japanese encephalitis, Lymphatic filariasis, West Nile fever
(Figure 1) Anopheles gambiae, primary malaria vector
The movement of insects towards a source of light radiation is referred to as positive phototaxis. The reasons for this behavior are not thoroughly understood and may or may not serve an evolutionary function in different species. Celestial navigation in nocturnal insects, where an individual uses a fixed point of light for orientation so it can fly in a straight line, results in a decreasing spiral
for terrestrial light sources. It has also been postulated that artificial light may appear to insects as an area of open sky, which may be either attractive or repellant depending on the behavioral goal of the insect at the time (when diurnal insects are disturbed at night, for example, they may seek areas of open sky as a means of escape).
Of particular interest is the influence of wavelength on positive phototaxis. Spectral (color) vision is dictated by the structure of the eye and the presence of certain pigments in the cell structures, and these vary by species. Many have peak visual sensitivity in the ultraviolet (UV) and blue portions of the spectrum, while some are also capable of seeing red and infrared (IR, or heat) wavelengths emitted by warm-blooded animals. Research and anecdotal experience suggest that flying insects have a strong phototaxis response to UV wavelengths. This has led to the development of UV ‘bug zappers’, which attract and kill flying insects, and also yellow incandescent ‘bug lights’, which (purportedly) do not attract as many insects because of the lack of blue light emitted by the source.