Edible Gardening Series: Question of the Week – insect vision and colors
By Carol Wyatt-Evens and Sarah Bostick
Gardening in Florida can be incredibly rewarding and incredibly frustrating, at the same time. If you are new to the region, you soon learn that gardening in the Sunshine State can quickly become a full-time job. While our subtropical climate is perfect for growing an abundance of different vegetables, fruits, and herbs, it also can present some overwhelming challenges.
We can help!
UF/IFAS Extension Sarasota County agents and staff have created an online edible gardening resource center. The website features short videos from our 25-episode “Edible Gardening Series” webinars, along with blog posts and resources list for episodes. Get help on an array of topics that befuddle many gardeners.
Each blog post in the Question of the Week blog series is associated with a specific EGS episode, but they are written for everyone to enjoy!
Question of the Week:
How do insects see?
Insects use their eyes in much the same way as humans do – to navigate, find food, and interact with the world around them. Sight is also used to help them find mates. insect eyes and human eyes are similar in that both have lenses and photoreceptors, but overall, the two are quite different.
The human eye consists of a single lens. The muscles of the eye allow it to move freely in the eye socket but can only look in one direction at a time. An insect’s compound eye has many lenses, but the eye is fixed. Insects, such as a predatory dragonfly, can have up to 30,000 lenses per eye. This is like having 30,000 tiny eyes looking in different directions! Each individual lens in the insect eye is part of a structure called an ommatidium. These structures are clustered together like a pack of drinking straws and collectively make up the insect compound eye. But, insects with compound eyes also have another type of eye that helps them ‘see’. These are the three simple eyes, ocelli, that are found on the top or at the front of the head in a triangle pattern. Ocelli are simple photoreceptors (light-detecting organs) that help the insect see shadows, see light and dark, detect movement, and orient themselves toward a light source (usually the sun).
Wavelengths and Color
The human eye has an iris that adjusts the amount of light entering the eye, allowing the eye to focus on objects with crisp detail. Insect eyes lack that light-adjusting ability and the end result is a fuzzy or out-of-focus image. With insects, the greater the number of ommatidia, the better the visual acuity or quality. However, even insects eyes with 30,000 lenses will never be able to ‘see’ objects the same way as a person. Life will always appear a bit fuzzy to an insect.
So, how do insects see color compared to humans? It’s all about wavelengths.
The visible spectrum, that spectrum with a famous name, ROY G. BIV, (red, orange, yellow, green, blue, indigo, violet) has a wavelength range from 700 nanometers (nm) for red to 400nm for violet. The visible light spectrum is part of the electromagnetic spectrum – a very small part – but the segment on the spectrum that corresponds to colors.
Insects and Ultraviolet Light
Humans see the entire visible spectrum from red to violet, but insects see from 650nm to 300nm. Yes, that is off the chart! Insects have the ability to see in the ULTRA-violet (UV) range on the spectrum. The range with higher frequency, shorter wavelength, and beyond where humans are capable of seeing. Within this spectrum, the colors that insects see really well are ultraviolet, green, and blue. They have difficulty seeing yellow and orange and they cannot see the color red at all.
When it comes to which wavelength insects are most attracted to – it’s ultraviolet (UV) light. People have taken advantage of insect attraction to UV light to eliminate insects around their home and yards. We use ‘bug zappers’ to kill unsuspecting insects that like to bite and suck our blood.These ‘zappers’ use UV light-emitting bulbs (a UV lightbulb or a black light) to attract insects to the contraption, only to be zapped to death when they come in contact with the wire frame surrounding the lightbulbs. Unfortunately, the number of biting insects (mosquitoes) killed by these zappers is extremely low compared to the number of non-biting or beneficial insects killed by the device.
Flowers also take advantage of ultraviolet light to attract a specific group of insects – pollinators. Flowers use patterns in their petals that act like a bullseye for the pollinators. The insects get a tasty meal while helping the flowers by moving extra pollen to the next flower. The flower pattern may not be obvious to us, but under UV light the pattern on the petals glows like a neon arrow directing the insect to the pollen.
Insects and the Color Yellow
Besides UV light, we also take advantage of colors to both attract and repel insects. Since yellow is at the far edge of the insect sight range, we use yellow lightbulbs outside our homes to keep from attracting insects. However, this will not completely eliminate insects since some light wavelengths in the blue range will still be emitted, a few insects will still be drawn in. To fully eliminate any insect attraction, red lightbulbs can be used. Unfortunately, red doesn’t offer our human eyes much usable light. It is very dim lighting and not very useful as a lighting source.Another way we use yellow is in the garden and landscape with yellow sticky traps. Now, I know, it seems counter to what has been said – they cannot see yellow. Yellow is a bright and very reflective color -it is very good at reflecting UV light. The yellow sticky card acts much like the patterned flower and will draw the insect toward the source emitting the UV light. In this case, to an unfortunate end for the insect, since the perceived light source is a yellow sticky trap.