The Intersection of Plants and People
In Israel’s hot, subtropical climate, agricultural researchers are constantly investigating ways to grow healthy plants despite the sun’s intensity, limited fresh water, and relentless insects. For two decades, scientists around the world have been testing how colored nets can positively affect the growth of crops beneath them, with several studies coming out of the Volcani Center, the agricultural research arm of the Israeli government. The netting offers many agricultural benefits, including reducing the amount of pesticides needed for crops and even controlling how a plant grows.
Black shade nets are already widely used to protect crops from the sun’s ultraviolet (UV) rays, hail, wind and birds. Dr. David Ben-Yakir, an entomologist at the Volcani Center, has studied the effects of colored netting on disease-spreading insects such as thrips, whiteflies and aphids. The netting used in his experiments, called ChromatiNet, was developed by Polysack Plastic Industries of Nir Yitzhak, Israel, in collaboration with the Volcani Center. Other companies, such as TENAX, headquartered in Viganò, Italy, are also creating colored shade nets for the agricultural industry.
Ben-Yakir has experimented with a variety of net colors, including yellow, red, blue, silver, white and pearl. These plastic nets can help deter insects by using what Ben-Yakir calls “optical manipulation,” meaning that the colors the insects perceive lead them to either not see the plants underneath or to distract them away from the plants. Yellow netting, for example, is irresistible to most insects.
“The information in itself is not new; the way we use it is new,” Ben-Yakir explains. “We know that yellow is a very attractive color for the landing of insects, because they perceive it as a dark green.”
Most insects only see from the ultraviolet to the green-yellow range. Once insects land on an attractive surface such as a smooth, plastic, yellow net, they try to feed. This period of attraction that the insect feels for an object — whether it’s a flower or something manmade — is called “arrestment.”
“Especially with small insects, they come on the last drop of their fuel,” Ben-Yakir says. “They fly and then they try to land when they really need to feed. They don’t have many reserves. So if they think they are on the plant and there is no food there, they either fly away if they still have energy, or they die.”
In a study published in 2012, Ben-Yakir and his colleagues reported that, even though whiteflies landed on a yellow net 40 more times than on a black or red net, only half of them passed through to the tomato plants below. The result was an 80-fold protection from whitefly infestation for the tomatoes under the yellow net. In addition, the tomato plants grown under red, yellow and pearl nets used in the study had 15-40 percent higher fruit production compared with plants under the black net.
To thwart insects that make it through the net to the plants, Ben-Yakir says that there are other possible ways to stop pests from destroying crops, such as using artificial yellow lights, yellow sheets with a sticky surface, or yellow sheets containing insecticide. Depending on the type of insect being targeted, another color might be a better way to distract a pest away from plants. For example, thrips — tiny, winged insects that, like botanical vampires, pierce plant tissue and suck out cell contents — are specifically attracted to the color blue.
Nets that greatly reflect the sun can also help keep away insect predators. An insect hovering over a highly reflective net will visually read the area as barren, since rocks and sand tend to reflect more sunlight.
Normally, plastics for outdoor agricultural use already contain some additives to protect the material itself from damaging UV rays. What Ben-Yakir and his colleagues have been investigating is how introducing color additives can further manipulate the light spectrum so that pests are distracted while certain growth properties in plants are enhanced. The goal, Ben-Yakir explains, is to reflect only the UV and the yellow light, while still allowing in enough of the blue and red light that plants need for photosynthesis.
In research published in Scientia Horticulturae in 2013, other scientists at the Volcani Center found that growing sweet peppers under pearl-colored nets improved the post-harvest quality of the fruit, compared with the same crop grown under black nets. The peppers had firmer, more elastic flesh, and had greater levels of ascorbic acid and antioxidants. While the air temperature and humidity under both the black and the pearl nets were relatively the same, the pearl netting’s ability to increase certain wavelengths of light and their intensity made a significant difference in how the sweet peppers fared once picked.
Other research has determined vast differences between colored netting. Blue netting, which reduces red and far-red light values, results in plants with darker, more compact foliage and a delayed bloom. Such traits are desirable to commercial growers who need to ship ornamental plants and cut flowers that can blossom while on display in retail stores. Interestingly, red netting tends to cause the opposite effect of the blue netting: a high growth rate. The root systems are more developed, stems are longer, plants flower earlier, and plants yield larger fruit that ripen more quickly.
Other research on colored nets has been ongoing, with the hopes of improving and controlling crop growth with a minimal use of pesticides and other chemicals. Understanding how colored nets affect plants, pests and beneficial insects could lead to more widespread use of these techniques, even in the home garden.