Green Filters Can Work Two Ways

The latest buzz word in combating agricultural pests sounds like a secret weapon from tomorrows battlefield where warring armies will "zap" their rivals communications systems, putting the enemy out of commission. The new strategy is based on disrupting natural communication lines between plants and insects by playing tricks on the optical faculties of insects eyes.

For more than a decade Israeli farmers have been employing specially-treated polyethylene sheeting in greenhouses where roses are grown, to block ultra-violet (UV) light in order to prevent rose petals from blackening in the strong sunlight of Israel. It was only by chance that plant virologists recently discovered that the UV sheeting can also provide a new way to fight pests.

While Dr. Hezi Antignus and his colleagues from the Volcani Institute were experimenting with the use of UV sheeting for another purpose, they noticed that the common cucumber virus, CYSDV, was less prevalent under UV sheeting. Upon closer inspection, they discovered that the development of the virus was not directly affected by UV rays but that greenhouses covered with UV-blocking plastic simply exhibited much lower infestation levels of some of the worst agricultural pests, such as aphids, white flies, thrips and leaf miners, which carry plant viruses.

When the phenomenon was examined under controlled conditions comparing conditions in greenhouses covered with UV-blocking plastic as opposed to those with other coverings it was found that not only was the insect population dramatically reduced when UV light was blocked, but those pests that permeated the greenhouse altered their behavior. They were much less active than "brother bugs" elsewhere, thus spreading fewer viruses as they moved about laying their eggs or feeding on plants.

The key is the way insects eyes operate. Insect eyes are fitted with photo-receptors that receive stimuli from various light wave lengths, not only "visible light" that humans see, but also UV light which we cannot perceive. Researchers postulate that the lack of UV light impedes the insects sight. Moreover, many flowers bear ultra-violet visual guide marks that reflect UV radiation, which help direct insects to nectar or pollen sites. In environments where the UV spectrum is filtered out, these "landing pads" become invisible to insects. And pests that do penetrate the greenhouses stumble around in the dark, so to speak.

The significance of the discovery is particularly marked in tomatoes where yellow leaf curl virus can decimate a crop. When tomatoes were experimentally grown in greenhouses fitted with UV-blocking plastic without employing any pesticides levels of infection were a mere one percent. In the control group grown under conventional plastic without spraying, infection was 80-90%. Until now the only way to fight viruses (for which there is no cure) was to keep the insect population low, primarily by spraying with pesticides, up to three times a day. In essence, UV sheeting can transform hothouse tomatoes into a high-yield "organic," pesticide-free crop.

What about open field crops? During the summer, Israeli farmers have adopted a stop-gap measure to avoid the massive use of pesticides. Field crops are covered with fine mesh nets bionets that keep most insects out. The findings on the effect of UV rays on insects sight prompted the development of "double-action" mesh nets that screen out both insects and UV light. Consequently, the UV-blocking strategy is expected to have an impact on a variety of crops, both in greenhouses and in open fields, providing an effective substitute for agrochemicals.

Dr. Antignus revealed that the only drawback to the UV system is that bees employed as natural pollinators exhibited the same "lethargy" refusing to leave their hives as other insects in a UV-free environment. Researchers found that only when hives were placed along the walls of greenhouses constructed of regular plastic so as to receive non-filtered light sources, did the bees venture out. Once in the open, they apparently compensate for their "loss of sight" by using other senses to find the flowers.

Innovative methods for eradicating pollutants in the environment are also being developed. Israeli agricultural researchers are cleaning up polluted waters by planting water lilies and pressing water ferns into service literally and figuratively. A team of botanists at the Hebrew Universitys Faculty of Agriculture, headed by Professor Elisha Tel-Or, have successfully utilized the bazaar "appetite" of the frail-looking but clearly hearty Azolla ferns and water lilies to absorb heavy metals without any detrimental effect to the plants.

Azolla plants have a long life and will absorb copper, cadmium, zinc, chromium, and nickel at 500 times their concentration in common effluents. Professor Tel-Or found that because the heavy metals bind to the cell walls of the plant, Azolla can be equally effective when dried and pressed. This attribute makes it possible to transform the Azolla into a product a biofilter rather than use it as a living plant that must be nurtured and attended. Biofilters can also be "planted" anywhere, especially close to the source of a potential pollutant. This reduces the amount of effluents that must be treated to a minimum and optimizes the efficiency of the biofilter, which can be targeted to treat one kind of metal, instead of a complex "chemical soup" of various compounds in a larger body of water.

Such biofilters are being tested in a number of industrial settings in Israel, including a nickel cadmium battery plant, a lead car battery factory and a plating facility using chromium. The same filters also proved effective in reclaiming gold in a jewelry-making establishment. Not only can the fern prevent pollution but when the Azolla is burned, its "naturally-enriched ash" contains six percent gold and four percent platinum which can be retrieved. Another potential use for Azolla biofilters is to strain out radioactive materials. Tests on radioactive uranium tracings in solutions originating in a nuclear research facility were 99% purified after being passed through an Azolla filter. The process is expected to significantly reduce the volume of radioactive wastes that must be stored. Moreover, radioactive isotopes can be recovered and "recycled" in this manner.

But the Azolla is not alone. Recently, Professor Tel-Or also discovered that water lilies are endowed with anatomical and physiological characteristics that allow them to literally thrive on water with a high concentration of heavy metals cadmium, mercury, nickel and cobalt. Under laboratory conditions the plant proved capable of absorbing metals at a rate of up to 16% of its dry weight! A pilot project at the Haifa municipal wastewater treatment plant aims to evaluate the lilys potential in an industrial setting.