We use Azamax (Azadirachta indica) for organic control which is systemic or Confidor (imidicloprid + procloraz).

The latter is available as a tablet for advanced trees and lasts many months

Hope you have some success in removing them

Cheers

http://influentialpoints.com/Gallery/Aphids_on_willow_Salix.htm

http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7404.html

you could try a color trap  , or you may try a several small tray around the greenhouse with drop of liquid soap with sugar added to water to attract and avoid the winged aphids. It was good to avoid new infestation when it is difficult to use insecticide

In an Experiment it is necessary to monitor the plants and remove the first aphids by Hand. If the aphids colonise, and you need clean plants for your experiment, an insecticide will help, e.g. 0.15 - 0.2 % Remados (systemic).

As I mention the site http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7404.html

where I found very clear for different type of controlling methods, and you could select one more suitable for your greenhouse situation, if its only few infested seedlings, you can collect the seedling and destroy them to avoid more infestation

Hope you have some success in removing them

Best regards

Chemical Control

When considering whether to apply insecticides for aphid control, remember that most larger plants can tolerate light to moderate levels of aphids with little damage. Larger aphid populations often rapidly decline due to biological control or when hot temperatures arrive. Often a forceful spray of water or water-soap solution, even on large street trees, when applied with appropriate equipment, will provide sufficient control.

If insecticides are needed, insecticidal soaps and oils are the best choices for most situations. Oils may include petroleum-based horticultural oils or plant-derived oils such as neem or canola oil. These products kill primarily by smothering the aphid, so thorough coverage of infested foliage is required. Apply these materials with a high volume of water, usually a 1 to 2% oil solution in water, and target the underside of leaves as well as the top. Soaps, neem oil, and horticultural oil kill only aphids present on the day they are sprayed, so applications may need to be repeated. Although these materials can kill some natural enemies that are present on the plant and hit by the spray, they leave no toxic residue so they don't kill natural enemies that migrate in after the spray.

These and other insecticides with contact-only activity are generally ineffective in preventing damage from aphids such as the leaf curl plum aphid or the woolly ash aphid, which are protected by galls or distorted foliage. Also, don't use soaps or oils on water-stressed plants or when the temperature exceeds 90°F. These materials may be phytotoxic to some plants, so check labels and test the materials on a portion of the foliage several days before applying a full treatment.

Supreme- or superior-type oils will kill overwintering eggs of aphids on fruit trees if applied as a delayed-dormant application just as eggs are beginning to hatch in early spring. (On plums dormant applications right after leaves have fallen in early November are preferred.) These treatments won't give complete control of aphids and probably aren't justified for aphid control alone but will also control soft scale insects if they are a problem. Common aphid species controlled with these types of oils include the woolly apple aphid, green apple aphid, rosy apple aphid, mealy plum aphid, and black cherry aphid.

Many other insecticides are available to control aphids in the home garden and landscape, including foliar-applied formulations of malathion, permethrin, and acephate (nonfood crops only). While these materials may kill higher numbers of aphids than soaps and oils, their use should be limited, because they also kill the natural enemies that provide long-term control of aphids and other pests, and they are associated with bee kills and environmental problems. Repeated applications of these materials may also result in resistance to the material.

Insecticides such as oils and soaps are also safer to use when children and pets may be present. Formulations combining insecticidal soaps and pyrethrins may provide slightly more knockdown than soaps alone yet have fewer negative impacts on natural enemies than malathion, permethrin, and acephate, because pyrethrins break down very quickly.

Systemic insecticides are also available for aphid management, primarily for woody ornamentals. These materials, including imidacloprid, are very effective and are especially useful for serious infestations of aphids such as the woolly hackberry aphid, which is often not effectively controlled by biological control or less toxic insecticides. Imidacloprid can have negative impacts on predators, parasitoids, and pollinators, so its use should be avoided where soaps and oils will provide adequate control. To protect pollinators, don't apply imidacloprid or other systemic insecticides to plants in bloom or prior to bloom.

Home-use soil-applied imidacloprid products are often diluted with water in a bucket and poured around the base of the tree or plant. Professional applicators can use soil injectors, which provide better control with less runoff potential. Applications are usually made in spring when aphids first become apparent.

Thanks to all for yours  valuable comments. I would like to  ask if we use some  insecticidal spray e.g PUUTARHA  does it will effect on heavy metals chemical analysis after harvesting the plant tissues  at the end of experiment.

I do not know Puutarha. Does it contain any of the metals you Analyse? If it is sprayed, it is supposed to stay only at the surface, where it does not interact with the metals taken up from the soil. If it is organic and systemic applied an interaction is not expected, but cannot be excluded.

Insecticidal soap works if applied regularly. It also has little residual. This may be good if you are using the plants for other assays. This may be of little value if you are using them as ornamentals or only interested in plant growth in the absence of herbivores. Power washing the plants can also work, as can dipping in hot water (though here you will need to test the plants for tolerance to hot water).