Certain insects, fungi and nematodes can acquire and spread virus during feeding. Virus spread can be prevented by control of the vectors. Vectors can be controlled using pesticides, insecticidal soaps and with ladybugs and other predators. Aphids can also be avoided by using silverized paper or plastic mulches. Migrating aphids are attracted to the color yellow but repulsed by dark green. Silverized mulches can repulse aphids. The yellow-sensitivity of aphids explains the ease that aphids have for finding infected plants and possibly why viruses cause symptoms such as chlorosis. Oil emulsions regularly applied to plants have been shown to prevent transmission of viruses by some airborne vectors. Control procedures for vectors are never completely effective and one must be dilligent. It takes only 1 insect to transfer the virus so by the time easily seen insect populations have developed, it may be too late. Virus-vector relationships are specific which means any one virus has only 1 type of vector (aphid, thrip, fungus). However, time needed for virus acquisition, distance of spread, number of plants that can be infected per insect and how long the vector retains the virus is dependent on the virus-vector combination. In the case of aphids, there are many different species that can transmit viruses such as DMV and CMV. Virus is acquired during a brief superficial probing of the plant but virus is retained for a short time (minutes to an hour). Also, aphids that do not feed or reproduce on dahlias are usually better at spread of DMV and CMV than species that colonize the plant.