1. I've seen them
2. I've seen their poop
3. Along the central vein of the affected leaves is a sheen

There are quite a few leaves on a few of my tomatoes (spreading around it seems) that are yellowing. They are all older leaves near the base, and I've been reading about a disease called Tomato Spotted Wilt Virus that is spread by thrips. Can this damage be caused by thrips alone, or is a disease setting in? I'm starting to get worried.

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"Tomato. Symptoms expressed on leaves, petioles, stems, and fruit will vary, depending on the stage plants are infected. Young leaves may show small, dark-brown spots and eventually die (fig. 11). Dark brown streaks also appear on stems and leaf petioles. Growing tips are usually severely affected with systemic necrosis and greatly stunted growth (fig. 12). The plant may exhibit one-sided growth. Tomato fruit set on severely infected plants will display very characteristic symptoms: immature fruit have mottled, light green rings with raised centers; the unique orange and red discoloration patterns on mature fruits make them unmarketable (fig. 13)."

http://vegetablemdonline.ppath.cornell.edu/PhotoPages/Tomatoes/Tom_SpWilt/Tom_SpWiltFS14.htm

From TAMU
http://aggie-horticulture.tamu.edu/tomatoproblemsolver/leaves/30b.html

From Auburn University

http://www.ag.auburn.edu/aaes/organicveg/OrganicVegetableProductioninAlabama_000.php

Pictures from Univ Florida

http://www.hos.ufl.edu/vegetarian/03/April/Apr03.htm