My neighbor was showing me what he thought was a spider mite on his tomato plant. It looked like an aphid. I know it wasn't a spider mite, but is was red. Are there red aphids??

yes, aphids can be many colors. Read on...

Recognizing aphids. Aphids range in size from 1.5 to 3.5 mm. in length although the largest aphid in northeastern America, the giant bark aphid (Logistigma caryae) is 6 mm. long (Borror et al. 1976). Aphids are pear-shaped, globose, ovate, spindle-shaped or elongate in shape and vary greatly in their body markings and color (black, grey, red, orange, yellow, green, brown, blue-green, white-marked, wax-covered, etc.). Single species may have several color forms and variable shapes. Aphids may be winged (alate) or wingless (apterous). Winged forms are usually triggered by environmental changes (i.e. decreasing photoperiod or temperature, deterioration of the host plant or overcrowding).

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Sandi
Texas Certified Nursery Professional
Texas Master Naturalist
Organic gardener
Tree-Hugger
Native Texan