Macrosiphum euphorbiae

Order: Hemiptera
Family: Ahididae

Those big red aphids on the tomato plants are probably potato aphids. This doesn’t mean potato aphids are always red.  Sometimes they are green, and sometimes an infestation will consist of a mixture of green and red aphids, but when they are on tomatoes, potato aphids are usually pink or red.  Potato aphids have a couple of hundred host plants, including many garden vegetables such as kale, spinach, turnips, lettuce, tomatoes, eggplant, and, of course, potatoes.  As with most aphids, potato aphids can reproduce without mating, a phenomenon known as parthenogenesis.  The females do not lay eggs; they simply give birth to small aphid nymphs.  In colder areas of the country, males occur in the fall and after mating, the females lay eggs, and this is the overwintering stage.  Males are rare here in the South, and overwintering occurs through “normal” parthenogenetic development on winter host plants.    
 
Control:  Don’t be too hasty to spray your tomatoes for aphids.  Low numbers of aphids cause little or no harm to healthy tomato plants, and low populations are often controlled by beneficial insects and diseases.  Fruitworms, hornworms and stink bugs are usually the most serious pests of tomatoes.  If heavy aphid infestations begin to cause excessive honeydew or distortion of leaves and terminals, spray with insecticides containing acetamiprid or malathion. 
 
See page 2 and pages 12-14 of Extension Publication 2347, Insect Pests of the Home Vegetable Garden http://extension.msstate.edu/sites/default/files/publications/publications/p2347_1.pdf for more information on aphid control.