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Time Frame
Temperature plays a significant role in the speed at which Tetranychus urticae develop from eggs to adults, but the longest-lived mites are the overwintering females. They go dormant in the fall, not feeding again until they emerge from their leaf or bark burrows in early spring.
Then, according to the Washington State Tree Fruit Research and Extension Center, they lay eggs for about 23 days before dying. Females born before the fall lay eggs for approximately 30 days.
In warm weather, eggs hatch in as little as 24 hours, and the larvae develop into adults in only 10 days. Under these conditions, the maximum lifespan for a Tetranychus urticae female is about 40 days.
Eggs
At just 1/150 of an inch in diameter, and with a translucency making them indistinguishable from the leaves on which they're laid, Tetranychus urticae eggs are invisible to humans. Older eggs gradually become yellow.
Larvae
The larvae have round, translucent six-legged bodies slightly larger than the eggs. Just prior to hatching, they develop red eye spots.
They begin feeding immediately, changing to light yellow or green with the dark back spots partially responsible for their name--"two-spotted red spider mites."
The rest of their name comes from the red-orange hue females take on before overwintering, and from the mites' lifelong ability to produce webs. Tetranychus urticae mites can't fly, but hanging on a silken strand from one plant lets them spread to others on the wind. The webs also protect them from predators.
Nymphs
Tetranychus urticae have two nymphal stages in between which they feed, rest and molt. More deeply colored than larvae, eight-legged nymphs have noticeable spots. The first nymphal stage, protonymph, is smaller than the second, deutonymph.
The mites' genders become recognizable in the deutonymph stage. Smaller than females, males have pointed abdomens.
Adults
Eight-legged adult females keep their pale green or yellow color, but males may have a brown or orange cast. Both genders keep the black spots, which vary in size because they're caused by food particles in the mites' digestive tracts.
When overwintering females turn red-orange, the spots vanish. They return with the females' normal green color after the females emerge in the spring.
Controlling Tetranychus Urticae
Because mites aren't insects, most commercial insecticides aren't effective at controlling Tetranychus urticae infestations. Look instead for a miticide meant (or registered) to be used on ornamental (flowering) plants for your garden. If a miticide isn't registered in this specific way, it may contain solvents which can harm or kill the plants you treat.