Some species reproduce asexually other species mate.
More Info: Most aphids reproduce asexually without having to mate, producing live offspring all year. The few species that do mate produce eggs. They mate in the fall or winter as the eggs are more hardy than the nymph and have a better chance of surviving the winter.
Adult aphids can reproduce as many as eighty offspring per week.
What Is the Lifecycle of an Asexual Aphid?
An aphid hatches from an egg at the beginning of spring. The aphid then devours its host plant for 7-10 days until the females mature and reproduce asexually through the process of cyclic parthenogenesis. Mature females give birth to 5-10 clones every day, and the clones give birth to more clones.
Once the host plant is devoured, the aphids migrate to a new plant and continue the process over again throughout the summer. As fall approaches, some of the females turn into males and start to reproduce sexually. The average life cycle of an aphid is 20-40 days.
What Is the Lifecycle of an Aphid that Mates?
Many aphid species have the capacity to mate during part of the year, usually in the autumn, and reproduce many generations of females rapidly via parthenogenesis in the spring. The environmental changes of autumn, such as decreasing temperatures, cause female aphids to produce both female and males partheogenetically; these males and females then reproduce sexually. After sexual reproduction, the female aphids will lay eggs. The aphid eggs hatch in the spring, beginning another cycle of asexual reproduction and another generation of parthenogenetic females.
 “Aphids.” The University of Rhode Island. N.p., n.d. Web. 22 June 2011. http://www.uri.edu/ce/factsheets/sheets/aphids.html
Backyard Gardener – Aphids, May 26, 1999.” College of Agriculture and Life Sciences. N.p., n.d. Web. 28 June 2011. <http://ag.arizona.edu/yavapai/anr/hort/byg/archive/aphids.html>.