Inducing the sexual forms and hatching the eggs of pea aphids

In temperate climates, pea aphids (Acyrthosiphon pisum) produce a single sexual generation each year in response to declining photoperiod and temperature. Mating occurs in the fall and the eggs have an obligatory winter diapause. Genetic recombination during the sexual phase is thought to be an important source of genetic variability within cyclically parthenogenetic aphid populations. Methods for reliably producing sexual forms and hatching the eggs of aphids are therefore central not only to the study of evolutionary change in aphid populations, but also for a general understanding of the origin of agriculturally important variation in destructiveness within pest species.Here, sexual forms of six pea aphid clones were induced in the laboratory and eggs were successfully hatched by creating conditions that closely mimicked those found in field situations. A declining photoperiod was produced by controlling artificial lighting using a timer with variable cycle length. Using these conditions, sexual forms were successfully produced for all six clones tested, which were then mated in all combinations. Eggs were exposed to a daily cycle of freezing and thawing in an incubator under a short-day photoperiod. Egg hatch averaged 60%, but was as high as 89% for some crosses. These methods will permit testing of evolutionary hypotheses and execution of detailed genetic studies of sources of variability within pea aphid populations. They are thus important tools for both evolutionary and agricultural studies.