In the last week (as you can see in the photo of my Department in Modena) I had to face several days of snowing and my aphids too… but where are aphids during the winter?
As you can see also in the blog Living with insects, insects can adopt different strategies to survive the winter. Many insects are in diapause, others (such as aphids) use eggs to wait for the spring arrival (in the photo of aphid eggs from Bug Guide).
Aphids reproduce primarily by viviparous parthenogenesis, a form of reproduction whereby adult females give birth to a female progeny without any male fertilization from spring to autumn. During autumn the presence of few hours of light (together with low temperatures) induces a shift in the life cycle of aphids to handle the challenge of winter. In particular, different forms of aphids are produced in place of parthenogenetic females, including special sexual form males and females that may lay eggs. After mating, females lays over-wintering egg in protected crevices in plants, often around buds. Eggs hatch the following spring and the normal life cycle resumes with the birth of parthenogenetic females.
In view of the need to protect eggs, some aphids alternate among host plants using some species for laying eggs and other one for food during spring and summer.
As you can read also here, some aphids are more lucky since some species of ants take care for them during winter. Indeed ants carry the aphid eggs at their home and protect them in their nests during winter. They store aphids at optimal temperatures and humidity and move them as needed when conditions in the nest change. In spring, when the aphids hatch, the ants carry them to a host plant to feed. For example the rosy apple aphid Dysaphis plantaginea has Malus sp. (suc as apples) as its primary host and Plantago lanceolata as its secondary or summer host.
As showed in the below figure (published in 2003 by Shingleton et al. BMC Developmental Biology 3:7), generally during the spring and summer aphid reproduction is by parthenogenesis and the asexually-produced embryos develop in approximately 10 days. In the fall there is a single generation of sexually-reproducing males and females which produce diapausing eggs that do not hatch until the following spring, more than 100 days later!
Frequently, some aphids become obligate parthenogenetic so that females and males do not occur in autumn. This shift in the reproductive mode makes their distribution limited to regions with warm winter since they can not lay overwintering eggs.
What effect will these cold days have on aphids? Probably, cold winter will delay the appearance of aphids that overwinter as adults due to the adverse effect of the cold weather on aphid survival. On the contrary, cold winter is likely to have less impact on the survival of aphids usually overwintering as eggs.