Thursday, July 26, 2007

Female insects that need no males

No rose grower need be told about the phenomenal rate of aphid reproduction. It may however surprise them that this is largely accomplished without male aphid intervention.

During the summer months, without benefit of mating, female aphids produce vast quantities of unfertilised eggs. These develop within the mother, who each day gives birth to 20-odd exact replicas of herself, each with their own eggs already maturing inside them. And each of these is the mother of a further generation of females.

In this way aphid populations expand rapidly. There are no males to take up food and space, and the females need not spend time mating. Every aphid reproduces and only cold weather, a predator such as a ladybird or a food short-age - or an insecticide - can stop them. Within days, the descendants of a single aphid can smother a rose bush.

In autumn, the pattern changes, and both male and female offspring are produced. This generation mates, laying fertilised eggs that spend the winter in crevices. The eggs hatch in spring, producing only females - and the cycle begins all over again.

Aphids owe their success to their dual method of reproduction. In the asexual phase, all individuals contribute to the next generation, while the sexual mode mixes and therefore refreshes the genes.

Oak gall wasps also reproduce by both methods. Having spent two winters encased in small galls, or swellings, on the roots of oak tress, wingless female surface and scramble up the trees to lay eggs in the buds. These females have not mated, and therefore reproduce asexually. The oak trees' tissues react by producing another gall that encases the eggs. By mid-summer its occupants are mature and winged, and fly into the world to find a mate.

Inseminated females burrow into the soil to lay single eggs in the roots of oak trees. There again galls are formed, from each of which, two years later, a wingless female emerges to climb up the tree and restart the cycle.

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