In winter there are only females

There’s an Oak Apple Gall above me on this oak tree. It’s probably five centimetres wide and inside it are anything up to thirty gall wasp larvae having a feast. Each Oak Apple contains only male or only female wasp larvae of the species Biorhiza pallida. It’s a small wasp, only about five millimetres long. In summer the flying wasps will eat their way out of the Oak Apple, find one another and mate.

I wonder whether there are males or females growing inside this one?

The males will not survive long, as they try to find a mate and then die. The mated females will head underground for the roots of an oak tree and lay their eggs on small rootlets. Here they will irritate the tree into developing the second generation of galls of the year. Inside of the root galls will develop only flightless females. There are no males.

These flightless females will crawl up the oak tree in spring and inject their eggs and venom into oak buds to continue the cycle as we see the Oak Apple Galls develop again amongst the oak leaves.

What allows a flightless female wasp to lay viable eggs on oak buds without ever mating with a male? What allows those females to produce eggs which will develop into either male or female wasps? What allows a flying female to mate with a male and then only produce eggs that will develop into females?

It’s all in the sex-determination method used by bees, wasps, ants and sawflies. All these insects, known as Hymenoptera (membrane wings) use the same sex-determination method. If a wasp has one copy of its genetic material (one set of chromosomes) it’s a male. If a wasp has two copies of its genetic material (two sets of chromosomes) it’s a female.

That means a wingless winter female can produce copies of itself, all of which are female, without there being a single male in existence anywhere in the world. It’s called parthenogenesis. That wingless winter female can also produce males by creating eggs which have only half the genetic material it has, by making sure each has only one working copy of every chromosome.

The subsequent summer winged females produce eggs after mating, all of which have both sets of genetic information, one from the male and one from the female, and therefore all of them develop as female. These wasps alternate a sexually reproducing generation, with males and females developing in the branches, and an asexually reproducing generation with only females developing in the roots.

Cool, isn’t it?

As with all sexual reproduction, there are only two sexes.