Diachasma alloeum is a small wasp in the family Braconidae

Braconidae

Braconidae is a family of parasitoid wasps and one of the richest families of insects. Between 50,000 and 150,000 species exist worldwide. The species are grouped into about 45 subfamilies and 1,000 genera, some important ones being: Ademon, Aphanta, Asobara, Bracon hebetor, Cenocoelius, Chaenusa,...

. It is a parasitoid

Parasitoid

A parasitoid is an organism that spends a significant portion of its life history attached to or within a single host organism in a relationship that is in essence parasitic; unlike a true parasite, however, it ultimately sterilises or kills, and sometimes consumes, the host...

 of Rhagoletis pomonella, the apple maggot

Apple maggot

The apple maggot , also known as railroad worm, is a pest of several fruits, mainly apples. The adult form of this insect is about 5 mm long, slightly smaller than a house fly, with a white dot on its thorax and a characteristic black banding shaped like an "F" on its wings...

. The wasp lays its eggs into third instar larvae of the fly, which then develop after the larvae have pupated. The immature wasps then eat the fly larvae and overwinter inside the fly puparia.

Diachasma alloeum wasps attacking Rhagoletis pomonella in apples appear to be undergoing a speciation

Speciation

Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...

 event in concert with their hosts. This is an example of sequential sympatric speciation

Sympatric speciation

Sympatric speciation is the process through which new species evolve from a single ancestral species while inhabiting the same geographic region. In evolutionary biology and biogeography, sympatric and sympatry are terms referring to organisms whose ranges overlap or are even identical, so that...

.