The Braconidae is a large family of parasitoid wasps. There are approximately 17,000 recognized species and many thousands more undescribed. One analysis estimated a total between about 30,000 and 50,000, and another provided a narrower estimate between 42,000 and 43,000 species.
Braconidae are currently divided into about 47 subfamilies and over 1,000 genera, which include Ademon, Aleiodes, Aphanta, Asobara, Bracon, Cenocoelius, Chaenusa, Chorebidea, Chorebidella, Chorebus, Cotesia, Dacnusa, Kollasmosoma, Microgaster, Opius, Parapanteles, Phaenocarpa, and Psenobolus.
These fall to two major groups, informally called the cyclostomes and noncyclostomes. In cyclostome braconids, the labrum and the lower part of the clypeus are concave with respect to the upper clypeus and the dorsal margin of the mandibles. These groups may be clades that diverged early in the evolution of braconids.
The morphological variation among braconids is notable. They are often black-brown (sometimes with reddish markings), though some species exhibit striking coloration and patterns, being parts of Müllerian mimicry complexes. They have one or no recurrent veins, unlike other members of the other famlily in Ichneumonoidea (Ichneumonidae), which usually have two. Wing venation patterns are also divergent to apparent randomness. The antennae have 16 segments or more; the hind trochanters have two segments.
Females often have long ovipositors, an organ that largely varies interspecifically. This variation is closely related to the host species upon which the wasp deposits its egg. Species that parasitize microlepidopterans, for instance, have longer ovipositors, presumably to reach the caterpillar through layers of plant tissue. Some wasps also have long ovipositors because of caterpillar defense mechanisms such as spines or hairs, or to reach deeply-burrowed coleoptera larvae in rotting tree trunks.
Most braconids are internal and external primary parasitoids on other insects, especially upon the larval stages of Coleoptera, Diptera, and Lepidoptera, but also some hemimetabolous insects such as aphids, Heteroptera, or Embiidina. Most species kill their hosts, though some cause the hosts to become sterile and less active. Endoparasitoid species often display elaborate physiological adaptations to enhance larval survival within the host, such as the co-option of endosymbiotic viruses for compromising host immune defenses. These polydnaviruses are often used by the wasps instead of a venom cocktail. The DNA of the wasp actually contains portions that are the templates for the components of the viral particles and they are assembled in an organ in the female'S abdomen known as the calyx. A 2009 study has traced the origins of these templates to a 100-million-year-old viral infection whose alterations to its host DNA provided the necessary basis for these virus-like "templates".
These viruses suppress the immune system and allow the parasitoid to grow inside the host undetected. The exact function and evolutionary history of these viruses are unknown. Sequences of polydnavirus genes show the possibility that venom-like proteins are expressed inside the host caterpillar. Through the evolutionary history of being used by the wasps, these viruses apparently have become so modified, they appear unlike any other known viruses today. Because of this highly modified system of host immunosuppression, a high level of parasitoid-host specificity is not surprising.
Parasitism on adult insects (particularly on Hemiptera and Coleoptera) is also observed. Members of two subfamilies (Mesostoinae and Doryctinae) are known to form galls on plants.
Larvae can be found on diverse hosts, including aphids, bark beetles, and foliage-feeding caterpillars. Many species are parasitoid wasps that attack host eggs or larvae; hence they are often used as biological pest control agents, especially against aphids.
The family seems to date from early Cretaceous (provided that Eobracon is properly assigned to this family). It underwent extensive diversification from mid or late Cretaceous to early Cenozoic, correlating with the radiation of flowering plants and associated herbivores, the main hosts of braconids.
Braconidae are distinguished from their sister group Ichneumonidae by these character combinations. In Braconidae, vein 2m-cu of the forewing is absent- this vein is present in 95% of Ichneumonidae. Vein 1/Rs+M of the forewing is 85% present in Braconidae, but absent in all Ichneumonidae. Vein 1r-m of the hind wing is in 95% of Braconidae basal to the separation of R1 and Rs (it is opposite or apical in Ichneumonidae). In Braconidae, metasomal tergum 2 is fused with tergum 3, (secondarily flexible in Aphidiinae) - 90% of Ichneumonidae have a flexible suture.
The species Microplitis croceipes possesses an extremely accurate sense of smell and can be trained for use in narcotics and explosives detection.
At least some Braconidae appear to be very resistant to ionizing radiation. While a dose of 400 to 1000 rads can kill an average human, a dose of 180,000 rads was required to kill a braconid of genus Habrobracon in an experiment.