A parasitic wasp (Hymenoptera: Braconidae), laying eggs in a cotton bollworm. Photo by W.Sterling.
Common Name: Parasitic wasp
Scientific Name: Varies
Order: Hymenoptera
Description: Adults of many species are very small (ranging from 1/100 to 3/4 inch long) and often go unnoticed. They vary in shape and coloration but usually have long, thread-like (filiform) antennae or they may appear elbowed, clear or colored wings with characteristic venation and a narrow “waist” between the thorax and abdomen. Females of many species have a spine-like egg-laying structure (ovipositor) at the tip of the abdomen. Larval stages are usually not observed unless they are dissected from hosts (internal parasites) or detected on the host (external parasites). They are usually cream colored, legless and tapered at both ends. Occasionally, caterpillars are observed with white silken cocoons of parasites (Braconidae) attached to their bodies. Stages of immature whiteflies parasitized with Encarsia formosa (Gahan) (Chalcidoidea, Encyrtidae) are darker or black when late in the parasite development compared to yellowish to creamy healthy ones. Aphids are hosts for species in the subfamily Aphidiinae (Braconidae) such as Aphidius spp. and others in the family Aphelinidae (Chalcidoidea). Parasitized aphids, called “aphid mummies”, appear puffed up, brown and hardened. The adult parasitic wasps chew a round hole in the abdomen to emerge.
Not all species in these groups are parasitic. For instance, the tiny (1/32 inch) fig wasp, Blastophaga psenes Linnaeus (Chalcidoidea, Agaonidae) and related species are essential pollinators of certain fig varieties, with larvae that develop only inside a gall in wild Caprifig flowers.
Life Cycle: Biology and details of development vary with species. Adult wasps emerge from pupae and females seek suitable host insects into or on which to lay eggs singly or in clusters. Usually, a larva hatches from an egg and develops through several stages (instars) before forming a pupa. However, some parasitic wasps, such as Copidosoma spp. (Chalcidoidea, Encyrtidae), undergo a process called polyembryony, whereby an egg inserted into a host divides and gives rise to hundreds of larvae. Most parasitic species have high reproductive capacity and develop rapidly. Several generations can sometimes develop during a single generation of the host, although some species have only a single generation per year.
Habitat, Food Source(s), Damage: Mouthparts of larvae and adults are for chewing. Larval stages of parasitic wasps develop inside or outside of a single host during one or more of the host’s developmental stages (egg, larvae, pupae or adult). Those that kill their hosts are called parasitoids. Most insect groups (including aphids, beetles, caterpillars, flies, sawflies, scale insects and true bugs) are attacked by parasitic wasps. Many species are host specific, developing in one or a limited number of related host species. For example, the species of Trichogramma (Chalcidoidea, Trichogrammatidae) primarily parasitize the eggs of caterpillars. Adults of some parasitic wasps feed on other insects to obtain energy to seek other suitable hosts into or on which to lay eggs. This behavior is called host feeding and also helps reduce populations of host insects. Parasitic wasps may emerge from host insects if they are reared. A number of parasitic wasp species are commercially available from insectaries and are purchased and released in augmentative biological control programs. Other species have been imported from other countries from which pests have been accidentally introduced without their natural enemies and released to reintroduce the natural enemy with its host, a practice called importation, or “classical” biological control and which occasionally results in sustained suppression.
Pest Status: Numerous tiny wasps parasitize various stages of other insects; many are beneficial parasites (parasitoids) that kill other insect pests, while some are considered harmful because they parasitize parasites (a condition called hyperparasitism) or other beneficial insects; all are medically harmless.
For additional information, contact your local Texas A&M AgriLife Extension Service agent or search for other state Extension offices.
Literature: Arnett 1985; Borror et al. 1976; Metcalf et al. 1962; Swan & Papp 1972.