Know Your Friends

The Fungus Gnat Parasitoid Synacra pauperi

Fungus gnats, Bradysia spp. are major insect pests in greenhouse crop production systems. The adults are a nuisance, and can carry spores of soil and foliar pathogens on their bodies. The larvae cause direct injury by feeding on plant roots, which reduces the plant’s ability to take-up water and nutrients. Also, the wounds created by larval feeding provide an entry site for secondary soil-borne pathogens. In addition, fungus gnat larvae have been shown to directly transmit soil-borne pathogens such as Pythium spp. to plants.

Two very good biological control agents are commercially available for controlling fungus gnats. These are the entomopathogenic nematode Steinernema feltiae and the soil-predatory mite Hypoaspis miles. Both have proven effective in controlling fungus gnats in commercial greenhouse production systems.

A figitid wasp trapped on a yellow sticky card.Another biological control agent is the naturally occurring parasitic wasp or parasitoid Synacra pauperi. This parasitoid is commonly found (entering from the outside) in greenhouses or conservatories that are unsprayed or avoid the use of conventional pest control materials. They are very noticeable on yellow sticky cards. In fact, they are more attracted to yellow sticky cards than adult fungus gnats.

S. pauperi (Family Diapriidae, Subfamily Belytinae) adults are approximately the same size as fungus gnat adults (1/8 inch long). The parasitoid has a noticeable constriction between the head and the thorax, and the thorax and the abdomen. The abdomen tapers to a sharp tip. Antennae are elbowed and the tips are dark and swollen. Females are reddish-brown with black eyes. Males are black with long antennae approximately the same length as the body.

Females insert eggs into fungus gnat larvae. They can develop in each of the three larval instars. S. pauperi is a solitary parasitoid, with only one parasitic wasp emerging from each fungus gnat pupa. The parasitized larvae live until pupation, then die, after which the wasp pupates. The parasitoid has a potential maximum rate of population increase that is higher than fungus gnats at 23C; however, the parasitized fungus gnat larvae may still cause plant damage during the interval between parasitization and death. The parasitoid has been demonstrated to be a useful biological control agent of fungus gnats in Swedish greenhouses. At the University of Illinois Plant Sciences Conservatory, S. pauperi (identified on yellow sticky cards) has provided seasonal control of fungus gnats to the point that insecticides are generally not needed.

— Raymond A. Cloyd, University of Illinois

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