Hosts or Prey
Elm leaf beetle and a few other leaf beetles
Identification
The most obvious evidence this tiny wasp is present is that the normally pale orange to yellowish eggs of elm leaf beetle become more dark and golden. When larvae of elm leaf beetle hatch they leave behind empty egg shells that are pale yellow to whitish. When adult O. gallerucae emerge from parasitized eggs the shells are brownish to golden and inside each shell are fecal pellets (meconium) of the last instar wasp.
Adults are shiny blackish to dark green wasps with clear wings. The adult, last instar (larva), and pupa are about 1/32 inch (0.8 mm) long. Eggs, larvae, and pupae occur hidden within host eggs. The pale, elongate oval egg and maggotlike first instar are 1/100 (0.25 mm) long. The last instar is oblong and plump. All instars have distinct segments. The first instar has hooked mouthparts and the last instar lacks distinct mouthparts. Pupae are initially pale, then blacken and develop distinct appendages folded against the body as they age.
Life Cycle
Oomyzus gallerucae develops through 4 life stages: egg larva, pupa, and adult. Adult females prefer to oviposit in freshly laid elm leaf beetle eggs but will oviposit in eggs up to 4 days old. Generally only 1 wasp egg per host is laid. If more than 1 egg is laid in a host only 1 wasp survives to adulthood. After hatching the wasp larva develops through 3 increasingly larger instars as it feeds inside a host egg.
In addition to the killing of host eggs by parasitization the adult female wasps also feed on host eggs. The female wasp punctures leaf beetle eggs with her ovipositor and consumes the oozing contents. Females host feed for several days before they start laying eggs and host feed periodically throughout their life. If flower nectar or honeydew and host eggs are available for nourishment the adult females live about 1 month. During her life each female oviposits in about 30 host eggs and kills about 60 additional eggs by feeding on them.
Egg to reproductive adult development occurs in about 2 weeks when temperatures are warm. Oomyzus gallerucae has multiple generations per year.
Habitat
Elm leaf beetle is a defoliating pest of elm trees that was inadvertently introduced in the eastern United States from Europe in the 1830s. It gradually spread westward and was first reported in California in the 1920s and become a major urban tree pest.
Entomologists from the University of California collected O. gallerucae (then named Tetrastichus gallerucae) from the Eastern United States, Europe, and the Middle East. After study in quarantine and mass rearing, approximately 224,000 O. gallerucae were released from 1974 to 1989 in 15 counties across a climatic gradient in northern California. Additional parasitoids were reared and released in Southern California in the 1970s and 1980s.
Oomyzus gallerucae now occurs in much of California where elm trees have been planted. However, it is generally not abundant. By comparison a larval-pupal parasitoid of elm leaf beetle, Erynniopsis antennata, is commonly abundant and an important parasitoid of elm leaf beetle in California.
To improve biological control
- Control ants because they attack natural enemies of various pests.
- Grow flowering insectary plants to provide nectar to nourish adult natural enemies.
- Reduce dustiness that disrupts the activities of natural enemies (e.g., periodically hose off small plants).
- Avoid the application of broad-spectrum, persistent insecticides for all pests because they are toxic to natural enemies.
See Protecting Natural Enemies and Pollinators for more information.
Species
At least three species of Oomyzus occur in California: O. brevistigma, O. gallerucae, and O. incertus. Oomyzus brevistigma is a pupal parasitoid of elm leaf beetle and is common in California. Elm leaf beetle is the only reported host of O. brevistigma. Oomyzus gallerucae parasitizes the eggs of at least three species of leaf beetles.
More Information
- Elm Leaf Beetle In Biological Control in The Western United States, UC Agriculture and Natural Resources
- Elm Leaf Beetle (Xanthogaleruca luteola [Muller]) (Coleoptera: Chrysomelidae) (PDF), In The Use of Classical Biological Control to Preserve Forests in North America, U.S. Department of Agriculture Forest Service
- Elm Leaf Beetle, Xanthogaleruca (=Pyrrhalta) luteola (Muller) (Coleoptera: Chrysomelidae), Encyclopedia of Entomology, Second Edition
- Establishment and Overwintering of Tetrastichus gallerucae (Hymenoptera: Eulophidae), an Egg Parasitoid of the Elm Leaf Beetle (Coleoptera: Chrysomelidae) in Northern California, UC Berkeley, Environmental Entomology
- IPM Helps Control Elm Leaf Beetle, UC Berkeley, California Agriculture
- Pest Notes: Elm Leaf Beetle, UC Integrated Pest Management Program
- Universal Chalcidoidea Database, Natural History Museum of London
Scientific classification:
- Phylum: Arthropoda
- Class: Insecta
- Order: Hymenoptera
- Family: Eulophidae
