Hosts or Prey
Walnut aphid and certain other aphidsIdentification
The most obvious evidence that T. pallidus or its Aphidiidae relatives are present is that the normally soft-bodied, yellowish green walnut aphids become crusty, tan, and swollen mummies due to a parasitoid inside. Some of the mummies may have a round emergence hole of the adult wasp. Prior to mummifying the aphid, an orangish wasp larva may be visible through the host covering.
The adult T. pallidus has a shiny black head and thorax and a long, slender, black and orangish abdomen. The adult body is 1/25 to 1/8 inch (1–3 mm) long and the slender antennae are nearly as long. The pale eggs, maggotlike larvae, and oblong pupae occur hidden inside parasitized aphids.
Life Cycle
Trioxys pallidus develops through four life stages: egg, larva, pupa, and adult. The female wasp lays an egg inside aphid nymphs and adults. Eggs can be laid in all instars of walnut aphid, but third and fourth instars are preferred.
The parasitoid egg hatches into a larva, which consumes the inside of the aphid as it develops through several increasingly larger instars. The mature wasp larva (prepupa) causes the host covering to become a brown to tan, crusty, puffy mummy. After pupating the adult wasp chews a rounded hole in the mummified aphid skin and emerges. The wasp has about 10 generations per year in California.
Habitat
Trioxys pallidus occurs in California wherever walnut aphids or its other hosts are present. It was introduced into California from Iran for biological control of walnut aphid. Since its establishment in the late 1960s it has virtually eliminated the walnut aphid as a pest in walnut orchards except when biological control is disrupted by the presence of ants or the application of broad-spectrum, persistent insecticides.
To increase the effectiveness of biological control
- Control ants that attack natural enemies of aphids and other pests.
- Grow flowering insectary plants to provide nectar and pollen 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 Relative Toxicities of Insecticides and Miticides Used in Walnuts to Natural Enemies and Honey Bees to learn which pesticides are most compatible with biological control. See Protecting Natural Enemies and Pollinators for more natural enemy conservation tips.
Species
At least seven species of Trioxys occur in California. Hosts of T. pallidus include blackmargined aphid, common oak aphid, hazel aphid, linden aphid, and walnut aphid.
More Information
- Biological Control of the Walnut Aphid in California: Impact of the Parasitoid, Trioxys pallidus, UC Berkeley, Hilgardia
- Biological Control of the Walnut Aphid in California: The Interrelationship of the Aphid and its Parasitoid, UC Berkeley, Environmental Entomology
- Factors Influencing the Abundance of Trioxys pallidus, a Successful Introduced Biological Control Agent of Walnut Aphid in California, UC Berkeley, Biological Control
- Trioxys pallidus (Haliday), The Hebrew University of Jerusalem
Scientific classification:
- Phylum: Arthropoda
- Class: Insecta
- Order: Hymenoptera
- Family: Aphidiidae
