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Inoculative releases of 3 parasitoids, Tachinaephagus zealandicus Ashmead, Sphalangia endius Walker, and Muscidifurax raptor Girault and Sanders, during December–April, 1969–70, in an enclosed poultry house in the interior area of southern California near Riverside produced maximum parasitization rates of 46% on Musca domestica L., but only 16% on Fannia femoralis (Stein) and F. canicularis (L.), in the absence of native predators. T. zealandicus was the most active species recovered during the coldest months, the other 2 species excelled in March and April when temperatures averaged ca. 2°C higher.

The synonymy, distribution, host range, and life history of the gregarious larval–pupal parasitoidTachinaephagus zealandicusAshmead, is discussed. Laboratory studies of its biology were conducted at 25° ± 2 °C usingMusca domesticaL. as host. Its developmental stages are described. Under laboratory conditions its life cycle lasted 23–27 days. Parasitoid development accelerated with higher average densities per host. Single standardized hosts produced 3–18 adult parasitoids. Mated females provided with hosts lived 50.4–67.2 h. The average length of the reproductive period and number of hosts parasitized were independent of host density; however, the average number of eggs deposited per host increased at lower host densities. Adult emergence displayed circadian rhythmicity independent of photoperiod over 3+ days.

Seasonal predatory and scavenger arthropod densities were studied at interior and coastal southern California poultry ranches. Though some seasonal population clumping occurred with some species, the distribution of predators and scavengers was fairly uniform within each ranch. Correlation analyses of key predators in theHisteridae, Staphylinidae, Hydrophylidae andDermaptera with the potential hosts,Musca domestica L.,Tinea fuscipunctellaHaworth, andFannia spp. suggested that predator activity was seasonally influenced. Possible periodic avoidance of a particular host's habitat was detected as significant negative correlations. The data tend to support the importance of different species of predators in different seasons and the need for natural enemy complexes rather than single species for biological control.

The native parasitoid complexes associated with four key muscids, Musca domestica L., Stomoxys calcitrans (L.), Fannia canicularis (L.), and F. femoralis Stein—and two predatory Diptera, Muscina stabulans (Fallen) and Ophyra leucostoma (Wiedemann)—are described and shown graphically for the major climatic zones of the southwestern United States. Three geographical groups of the parasitoid species identified were (1) those whose populations extended into most climatic zones below 2,000 meters (Muscidifurax species and Spalangia nigroaenea Curtis); (2) those whose populations were restricted to the warmer climatic areas (S. endius Walker and S. cameroni Perkins); and (3) those that were largely restricted to higher latitudes and colder winter climates (S. nigra Latreille and Aleochara spp.). Considerable yearly, seasonal, and climatic variations were shown in the abundance of both host and parasitoid demes. These factors, combined with others, probably account for the high degree of instability among demes. This instability may be important in determining the panmictic nature of most populations. Some parasitoids such as the Diapriidae and Staphylinidae (Aleochara spp.), which exist in small demes, appeared to be local species that may have adapted to invading Diptera. The Stilpnus species were specific to Fannia. Parasitoid species appear to be irreplaceable mortality factors in the natural control of populations of their dipterous hosts.

Parasitization of the house fly, Musca domestica L.; the stable fly, Stomoxys calcitrans (L.) ; and species of Fannia, Muscina, and Ophyra in animal dung is recorded at collection sites through portions of the Palearctic, Ethiopian, and Australasian Regions, and the Pacific area. Comparisons are made with similar collections in the Western Hemisphere. The importance of parasites (parasitoids) in host population regulation and their world exchange as biological control agents is discussed.

Cull cantaloupes and watermelons were a major breeding habitat for Musca domestica L. in the Palo Verde Valley, California. By mid-July the average pupal density on decayed watermelons ranged 13.2-79.4/melon over three years. Parasitization never exceeded three percent through the combined action of Spalangia endius Walker, S. cameroni Perkins and S. nigroaenea Curtis. These species and S. drosophilae Ashmead were reported for the first time from an otitid, Euxesta notata (Wiedemann), also in decaying melons. Destruction of breeding habitat by mechanically breaking cull melons appears to be the only practical solution to the house fly problem.