Psyttalia humilis (Silvestri, 1913)

For additional information, see the Psyttalia and Psyttalia concolor pages.
Taxonomic History / Nomenclature
This species was originally described as Opius humilis by Silvestri (1913, 1914).
Fischer (1987) placed humilis in the subgenus Psyttalia within Opius. Psyttalia was subsequently elevated to generic rank by Wharton (1987). The combination Psyttalia humilis (Silvestri, 1913) was first used by Wharton (1989). The species name was misspelled as humilus in Wharton and Gilstrap (1983).
The original description of this species was based on material collected in South Africa (Silvestri 1914) from medfly infesting pears. Introduced to and established in Hawaii in 1913, but vanished from the islands sometime between 1933 and 1949. All other locality information requires verification and resolution of taxonomic problems mentioned in the identification section below.

Several attempts were made to redistribute P. humilis from Hawaii to other parts of the world for control of medfly, C. capitata, during the 1920s and 1930s. Shipments made to Israel in 1925/26 and Spain in 1933 either arrived dead or failed to establish in culture (Bodenheimer 1951). Other shipments were apparently equally unsuccessful (Clausen 1978, Wharton 1989).

In addition to the distribution information noted below, Psyttalia humilis has been recorded from Australia (Cameron 1941), Benin (Kimani-Njogu et al. 2001), Bermuda (Thompson 1953, Cameron 1941, Waterson, 1937, Russell 1934, 1935, 1936), Fiji (Lever 1938), Japan (Clausen 1978), and Kenya (Bianchi and Krauss 1936, 1937, Cameron 1941, Le Pelley 1959, Gilstrap and Hart 1987). Verification of these distribution records is complicated by the species identification problems noted below and elsewhere. For example, P. humilis was described from South Africa and P. concolor from North Africa. Since the two are nearly identical, it has been difficult to place material from intermediate areas in Africa (Rugman-Jones et al. 2009). Similarly, P. humilis was shipped from Hawaii to parts of the Mediterranean Region and the Middle East where P. concolor is native. Thus, Psyttalia concolor was initially described from Tunisia and also occurs in Spain, but there are published records of Psyttalia humilis from both localities (Thompson 1953, Cameron 1941).

Identification of Species and Subspecies
During the exploration phase of the Oriental fruit fly program in the late 1940s and early 1950s, several of the opiines from Kenya were variously identified as color varieties of Psyttalia concolor or as Psyttalia perproxima (Clausen et al. 1965). Material from the same localities had been identified as either P. humilis or P. perproxima during an earlier sampling program (Bianchi and Krauss, 1936). Difficulty in identification of these three species is still a problem, and uncertainty over whether or not they are distinct makes it difficult to correctly associate previously published host records. Efforts to resolve this problem have been long-standing (Fischer 1958, 1972, Wharton and Gilstrap 1983, Wharton et al. 2000, Kimani-Njogu et al. 2001) and are still on-going. Rugman-Jones et al. (2009), for example, showed that there were several genetically distinct subsaharan populations that could not be readily distinguished morphologically from the Mediterranean Psyttalia concolor. Kimani-Njogu et al. (2001) noted that at least some of these subsaharan populations from Kenya could be hybridized with populations of P. concolor from Italy. Given these disparate results, Rugman-Jones et al. (2009) suggested that the name Psyttalia humilis could be applied to several of the subsaharan populations used in biological control should a separate name be needed to denote genetically and perhaps biologically distinct entities. On the basis of this suggestion, the name Psyttalia humilis has been adopted for use in the California olive fly biological control program when referring to populations introduced to California from subsaharan Africa (Yokoyama et al. 2011). Psyttalia concolor is the appropriate name to use for populations from the Mediterranean region.
Biology and Behavior
See Yokoyama et al. (2011) for detailed information on populations introduced to and established in California. See also the P. concolor species page.
Biological Control
Psyttalia humilis, originally collected in South Africa, was successfully established in Hawaii in 1913 against medfly. The efficacy of Psyttalia humilis and three other introduced species was extensively documented over a 23 year period (Back and Pemberton 1915, Pemberton and Willard 1918, Back and Pemberton 1918, Willard and Mason 1937), and detailed biologies were published by Pemberton and Willard (1918). All four species were still established in 1933, the last year for which data are available from this program, with Psyttalia humilis and Diachasmimorpha tryoni (Cameron) exhibiting roughly equivalent levels of parasitism on medfly. The primary benefit of these introductions was the considerable reduction in infestation levels in coffee, which had previously been so badly infested that the coffee berries could not ripen (Willard and Mason, 1937). Infestations in other fruits was also reduced, though not as much in large, fleshy, preferred fruits such as mango. Nevertheless, reduced infestations made it possible to integrate other control measures more successfully, and eliminate medfly in some non-preferred fruits. Psyttalia humilis was last recorded from Hawaii in 1933 and subsequently disappeared for reasons unknown. It is interesting that humilis was established in Hawaii from three females bred from the initial small lot of 5 individuals that survived Silvestri’s voyage to Hawaii from Africa (Fullaway 1914). Cultures were lost after the initial release in Kona of these 3 females and several males.

Kenyan populations were introduced to California in the 2000s and have become established (Yokoyama et al. 2011). Details of establishment and rates of parasitism are provided by Yokoyama et al. (2011).