Encarsia species of Australia and the Pacific Islands
attacking Bemisia tabaci and Trialeurodes vaporariorum

The silverleaf or sweetpotato whitefly, Bemisia tabaci (Gennadius)
(=B. argentifolii Bellows & Perring, 1994) in Australia

In Australia the silverleaf whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) biotype B was first detected in Darwin, Northern Territory, in October 1994 (Gunning et al., 1995). Subsequent surveys found the whitefly to be establishin nurseries across northern New South Wales and Queensland (De Barro, 1995). The silverleaf whitefly is now well established in cropping areas along the Queensland coast from Cooktown to northern New South Wales with scattered populations in the Queensland cotton growing towns of Emerald, Biloela, Warra, St George, Dalby and Oakey. This pest is highly polyphagous and colonises numerous hosts including cotton and ornamental, vegetable, and weed species. Damage is caused, a) by direct feeding which may induce irreversible physiological disorders in certain plant species as well as yield decline, b) by contamination with honeydew and sooty mould, and c) by the vectoring of geminiviruses (see De Barro, 1995, for a review).

One of the key management challenges posed by the silverleaf whitefly is its ability to develop resistance against insecticides. This is further compounded by the shortage in Australia of effective insecticides and difficulties in obtaining minor use registration for new effective products. For this reason reliance on insecticides as the sole means of managing infestations was considered at best a short-term solution. It is generally considered that long term sustainable management of silverleaf whitefly requires an integrated approach in which a range of management strategies is combined to control the pest. One of the key components to achieving this elsewhere has been the use of natural enemies.

Australia has never before had a serious whitefly pest of outdoor crops. As a consequence, there was very little research experience present in Australia capable of dealing with the pest. It was therefore concluded that research into management of this pest should commence before problems occurred so as to build the necessary research capability. One of the key areas targeted was biological control. Research overseas indicated that parasitoids offered the best potential (Gerling, 1986; Osborne et al., 1990; Goolsby et al., 1996; Kirk & Lacy, 1996; Lacy et al., 1996; Legaspi et al., 1996; Nordlund & Legaspi, 1996; Goolsby et al., 1998; De Barro, 1995).

Our knowledge of the parasitoid fauna in Australia that may contribute to the control of the silverleaf whitefly was virtually nil. There was however, at least one indigenous biotype of B. tabaci that was widespread across the northern half of Australia (De Barro & Driver, 1997; De Barro et al., 1998) as well as several other related indigenous species of Bemisia (Martin, 1999). It was therefore concluded that agents capable of contributing significantly to the biological control of this pest may already be in Australia.

Encarsia in Australia

Encarsia Förster, 1878, is a large genus of the chalcidoid family Aphelinidae, with currently about 280 described species (Polaszek et al., 1999). About 110 Encarsia species are known to parasitize whiteflies (Babcock & Heraty, 2000). Prior to this survey only a single species attacking B. tabaci and T. vaporariorum, Encarsia formosa Gahan, was known from Australia (Wilson, 1960).

Despite considerable efforts, there are many geographical regions where the Encarsia fauna is still very poorly known. The results of this study and our current research efforts indicate that Australia has a high diversity of Encarsia species. This notion is supported by the high species richness of host taxa, in particular whiteflies (Martin, 1999), and the high number of Australian species described by early authors: between 1894 and 1939 about 35 Encarsia species have been described, the majority of them by A.A. Girault (Noyes, 1998). Most of these species are insufficiently described and they are usually only known from the type specimens which are often in very poor condition. The taxonomy of most of the early described species can not be clarified until the Australian fauna is better known, and until redescriptions of freshly collected and slide mounted material have been made.

Molecular approaches to Encarsia taxonomy

In addition to traditional morphological methods, molecular approaches are more and more used and provide an important tool to investigate the status of closely related species and to infer phylogenetic relationships (Babcock & Heraty, 2000, Babcock et al., 2001). The D2 expansion region of the 28S ribosomal DNA showed rates of sequence divergence, expressed as the number of pairwise differences divided by the number of shared nucleotides, of, on average, 10.8% between species and 2.4% within species and was found to be most suitable to characterize Encarsia species genetically and to develop molecular markers which allow rapid identification of species which are morphologically difficult to distinguish (Babcock & Heraty, 2000).

The Study

The study was mainly based on material collected over a period of three years on the Pacific Islands (1996-97) and in Australia (1996-98). Most of the specimens were reared from either B. tabaci or T. vaporariorum, although some were obtained from related Bemisia spp. as well as species of Lipaleyrodes Takahashi and Aleurocanthus Quaintance & Baker. Each sample was given a unique code number and the host plant, host whitefly species, date, location and collector were noted. Nymphs of parasitized hosts were kept in emergence chambers and the parasitoids transferred to gelatine capsules or 94% ethanol, where they remained at room temperature until further examination or DNA analysis. Whiteflies were identified to species level using the fourth instar pupal case from which the parasitoid had emerged (Martin, 1987). Bemisia tabaci biotypes were identified using adults collected along with the parasitized nymphs according to the method described in De Barro & Driver (1997).

Identification of Encarsia species

The identification of Encarsia species is often difficult because of their small size and the necessity to prepare slide mounts. New species are being continuously added to the approximately 280 described species and there is evidence for the presence of complexes of cryptic species within several of these described species (Polaszek et al., 1999). Many Encarsia species have a very wide or even cosmopolitan distribution, complicating taxonomic revisions on a local scale. Only four of the 16 species treated in this study seem to be restricted in their distribution to Australia, whereas at least seven species have either a wide geographical distribution embracing several major zoogeographical regions (E. azimi, E. bimaculata, and E. pergandiella) or are virtually cosmopolitan (E. formosa, E. lutea, E. protransvena, and E. sophia) (Huang & Polaszek, 1998, Heraty & Polaszek, 2000).

Males are often very difficult to identify without accompanying females. In several species males are not known. Therefore the pictorial key is designed only for females, but male description are provided where possible to aid identification of males in samples where males and females are present.

Measurements of quantitative characters given in the species descriptions are based on specimens which were available at the time this study was conducted and additional material or abnormal specimens may reveal values which lie slightly outside the given ranges. This is in particular the case for species which were recorded by only a few specimens.

Slide prepartion of Encarsia specimens and their examination

All specimens used in this study were slide mounted as described by Noyes (1982) with the following modifications: specimens were placed in 10% KOH for 5-8 min (depending on whether the specimen was dry or preserved in ethanol) and incubated at 97° C using a block heater. Detailed instructions can be found here. The terminology follows Heraty & Polaszek (2000). All measurements of antennae and legs refer to the maximal length of the morphological structure in lateral view. Lengths of antennal segments were taken excluding the intersegmental membranes because they can vary depending on how much the antenna was stretched during slide preparation. Fore wing length is the distance between its most apical point and the proximal end of the submarginal vein, excluding the tegula (see Glossary). The length of the ovipositor was measured as the distance between the proximal margin of the basal ring to the extreme apex (cf. fig. 5 in Huang & Polaszek, 1998: 1828, fig. 1B in Heraty & Polaszek, 2000: 145). This is different from Hayat (1998) who measures the ovipositor length as the combined lengths of second valvifer and second valvula (cf. fig. 8 in Hayat, 1998: 272). Care should be taken if specimens are distorted because this can affect measurements, in particular measurements of the ovipositor (Heraty & Polaszek, 2000). When taking measurements it is necessary that all reference points of the structure to be measured are equidistant from the objective of the microscope.