Tamarixia radiata, is an arrhenotokous ectoparasitoid of the Asian citrus psyllid (ACP), Diaphorina citri, vector of citrus greening disease or huanglongbing (HLB). Tamarixia radiata was released in Réunion, Taiwan, and Guadeloupe (Aubert & Quilici 1984; Chien 1995), and was credited with reducing the populations of D. citri sufficiently in Réunion to mitigate the impact of HLB (Étienne & Aubert 1980; Chien & Chu 1996; Étienne et al. 2001).

Tamarixia radiata was first imported to Florida from Taiwan and Vietnam in 1998 and released in 1999–2001 (Hoy & Nguyen 2001). Tamarixia radiata is well established in citrus orchards throughout the state according to a survey conducted in 2006–07 (Qureshi et al. 2009). Incidence was highest in fall but low early in the growing season, presumably due to poor survivorship in the winter when new flushes necessary for development of host nymphs are scarce. This bottleneck in the life cycle of T. radiata has only been exacerbated by the now widespread practice of “dormant sprays” of broad-spectrum insecticides during the months preceding the spring flush (Qureshi & Stansly 2010). Thus, successful biological control using T. radiata would require augmentative releases to jump-start parasitoid populations early in the growing season. Mass-rearing of this parasitoid to obtain adequate numbers for release would be essential and entail temporary storage and stockpiling of the wasps. During the holding period, food provided to females may affect the number of eggs formed in ovaries, which would influence efficiency upon release.

Oogenesis is a nutrition-limited process, and nutrition is obtained during both the larval and adult stage. Tamarixia radiata has been characterized as synovigenic-autogenous, meaning that some eggs become mature in the newly emerged wasp without feeding, but that host- feeding is later required to mature more eggs (Chien et al. 1994a).

Insufficient nutrients always affect egg production (Wheeler 1996). Most synovigenic parasitoids, including T. radiata, can resorb eggs when hosts are absent or scarce (Chien et al. 1994a). Carbohydrates are the major energy source for most insects, but they are not the main nutrient that triggers egg formation in female insects. Egg resorption can occur in T. radiata at either 15 °C or 25 °C when only honey is provided, and egg resorption is positively related to the length of deprivation time (Chien et al. 1991, 1994a). Once suitable hosts are fed upon, new mature eggs can be produced in the ovary and oviposition can proceed normally (Chien et al. 1994a). Varley & Edwards (1957) reported that female Nasonia vitripennis (Walker) (Pteromalidae) fed on carbohydrates survived, but resorbed eggs, resulting in gradual decrease of eggs in the ovaries. Bownes & Blair (1986) found that oviposition and formation of vitellogenic oocytes in ovaries of Drosophila melanogaster (Meigen) were reduced significantly on sugar diets due to reduced fat body transcription of yolk protein and consequent low availability for oogenesis.

Host hemolymph is rich in protein and an important source of nutrients to many hymenopterous female parasitoids for increasing fecundity (Howard 1910; Rockwood 1917). Varley & Edwards (1957) observed that egg development by female N. vitripennis was initiated when or shortly after host hemolymph was consumed. Leius (1961) reported that female Itoplectis conquisitor (Say) (Hymenoptera: Ichneumonidae) fed on host body fluids laid more eggs than those fed on carbohydrates alone. Additionally, Leius (1961) found that pollen in diets significantly increased fecundity of I. conquisitor females, and females fed on a mixed diet (host body fluid and carbohydrates) deposited even more eggs. Tamarixia radiata females fed on a diet of honey and yeast extract significantly decreased host-feeding while maintaining or improving intrinsic rate of increase (0.2976 to 0.3014 per day) and net reproductive rate (140 to 187 female eggs per female), respectively (Chien et al. 1994b).

The objective of this experiment was to investigate the effects of food supplements on oogenesis with the ultimate goal of maintaining or increasing reproductive potential during storage, and presumably optimizing parasitism rate immediately upon release.

Materials and Methods

Egg Load

Tamarixia radiata females from each different treatment were dissected after 5, 10, 15, and 20 days as well as newly emerged T. radiata under a dissecting microscope (Olympus SZX 16, Olympus Corporation). Ovaries were photographed using an Olympus DP 21 camera, and the number of mature eggs was recorded.

Table 1.

Number of eggs (Mean ± SE) per female by dietary treatment obtained by dissecting Tamarixia radiata females after 5, 10, 15, and 20 days on each diet at 17 C.

Results

Newly emerged T. radiata females averaged 4.6 ± 0.65 eggs each. Significant treatment effects on egg load were seen at all 4 dissection intervals (for 5 days: F _{7, 48} = 11.1702, P < 0.0001, for 10 days: F _{5, 35} = 12.72, P < 0.0001, for 15 days: F _{5, 30} = 7.8989, P < 0.0001, for 20 days: F _{5, 26} = 10.28, P < 0.0001) for 5, 10, 15 and 20 days respectively. All of the wasps fed on just water, or water and Nu-Lure died within 10 days. Survivorship of females fed honey for 20 days was 40%, the same as wasps fed honey + Nu-Lure.

The addition of Nu-Lure increased the egg load associated with every diet more than honey. Significantly more eggs formed with honey + Nu-Lure compared to honey alone on all 4 dates, whereas no significant differences were seen between females provided with honey+ Nu-Lure compared to honey + nymphs until 15 days (Table 1). NuLure + nymphs was significantly better than nymphs alone except at 10 days, whereas, nymphs + honey was not better than nymphs alone until 20 days. Nu-Lure + nymphs was better than honey + nymphs over all treatment periods, but nymphs + Nu-Lure + honey was never better than nymphs + Nu-Lure. Survivorship at day 20 from diets of nymphs, honey+ nymphs and Nu-Lure + nymphs were all 80%, whereas survivorship of the females fed on honey+ Nu-Lure + nymphs was 90%.

Discussion

An average of 4.6 eggs were observed in the ovaries of newly emerged T. radiata females, despite no supplemental feeding, indicating that nutrition acquired during the larval stages was used to form the first clutch of eggs. Mating was not necessary for egg formation.

This study showed that honey alone was enough to keep wasp females alive, but egg resorption took place within 5 days after emergence in the absence of food. On the other hand, NuLure alone with water did not support survival beyond 5 days. These results indicate that sugars provided by honey are necessary but not sufficient for survival, since nutrients from egg resorption were apparently also required. Addition of Nulure to the diet appeared to forestall egg re sorbtion although it did not allow the number of eggs to increase. This result indicated that additional amino acids provided by Nu-Lure satisfied nutritional needs of the wasp but were insufficient for oogenesis. Access to nymphs allowed for a limited degree of oogenesis that was enhanced by the addition of honey, but even more so by the addition of Nu-Lure. This result indicated that while nutrients provided by host feeding were required for egg production, additional carbohydrates and especially amino acids were still beneficial. The fact that no additional benefit was conferred by the addition of honey to wasps provided with nymphs (Fig. 1) indicated that carbohydrates were not a limiting factor when hemolymph and an additional source of amino acids were provided.

Skelley & Hoy (2004) showed that T. radiata stored for up to 35 days at 17 °C with honey and yeast suffered less than 5% mortality. Chien et al. (1994a) reported that females stored for 20 days at 25 °C were able to lay a total of 156 eggs, compared with 98 eggs when stored at 15 °C. Thus, although less mortality was experienced at low temperatures, production suffered. Therefore, the ideal storage temperature should be determined according to specific objectives (establishment or augmentation) and conditions (host availability). Clearly, both honey and Nu-Lure or some facsimile are valuable supplements for wasps in storage at 17 °C. However, maximum benefit will be obtained if hosts can also be provided. Nu-Lure or other amino acid supplements could also enhance the efficiency of mass rearing. Studies are planned to evaluate benefits of providing carbohydrate and amino acid supplements to ovipositing T. radiata by assessing effects on fecundity and host feeding.

Fig. 1.

Paired Tamarxia radiata ovaries of females that had been fed Nu-Lure + honey + Diaphorina citri nymphs for 5 days.