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• Materials and methods
• Host plants
• Host aphid
• Parasitoid
• Experimental units
• Experimental procedure
• Data analysis
• Results
• Discussion

 The cotton aphid, Aphis gossypii and the green peach aphid, Myzus persicae are major pests on cucumber and sweet pepper in greenhouses (Van Schelt, 1993), and these occurrences in other greenhouse crops are increasing (Van Steenis, 1995). As the cotton aphid and green peach aphid have developed resistance to many commonly used pesticides (Gubran et al., 1993), exploring natural enemies that can be used in the biological control of these aphids is quite an interesting study. The purpose of this study is to determine whether A. varipes could supplement the aphidiinae species (Braconidae) that are used in the biological control of cotton aphid and green peach aphid in greenhouses today.

 Few aphelinids are known to parasitize the cotton aphid and the green peach aphid successfully. Only Aphelinus asychis and A. abdominalis have been used for biological control of the cotton aphid (Wyatt, 1967). Van Steenis (1995) collected life history data from a strain of A. varipes from Cameroon with the cotton aphid as host. The data concluded that A. varipes could be useful in biological aphid control programs, especially when used together with Aphidius colemani Viereck (Hym., Aphidiidae).

 Different strains of one parasitoid species can have different life history parameters. Due to intraspecific variability, choosing the right strain is of crucial importance for the success of biological control (Haardt and Holler, 1992). In this study, some important life history traits of a Korean strain of A. varipes parasitizing the green peach aphid are described. The traits studied were host feeding, mummification, pupal mortality, developmental period, and sex ratio when green peach aphids in different stages were presented to A. varipes. Green peach aphids are large and dark green at lower temperatures, but they are smaller and light green (Blackman and Eastop, 1984; Aldyhim and Khalil, 1993) at higher temperatures. The experiment was performed at 20℃, 25℃, 30℃, and 35℃ to find out whether this morphological variation had any influence on the traits studied.

Materials and methods

Host plants

Sweet pepper (Capsicum annuum L.) were grown in a greenhouse at 25±5 ℃ and moderately fertilized. We inoculated aphids on the plants cultured for 3-5 weeks (about 10-15 cm). The plants inoculated were maintained in acrylic cage (500 mm3) and transferred to the controlled climate room. To avoid contamination of other pests to the plants, the plants were cultured in net cages (1,000 × 2,000 × 1,500 mm). 

Host aphid

 We collected the green peach aphids on greenhouse peppers at Yuseong, Daejeon, Republic of Korea in year 2000. They were transferred to laboratory and were reared successively under the described conditions. To avoid their contamination by other natural enemies, they were isolated with acrylic cages (500 × 500 × 500 mm). The acrylic cage had the draft hull with a net (80 mesh).

Parasitoid

Aphelinus varipes is distributed in Korea (Paik, 1978). This species was collected from the colonies of wheat aphid, Rhopalosiphum padi L. on barley field, Yuseong, Daejeon, Republic Korea in March 2000. They were transferred to green peach aphids and cultured on sweet pepper. The experiments were performed 4 years later. 

 The rearing of A. varipes was held in cages at 25±2℃ temperature, 16L:8D light. The parasitoids used in the experiments were 24-hour-old mated females placed in emergence vials (Ø1.6 cm × H6.3 cm). They had some host experiences from green peach aphids placed in the emergence vials for host feeding (2-3 aphids per parasitoid) the day before the experiments.

Experimental units

At first, petri dishes (Bock, Art. Nr. 41113, 31 mm in height; 77 mm in diameter) that Van Schelt (1993) indicated were used despite the insignificant diameter and height. Therefore, we used petri dishes with a diameter of 90 mm and a height of 50 mm for the experiments. Nets (80 mesh, 40 mm in diameter) were incorporated into the lids for air exchange. In order to adhere to leaf disc for each petri dish, Van Schelt (1993) and Röhne (2002) used water agar but the moisture was not maintained in petri dish. Therefore, we used square absorbent cotton (40 mm × 40 mm) instead of water agar. Square absorbent cotton wetted with distilled water was adhered to bottom of each petri dish and a punched sweet pepper leaf disc was placed upside down on the square absorbent cotton. The petri dishes were placed upside down on shelves (with ventilation hole) to simulate a more natural condition for the aphids and prevent the leaf from being contaminated by honeydew and mold (Van Schelt, 1993). 

Experimental procedure

Experiments were carried out in all five host stages and at four different development temperatures: 20℃, 25℃, 30℃, and 35℃. The climate cabinets with 16L: 8D were used. Ten adult green peach aphids were placed in each petri dish for 24 hours. When the offspring of these aphids had reached the desired stages, 50 aphids per dish were selected, while the surplus was removed. Three replicates (dishes) were found for each of the 20 combinations of aphid stages and temperatures. 

 Two female parasitoids were allowed to host feed and parasitize in each petri dish for 12 hours at different temperatures. The parasitoids were then removed from each petri dish. The proportion of aphids mummified was used to measure the host acceptance of A. varipes and the suitability of the host in a no-choice situation. Host feeding, pupal mortality, sex ratio, and development time from egg to mummy (pupa) and from mummy to adult were measured. The mummies were transferred to glass vials (50 mm height; 20 mm diameter) for enclosure.

Data analysis

The data were examined using a two-way analysis of variance (ANOVA) and checked for interaction. Data on development time were analyzed using the averages for each petri dish. Multiple comparisons were made using an LSD-procedure (α= 0.05) with Bonferroni adjustment (Reimer, 1959). 

Table 1. Developmental Periods of A. varipes Parasitized from 1st to 4th Instar of M. persicae at Four Different Temperatures

Results

The total developmental period from egg to an emergence of A. varipes adult parasitizing the green peach aphid was 20℃ (18.3 days), 25℃ (14.7 days), 30℃ (10.4 days), and 35℃ (9.3 days) while the shortest is at 30℃ and 35℃ (Table 1). The developmental period for Korea strain of A. varipes was in accord with the Norwegian strain parasitizing the cotton aphid (Röhne, 2002) and was about a day longer than the African strain (Van Steenis, 1995). The pupal mortality of A. varipes was 20℃ (0 %), 25℃ (1.6 %), 30℃ (2.8℃), and 35℃ (10.4 %), and the highest at 35℃ (Fig. 1). These results were in accord with many experiments where the pupal mortality of Aphelinus species was not influenced by temperature of up to about 32℃ (Force and Messenger, 1964a; Tang and Yokomi, 1995). Therefore, the optimum temperature of development of A. varipes parasitizing the green peach aphid was inferred from 30℃ to 35℃. 

 Table 2 shows that the mummification rate of A. varipes parasitizing the green peach aphid at different developmental stages and at different temperatures was significantly different. The experiments indicated that A. varipes like other aphelinid species (Stary, 1988; Tang and Yokomi, 1966) have the highest parasitization rate for the earlier host instars. In case of 20℃, the mummification rate of A. varipes parasitizing the green peach aphid was 1st (52.2 %), 2nd (58.3 %) at 20℃ and 1st (60.0 %), 2nd (48.9 %) at 25℃. The result in 30℃ (df = 14, F = 36.60, P < 0.0001), A. varipes shown the higher oviposition compared with other temperatures and was the highest at 3rd (85.0 %). In 35℃ (df = 14, F = 206.85, P < 0.0001), A. varipes preferred 3rd (84.4 %) instar to the other stages. This Korean strain of A. varipes preferred 1st and 2nd in low temperatures (20 and 25℃), but 3rd in high temperatures (30 and 35℃). These results were in accord with the Norwegian strain of A. varipes (Röhne, 2002).

Fig. 1. The mortality of Aphelinus varipes parasitizing Myzus persicae at four different temperatures.

Table 2. Percentage Mummified Green Peach Aphids

Table 3. shows the life history parameters of Aphelinus varipes with green peach aphid, Myzus persicae as host, parasitized in five different host stages. Two parasitoids were allowed to feed hosts and parasitize in each dish for 6 hours

 Table 3 shows the life table of A. varipes. Aphids killed by host feeding were easily recognized due to a red-brown color and a shrunken appearance. The rates of host feeding by A. varipes at different temperatures were 12.5 %, 17.3 %, 16.0 %, and 10.9 %. The host feeding of Korean strain A. varipes was higher than Norwegian strain at 10% (Röhne, 2002). The averages of host feeding rates by A. varipes at different stages of green peach aphid were 20℃ (12.5 %), 25℃ (17.3 %), 30℃ (16 %), and 35℃ (10.9 %) respectively, and the higher temperature, the higher host feeding rate but it decreased at 35℃. The highest rates of host feeding were 1st at 30℃ (23 and 23.3 %), and 2nd at 25℃ (23.5 %). The smaller the side of the aphid, the higher the rates will be for the host feeding in different temperatures. The pupal mortality of A. varipes was in proportion with the increase of temperature but increased up to 30℃ and increased rapidly at 35℃. The sex ratios (female) of A. varipes at different temperatures were 20℃ (48.5 %), 25℃ (46 %), 30℃ (68.9 %), and 35℃ (59.5 %), respectively. The highest sex ratio of A. varipes female was 2nd at 30℃ (87.9%). This result was lower compared with Röhne's result at 74 – 95 %.

Discussion

The amount of host feeding of A. varipes was similar to other Aphelinus species (Cate et al., 1977; Kuo, 1986; Tang and Yokomi, 1996; Tokumaru and Takada, 1996). In size and color, the variation of green peach aphids that reared at different temperatures only seemed to have an effect on the sex ratio allocation of the egg-laying A. varipes. The number of aphids killed by host feeding had no great variation between the rearing temperatures but a great variation of the parasitization rate was observed. The pupal mortality of A. varipes increased with the temperature. However, Aphidiinae-species have a higher juvenile mortality at higher temperatures (Force and Messenger, 1964b). 

The experiments indicate that A. varipes, like other aphelinid species, have the highest parasitization rate for the earlier host instars (Stary, 1988; Tang and Yokomi, 1996). Many Aphidiinae species, on the other hand, show a preference for second and third instar aphids (Hagvar and Hofsvang, 1991), or show no clear host-stage preferences (Völkl et al., 1990). The host stage preference and host stage suitability are often correlated with parasitoids (Godfray, 1994). The lower rate of parasitization is more likely in the later host stages due to a more effective defense against parasitoids, thus making successful parasitization of older aphids more difficult (Force and Messenger, 1965; Cate et al., 1977; Gerling et al., 1990). In A. asychis, the host stage did not seem to influence the juvenile mortality of the parasitoid (Cate et al., 1977). 

 Green peach aphids are large and dark green at lower temperatures, while they appear smaller and light green (Blackman and Eastop, 1984; Aldyhim and Khalil, 1993) at higher temperatures. This morphological variation of aphid influences the mummification, the host feeding, and the sex ratio of A. varipes.

 A. varipes could be a useful supplement to the aphidius species that are used in the biological control of the green peach aphid and cotton aphid in greenhouses today, as A. varipes. Unlike Aphidiinaes, host feeds on aphids in addition to parasitization, are less affected by high temperatures. A. varipes also prefers earlier aphid stages, and this may reduce potential competition between the parasitoid species if used together. The short developmental period at 20℃, 25℃, and 30℃ and the high proportion of females are properties that make the Korean strain of A. varipes an interesting alternative for the biological control of the green peach aphid and cotton aphid in greenhouse.