Soybean aphid, Aphis glycines (Hemiptera: Aphididae), developmental and reproductive capacity on white clover, Trifolium repens (Rosales: Leguminosae), in northeast China

Nymphs of Aphis glycines Matsumura were individually reared to adults in the laboratory on detached leaf discs of Trifolium repens L. (white clover) mounted on agar medium. Adults of A. glycines were fed T. repens within small clip cages in the field. Development, reproduction and intrinsic rates of increase of A. glycines were studied. These data were compared to those of controls fed known host plants including cultivated soybean Glycine max (L.) Merr. and the wild soybean species Glycine soja Sieb & Zucc. The results demonstrated that nymphs of A. glycines successfully developed into adults and reproduced efficiently when reared on T. repens in the laboratory. The lower development temperature threshold for nymphs fed T. repens was estimated as 8.27 °C, and the effective cumulative temperature for A. glycines development from nymph to adult was 90.91 degree-days. Adults of A. glycines could also survive on T. repens in the field, but only a few nymphs were produced.


Introduction
The soybean aphid, Aphis glycines Matsumura, is an insect pest commonly found on soybeans in Asia. In 2000, A. glycines became established in North America (Ragsdale et al. , 2011. By 2003, this pest had infested over 21 America states and three Canadian provinces (Venette and Ragsdale 2004). Soybean aphids can cause direct damage to plants by sucking fluids from leaves and stems (Liu and Zhao 2007;Wu et al. 2004). Additionally, they are also capable of transmitting a variety of plant viruses (Davis et al. 2005;Mueller and Grau 2007).
Secondary hosts of A. glycines include cultivated soybean Glycine max (L.) Merr., and the wild soybean species Glycine soja Sieb & Zucc (Wang et al. 1962). It was determined that the aphid could likely utilize horsenettle, Solanum carolinense L. (Clark et al. 2006), and Japanese Metaplexis, Metaplexis japonica (Thunb.) Makino (Chen et al. 2015) as hosts. Results from a previous study showed that Trifolium repens L. was a poor host for soybean aphid (Hill et al. 2004). A later study showed that clover variety significantly affected aphid density, and A. glycines could achieve highest population growth on Ladino, a variety of white clover (Swenson et al. 2014).
T. repens is a common legume in natural landscapes and cultivated fields and has a wide distribution in northeast China. In this latter region, it is still questionable whether A. glycines can utilize T. repens as a host. Understanding the role of this widely distributed plant as a host for A. glycines is important for the effective management of this insect in northeast China.
In the current study, nymphs of A. glycines were individually reared to adults on detached leaves of T. repens in the laboratory. Adults of A. glycines were fed T. repens in the field while contained in small clip cages. Development, reproduction, and intrinsic rates of increase of A. glycines were studied. These data were compared to those of controls fed the species' known hosts G. max and G. soja.

Aphid source and host plants
Soybean aphids were taken from a soybean field at the Xiangfang Experiment Station, Northeast Agricultural University (NEAU), Harbin, Heilongjiang Province, northeast China (126.75°E, 45.72°N). The colony was maintained on G. max (variety Heinong 51) in an environmental chamber at 25 ± 1 °C, 70 ± 5% relative humidity (RH), and a photoperiod of 14:10 h (light:dark; L:D) with artificial light of 12,000 lx. G. max was grown in a chamber at 28 ± 1 °C with six to ten seeds per pot in 10 × 10-cm (diameter × height) plastic pots under the same humidity and photoperiod as described above. Seedlings 15-20 cm tall at the V2 growth stage (Fehr and Caviness 1977) were used for experiments. T. repens (variety Rivendel), were collected from a lawn in NEAU (126.75°E, 45.72°N) and were transplanted into a 50-m 2 experiment plot. These plants were used for experiments when they were at a vegetative or generative growth phase. Seeds of G. soja were collected from a field near Limin, Harbin (126.61°E, 45.87°N) and were planted in the same plot. These plants were allowed 4-5 weeks to germinate and mature before being used for experiments.

Development of nymphs
About 200 apterous adult aphids were transferred from the stock colony onto ten pots of soybean plants (approximately 20 aphids per pot). The plants were placed in an environmental chamber at 25 ± 1 °C, 70 ± 5% RH and a 14:10-h (L:D) photoperiod for a 24-h reproduction period, after which, all adults were removed. Using a small brush, newly deposited nymphs were individually removed from the plants. Detached leaves of G. max, G. soja, and T. repens were cut into 1.5 cm-diameter leaf discs using a hole punch. Solid agar media were prepared in 45-mL, 4 × 4.5-cm (diameter × height) glass beakers. Each nymph was placed on the reverse side of a leaf disc adhered to the surface of the medium. The beaker was then placed upside down on a 5-cm-diameter Petri dish (leaf disc method) ( Fig. 1) (Liu 1987;Liu et al. 2003). For each host plant treatment, 50 nymphs were tested at each temperature. Beakers were placed in environmental chambers at 13, 18, 23, 28, and 33 ± 1 °C, 70 ± 5% RH, and a 14:10-h (L:D) photoperiod. Individual aphids were checked daily for ecdysis and survivorship. Leaves and media were replaced every 5-7 days when the old leaves became yellowish or upon observation of fungal growth.

Adult longevity and reproductive capacity
Adults which had been reared from nymphs at 13-33 ± 1 °C were maintained in the same conditions as the immature insects. Nymphs deposited by each female were counted and removed daily. Adult longevity was recorded daily until the death of each adult. Leaves and media were replaced every 5-7 days when leaves became yellowish or upon observation of fungal growth.

Adult development and fecundity in the field
A small clip cage was designed for this experiment. Pieces of polystyrene KT board, 2.0 cm × 2.0 cm × 0.5 cm (length × width × height), were used for the clip cages. Firstly, a circular 1.0-cm hole was cut in the middle of this piece with a hole punch. Then the hole was covered with polyester net with 502-glue (Tonglin Glue Manufactory, Harbin, China). Finally, a T-form frame consisting of elastic steel wire was set into the piece (Fig. 2).
Another 100 A. glycines nymphs were fed soybean using the leaf disc method at 23 ± 1 °C, 70 ± 5% RH and a 14:10-h (L:D) photoperiod. When they developed into adults, 27, 32, and 31 soybean aphids were respectively transferred onto leaves of G. max, G. soja, and T. repens growing in a 10-m 2 plot. Each aphid was covered by a clip cage and surveys were conducted from 24 August to 19 September 2012. Nymphs deposited by each adult were counted and removed daily with a small brush. Adult longevity was recorded daily until the death of each adult.

Data analysis
Raw data for all individuals reared at 13-33 ± 1 °C were analyzed according to the age-stage, two-sex life table theory (Chi 1988). Nymph stage, adult longevity, adult fecundity were calculated using the bootstrap method (Efron and Tibshirani 1993) in TWOSEX-MSChart software (Chi 2015). Differences in these parameters among hosts at each temperature were analyzed by general linear model (GLM) and Tukey's honest significant difference (HSD) tests. Differences in longevities and fecundities of adults among field-grown hosts were also analyzed by GLM and HSD tests. Analyses were conducted with SAS 8.1 software (SAS Institute 2000). Intrinsic rates of increase for A. glycines on different hosts at each temperature were calculated using the bootstrap technique (Efron and Tibshirani 1993) in the TWOSEX-MSChart program (Chi 2015). Because bootstrap analysis uses random resampling, a small number of replications will generate variable means and SEs. To reduce the variability of the results, we used 100,000 bootstrap iterations. We then used the paired bootstrap test to examine the differences among the three hosts at each temperature (Efron and Tibshirani 1993).
To estimate the lower developmental temperature threshold and effective cumulative temperature for nymph development on each plant, linear regression of the mean developmental rate y (the reciprocal of development time to adult) on temperature x was applied to each temperature from 13 to 28 °C (Murai 2000), and was performed with the GLM.

Development of nymphs
Nymphs of A. glycines successfully developed into adults when they were fed G. max, G. soja, and T. repens at temperatures of 13-33 °C. Significant linear relationships between mean developmental rate and temperature (between 13 and 28 °C, inclusive) were evident for nymphs feeding on different plants (Fig. 3).

Adult longevity, reproductive capacity, and intrinsic rates of increase
Adult longevities, fecundities, and intrinsic rates of increase of A. glycines fed the three plant species at five constant temperatures are shown in Table 1.

Adult development and fecundity in the field
The data demonstrated that A. glycines adults could survive on T. repens in the field. Adult longevities on T. repens were similar to those on G. max, but were shorter than those on G. soja (ANOVA; F = 6.35; p < 0.01). Adults of  A. glycines fed T. repens had lower fecundities than those fed G. max and G. soja (ANOVA; F = 23.58; p < 0.01) ( Table 2).

Discussion
There has been controversy as to whether A. glycines could utilize T. repens as a host. Results from a previous study showed that T. repens was a poor host for soybean aphids (Hill et al. 2004). High levels of A. glycines population growth were observed on the Ladino variety of white clover (Swenson et al. 2014). Morphological variations of A. glycines were studied when they were fed T. repens (Chen et al. 2015). This study was conducted to determine the developmental and reproductive capacities of A. glycines on T. repens, and our results indicated that A. glycines could feed on T. repens in northeast China (Chen et al. 2015).
In our study, the lower development temperature threshold for nymphs on soybean was estimated at 5.44 °C, which differs from the previous estimates of 1.66 °C (Xu et al. 2011) and9.50 °C (Hirano et al. 1996). The effective cumulative temperature for A. glycines development from nymph to adult was estimated at 90.91 degree-days, which also differs from the previous estimates of 57.10 degreedays (Hirano et al. 1996) and 202.79 degree-days (Xu et al. 2011).
Interestingly, there were differences in fecundities of A. glycines when fed G. max, G. soja, and T. repens. The fecundities also differed when they were reared between 13 and 28 °C. Fecundities of A. glycines fed T. repens were lower than those fed G. max at 13 and 18 °C; nevertheless, they were similar to those fed G. max at 23 and 28 °C (Table 1).
We undertook this study to compare the adaptabilities of A. glycines to different plants. Although A. glycines could survive (Fig. 3) and reproduce well (Table 1) on T. repens,  experiments were only conducted at constant temperatures. Importantly, the aphids that we utilized were from field populations and there were likely differences among the adaptations of individuals for different hosts. To ascertain more significant experimental conclusions, it would be necessary to conduct experiments with aphids derived from a monoclonal population. In addition, soybean aphids used in this study were apterous ones. Only alate aphids migrate between primary and secondary hosts in the autumn or spring (Liu and Zhao 2007;Wu et al. 2004). With data derived from a study on alate A. glycines feeding on T. repens, further research is required to address whether A. glycines could feed on T. repens in northeast China.