The flea beetle, Agasicles hygrophila Selman and Vogt, was introduced into China in 1987. For a more comprehensive understanding of the effect of elevated CO2 concentration on the population dynamics, we collected the life table data of the flea beetle, A. hygrophila, at two different CO2 concentration conditions, i.e., ambient (420 µl/liter) and elevated (750 µl/liter). The raw data were analyzed using the age-stage, two-sex life table theory. At 750 µl/liter CO2, shorter developmental durations of the egg, first instar, and pupa were observed, while the duration of the third instar and the total developmental duration of the larva were prolonged. The generation length of A. hygrophila was significantly shortened at the higher concentration. It was observed that the intrinsic rate of increase (r), finite rate (λ), and net reproduction rate (R0) were higher and the mean generation time (T) was shorter at 750 µl/liter compared with that at 420 µl/liter. The bootstrap techniques were adopted to estimate the variances and standard errors of the developmental time, longevity, fecundity, and the population parameters. The bootstrap technique generated a normal distribution that was consistent with the central limit theorem and critical for following statistical analysis and comparison. Population projections based on age-stage, two-sex life tables could reveal the stage structure of A. hygrophila population and the leaf consumption capacity. Data collected in this study can potentially be used to evaluate the efficacy of A. hygrophila as a biological control agent of the alligator weed.
Alligator weed, Alternanthera philoxeroides (Mart.) Griseb (Amaranthaceae), originated in the Parana River region of South America (Maddox 1968, Vogt et al. 1979). It was introduced into China in the 1930 s, and was listed as one of the 16 most important invasive species in China (Ministry of Environmental Protection of the People's Republic of China, Chinese Academy of Sciences 2003). The flea beetle, Agasicles hygrophila Selman and Vogt, has been studied as a biological control agent against the alligator weed in the United States, Australia, New Zealand, and China (Spencer and Coulson 1976, Julien 1992, Julien et al. 1995, Guo et al. 2012). The flea beetle population was established in southern China after an accidental release in 1989 in Fuzhou, Fujian province, when A. hygrophila was accidentally introduced from South America (Wu 1997).
The average level of CO2 has increased from ∼280µl/liter immediately before the Industrial Revolution to a daily average of 380 µl/liter in 2005 and is increasing at a rate of about 2 µl/liter per year. A value of 570 µl/liter atmospheric CO2 is expected by the end of this century, and global levels of CO2 are predicted to double (average of 770 µl/liter) by the year 2200 (Intergovernmental Panel on Climate Change [IPCC] 2007).
Plant chemistry can be strongly affected by elevated CO2 concentrations. Elevated CO2 may increase rates of photosynthesis, which could enhance plant growth, biomass accumulation, and plant size (Frenck et al. 2011, Klaiber et al. 2013a). Thus, these changes could directly impact plant–insect herbivore interactions (Agrell et al. 2006). So, the elevated CO2 concentrations not only directly affect the physiology of plants but also affect phytophagous insects indirectly. Entomologists have recognized that elevated CO2 influences the distribution, abundance, and performance of herbivorous insects (Lincoln et al. 1984, Fajer 1989). Guerenstein and Hilderbrand (2008) reviewed the effect of CO2 on insect life. Chen et al. (2005) showed that the body weight and growth rate of the cotton aphid, Aphis gossypii (Glover), were increased at higher CO2 concentration. Yin et al. (2009), however, showed that elevated CO2 had variable effects on the population parameters of Helicoverpa armigera Hübner (Lepidoptera: Noctuidae).
Most studies on elevated CO2 have been focused on the growth, reproduction, or food consumption of specific stages (Wu et al. 2006, Sun et al. 2009, Huang et al. 2013, Klaiber et al. 2013b). Because studies focused on specific stages of a species do not reveal the overall effect on an insect population, the above studies contributed only limited knowledge to the consequences that elevated CO2 has at the population level. In order to comprehensively understand the overall effect of an environmental factor on an insect population, it is necessary to detect its effect on the survival rate, developmental rate, and fecundity throughout their entire life span, i.e., the life table. Because the traditional female age-specific life table (Birch 1948, Carey 1993) ignores male individuals and cannot describe the stage differentiation, their application to two-sex populations will usually result in a number of problems (Huang and Chi 2012a). In the present research, the direct effects of elevated CO2 (750 µl/liter) on the life table of the flea beetle, A. hygrophila, reared in a closed-dynamic CO2 chamber were compared with the life table data of the untreated control group reared in ambient CO2 (420 µl/liter). Data obtained from using the age-stage, two-sex life table (Chi and Liu 1985, Chi 1988) were then utilized to simulate the population growth of the flea beetle and its efficacy as a biological control agent against A. philoxeroides.
Materials and Methods
Insect Rearing
The initial culture of the flea beetle, A. hygrophila, was obtained from the Institute of Plant Protection, Fujian Academy of Agricultural Sciences (IPP, FAAS), China, and maintained in a phytotron under controlled conditions of 25 ± 1°C, 80 ± 5% relative humidity, and a photoperiod of 12:12 (L:D) h. The insects used in this experiment were reared in elevated CO2 concentrations (750 ± 28.8 µl/liter) or at normal atmospheric concentrations (420 ± 20.1 µl/liter) in a closed-dynamic CO2 chamber for one generation prior to the experiments.
Plant Growth Conditions
The host plant, alligator weed (A. philoxeroides), used in this experiment was collected from a field greenhouse at the IPP, FAAS. Plants were watered twice a week throughout the study to maintain soil moisture. Pest-free alligator weeds (20–30 cm in height, stem diameter 2–2.5 mm) with leaves intact and without flea beetle damage were used in the experiments.
Life Table Study
Newly emerged adults were paired and placed in separate plastic boxes (18 cm in length, 11 cm in width, and 7 cm in height). Eggs laid within a 24-h period were removed, placed in glass Petri dishes (11 cm in diameter) for life table studies, and kept in growth chambers. In total, 200 eggs were used for the life table study in each of the two CO2 concentrations. The number and hatch rates of eggs were recorded daily. The first-instar larvae were individually transferred to separate glass Petri dishes (9 cm in diameter containing a filter paper moistened with water) provided with a fresh leaf of A. philoxeroides, and maintained through the third instar. The number of surviving larvae and the developmental stage were recorded, and the leaves replaced daily. At the end of the third instar, the larvae stopped feeding and were ready for pupation. A section of plant tip with three stem nodes (∼4–5 cm in length) without leaves was supplied for pupation. Each stem was inserted into a piece of floral foam soaked with water to avoid dehydration, placed into a plastic tube (2.5 cm in diameter, 12 cm in length), and covered with fine mesh net. The pupal duration and the number and sex of emerging adults were recorded daily.
As adults emerged, male and female beetles were paired and moved to a new glass container (5 cm in diameter, 8 cm in height) for oviposition. The fecundity and survival of the beetles were recorded for each individual until the deaths of all adults. If a beetle died earlier than its mate, another of the same sex was supplied from the mass-rearing colony. The data of these recruited individuals were excluded from analysis. Insects were kept separated in each of the growth chambers set at respective CO2 concentration for the entire life table study period.
Life Table Data Analysis
The raw life history data for survival, longevity, and female daily fecundity of A. hygrophila individuals were analyzed using the TWOSEX-MSChart (Chi 2015) program, based on the age-stage, two-sex life table theory and the method described by Chi and Liu (1985) and Chi (1988). The TWOSEX-MSChart program is available at http://140.120.197.173/ecology/. The survival rate (sxj; x = age, j = stage), which is the probability that a newly laid egg will survive to age x and stage j, and fecundity fxj, which is the number of hatched eggs produced by a female adult at age x, were calculated. According to Chi and Liu (1985), the specific survival rate (lx) is then calculated as:
where m is the number of stages. To take individuals of different stages at age x into account, the age-specific fecundity (mx) is calculated as:
The total number of offspring that an individual can produce during its lifetime, i.e., the net reproductive rate (R0), is calculated as:
The intrinsic rate of increase (r) using the Lotka–Euler equation with age indexed from zero (Goodman 1982) is calculated as:
The mean generation time (T) represents the period that a population requires to increase to R0-fold of its size as time approaches infinity and the population growth rate settles down to the intrinsic rate and finite rate. Mean generation time is calculated as:
According to Chi and Su (2006), the age-stage-specific life expectancy (exj), i.e., the time that an individual of age x and stage j is expected to live, is calculated as:
where s′iy is the probability that an individual of age x and stage j will survive to age i and stage y. Fisher (1930) defined the reproductive value as the contribution of individuals of age x and stage y to the future population. According to Huang and Chi (2011) and Tuan et al. (2014), the reproductive value in the age-stage, two-sex life table is calculated as:
The standard errors of developmental time, longevity, fecundity, and population parameters were calculated by using the bootstrap method with 200,000 bootstrap replicates (Efron and Tibshirani 1993, Huang and Chi 2012b). Differences between treatments were compared using the paired bootstrap test (Efron and Tibshirani 1993, Smucker et al. 2007, Polat Akköprü et al. 2015).
Population Projection
To reveal the dynamic change of stage structure and leaf consumption of the populations of A. hygrophila under different CO2 concentrations, we projected the population growth and leaf consumption based on the age-stage, two-sex life table (Chi and Liu 1985, Chi 1990) by using the computer program TIMING-MSChart (Chi 2015). At 420 µl/liter CO2, the daily mean leaf consumption of the first instar, second instar, third instar, and adult were 3.12, 13.07, 41.96, and 49.95 mm2 per insect, respectively, while they were 4.93, 10.65, 37.68, and 37.83 mm2 per insect, respectively, at 750 µl/liter CO2 (unpublished data of authors). For comparative purpose, the same initial population of 10 newborn eggs was used for the simulation at different CO2 concentrations. The data file for TIMING-MSChart was generated by the TWOSEX-MSChart program. Because the age-stage, two-sex life table is capable of describing the stage differentiation during population growth, we calculated the increase rate of stage j from time t to t + 1 using the common logarithm according to Akca et al. (2015) as:
where nj,t is the number of individuals in stage j at time t. According to Akca et al. (2015), we also used the natural logarithm to calculate the increase rate of stage j from time t to t + 1 as:
When the individual number of a stage is 0 (nj,t = 0 or nj,t + 1 = 0), logarithmic transformation is impossible. Therefore, we used nj,t + 1 and nj,t + 1 + 1 in the calculation of øj,t and rj,t (Akca et al. 2015).
Table 1.
Developmental time, longevity, APOP, TPOP, and fecundity (mean ± SE) of A. hygrophila under different CO2 conditions
Results
Development Time and Fecundity
The developmental times of the egg and the first instar in 420 µl/liter CO2 were significantly longer than those in 750 µl/liter CO2, while the developmental time of the third instar in 420 µl/liter CO2 was shorter than that in 750 µl/liter CO2 (Table 1). However, the total developmental time for the preadult stages was 24.13 d in 420 µl/ liter CO2, which was significantly longer than the 22.22 d in 750 µl/ liter CO2. The total longevities of both sexes in 420 µl/liter CO2 were, however, significantly longer than those in 750 µl/liter CO2. When the longevities of all individuals were used to calculate the mean longevity, there was no significant difference between the two treatments.
There was also no significant difference in adult preoviposition period (APOP) between the two CO2 concentrations. There was, however, significant difference in the total preoviposition period (TPOP) between 420 and 750 µl/liter CO2. The mean fecundity of 117.60 eggs per female in 750 µl/liter CO2 was not significantly different than the 93.67 eggs produced in 420 µl/liter CO2.
The standard errors of developmental time, longevity, and fecundity were estimated using the bootstrap method. As Polat Akköprü et al. (2015) observed, the general statistical procedure uses all individuals in a cohort to calculate the mean, variance, and standard errors, which generates a nonnormal frequency distribution (Fig. 1), while the bootstrap method with 100,000 replications, however, generates a normal distribution, which is important for further statistical analysis and comparison. In this study, the frequency distributions of mean fecundities (B = 200,000) of both treatment are normally distributed (Fig. 1); although the SEs estimated by using general statistics were similar to those estimated by using the bootstrap method, there were huge differences in variances.
Life Table and Population Parameters
The detailed age-stage-specific survival rates (sxj) of A. hygrophila in the two different CO2 conditions are plotted in Fig. 2. The parameter sxj represents the probability that an egg of A. hygrophila will survive to age x and stage j. Overlapping among stages can be observed. In the 420 µl/liter CO2 treatment, the probability that an individual surviving to the pupal stage is lower than that it is in the 750 µl/liter CO2 treatment. However, those individuals developing to adult stages survived longer in 420 µl/liter CO2 than in 750 µl/liter CO2, which is also consistent with the longer total longevities (42.23 d for female and 45.13 d for male) in 420 µl/liter CO2 shown in Table 1.
When the survival rates sxj of different stages are pooled, the age-specific survival rate (lx) produces a simplified overview of the survival history of the entire population (Fig. 3). Significant differences can be observed between the two different CO2 conditions. In the 420 µl/liter CO2 treatment, 34.5% of A. hygrophila survived longer than 20 d, while 42% survived longer than 20 d in 750 µl/liter CO2. However, for those insects developing to the adult stage in 420 µl/liter CO2, the survival rate curve of adult females and males extended to 73 and 90 d, respectively, while the survival rates of females and males in 750 µl/liter CO2 ended at age 74 and 66 d, respectively (Fig. 2). In 420 µl/liter CO2, the first reproduction occurred at age 26 d and reached its peak between ∼28–31 d; however, in 750 µl/liter CO2, reproduction began on age 24 d and reached peak fecundity at age 27 d. In 750 µl/liter CO2, a few females produce more eggs during the age interval ∼45–66 d.
The population parameters of A. hygrophila are listed in Table 2. The intrinsic rate of increase (r = 0.0798 d–1), finite rate of increase (λ = 1.0831 d–1), and net reproductive rate (R0 = 14.05 offspring) in 420 µl/liter were all lower than those in 750 µl/liter (r = 0.0954 d–1, λ = 1.1000 d–1, R0 = 20.58 offspring). In 420 µl/ liter, the mean generation time (T = 33.10 d), however, was longer than that in 750 µl/liter (T = 31.71 d).
The age-stage-specific life expectancy (exj) of A. hygrophila indicates the amount of time that an individual of age x and stage j is expected to live after age x (Fig. 4). The life expectancy of a new born egg (e01) is exactly the same as the mean longevity of all individuals in the cohort, e.g., the life expectancy of a new egg was 24.04d in 420µl/liter CO2 and 22.23d in 750µl/liter CO2 (Table 1). Due to the higher mortality found in the first instar and pupal stage, the life expectancy declined in the larval and pupal stage. When an individual survived to the adult stage, the life expectancy jumped to a high peak then declined gradually with aging. In general, our results showed that the life expectancy (exj) of A. hygrophila decreased with increasing concentration of CO2 from 420 to 750µl/liter.
The reproductive value is defined as the contribution of an individual at age x and stage j to the future population (Fisher 1930). The reproductive values (vxj) of A. hygrophila are presented in Fig. 5. The reproductive value increased significantly when A. hygrophila began to produce viable eggs. In 420 µl/liter CO2, the reproductive value jumped to 42.02 at age 22 d and reached a peak of 60.28 at age 27 d (Fig. 5). In 750 µl/liter CO2, the reproductive value reached 46.11 at age 21 d and peaked at 71.80 at age 27 d, the vxj value jumped again to 62.78 at age 42 d and remained at high values for an additional 20 d.
Population Projection
Huang and Chi (2013) pointed out that there are two types of information that can be obtained through life table study: the basic data (sxj, fxj, etc.) and the derived parameters (r, λ, T, etc.). Both intrinsic rate (r) and finite rate (λ) are the derived parameters and are calculated by assuming the population settles down to a stable age-stage distribution as time approaches infinity. As shown in Fig. 6, it is evident that neither of the A. hygrophila populations reached the “stable age” or “stable age-stage” distribution after 100 d when starting from an initial 10 eggs; therefore, it is inappropriate to predict the population size and stage structure using only the intrinsic rate or finite rate. The population projection based on the basic data (sxj and fxj) of a life table, however, offers a comprehensive understanding of the age and stage composition of a population during its growth.
Fig. 3.
Age-specific survival rate (lx) and fecundity (mx) of A. hygrophila reared in different CO2 conditions.
The population projection showed that A. hygrophila reared in 750 µl/liter CO2 concentrations would grow faster than in 420 µl/ liter CO2 (Fig. 6). Beginning with 10 eggs and reared under 750 µl/ liter CO2 conditions, the population would go through four generations, with the total population exceeding 41,000 after 100 d, while the flea beetle would go through four generations under 420 µl/liter CO2 conditions, but would only attain a final population size of ∼13,000 individuals.
Because of the rapid population increase potential of A. hygrophila, and the theoretical population size increasing to >1,300-fold of the initial 10 eggs within 100 d, we used equations 8 and 9 to describe the growth and dynamic of each life stage in logarithmic scale. The positive rate indicates an increase of a stage from time t to t + 1, and the negative rate represents a decrease in stage size. Because the intrinsic rate (r) reflects the propagation potential of a population under ideal conditions when the population approaches stable age-stage distribution, the growth rate of all stages will approach the intrinsic rate (r = 0.0798 d–1 in 420 µl/liter and 0.0954 d–1 in 750 µl/liter; Fig. 7).
Leaf consumption increased faster at the higher 750 µl/liter CO2 concentration than it did at the 420 µl/liter CO2 concentration. Total consumption increased from 36.07 mm2 at 37 d to 500.78 mm2 at 74 d under 420 µl/liter CO2, but increased from 55.26 mm2 at 37 d to 962.42 mm2 at 70 d under 750 µl/liter—demonstrating that A. hygrophila would potentially be more effective against alligator weed by consuming a higher amount of the weed at 750 µl/ liter CO2, than at the lower concentration (Fig. 6).
Table 2.
Population parameters (mean ± SE) of A. hygrophila under different CO2 conditions
Discussion
Global warming has been a timely topic, and its effects on individual organisms and ecosystems have drawn international attention (Hughes 2000, Parmesan and Yohe 2003, Zvereva and Kozlov 2006). One of the major factors influencing global warming is the generation of greenhouse gases, especially CO2. Because insects are ectothermic organisms, the effects of elevated temperature on their developmental rate are well documented and have been extensively studied (Yang and Chi 2006, Zhao et al. 2009, Hou and Weng 2010). Guo et al. (2012) reported that the population distribution and migration of flea beetles were affected by rising atmospheric temperature during the previous 20 years in China. Elevated CO2 may profoundly interfere with the physiology and ecology of insects not only directly but may also act on them indirectly by affecting their host plants (Meng and Li 2005, Ge and Chen 2006, Guerenstein and Hilderbrand 2008). Although indirect effects of elevated CO2 on insects merit further study, it is essential that the direct effects be documented to discover the effect of elevated CO2 on insect occurrence and distribution, in order to predict possible shifts in insect population dynamics and community interactions in future environments (Guerenstein and Hilderbrand 2008).
Fig. 5.
Age-stage-specific reproductive value (vxj) of A. hygrophila reared in different CO2 conditions.
Ge et al. (2010) showed that elevated CO2 could prolong the larvae duration and delay the development of cotton bollworm larvae. Chen et al. (2006) reported that elevated CO2 shortened the preoviposition period and the generation time, while accelerating the growth of the aphid, Sitobion avenae (F.) with increasing CO2 concentration. Qian et al. (2015) and Li et al. (2013) reported that the egg stage and first-instar larval stage of Frankliniella occidentalis (Pergande), and the pupal duration of Cnaphalocrocis medinalis (Guenée) were significantly decreased in an elevated CO2 concentration treatment. Similarly, the developmental times of the egg, first instar, and pupa of A. hygrophila were shortened in 750 µl/liter, while the developmental time of the third-instar larvae was increased in 750 µl/liter compared with those reared in 420 µl/liter. In summary, the total larval developmental duration was prolonged in 420 µl/liter CO2. Akey and Kimball (1989) reported that the longevities of both sexes of adult beet armyworm, Spodoptera exigua (Hübner), were shortened in elevated CO2. Fajer et al. (1989) reported similar phenomena in the buckeye butterfly, Junonia coenia Hübner. When generation times are shortened, the number of annual generations will often increase, as reported for the aphid, S. avenae, when exposed to high CO2 concentration (Chen et al. 2006). In the present study, we demonstrated that the TPOP is a more appropriate statistic from the point of view of demography, because it reflects the effect of the first reproduction from birth onward on the population parameters. The TPOP was 28.08 d in 420 µl/liter and 26.55 d in 750 µl/liter. As Lewontin (1965) demonstrated that earlier reproduction would result in a higher intrinsic rate, the faster population increase noted in A. hygrophila was observed in the intrinsic rate (Table 2) and computer simulation (Fig. 6).
Fig. 6.
Computer simulation of population growth and leaf consumption of A. hygrophila reared in different CO2 conditions.
The response of insects to atmospheric CO2 concentration may also be reflected in an increase or decrease in fecundity. The fecundity of S. avenae was improved when reared in an elevated CO2 environment (Chen et al. 2006). Our study demonstrated that the fecundity of A. hygrophila also increased under the elevated CO2 concentration, which may be conducive to the proliferation of A. hygrophila population and its control effect on alligator weed A. philoxeroides. Moreover, the total preoviposition period in elevated CO2 (750 µl/liter) was 1.53 d shorter than in the ambient CO2 condition.
This study demonstrated the advantages of using the age-stage, two-sex life table theory in describing demography (Chi 1988, Yu et al. 2005). The stage differentiation can be observed in the sxj, exj, and vxj curves (Figs. 2, 4, and 5). The two peaks of vxj showed that the population of A. hygrophila could increase faster at elevated CO2 concentration than that at ambient CO2 concentration, which is consistent with the fecundity shown in Table 1. As the traditional female age-specific cannot describe the stage differentiation and ignores the male population, the practical applications of traditional female age-specific life tables in population ecology and pest management are limited. The problems associated with of the traditional female age-specific life table are discussed in detail in Huang and Chi (2011).
Population projections based on life tables and stage-specific consumption rates can reveal the stage structure and damage potential of a pest population as shown in Tuan et al. (2014), Hou and Weng (2010), and Akca et al. (2015). Predicting the population size, stage structure, and leaf consumption capacity of A. hygrophila is important to the management strategy of the alligator weed A. philoxeroides. These parameters provide useful information for understanding the potential control efficacy of A. hygrophila against A. philoxeroides.
The daily leaf consumption of A. hygrophila larvae was similar at 420 µl/liter and 750 µl/liter, i.e., 23.45 and 23.77 mm2/d, respectively, while the daily adult consumption under 750 µl/liter (37.83 mm2/d) was significantly lower than that under 420 µl/liter (49.95 mm2/d). Population projection incorporated with leaf consumption showed, however, that the consumption of the population under 750 µl/liter was higher than that of 420 µl/liter due to the higher growth rate. Our results demonstrated that the control efficacy of natural enemies against a pest species should not be evaluated based solely on individual consumption. It is essential that the growth rate of natural enemies should be taken into consideration as well. This, once again, demonstrates the advantage of using the two-sex life table in life table analysis and its application in biological control programs.
Acknowledgments
Sincere thanks are due to the many colleagues who have worked to improve management of A. philoxeroides. We are grateful for the support of the National Natural Science Foundation of China (31572065) and the Natural Science Foundation of Fujian about the effects of CO2 on A. hygrophila (2013J06008).
