3.1.1. Two-Sex Life Table of A. chinensis Fed on Third Instar S. subpunctaria
The individual number and mean developmental periods of
A. chinensis fed on
S. subpunctaria in each stage were recorded in the whole living period of 100
A. chinensis eggs at 25 °C.
Table 1 summarizes the mortality/survival, developmental duration, adult longevity, female fecundity, and sex ratio of
A. chinensis at each age. As a natural enemy,
A. chinensis could prey on third instar
S. subpunctaria and complete its life history. Including the seven different developmental stages, the maximum life span of
A. chinensis was 104 d. The mean developmental period of the early ages (eggs stage to the fourth instar nymph) was nearly the same level (5–8 d), and the adult life period was relatively longer than the life period of the early stages. The mean time for the egg stage was 8 days and 82 eggs survived to the first instar nymph. A total of 61 of them lived through this stage. The first nymph had the shortest developmental period (5.03 d). The developmental durations for the second, third, and fourth instar nymphs were steadily around 7 d. The total preadult stage was accumulated to 45.42 d and was significantly longer than that of the remaining adult stage in the life cycle. The mean living period for the female adults was 26.67 d and was longer than the male adults (19.00 d), though with no significant difference. The sizes of the females were slightly lower and the sex ratio for the adults (female/male) was 0.94.
The adult preoviposition period (APOP) of A. chinensis was approx. 13.56 d and the oviposition duration was 5.67 d. Six of the fifteen adult females of A. chinensis died before oviposition, so the mean fecundity of A. chinensis was 56.60 for the fifteen females and 94.33 for the nine maternal females that completed their reproductive life.
The bootstrapping results of the population parameters of
A. chinensis from a cohort of 100 eggs are shown in
Table 2. The population dynamics of
A. chinensis were exponential with the intrinsic rate of increase (
r = 0.0321), and the values of the finite rate of increase (λ) were 1.0327. The net reproductive rate
R0 is the number of offspring an individual is expected to produce during their lifetime (a net reproductive rate of one means that the population is at its demographic equilibrium). This definition envisions the generation time as a renewal time of the population. The
R0 for
A. chinensis was 8.49, where 8.49 of the offspring were expected to produce during the
A. chinensis lifetime. The mean generation time (
T) was measured as 66.55, which was the average time between two consecutive generations of
A. chinensis in this research.
The expected changes in the survival rate of 100
A. chinensis eggs in each stage of the entire lifetime are projected in
Figure 3. The line of
Sx1 for the egg stage represented that the cohort hatched or died at day 8 and approx. 82% successfully lived to the first nymph stage. The other
sxj curves of each stage climbed from the day of emergence of the
j stage and were then eliminated in the following time due to the death or stage transition to
j + 1. Therefore, the overlap of
sxj for the different stages indicated the phenomenon of the nymphs of various stages appearing at the same age. For example, nymphs covered the L2–L5 stage on day 27 and the overlap of the L3, L4, and L5 nymphs, and adults could be found during days 33–34. This might have been because the development speeds of the
A. chinensis individuals were relatively low and different at an older age. The adults, especially the females, possessed a long-term trend of low survival rates (
Figure 3A).
Further, the age-specific survival rate (
lx), female age-specific fecundity (
fx), age-specific fecundity (
mx), and age-specific net maternity (
lxmx) of
A. chinensis are illustrated in
Figure 3B. An overall decreasing trend of
lx during the observed 104 d, however, decreased quickly to 54% on the 15th day. The fecundity is the probability of achieving a live offspring within a single cycle. The female age-specific fecundity (
fx) denotes the number of eggs produced by a female adult (15) at age
x and the age-specific fecundity (
mx) is the number of eggs produced per individual (31 adults) at age
x. Both curves showed similar patterns and
fx doubled the value of
mx due to the equal ratio of the sexes (0.94) of
A. chinensis (
Table 1). The reproduction began at age 56 d, and the highest peak (10.67 eggs) was observed at the age of 63 d. The
lxmx of
A. chinensis was related to
lx and the peak number was 0.96.
The age-stage-specific life expectancy
exj is mathematically the mean number of subsequent lifetimes for
x-aged
A. chinensis, or the time remaining at a given age
x during stage
j (
Figure 3C). It was acquired by assuming that the age-stage-specific mortality rates remained at the measured level of the observed cohort (
Table 1). The
exj of
A. chinensis was lower at the egg and first nymph stages and higher at the L3, L4, and L5 nymph stages, showing a general decline at each stage
j. However, there were fluctuations for
ex3,
ex5, and
ex7 at the second nymph, fourth nymph, and adult stage, respectively. The life expectancy value of the newly laid eggs of
A. chinensis was 34.02, representing the mean longevity of the cohort (
Table 1) surveyed. The maximum longevity of
A. chinensis was 104 d. The highest statistical life expectancy of
A. chinensis was
e27.6 (40.38), appearing at the 27-day aged fifth nymph stage. The female adult
A. chinensis had a higher life expectancy and a longer age than the male adults. In addition, there was an obvious overlap
exj for the fourth nymph, fifth nymph, female, and male stages aged 36 to 44 days.
The age-stage-specific reproductive value (
vxj) describes the contribution of an individual of age
x and stage
j to the future population (
Figure 3D). The reproductive value increased in the successive developmental stages and reached a peak value of 64.40 (
V60.7) at day 60 for the females. The time for the maximum
vxj at the adult stage was naturally close to the time of the TPOP (61.33) (
Table 1) [
45].
3.1.2. Two-Sex Life Table of S. subpunctaria Fed on Tea
To improve the understanding of the biology of
S. subpunctaria and assess its damage to the tea plant, we investigated the life table of
S. subpunctaria fed on the tea branch at 25 °C (
Table 3). Similarly, the individual number and mean developmental periods of
S. subpunctaria in each stage were recorded starting from a cohort of 100 eggs (laid within 24 h).
Table 3 summarizes the mortality/survival, developmental duration, adult longevity, female fecundity, and sex ratio of
S. subpunctaria separately at each age.
A total of 74 of the 100 S. subpunctaria eggs hatched and then stably survived to L5 through the whole life cycle, except for the size reduction when transited to pupa (72). The mean developmental period was relatively short in the early age, and the preadult stage lasted for approx. 32.86 d. The female adult S. subpunctaria had the longest developmental period (28.28 d) and the fourth instar larva (1.89 d) was the shortest. An exact equal ratio of the sexes was observed where the female and male moths were both 36. However, the females lived longer than the males. The adult preoviposition period (APOP) of S. subpunctaria was approx. 5.29 d, the oviposition period was 19.18 d, and the mean fecundity of the 36 female adult S. subpunctaria was 200.64.
The population parameters of
S. subpunctaria are shown in
Table 4. The intrinsic rate of increase (
r) was 0.0951 d
−1, and the finite rate of increase (
λ) was 1.0998 d
−1. The net reproductive rate (
R0) was an average of 72.23 offspring from an individual, and it would take an average of 45 d to complete one generation of
S. subpunctaria.
The projected age-stage survival curves (
sxj) of the
S. subpunctaria populations in each stage of the survey are illustrated in
Figure 4A. The probability of the hatched egg survival to the adult stage of
S. subpunctaria was stable at around 72–74%. The
sxj of the female and male adult
S. subpunctaria was comparable before 47 days. After that, the females would display a higher survival rate and a longer living period.
From
Figure 4B, the age-specific survival rate (
lx) of
S. subpunctaria showed an overall decreasing trend. The
lx curve of
S. subpunctaria dropped rapidly to 74% on day 7 and then remained at a static level of 74% up to the first 37 days. In other words, there were no population losses during this period until the pupa stage. The
fx curve and
mx curve for
S. subpunctaria showed the same increasing and decreasing trend from 34 to 75 days and reached a maximum value (16 and 7.9) at 40 days. The
fx value based on the female was exactly twice as large as the
mx based on all the adults since the two-sex individuals were equal (37) for
S. subpunctaria in this surveyed cohort.
The age-stage-specific life expectancies (
exj) of
S. subpunctaria fed on tea were projected in
Figure 4C. Due to the stable survival rate and short stage time during L1–L5, the
exj value decreased continually in the successive developmental stages. The value
e01 represented the life expectancies of the initial 100-egg cohort and was identical to the mean longevity of all the individuals (44.53) (
Table 3). In addition, it was close to the mean generation time (45 d) (
Table 4). The life expectancy of all the age stages of
S. subpunctaria decreased during stage
j (
Figure 4C), and the highest statistical life expectancy of
S. subpunctaria was
e6.2 (51.72), appearing at the 6-day aged first larvae stage. Furthermore, the female adult
S. subpunctaria had a higher life expectancy and a longer age than the male adults. The maximum longevity of
S. subpunctaria was 76 days, and there was an obvious overlap
exj for the pupa, female, and male stages from ages 32 to 40 days.
Conversely, the reproductive value (
vxj) (
Figure 4D) increased in the successive developmental stages and peaked at day 38 with a maximum of 100.26 (
V38.8), with a value close to the TPOP (38.15) (
Table 3).
3.1.3. Predation Rate
As shown in
Table 5, the stage total predation rate of the
A. chinensis individuals increased with the developmental stage. In the preadult stage, the total predation rate
Pj of the
A. chinensis individuals was 69.26 prey/predator. However, if the individuals that died in the preadult stage were included, the total predation rate
Uj was 26.19 prey/predator. Overall, the predation rates of
A. chinensis in the adult stage were higher (157.29 prey/predator) than in the preadult stage, and the predation of the females was higher than that of the males.
From the predation parameters shown in
Table 6, it can be found that the net predation rate (
C0) of
A. chinensis on the populations of third instar
S. subpunctaria was 74.95, and the finite predation rate (
ω) was 0.98 d
−1. The transformation rate (
Qp) of
A. chinensis was 8.83, which was the number of
S. subpunctaria required for
A. chinensis to lay one egg.
In the lab survey,
A. chinensis of first nymph stage was too small to prey on third instar
S. subpunctaria and was raised using 10% hydromel. Therefore, the age-stage-specific predation rate
Cxj data was measured from the second instar
A. chinensis nymph. There was an overall increasing tendency in the predation potential from the second instar nymph to the adult stage. Specifically, the predation rate of female
A. chinensis was overwhelmingly higher than that of the males (
Figure 5A).
The age-specific survival rate (
lx) of
A. chinensis fed on third instar
S. subpunctaria decreased dramatically during the early age and continuously declined in the following survey period. The maximum longevity was 104 days (
Figure 5B). However, the age-specific predation rate (
kx) increased during all the successive developmental stages and presented a high predation rate during the adult stage. However, when
lx was considered, the age-specific net predation rate (
qx) was much lower than the age-specific predation rate (
kx) and showed an increasing and then decreasing trend. The maximum values of the age-specific predation rate (
kx) (9.5) for
A. chinensis predation on
S. subpunctaria occurred on days 91–92 and the age-specific net predation rate (
qx) (1.96) on day 53. There was no overlap during the first 11 days since no predations occurred during the egg stage and the first nymph stage of
A. chinensis.