Abstract: Survivorship and fertility of the White Backed Plant Hopper (WBPH), Sogatella furcifera were assessed under laboratory conditions in Myanmar. A pair of five days old WBPH was released into a wooden cage covered with wire mesh sieve. Thirty days old rice plant in a pot was placed on the floor of the wooden cage. The single sex method was applied in the life table study. Life tables and population parameters were constructed based on unlimited food supply and a natural enemies-free environment. Results showed that the highest mortality occurred in the immature stages, especially in the first and second instars. The life table analysis showed that population densities of S. furcifera decreased gradually. The proportion of male to female observed was 1:0.88. The females could live for a maximum of 12 days. The trend of oviposition showed a peak at about the 10th day of the female life span. The mean number of eggs produced per female was 8.75. The intrinsic rate of increase (rm) was 0.06999 per female per day and daily finite of increase (λ) was 1.0255 per female per day, with a mean generation time (T) of 34.97 days. The net reproductive rate (Ro) of the population was 9.2732. The population Doubling Time (DT) was within 10.88 days. It could be concluded that the survivorship curve reflected a modest rate of mortality during the early life stages and a gradual reduction when approaching adulthood. All the surviving nymphs underwent four moults. The life table showed that about 37.26% of S. furcifera eggs successfully emerged as adults and high mortality occurred during the early immature stages. This type of survivorship is commonly classified as type II.
INTRODUCTION
Pests and disease problems are major constraints for increasing rice production. Insect pests including plant hoppers can cause serious damage to rice crop. One such example is the White Backed Plant Hopper (WBPH), Sogatella furcifera (Horvath) which has become a serious threat to rice production throughout South and South East Asia since the early 1970’s. Loss of crop yield due to WBPH is estimated at between 10 to 30%. Serious damage usually occurs during the early stages of plant growth with symptoms of hopper burns due to intensive sucking by the insect (Dale, 1994).
The construction of life tables is an important tool for understanding the population dynamics of an insect. The basic life table construction is one of the most important conceptual and analytical tools in entomological research. Age-specific life table serves as a framework for organizing dates on mortality and natality and also provides detailed transparent descriptions of the actual properties of a cohort. It generates simple summary statistics such as life expectancy and natility rate and also has a basic form that can be expanded, condensed, or modified for analyzing different types of data such as mortality by some factors (Carey, 2001). Analysis of life tables is the most suitable method to account for natality and reproduction of a population (Begon and Mortimer, 1981; Price, 1997). Deevey (1947) reported that life table is a concise summary statement for every interval of age, the number of deaths (dx), the number surviving at the beginning of age class x (lx), the rate of mortality (qx) and the expectation of life remaining for individuals of age x (ex). The table also includes numbers living between age x and x+1, i.e., the age structure (Lx). In such studies, development times and survival rates of each stage, longevity of adults and the daily fecundity of females are recorded for every individual (Chi, 1988). Therefore, the aims of this study was to construct the life tables of S. furcifera fed on rice plants for demographic analysis and to determine the survivorship and rate of increase of the plant hopper.
MATERIALS AND METHODS
The insects used for the experiments were obtained by culturing in the Laboratory. The experiments were conducted in the laboratory and in a smallholder pesticide free rice field in Hmaw Be, Upper Myanmar. The rice plants were grown in a pot. Five seeds of rice were placed in each pot (10 cm diameterx5 cm in length). The soil was provided up to the height of 13 cm prior to seeding. The excess seedlings were thinned out 15 days after seeding. One or two seedlings were left in each pot. Manawthukha rice variety was selected for in this experiment. The single sex method was applied in the life table study (Southwood, 1978). A wooden cage (length 34 cmx breadth 34 cmxheight 61 cm) covered with wire mesh sieve on each site was constructed and a pair of five days old WBPH was released into the wooden cage. Thirty days old rice plant in a pot was placed on the floor of the wooden cage.
The construction of the life tables followed the procedure described by Southwood (1978). In order to construct the age-specific life table, 27 females and 23 males were paired in the field into 5 cages. The adult hoppers were taken out from the wooden cage three days after releasing. Data of the survival rate of eggs, 1st to 5th nymphal instar and adults were recorded daily for calculations. The mortality rates of all nymphal and adult stages were also recorded. Total numbers of eggs laid were recorded until all the released plant hopper females died. Data analysis was carried out following the single sex method and the life table was then constructed using the following parameters:
| X | : | The pivotal age for the age class in units of time (days) |
| lx | : | The number of surviving individual at the beginning of age class x |
| Lx | : | The number of individual alive between age x and x+1 |
| dx | : | The number dying during the age interval x |
| 100qx | : | Percent apparent mortality |
| Sx | : | Survival rate within stage |
| Tx | : | Total number of age x units beyond the age x |
| ex | : | Life expectancy for individual of age x |
| mx | : | Age specific fertility, the number of living females born per female in each interval class |
| Ro | : | Net reproductive rate, equal to the sum of the lx mx products, or Ro = ∑ lx mx |
| Tc | : | Cohort generation time (in days), approximated by the following formula: |
Tc =∑ Xlx mx/∑ lx mx |
| rc | : | Innate capacity for increase, calculated by, rc = ln Ro/Tc |
| rm | : | The maximum population growth, the intrinsic rate of natural increase or the innate capacity for increase, calculated by iteration of Euler’s equation: |
∑e-rm.x lx mx = 1 |
| T | : | The corrected generation time, T = ln Ro/ rm |
| λ | : | The finite rate of increase, number of female offspring per female per day, calculated by: |
λ = er m |
| DT | : | Doubling time, the number of days required by a population to double, calculated by: |
DT = ln2/ rm |
| b | : | Intrinsic birth rate, 1/∑ e-r mx lx |
| d | : | Intrinsic death rate, b - rm |
To estimate the population parameters of S. furcifera, three sets of data from different cohorts were obtained on different dates (7 July, 20 July and 25 July 2007)
RESULTS
Age-specific survival life table: The survivorship (lx) of S. furcifera for three different cohorts (Fig. 1a-c) in general shows similar pattern with high mortality occurring during nymphal growth particularly in the early instar which gradually decreases throughout the life span. The first emerging adults occurred on days 22, 23, 22 and the maximum life spans (from hatching egg to death of adult) were 33, 36, 36 days for cohorts 1, 2 and 3, respectively. The patterns of survivorship observed indicated that the young immature stage is susceptible to physical disruptions and unsuitable food quality. This survivorship curve reflected a modest rate of mortality during the early life stages and a gradual reduction when approaching adulthood. The population assumed a near type II diagonal survivorship curve following the classification of Speight et al. (1999).
Table 1 shows the pooled life table concerning mortality, the description of one pathway of population change. All the surviving nymphs underwent four moults. The life table showed that about 37.26% of 378 S. furcifera eggs successfully emerged as adults and high mortality occurred during the early immature stages. This type of survivorship is commonly found in most insect species (Begon and Mortimer, 1981).
| Fig. 1: (a-c) | Patterns of survivorship curve (lx) of S. furcifera for three different cohorts |
| Table 1: | Pooled life table of S. furcifera on rice |
| X: Developmental stage (days), Ix: No. entering stage, Lx: No. a live between stage x and x+1, dx: No. dying in stages, 100qx: Percent apparent mortality, Sx: Survival rate within stage, Tx: Total number of age x units beyond the age x, ex: Life expectancy | |
Age-specific fertility life table: The age-specific survival and fecundity of S. furcifera females were shown in Fig. 2 and the detailed data in Table 2. The first female emerged on day 23 and the first death on day 30. The survival of the immature stage from egg to adult emergence was 0.92. The proportion of male and female recorded was 0.506:0.494 (Table 2). The last females died on the 42nd day. On average the females could be alive for a maximum of 35 days. Females started laying eggs from the 29th day or about 6 days from their first emergence. The numbers of eggs deposited were low in the early period and higher during the final period of their life span. The highest numbers of eggs were laid at 34 days of age with the number of eggs per female being 9.
| Fig. 2: | Daily age- specific survival (lx) and fecundity (mx) of S. furcifera females fed on rice |
| Table 2: | Age-specific survival and fecundity table of S. furcifera fed on rice |
The population and reproductive parameters of S. furcifera females are summarized in Table 3. The intrinsic rate of natural increase (rm) of S. furcifera was 0.0699 per female per day and the daily finite rate of increase (λ) was 1.08 female offsprings per female per day. The mean generation time (Tc) was 31.86 while the net reproduction rates (R0) of the population was 9.27. The Doubling Times (DT) was within 10.88 days. The rm, Tc and DT are useful indices of population growth under a given set of crop growing conditions.
| Table 3: | Population and reproductive parameters of S. furcifera fed on rice |
DISCUSSION
The survivorship (lx) of S. furcifera (Fig. 1) showed that higher mortality was found during the nymphal growth stages particularly in the early stages. The population densities subsequently decreased gradually through out the life span during this study period. Velasco and Walter (1993) reported that survival of insects, growth of nymphs and reproductive phase were highly influenced by food quality. The pattern of survivorship of S. furcifera observed indicated that the nymphal stages were susceptible to poor food and nutrition quality. It was also influenced by the density dependent factor of food change during captivity for S. furcifera. Affects by variations in food sources on population parameters were observed in Earias vitella fed on different host plants (Satpute et al., 2005), Diaphorina citri fed on four different host plants (Tsai and Liu, 2000) and Orius albidipennis fed on various arthropod preys (Chyzik et al., 1995). The survivorship curve for S. furcifera was also classified as type II following that of Speight et al. (1999) and Schowalter (2006) since it was very poor for the nymphal immature stages but much higher for the older individuals. The experiments in this study were conducted in July 2006 coinciding with rainy season which could influence the mortality of the early nymphal stages of S. furcifera. Schowater (2006) reported that insect populations are highly sensitive to changes in abiotic conditions, such as temperature, rainfall and relative humidity. Any changes in parameters could affect the growth of insects and their survival.
The pooled life tables (Table 3) showed that the population changes according to the death and birth rates. The mortality of eggs of S. furcifera recorded was 9.52%. The mean net reproductive rate (R0) of S. furcifera was 9.27. According to Birch (1948) the comparison of two or more populations by their net reproductive rate (R0) might be misleading unless the mean values of the generation time are the same.
The intrinsic rate of increase rm,, mean generation time Tc and Doubling Time (DT) of the population were useful indices of population growth under a given set of growing conditions. Longevity, reproductive rate, growth rate and population fluctuation could be influenced by their food sources (host plants or host preys) and also by environmental conditions such as temperature (Ellers-Kirk and Fleischer, 2006). If the tested host plants are different, the results of the population parameters will be varied. Life tables giving data on the rm of a particular species provide insight into the characteristic life patterns of different species (Satpute et al., 2005). There is a range of innate capacity for individual of a population (Gill et al., 1989).
The values of the population parameters may be varied according to field and laboratory conditions. It appears that the survival of different stages of plant hoppers under field conditions and laboratory conditions (testing in cages) may be varied due to more stress during life span of the plant hoppers released in the cages. Such variations have been known to cause some effect on population parameters of Acalymma vittatum (Coleoptera: Chrysomelidae) fed on cucurbits (Ellers-Kirk and Fleischer, 2006) and Nasonovia ribisnigri (Homoptera: Aphididae) fed on lettuce (Diaz and Fereres, 2005). Environmental conditions such as temperature, relative humidity, wind speed, solar radiation and the overall microclimate are expected to vary considerably between field and caged conditions.
CONCLUSION
The survivorship curve reflected a modest rate of mortality during the early life stages and a gradual reduction when approaching adulthood. All the surviving nymphs underwent four moults. The life table showed that about 37.26% of S. furcifera eggs successfully emerged as adults and high mortality occurred during the early immature stages. This type of survivorship is commonly classified as type II.
ACKNOWLEDGMENTS
This study was supported by funds from Third World Organization for Women in Science (TWOWS) to Ms San San Win.