Life table of Phenacoccus solenopsis Tinsley ( Pseudococcidae : Hemiptera ) on various phenological stages of cotton

A study of the life table of Phenacoccus solenopsis Tinsely on cotton (Gossypium hirsutum L.) for two constitutive seasons at Entomological Farm, Punjab Agricultural University (PAU) Ludhiana revealed that among three phenophases (vegetative, flowering and maturation stage), gross reproductive rate (GRR), net reproductive rate (Ro), intrinsic rate of increase (rm), precise value of rm and finite rate of increase (λ) were maximum (329.83 and 349.10, 65.29 and 278.46, 0.169 and 0.242, 0.183 and 0.246, 1.184 and 1.274) at vegetative stage during 2008 and 2009, respectively. The total life cycle was completed in 32 days at vegetative stage while took maximum (39.00) days at maturation stage. During 2008, apparent mortality (100qx) was maximum (74.33 and 77.31%) at vegetative and flowering, respectively. Survival rate (Sx) was (0.26 and 0.23) in second instar at vegetative and flowering stage, whereas, at maturation stage the corresponding values for apparent mortality and survival rate were 40.69 and 0.59 in third instar. During 2009, maximum apparent mortality (76.35 and 56.91) and minimum survival rate (0.24 and 0.43) were recorded in adults at vegetative and maturation stage respectively. Heavy rainfall, Aenasius sp. (Hymenoptera: Encyrtidae) and Coccinella sp (Coleoptera: coccinellidae) contributed maximum mortality at vegetative and flowering stage, while rain fall and low temperature were major factors for mortality during maturation stage. The study suggested that P. solenopsis population is more affected by the biotic and abiotic factors in the vegetative stage. Therefore, the application of control measures at this stage could be drastically further reduces the population and thus prevents the buildup of population at later in the season.

Despite this biological and chemical control, the risk of spreading P. solenopsis remains high due to its polyphagous nature and high fecundity.Some other crops that are most susceptible to attack of P. solenopsis are cotton, Trianthema monogyna L. (Shanti or itsit), Xanthium strumarium L. (gutputna), Achyranthus aspera Linn.(puthkanda), ornamental plant (Hibiscus rosasinensis L.), okra, tomato, brinjal and chilli (Tanwar et al., 2008).Scientists at the National Centre for Integrated Pest Management, New Delhi have reported the Spatiotemporal distribution of host plant of P. solenopsis in India.Based on the report, the P. solenopsis had spread all over India (South > Centre >North region) (Anonymous, 2010).
P. solenopsis feed on 154 species of field vegetables, ornamental plants and weeds.Four parasitoid species and four predator species have reported against P. solenopsis.Some of these natural enemies such as Aenasius sp and Cryptoleamus montrozieiri (Coleoptera: coccinellidae), Coccinella septumpuntata have reported, identified and used to control P. solenopsis.Because of the polyphagous nature of P. solenopsis, researchers used different host plants for the study of biology (David et al., 2009;Abbas et al., 2010;Sana-Ullah et al., 2011).The variations in the methodologies complicated the efforts in estimating the life table parameter of P. solenopsis.
Although number of chemicals and biological control agents are effective against cotton mealy bug (P.solenopsis).Parthenogentic reproduction of this pest can give birth to some young ones which may act as biotypes of this pest and may lead to resistance against these insecticides and biological control agents.Therefore, there is need to investigate the life table parameters of P. solenopsis and identification of the ecological factors (including both biotic and abiotic factors like predators, parasitoids, rainfall, relative humidity, etc.) associated with this pest as no such factors are reported prior to this study.This investigation presents concise information on the life table of P. solenopsis conducted on cotton (Gossypuim hirsutus L.) at three stages (vegetative, flowering and maturation stage).We determined the life table parameters, age-schedule of survival (lx), gross reproductive rate (GRR) (∑mx), net reproductive rate (R o ), mean length of generation (T), intrinsic rate of increase (rm), finite rate of increase/ day (λ) and doubling time (DT) of P. solenopsis under natural conditions.The main objective of this study is to provide a better understanding of life table of P. solenopsis, and to provide information about the favorable phenological stage for development and prediction of P. solenopsis distribution.

Maintenance of insect colonies
Nymphs and adults of P. solenopsis were collected from different host and non-host plants in cotton fields of Southern region of Punjab State and from the University fields.These stages of P. solenopsis were maintained and mass multiplied on the various hosts (Congress grass, Gutputna, Hibiscus sp. and cotton) in earthen pots under screen house (4 × 3× 2 m) of 20 cm square mesh size at Entomological Farm, Punjab Agricultural University (PAU), Ludhiana.These hosts were selected for mass multiplications due to their preference by P. solenopsis.From these colonies, gravid females of P. solenopsis were transferred to cotton (RCH 134) plants for conditioning (3 to 4 days) in another screen house of same dimensions, so that of P. solenopsis adapt to the cotton plant and then produce the offspring /ovisacs on cotton plant.Colonies of P. solenopsis on cotton plants were used in experiments.
In winter, the colonies of P. solenopsis were reared on cuttings of tender parts of Hibiscus sp. in battery jars in the laboratory.The old cuttings replaced with newer when required.Temperature was control by using electrical heater to provide better condition for development and survival of P. solenopsis.Prior to the oncoming of main season (April to December), P. solenopsis were transferred to the cotton plant for conducting the experiments.

Methodology adopted
Seeds of cotton hybrid RCH 134 were procured from Rasi Seed Pvt. Ltd.Madhya Pradesh, India, and were sown in the 10 × 10 m 2 plots with three replication in randomized design at Entomological Research Farm, PAU, Ludhiana during 2008 and 2009 by following recommended guidelines for cotton cultivation (Anonymous, 2008).The experiments were conducted at three different phenophases that is, vegetative, flowering and maturation stage during June-mid July, mid-July -August and September -October, respectively, in both the years.P. solenopsis feed on the apical portion of cotton plant, thus the crawlers released on the upper canopy.Ten plants have tagged in each plot; hence thirty plants were used for one phenophase of the cotton crop to record the data.Ten newly hatched crawlers were collected from the insect colonies and transferred on the tagged cotton plants.Crawlers on each plant represented a cohort and each plant was act as replicate in the experiment under field conditions.Each plant and cohort has observed after 24 h and data on the life table parameters recorded.The presence of exuviae indicated the successful development from one instar to another.Various biotic (predators and  (Atwal and Bains, 1974).Gross reproductive rate (GRR) = ∑mx, Net reproduction rate (R0) = ∑ lx mx, Mean length of generation (T) = ∑ x lx mx / Ro, Doubling time (DT) = loge 2 / rm, Intrinsic rate of increase (rm) = loge Ro / T, Finite rate of increase (λ) = e rm  To calculate the precise value of rm, we followed Southwood (1978) graphical method by using the equation mentioned below: ∑e (7-rmx) .(lxmx) = 1097 Along with life table parameters, data on biotic (natural enemies) and abiotic (weather conditions) were also recorded.In the beginning, 300 nymphs used and then with the passage of time the mortality assessed along with the cause of death.The different observations recorded were (Atwal and Bains, 1974) pivotal age (x), number of individuals in the beginning (lx), number of individuals died (dx), factors responsible for death (dxf).Based on these observations, apparent mortality (100qx) and survival rate (Sx) were computed by using formula given below: Apparent mortality (100qx) = dx/lx × 100 Survival rate (Sx) = lx of subsequent stage / lx of particular stage

RESULTS
The life table parameters of P. solenopsis show clear cut differences at three phenophases viz.vegetative, flowering and maturation stage (  1 and 2. Quite similar results were obtained during both years.Pivotal age (x in days) to complete the life cycle was least (approx.32.5 days) at vegetative stage, while, maturation stage took maximum (approx.39.5 days) time to complete one generation.This difference in the pivotal age may be due to hardening of the plant parts as it reaches to maturity.Precise value of rm was calculated only at vegetative stage as the intrinsic rate of increase has maximum value in this stage.The corresponding values for precise rm were 0.183 and 0.246 during 2008 and 2009, respectively (Figure 3).
During 2008, heavy rainfall (199 mm) responsible for 74.33% mortality in second instar nymphs developing on cotton plants in vegetative stage, while no substantial mortality in other developmental stages (Table 2).At flowering stage, heavy rainfall (161.8 mm), Aenasius sp. and Coccinella sp contributed 77.31% mortality in second instar nymphs, while it was 71.21% in the third instar.At maturation stage, the highest percent mortality (40.64%) observed in the second instar nymphs due to heavy rainfall (64.2 mm) coupled with high relative humidity     (231), where as it was least in Clerodendron.However, the intrinsic rate of increase after adjustment was highest on eggplant while vegetative stage of cotton have maximum intrinsic rate of increase among the three phenophases studied in this research.In this study, heavy rain fall and high relative humidity are the major abiotic factors responsible for the mortality of P. solenopsis on cotton.Our study is in line with the findings of Suresh and Kavitha (2008) in which they concluded that high rainfall reduced the population of P. solenopsis on Parthenium hysterophorus.They further reported that for every unit increase in maximum temperature, evening relative humidity, rainfall caused significant population reduction (8.9, 0.46 and 0.96 units, respectively).Similarly, Akintola and Ande (2009) concluded that the mealy bugs are abundant on host plant during the dry season and more or less absent in rainy season.During the rainy season, crawlers were dislodging by the effect of rainfall and wind.
Survivorship and fertility of Helopeltis antonii measured under laboratory and field condition.The highest mortality occurred in the immature stages (first and second instar).The females lived for a maximum of 24 days.The intrinsic rate of increase (rm) was 0.092 per female per day and daily finite rate of increase (λ) was 1.097 females per female per day, with mean generation time of 27.70 days.
The net reproductive rate (R o ) of the population was 12.84 (Siswanto et al., 2001).Joshi et al. (2010) reported the bionomics, natural enemies and host range of P. solenopsis under controlled and laboratory conditions.The total life cycle completed in 16 to 38 days, whereas, in this study, 32.5, 37.0 and 39.5 days (approx.) at vegetative, flowering and maturation stage, respectively for both the seasons.
At the vegetative stage of cotton crop, the average temperature was varied between 23.7 to 40.3°C during 2008 to 2009.So this study is also compare with the studies conducted at control temperature too, although, R o , T and λ were (65.29 and 278.46, 0.169 and 0.242, 1.184 and 1.274, respectively) at vegetative stage for 2008 and 2009 (in this study).However, in results on age specific cohort life table on P. solenopsis at 27 ± 2°C and 65 ± 5% RH (Fand et al., 2010) these were 123.41 females/female/generation, 28.34 days and 1.185 females/female/day, respectively.The mortality rate was high for the first instar crawlers, which declined sharply in subsequent instars.Study also revealed that R o and T were higher than (15.51 females/female/generation and 34.23 days, respectively) reported in Persad and Khan (2002).The estimated intrinsic rate of increase (rm) at vegetative stage was 0.169 and 0.246 females/female/generation which was more than 1.4 to 2 times than those achieved at 25 and 27°C by M. hirsutus (Chong et al., 2008).This difference with the results of Chong et al. (2008) could be because of controlled conditions (at constant temperatures) and qualitative differences between cotton and hibiscus plant.
An understanding of the life table of P. solenopsis has important implications in management.Vegetative stage is most suitable for the production of its parasitoids and predators (e.g.Aenasius sp. and Coccinella sp.) to take advantage of high development rate, more fecundity and less mortality.A comparison between the life table parameters of P. solenopsis and its natural enemies, which not investigated in this study, is also helpful in selecting the most appropriate biological control agents especially these having high rm relative to this pest.Therefore, this study provides a pathway to the worker for comparing the life tale of natural enemies and P. solenopsis, to achieve efficient biological control.This study also provides farmer with information about the biotic and abiotic factor especially, rainfall and low temperature, which are detrimental for the growth of P. solenopsis.Such information used for the successful management of this pest under the Indian conditions.

Table 1 .
Effect of different phenophases on the life table parameters of P. solenopsis on cotton (RCH 134).
Life table analysisThe effect of three phenophases (vegetative, flowering and full bloom) on the population growth and age structure of P. solenopsis was determined by assessing different life table parameters.Data measured on the survival and reproduction were used to estimate lx (age-schedule of survival) and mx (age-schedule of female birth).Age-specific survival is the fraction of initial cohort alive at age x, while age-schedule of female birth was the mean number of females produced by each female at a pivotal age in days (x).The formulae for estimating life table parameters of female P. solenopsis at each phenophase were

Table 1 )
. Gross reproductive rate (GRR) was maximum (329.83 and 349.10) at vegetative stage followed by flowering stage(328.65 and 327.80)and minimum (231.17 and 246.54) at maturation stage during 2008 and 2009, respectively.Same trend was observed in finite rate of increase (λ) with corresponding values 1.184 and 1.274, 1.114 and 1.123, 1.156 and 0.154 at vegetative, flowering and maturation stage during both seasons, respectively.Maximum (65.29 and 278.46)net reproductive rate (R o ) and intrinsic rate of increase (rm) (0.169 and 0.242) was recorded at vegetative stage followed by maturation stage and minimum (18.69 and 28.68)net reproductive rate was recorded at flowering stage.Similar results were recorded in precise value of rm as in net reproductive rate.Mean length of generation (T) increased as the cotton phases goes towards maturation stage.

Table 2 .
Life table of P. solenopsis at different phenophases under natural conditions in field during 2008.

Table 3 .
Life table of P. solenopsis at different phenophases under natural conditions in field during 2009.