Introduction

Vegetables constitute an essential component of a balanced diet and play a vital role in the maintenance of good human health. Pest infestation and related diseases are major constraints in vegetable production. The warm temperature of spring and summer brings a flush of new foliage growth which attracts a wide variety of pestslike sap suckers foremost being whiteflies which can cause serious damage to crops worldwide (Peric, 1994).

The whiteflyis an important insect pest on a global scale, attacking a wide variety of agricultural commodities especially in vegetable and ornamental crops including citrus, squash, poinsettia, potato, cucumber, grape, tomato, hibiscus etc. (Van Lenteren and Martin, 2000). Trialeurodes vaporariorum (Westwood) commonly known as glasshouse whiteflyis a polyphagous pest. It was first described as Aleurodes vaporariorum (Westwood, 1856) from whiteflies collected from tomato in glasshouses throughout Europe (Quaintance, 1900). In India, it was reported at Thumantty (Nilgiris) infesting potatoes (David, 1971). The nymphs and adults suck the vital sap from the foliage which reduces photosynthetic activity of the plants (Yamada et al., 1979). The damage is inflicted by yellowing of leaves which later fade and dry away (Baker, 1922). The honeydew secreted by nymphs and adults of whitefly results in sooty mould which makes plants unsightly and valueless (Garman and Jewett, 1992; Johnson et al., 1992; Omar et al., 1992; Liu et al., 1993) and also reduces photosynthetic activity of the plants (Yamada et al., 1979; Tosh and Brogan, 2015).

Although whiteflies themselves can cause significant crop damage, T. vaporariorum-vectored viruses can cause losses that are much more economically damaging than those resulting from vector feeding alone (Byme et al., 1990). The viruses have been found to be transmitted by Trialeurodes, all within the genus Crinivirus (Wisler et al., 1998; Wisler and Duffus, 2001).

The whitefly species like Bemisia tabaci (Gannadius) and T. vaporariorum are serious pests of large cultivated crops throughout the world and in India, it poses potential threat to the cultivation of highly remunerative crops like tomatoes, beans and cucurbits. Since French bean is a short duration (85-90 days) crop, it fits as a component of intercropping and sequence cropping in many of the agro-climatic zones.In India, French bean is cultivated in 137.54 thousand hectare with production of 1370.21 thousand tones and productivity of 478 kg ha^-1 (Saxena et al., 2015). The pests that attack French beans at different growth stages include white flies, pod borers and leaf miners (Lohr, 2006). Since, the construction of life fertility tables is vital for determining the inherent capacity of an insect to increase in numbers and understanding population dynamics of a species (Phadke,1982). Hence, keeping in view the serious nature of the pest and potential threat it poses to the cultivation of highly remunerative crops like tomatoes, beans and cucurbits, an attempt was made in the present study to critically assess the reproductive potential of T. vaporariorum on French bean cv. Contender. The calculation of growth rate statistics is, therefore, a value for determining the growth potential of whitefly population under a given set of environmental conditions. The present study will enable us to know their potential in different temperature conditions.

Materials and Methods

All experiments were conducted at the Department of Entomology, Dr. Yashwant Singh Parmar University of Horticulture and Forestry, Nauni, Solan, Himachal Pradesh in climatic chambers operating under controlled conditions (15, 20, 25 and 30 °C). The culture of whitefly was raised in the laboratory on French bean cv. Contender. The French bean seedlings were grown in disposable cups and after 6-7 days of emergence, transferred to rearing cages (40×30×40 cm³). The adults were released on the seedlings. The pots were taken out and leaves were observed under the microscope for egg laying activity of the insects.

After egg laying, the eggs were marked withpointed tip permanent Faber castle make markers,then the seedlings were taken out and covered with glass chimney (20×15 cm²). The top of the chimney was covered with muslin cloth. The effect of temperature on the development of whitefly was studied in BOD incubators maintained at four constant temperatures i.e.15, 20, 25, 30 °C. A photoperiod of 14:10 (L:D) and relative humidity of 70% maintained at all the temperatures and replicated ten times. Daily observations at all temperatures were taken. The age specific survival (l_x) and age specific fecundity (m_x) at each pivotal age (x) were worked out daily for entire reproductive period to prepare fertility table as per the details given by Andrewartha and Birch (1954) and Southwood (1978).

Life table was constructed using the following column of parameters:

X: the pivotal of individuals (days)

l_x: the number of surviving individuals at age X

m_x : the number of living females born per female in age interval  X(fecundity rate)

The reproduction rate: GRR =Σ m_x

Net reproduction rate of increase: R_o = Σ l_xm_x

Approximate generation time (in days) T_c =∑ X l_xm_x/R_o

True generation time (in days): T =log_eR_o/r_x

The innate capacity for increase: r_c = log_eR_o/T_c

True Intrinsic rate of increase: r_m = ∑ e^7-r_m X l_xm_x

Finite rate of increase: λ = antilog_er_m

Population doubling time: DT = log_e 2/r_m

Weekly multiplication rate: WM = antilog_e r_m⁷

Results and Discussion

The age specific survival and age specific fecundity is illustrated graphically (Figure 1-4), the perusal of which revealed reproductive potential of T. vaporariorum at different temperatures. The whitefly had a maximumlife span of 58 days at 15 °C and pre-oviposition period was of 39 days. The oviposition started on 43^rd day and maximum fecundity (6.80) was on 49^th day (Figure 1). At 20 °C, lifespan was 42 days of which immature stage occupied 24 days. The oviposition started on 27^th day and maximum age specific fecundity (9.66) was attained on 34^th day, respectively (Figure 2).The fertility statistics at 25 °C revealed that oviposition lasted for 15 days and female contributed highest egg production (9.98) on 7^th day of oviposition. The egg laying ceased on 41^st day of pivotal age (Figure 3). At 30 °C, the total lifespan of T. vaporariorum  was 32 days and immature stages  lasted for 20 days. The oviposition started on 22^nd day and lasted for 8 days, the maximum fecundity (4.35) was on 25^th day (Figure 4).

Figure 1: Daily age specific survival and age specific fecundity of T. vaporariorum on French beans cv. Contender at 15±1˚C

Figure 2: Daily age specific survival and age specific fecundity of T. vaporariorum on French beans cv. Contender at 20±1˚C

Figure 3: Daily age specific survival and age specific fecundity of T. vaporariorumon French beans cv. Contender at 25±1 ˚C

Figure 4: Daily age specific survival and age specific fecundity of T. vaporariorum on French beans cv. Contender at 30±1 ˚C

At all temperatures, the mortality mostly acts heavily on old individuals and was almost constant with time. Works done by Verma et al. (1990) are in consonance with the present work. Fertility table statistics of T. vaporariorum on French bean at different temperatures presented in Table 1 revealed that the higher net reproductive rate (R_o) was at 20 °C and 25 °C.

Table 1: Fertility table statistics of T. vaporariorumon French beancv. Contender at different temperatures.

The temperature below 20 °C and above 25 °C decreases the whitefly population. Rodriguez et al. (2005) made a generalization regarding climatic conditions that T.vaporariorum is absent in the area where RH is higher than 80% but other whitefly species like Bemisia tabaci (Gennadius) biotype B are present on vegetable crops.

This observation is supported by findings of Burnett (1949) who  reported  the maximum  fecundity between 18 and 21 °C and  maximum female  longevity at 15 °C with very short survival at 9°C and temperatures over 27 °C. These results are corroborated by the work done byVerma et al. who studied the life cycle of the whitefly and concluded that temperature  ranges from 23 to 30 ËšC provided optimal conditions for development of T. vaporariorum .

The gross reproductive rate (GRR) of T. vaporariorum was 55.71, 87.69, 100.22 and 19.34 while net reproductive rate was 19.99, 45.86, 40.83 and 11.50 at 15, 20, 25 and 30 ËšC, respectively. The difference in gross and net reproductive rate may be attributed to mortality of the parent female before reaching the maximum reproductive age at the respective temperature. The whitefly completed one generation (T_c) in 34.78 days at 15 °C, 31.66 days at 20 °C, 31.03 days at 25 °C and 24.90 days at 30 °C. The temperature was inversely proportional to duration of development. Yano (1988) while working on survival rate of T. vaporariorum adults also found that at low temperatures the survival capacity of the fly reduced considerably.

The innate capacity of natural increase (r_c) gives the actual rate of a species with no overlapping of generation and is derived from survival rate and reproductive performance of a cohort of females. It was found that the r_c was maximum (0.120) at 20 and 25 °C. This was attributed to the fact that the intrinsic rate of increase (r_m) gives response of an organism to a particular set of environmental conditions and therefore, it gives a precise estimate of the reproductive capacity of a species (Birch, 1948; Bursell, 1964; Macfadyan, 1963 and Messanger, 1964). It was observed that maximum r_m value (0.123) was at 20 ËšC and the nearly same value of r_m (0.121) was at 25 °C. Calvitti and Buttarazzi (1995) also reported the similar r_m value (0.121) of T. vaporariorum population on marrows.

The higher rate of finite increase (λ) was 1.13 per female per day at 20 and 25 °C. The population multiplied weekly 1.57, 2.31, 2.33 and 1.99 times at 15, 20, 25 and 30 °C, respectively and doubling time was maximum at 15 ⁰C (10.83) and minimum  (5.64) at 20 °C. No such work appears to have been carried out at constant temperature. However, Yano( 1989) studied  the effect of two temperature regimes  24 and 30 °C during the day time  and 10 and 25 °C during  night  on population growth of T. vaporariorum on tomato and concluded that low  temperature regimes  decreased survival rate of eggs and larva and the higher temperature increase the intrinsic rate of natural increase. Yano (1988) also found that survival rate of T. vaporariorum adults reduced at low temperature. Recently, it was found that T. vaporariorum possesses less tolerance to higher temperatures than B. tabaci biotype B, probably linked to the expression of heat shock protein genes (Wan et al., 2009).

Conclusion

This study provides important data for understanding the population dynamics of T. vaporariorum,for development of potential population dynamic models and pest forecasting. Further, the whitefly population growth model can be used to evaluate effects of intended changes in the cropping system, e.g., of changes in climate and crop species or cultivar on whitefly development. Result of this study indicated that optimum temperature for population growth of T. vaporariorum was between 20 to 25 °C, so early sowing of French bean can help the crop escape the onslaught of this pest.

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