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White backed planthopper (WBPH), Sogatella furcifera (Horvath) on rice is one of the most important sucking pests in India in many rice growing tracts. Some of the factors responsible for severe infestaton levels are introduction of high-yielding but susceptible varieties (HYVs), high input use especially use of nitrogenous fertilizers and use of broad spectrum synthetic pesticides. Serious incidence of WBPH coincides with maximum tillering stage with appearance of ‘hopper burn’ symptoms towards crop maturity stage. WBPH nymphs and adults are found on the upper portion of the stems, with the majority at the junction between leaves and stems. Both nymphs and adults suck sap and actively multiply under congenial weather conditions. Planting time, which is dependent on the incident rainfall or water availability for irrigation has a significant bearing on the timing of initial population, its build-up and coincidence with susceptible crop growth stage with conducive temperatures and relative humidity. Temperature is a key driver of WBPH development, survival and reproduction. User can forecast the timing of pest generations by providing inputs such as daily temperature (max and min) for the location since the time of pest observation under field conditions (date) and pest stage observed. Pest forecast can be applied to the location by selecting the State and District as well.
Description
White backed planthopper (WBPH), Sogatella furcifera (Horvath) on rice is one of the most important sucking pests in India in many rice growing tracts. Prior to 1991,WBPH was confined mostly to the northern states like Punjab and Haryana. However, WBPH infestation along with another plant hopper (BPH) started to increase in the southern states since 1995. These plant hoppers cause severe crop loss in certain years [1]. Some of the factors responsible for severe infestaton levels are introduction of high-yielding but susceptible varieties (HYVs), high input use especially use of nitrogenous fertilizers and use of broad spectrum synthetic pesticides. Severe incidence by both the hopper species can lead to plant death and hopperburn symptoms [2-5]. One distinguishing feature between the two hopper species is that WBPH occurrence coincides mostly with the vegetative stage while BPH severity coincides with the panicle initiation stage. WBPH is monophagous. Feeds mostly on cultivated (Oriza sativa L.) or wild rice species. However, in reports on no-choice experiments, WBPH could feed on several grass species such as Leersia hexandra Sw.(cut grass), Choris barbata, Paspalum distichum (knotgrass), Brachiaria mutica (Paragrass), Echinocloa crusgalli (barnyard millet), Cyperus difformis (sedge), Eleusine indica(Indian goose grass), Dinebra sp. (viper grass), Leptochloa chinensis and Digitaria ciliaris (crab grass). WBPH occurs in all rice environments and is distributed widely in Asian countries, Australasia, and the Pacific Islands. In Asia, it is found in Bangladesh, Cambodia, China, Hong Kong, India, Indonesia, Japan, Korea, Laos, Malaysia, Myanmar, Nepal, Pakistan, the Philippines, Ryukyu Islands, Sri Lanka, Taiwan, Thailand, Vietnam, and in the former Soviet Union. In Australasia and the Pacific Islands, it is distributed in Australia, Caroline Islands, Fiji, Irian Jaya, Marianas and Marshall Islands [7-8]. WBPH is not found in America and Africa. The life cycle of WBPH comprises three stages i.e., egg, nymph and adult. The duration of each stage depends on temperature and host cultivars. Its occurrence is throughtout the year and generally 6 to 8 generations have been recorded. Adult Adult male and female moths exhibit two winged forms depending on the stage of crop and crowding density. Short winged or brachypterous sedentary forms are mostly found settled on infested plants during seedling, panicle initiation and dough stages. Long winged or macropterous forms are found at the time of crop maturity and take to flight (emmigrate). The short winged adult females exhibit higher fecundity compared to the migratory long-winged forms. Macropterous adults can migrate over long distances to find a new favourable habitat. Adult longevity in general varies from 10 to 20 days depending on temperature.
Egg Females lay eggs in groups by inserting in plant tissue with the ovipositor. In most cases, eggs are thrust in a straight line generally along the mid-region of the leaf sheath. Sometimes eggs are laid in clusters of 4-10 in longitudinal rows within the leaf midribs. Newly laid eggs are whitish and turn darker prior to hatching. A red eye spot appears towards the egg maturation stage. Egg period varies from 7 to 10 days depending on temperature.
Nymph The newly hatched first-instar nymph is white and turns strongly mottled dark grey or black and white in color and are devoid of functional wings. Wing buds appear during the fifth instar. The nymphal stage passes through 5 instars and takes 12 to 17 days in the temperature range of 20-30°C to complete development.
WBPH is one of the most important sucking pests of rice crop. WBPH is found on the upper portion of the stems, with the majority at the junction between leaves and stems. From 1979 and into the 1980s, BPH and WBPH became epidemic or outbreak pests in the South and Southeast Asia, where traditional rice varieties were markedly replaced by improved HYVs [9-13]. It has also been well documented that the simultaneous introduction of broad-spectrum insecticides induced a serious resurgence of planthopper populations [14-15]. Kuno [16-17] compared the population growth characteristics of the two planthopper species and showed that WBPH has a much lower rate of population growth than BPH. WBPH feeds on the phloem and causes decrease in leaf area, plant height, dry weight, leaf and stem nitrogen concentration, chlorophyll contents and photosynthetic rate [18-19], which subsequently results in yield losses. In addition, both adults and nymphs while sucking the sap inject their toxic saliva into the plant which produces “hopper burn” resulting in drying of leaves and tillers. WBPH under favourable conditions can cause 35-95% yield loss [20]. Serious damage usually occurs during the early stages of plant growth with symptoms of hopperburn due to intensive sucking by the insect [21]. In addition, rice plants show discoloration of the leaf sheaths caused by intensive WBPH oviposition [6].
Among all the climatic factors, temperature has probably the greatest effect on insect development. Temperature influences various biological characteristics of insects such as sex-ratio, adult life span, survival, fecundity, and fertility. As a result temperature profoundly affects colonization, distribution, abundance, behavior, life history, and fitness of insects. Therefore, information on the thermal requirements of insect pest development has important implications for control programs as temperature determines the population growth and size of pest populations and their variation under different environmental conditions. The degree day model (thermal summation model) was used to estimate the linear relationship between temperature and the rate of development of WBPH [22]. The reciprocal of developmental period for each stage was calculated to obtain the rate of development (1/d) at each temperature.
Linear models Two models were evaluated to estimate the linear relationship between ecologically relevant temperatures and the rate of development of the pest. The first was the thermal summation model [22] which is given by the expression, , where, r is the rate of development (=1/ Development time (D) in days), T is ambient temperature (oC); intercept (a) and slope (b) are the model parameters. Thermal constant, k (= 1/b), is the number of degree-days (DDs) or heat units above the threshold needed for completion of an instar. Lower temperature threshold (Tmin) was determined as the x-intercept (= - a/b) which is the estimated lower temperature at which the rate of development is either zero or no measurable development occurs. The second linear model by Ikemoto and Takai [23] is given as, , where, DT is the product of the duration of development, D(days), and temperature, T(oC), k is thermal constant and Tmin is the lower developmental threshold. Nonlinear models Two empirical nonlinear models were fitted to the instar specific developmental rate data to estimate the optimum temperature threshold (Topt) and upper temperature threshold (Tmax). Topt is the threshold temperature at which developmental rate is maximal, while Tmax is the lethal threshold at which development ceases. Lactin-2 model [24], Briere-1 model [25] were applied to assess the nonlinear relationship. In addition, a thermodynamic model referred to as the Sharpe-Schoolfield-Ikemoto model (SSI model) was used to estimate intrinsic optimum temperature for the pest [26]. All the nonlinear models described the relationship between developmental rate (1/D) and temperature (T). The Lactin-2 model [24] is given by the expression, , Where, D is the mean development duration in days, ρ is the composite value for critical enzyme-catalyzed biochemical reactionsas T increases to Topt, Δ is the difference between Topt and Tmax when thermal breakdown becomes the overriding influence and λ is a fitted coefficient that forces the nonlinear curve to intersect the x-axis and allows the estimation of lower developmental threshold. The Briere-1 model [25] is given by the expression, , where, r is the developmental rate as a function of temperature (T), and ‘a’ is an empirical constant. The nonlinear thermodynamic SSI model [26] is given by the expression: , where, developmental rate (r) is a function of temperature, T (in absolute temperature, K) (273.15K = 0°C), R is the gas constant (1.987 cal/deg/mol), Δ HA is the enthalpy of activation of the reaction that is catalyzed by the enzyme (cal/mol), ΔHL is the change in enthalpy associated with low-temperature inactivation of the enzyme (cal/mol), ΔHH is the change in enthalpy associated with high-temperature inactivation of the enzyme (cal/mol), TL is the temperature at which the enzyme is half active and half low-temperature inactive (K), TH s the temperature at which the enzyme is half active and half high-temperature inactive (K), TΦ is the intrinsic optimum temperature at which the probability of enzyme being in the active state is maximal (K), and ρΦ is the mean development rate at the intrinsic optimum temperature (TΦ ) assuming no enzyme inactivation (day-1). Survival and fecundity rates are used in construction of life tables to predict the potential population size and trend index under input weather conditions at the location of interest.
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