Effect of elevated CO 2 and temperature on thermal constants and lower threshold temperatures of maize aphid , Rhopalosiphum maidis ( Fitch . ) ( Aphididae : Hemiptera ) on maize , Zea mays ( Linn . )

The objective of this study was to examine the development of corn leaf aphid,Rhopalosiphum maidis Fitch (Aphididae: Hemiptera) on maize Zea mays Linnaeus at elevated and ambient concentrations of CO2 (550 and 380ppm ± 25 ppm, respectively) at six temperatures (20, 25, 27, 30, 33 and 35°C) and to estimate thermal constants and lower temperature thresholds for the forecasting models based on heat accumulation units which could be developed for use in forecasting. The duration of different growth stages of R.maidis were reduced with an increase of temperature from 20°C to 35°C under both ambient and elevated CO2 conditions. The lower development threshold for first nymphal instar,second nymphal instar, third nymphal instar, fourth nymphal instar, adult duration and total development period required 10.1, 5.04, 13.42, 26.96, 10.9, 23.22 and 20.20°C under eCO2 whereas it was 13.32, 9.41, 19.13, 30.48, 16.38, 22.88 and 20.89°C under aCO2 conditions,respectively. The mean lower temperature threshold for nymph was slightly higher (16.38°C) at aCO2 compared to that of eCO2 (10.90°C) whereas for adult the mean lower temperature threshold was slightly higher (23.22°C) at eCO2 compared to that of aCO2 (22.88°C). The thermal requirement of R. maidis from first nymphal instar to adult (total development period) was found to be 100.00 degree days (DD) under eCO2 conditions as against 111.11degree days under aCO2 conditions. These estimated temperature thresholds and thermal constant can predict the pest scenarios and population dynamics of R. maidis.

changes in temperature and CO 2 concentration affect the population dynamics and the status of insect pests of various crops. Plants with C 4 photosynthesis will respond little to rising atmospheric CO 2 because a mechanism to increase the concentration of CO 2 in leaves causes CO 2 saturation of photosynthesis at ambient conditions. Numerous studies have shown that the annual average temperature of the earth will increase 1°C by 2025 and the probable rise in temperature by the end of the century is expected to reach 3°C (Solomon et al., 2007). The parameters associated with life table are crucial for understanding population growth potential and for establishing effective management tactics to pest control in general because they provide information on development, reproduction and mortality of a pest population (Maia et al., 2000).
So far, there is no published report from India on the effect of eCO 2 and variable temperatures on R. maidis.
Hence, in the present study, life table parameters of R. maidis were examined at two levels of CO 2 and six different temperatures to estimate the temperature thresholds and thermal constants which would be useful in status of the pest populations.

Maintenance of Rhopalosiphum maidis culture
The corn leaf aphids, R. maidis were collected from the field and maintained at an optimum temperature of 27 ± 1°C and 75 ± 5% RH in the insectary of Entomology section, CRIDA, Hyderabad. The nymphs and adults were reared individually in petridishes (110 X 10 mm) to obtain bulk population for experiments. Light intensity of 30, 000 Lx was provided by 26 W florescent bulb inside the chambers during the 14 hours light period with a relative humidity of 60 % (day) and 70 % (night). Light illumination is provided through fluorescent lamps horizontally mounted in pairs above each shelf. Air circulation inside the chamber was maintained from a specifically designed air diffuser. The period of light, CO 2 concentrations and temperature levels were automatically monitored and controlled using Intellus Ultra Controller. The maize plants (DHM-117) and insects were maintained in open top chambers (OTC) and CO 2 growth chambers under respective set conditions at elevated and ambient concentrations of CO 2 (550 and 380ppm ± 25 ppm, respectively) at six temperatures of 20, 25, 27, 30, 33 and 35±1°C. Fully grown foliage (30 days after sowing) obtained from respective treatments was used for feeding trials and leaf quality analysis. The crop was maintained at insecticide free condition throughout the experiment to understand the impact of eCO 2 and temperature on insect pests. Experiments on life-tables of R. maidis were conducted by adopting cut leaf method at elevated and ambient concentrations of CO 2 (550 and 380ppm ±25 ppm, respectively) at six temperatures of 20, 25, 27, 30, 33 and 35±1°C and a photoperiod of 14L:10D. The cutcorn leaf (6x6 cm) from the top of corn seedlings of 1-2 months old was detached and placed in a petridish with a moist cotton swab on one side of the leaf margin. The cotton swabs were moistened daily to keep the leaf fresh and the leaves were changed on alternate days (Srinivasa Rao et al., 2013). In order to construct life-tables, newly hatched 50 first instar nymphs were collected carefully from the stock culture with the help of wet camel hair brush and transferred individually into each petridish containing maize leaves obtained from respective set conditions with 50 replications per each treatment. The data on durations of first instar, second instar, third instar, fourth instar, adult and total development period at each temperature under eCO 2 and aCO 2 conditions were recorded.

Calculation of lower temperature thresholds and thermal constants
Quantification of the relationship between insect development and temperature is useful to predict the seasonal occurrence and population dynamics of the insects. The degree day model (thermal summation model) was used to estimate the linear relationship between temperature and the rate of development of insect pests (Campbell et al., 1974). The reciprocal of developmental period for each stage was calculated to obtain the development rate (1/day) at each temperature. Development threshold and thermal constant were determined by regressing development rate on temperature. Thermal constant is estimated by the formula Thermal constant (K) = (T -T0) x D; where, where, T -Temperature at which insect species is reared, T0 -Development threshold temperature, D -Duration of development, K was estimated as reciprocal of regression coefficient (b) between development rate and temperature. K = 1/ b.
To was determined as ratio of regression intercept 'a' and 'b' T0 = -a /b. The life table parameters were estimated for eCO 2 and aCO 2 separately at abovementioned six temperatures and were plotted against the tested temperatures to compare the lower threshold temperatures and thermal constant under both eCO 2 and aCO 2 conditions.

RESULTS AND DISCUSSION
The durations of different growth stages R. maidis were significantly affected by eCO 2 and temperature conditions. The results pertaining to the variation in durations of first instar, second instar, third instar, fourth instar, adult and total development periods of R. maidis on maize at elevated and ambient concentrations of CO 2 (550 and 380ppm ± 25 ppm, respectively) at six different temperatures of 20, 25, 27, 30, 33 and 35±1°C are presented inTable 1.
The first nymphal instar duration (3.00 to 1.00 days), second nymphal instar (1.92 to1.00 days), third nymphal instar (1.08 to 1.00 days), fourth nymphal instar (1.00 day), adult duration (23.1 to 1.20 days) and total development  All values are mean ± standard deviation; ** Significant @ 1% level of significance NS = Not-significant  (Fig 1a  &1b). The total development period was decreased with increasing temperature from 20 to 35°C under both eCO 2 and aCO 2 conditions. The present results are in conformity with the findings of Kuo et al. (2006), who reported that the duration of total immature stages of R. maidis from birth to adult decreased as temperature increased from 6°C (51.7 days) to 30°C (5.2 days). The total immature stages of R. maidis from birth to adult fed on barley and maize foliage, the estimated low developmental threshold (4.44°C and 6.1°C) and the shortest developmental time at 26°C and 30°C (Elliott et al., 1988), respectively. Total immature developmental duration of Nilaparvata lugens decreased from 74.4 days at 15°C to 22.4 days at 30°C (Vattikuti et al., 2019).This means that R.maidis have a wider temperature range for immature development on corn leaves than on barley leaves.The present results indicated that the lower degree days requirement for completion of nymphal stage at eCO 2 and might be due to shortening of development time.
According to lower temperature thresholds and thermal constants, the results were inconformity with the findings of Kuo et al. (2006) who reported that the lower thermal constants for R. maidis from 33.15 DD for first instars to 25.96 DD for fourth instars. Similar results were observed by Srinivasa Rao et al. (2017) who reported that the thermal requirement of nymph varied from 74-102 DD on eCO 2 with temperature in the range of 20-30°C as against 90-130 DD on aCO 2 and the results indicated that the lower degree days requirement for completion of nymphal stage at eCO 2 . The lower thermal constants of A.craccivora at eCO 2 conditions might be due to reduction of DT as reported earlier (Berberetet al., 2009). Similar kind of variation of 513-290 DD for soybean aphid was reported by McCornack et al. (2004) in the 20-35°C temperature range. These lower thermal requirements might be due to feeding on low nutritious maize foliage obtained from eCO 2 conditions. It was suggested that the thermal constant is influenced not only by the temperature but also by the biochemical constituents present in the host plant viz., nitrogen, proteins, aminoacids and tannins (Marchioro and Foerster, 2011). The estimated temperature thresholds and thermal constants are useful in the prediction of population peaks (Taveras etal., 2004) and the present data will be relevant in future climate change scenarios to understand the distribution and abundance of insect pest.