on high incidence of mustard aphids, Brevicoryne brassicae (L.) and Myzus persicae (Sulzer) on Gobhi Sarson during warmer and drier winters at Palampur, Himachal Pradesh*

Oilseeds are an important component of the agricultural economy, next to food grains, in terms of area, production and value. India is among the top few vegetable oil economies of the world and contributes about 28.3% acreage and 19.8% production of the world oilseeds. Gobhi sarson (Brassica napus), a long duration crop, is the new emerging oilseed crop in Himachal Pradesh. It has good yield potential, wide adaptability and possesses high oil content of good quality (Shekhawat et al., 2012). But, several biotic and abiotic factors pose as constraints in the production of this crop.

The pest development is favored by warmer and drier conditions. Therefore, present investigation was aimed to assess the aphid populations and their weather relations in Gobhi sarson under two different thermal environmental conditions.
In order to assess the aphid incidence, build up and their weather relations in three Gobhi sarson varieties (HPN-1, HPN-3 and Hyola-401), two field experiments were conducted in randomised block design with three replications in plot size of 4.5×3 m 2 each at CSK HPKV, farm, Palampur during two rabi seasons (2010-11 and 2011-12).
The crop was sown on three dates representing three thermal environments i.e., October 25, November 2 and November 10 in each season under unprotected conditions. Recommended package of practices were followed to raise the crop. For aphid population count in each treatment, 10 plants were randomly selected, tagged and aphids were counted from 10 cm top terminal central shoot of each plant (Singh and Lal, 1999) at 3-5 days interval from first date of incidence. Seven observations starting from Feb 21, 23, 27, March 2, 7,11 and 15 during 2010-11 and thirteen observations from Feb 21, 25, 29, March 3, 7,12, 16, 20, 25, 30, April 5, 10 and 16 during 2011-12 were recorded and average population per plant was calculated.
Accordingly, dates of incidence, economic threshold level, ETL (30 aphids plant -1 ) and peak population were calculated. Daily weather data on maximum and minimum temperatures (Tmax and Tmin), morning and evening relative humidity (RH I and RH II) and rainfall three days prior to incidence, ETL and peak population were collected from the records of agrometeorological observatory of the university and used for the study. Seed yields (kg ha -1 ) from each treatment were recorded at harvest and analysed statistically as per Panse and Sukhatme (1985).
During 2010-11, aphids appeared on 21 st February in all the dates of sowing, however, the population was higher in crop sown on third date. First ETL in crops occurred on 23 rd February whereas peak activity of B. brassicae (mean of three dates, 269.3/10 cm terminal shoot plant -1 ) and M. persicae (93/10 cm terminal shoot plant -1 ) was noticed on 2 nd March ( Table 1). ETL was found to be associated with Tmax of 18.0 o C (17.0-19.0), Tmin 6.2 o C (5.5-7.5), Tmean 11.6 o C (10.8-12.8), RH I 72 % (69-74), RH II 65 (47-93) and RH mean 68 % (60-81). Peak incidence of aphid was associated with  Incidentally, aphid incidence during 2011-12 was also noticed on 21 st February, 2012 and on the same day the population was found to be above ETL. Incidence of aphid and ETL were associated with Tmax of 17.2 o C (16.5 -18.5), Tmin 7.2 o C (6.5 -8.0), Tmean 12.2 o C (11.8 -12.8), RH I 85 % Compared to 2010-11, high aphid activity for longer period was noticed during 2011-12 and this was attributable to favourable weather conditions during the season for population build up. The aphid population was considerably higher because of low rainfall and prevailing high temperature during second fortnight of February. Peak population of B. brassicae (mean of three dates of sowing, 1141.3/10 cm terminal shoot plant -1 ) and M. persicae (724.1/10 cm terminal shoot plant -1 ) was recorded on 3 rd March. Higher Tmean and lower humidity by 3 o C, 15-27% and 2-4 o C, 7-21% during III and IV observations (between 25 th February-3 rd March) and X, XI and XII observations (between 6 th -16 th March) respectively, appeared to be responsible for higher incidence of the pests.

A study on high incidence of mustard aphids, Brevicoryne brassicae (L.) and Myzus persicae (Sulzer) on Gobhi Sarson during warmer and drier winters at Palampur
Rainfall during second fortnight of February (14.4 mm) and after aphid incidence during aphid activity was lower (69.1 mm) during 2011-12 compared to 2010-11. The corresponding values for 2010-11 were 110.8 and 143.7 mm, respectively.
Lower rainfall and prevailing higher temperature during second fortnight of February produced considerably higher aphid population of both the species. Also critical periods (having greater than 30 aphids plant -1 ) of exposure of crops to aphid infestation prevailed in first week of March during both the seasons. Raj and Sharma (1991) had also reported similar results. The incidence of mustard aphid is affected by weather conditions prevailing during the growing season of Brassica crop (Dhaliwal and Hundal, 2004). Besides, higher maximum temperature, minimum and mean temperature and higher morning humidity also favoured high build up of aphid during 2011-12. This revealed that infestation of aphid on Gobhi sarson was largely governed by the temperature and relative humidity. The results are in conformity with earlier reports of Saxena et al. (2012).
Maximum temperature (>15 o C) three days prior to incidence favoured higher population build up. Same temperature regime observed earlier during second fortnight of February also favoured aphid incidence and build up under mid hill conditions of the state (Anonymous, 2011).
During both the seasons, 25 th October date of sowing gave significantly higher seed yield whereas 10 th November date of sowing gave significantly lower yield during 2011-12 (Table 2). With delay in sowing, yield decreased progressively from 1372 to 456 kg ha -1 (66.8 %). HPN 1 (1074 kg ha -1 ) out yielded the other two varieties during 2011-12 though, remained statistically at par. Warmer and drier weather conditions during 2011-12 were found to be favourable for the pest development which ultimately led to the severe reduction in yield.
Compared to 2010-11, the average reduction in seed yield was found to be 21.4, 32.7 and 67.5 % in first, second and third date of sowing respectively, with the overall reduction in yield by 40.5 %. Among varieties, the reduction was highest (51.5%) in Hyola 401 followed by HPN 3 (38.4 %) and HPN 1 (23.2 %) indicating that hybrids are more tolerant to aphid attack. Aphid population was also high on the crop sown on third date of sowing followed by second and first date during both the seasons. Earlier Dhaliwal et al. (2008) had also reported similar observations. Based on the present study, it was concluded that the mean temperature 10.8-12.2 and 10.2-13.3 o C, mean RH 66-71 and 47-74 % were conducive for aphid incidence and peak population build up, respectively. Since the aphid population was higher in late sown crops as compared to early sown, timely sowing of crops would be one of the important