56 Population change of Helopeltis theivora and pattern of insecticides use in the Dooars region of North Bengal: a historical perspective. •!• Records and evidences of population change of Helopeltis theivora in the Do oars Data on infestation of the tea mosquito bug (H. theivora Waterhouse) were collected for 24 years (1980-2004) from different tea estates (T.E.) situated in the different tea subdistricts of the Do oars region of North Bengal. The intensity of incidence pattern was divided into three categories, low: (1-10% infestation), moderate (ll-20% infestation), and severe (above 20%). The study in general indicated that intensity of H. theivora infestation was low in 70 to 90% T.E. from1980 to 1990. During this period only very few T.E. suffered severely from attack; of this obnoxious pest. After that, tlie severity of the infestation of H. theivora in the Do oars increased from year after year. During 1991 to 1995, 20 to 35 percent tea estates suffered from moderate type of damage by the pest. Outbreaks of H. theivora occurred more frequently causing major damage of tea in the Dooars from the year 1997. It was noted that 70 to 83% tea estates in the Dooars could be considered under moderate to severe categories during 2000 to 2004. Since then the severity of the infestation of H. theivora in the Do oars showed an ever-increasing trend (fig.1 ). 0 By and large tea estates of Kalchini, Damdim, Nagrakata and Binnaguri subdistricts suffered severe damage by this pest. The rest subdistricts of the Dooars such as Chulsa and Damdim showed low incidence pattern of H. theivora (Fig. 2 to 7).' Among the different subdistricts of the Dooars tea plantation, the tea estates 0f 0 Kalchini subdistrict were marked as high Helopeltis prone zone. H. theivora was 57 1 00% .,---,-r-1 80o/o Cf) ~ ~ 60°/o Cf)~ ~ 40 °/o < ~ ~ 20o/o 0°/o +--'- '80- '86- '91 '92 '93 '94 '9 5 '96 '97 '98 '99 00 '0 1 '02 'Q3 '04 '85 ' SO o Lo\v infested T E o IVI oderte infested T. E • Severe infested T. E. Fig 1. Percent of tea estate (T E.) sho\v1ng d1fferent levels of 1nfestabon by H thezwra frorn the Do oars tea pl antatlon more or less ubiquitously distributed throughout the tea estates of Kalchini subdistricts of the Dooars. During 1980 to 1985 among the nineteen different T.E.s of ' Kalchini subdistrict, infestation of H. theivora was quite low, excepting in some, where it extended to moderate level (15.79%). Till that point of time no severe infestation was reported. The complexion however changed during the period 1986 to 1993 when 10 to 15 % of the T .E. showed severe attack by H. theivora. After that the severity of infestation gradually increased till the year 2001. During this time more than 50% of the T.E. ofKalchini have suffered from severe attack by H. theivora and rest ofthem maintained moderate level of infestation (Fig.- 5). The tea estates namely Central Dooars, Bhatkawa, Rajabhatkawa, Bharnobari, Satali, Radharani, Turturi, Rydak and Chuapara were marked as high prone to H. theivora attack. Depending on the intensity of damage Nagrakata, Dalgong and Binnaguri could be placed next to Kalchini subdistrict (Fig. 3, 4 and 2). An overall observation of the three subdistricts in question depicted a steady growth of infestation from low densities in 1980 to 1999 to a peak by 2002-2003. The general trend and data since 2003 indicated that severity of H. theivora infestation was more or less similar in Kalchini, Nagrakata and Dalgong subdistrict (F. 5, 4 and 2). The tea estates such as Dalmore, Dalgong, Hantapara, Lankapara, Garganda, Makrapara and J oybirpara tea estates in the Dalgong subdistrict; Jiti, Chengmari, Hope, Looksan, Grassmore, Ghatia and Nagrakata tea estates in Nagrakata subdistrict, and Ambari, Binnaguri, Newdooars, Banarhat, Maraghat, Karbala, Red ballk and Dina tea estates in the Binnaguri subdistrict regularly suffered from the ravage of H. theivora in most season of the year. 58 ,- I uun;., • ill • I - I I • (/) xu•y,, "" I I:-;; ....v I ....~ (o ll 0/o (/) f1 v I• ~n ·v.. I v~ ~ :! U% II u• y,, . ,,~ ~ ~II- Xh- 'II ·tn 'l-l IJ' ·~ . .,,- IJX ·~J I~~ ... II~ ... n-l X" "\}1) Year 0 Lm' 0 1udt<ra tc • St<\t>l~ Fig .., Percent of tea e tate (T E) sho\\i ng different le,el oftnfestatt on l1\ H lhetmru from tea plantati on of Btnnaguri subdt stn ct 0 ,.. ,.. I UU / o ~ II • • I - I I XU'% - ' ltl (/) ....v 6U' Y,, ~ .... I (/) i '-I ~U' Yo ~ ' ~ ~ 2U'Yn 1- - I- 1-- 1- 1- .., ~ U'Yo - - - - - - Year 0 LH\\ 0 1\1mil • rate • l'\ Cl~ Fi g 3 Percent of tea e tate ( T E.) sho'' i ng different Ie' el of i nfe tati on b\ H 1/lemmt from tea plantati on of agrakata subdi ·tri er l tttt• .. ~ 1 • • I • I 'n I I • kU I I 11 (/) ()U o ! ~ I v ..... ! ....~ -tU uu (/) '-I 2H" o ~ ~ ...., ~ n•o 1- - h. h- - - - - I- - - - - ,, '~ '" tH Year 0 um 0 1\l mkra te • C\C I~ Fi g. -t . Percent of tea estate (T.E.) showin g different levels of infestati on b) H.1hetmru from tea plantati on of Dalgong subdistri ct 100% ,.. _. 80% f.- • I I • I • 60% UJ 40% ...,:. 20% 1- - 1- 1- !-.,- 0% "- 80- 86- 91 9~ 93 94 95 96 9- 98 99 oo 01 02 ·o3 04 85 90 Year O low o Moderate • Severe Fig .:; Percent o ft~n e tnte IT E) ~hO\\Ing d1fferent le ,e l ~ t)flnfe-..tati on b' H then·oru f10m tea plnntnti on of Kalc l11ni subd1 stnct ,... ,- ,- t::: ,... ,- 100% - - r- - • • • I I r- 80% • II f- f- l . 60% f- UJ 40% ~ 20% ho 1- '-1- '- 1- 1 - ~-~-- ... o......c..........,:.-.'- I;::.., 0% 80- 86- 91 92 93 94 95 96 9- 98 99 00 01 02 03 04 85 90 Year O low 0 Moderate • Se vere F1 g b Percent of tea est<l te ( T E ) sho" i ng eli ffe1en t I e' el s of 1nfe ~ tati on h\ H thl'twru from rea plantation of Damd1m subdi strict ... ... r"" "" - - "" "" II - • xuu .. , r) uo.. UJ 11 ,_:..ttl u 1,2 ,,- 'IX I~ I Ill n~ 111 1'-l XII- Xh- " I ·n ' '-l "" 'K' ·~· X-\ ·~· Yea•· 0 L~m 0 lode rate • e' ere Fi g 7 Percent of tea estate (T E) shO\\ ing different le\ el of infestation b~ H thetmru from tea plantati on of Chulsa Subdistrict Some relief from H. theivora attack was evident m the Damdim and Chulsa subdistricts (Fig. 6 and 7). In general intensity of H. theivora infestation increased by step from year to year from the extreme western Dooars to the eastern Dooars. History of advent of the tea mosquito bug (H. theivora) in North Bengal has become the central problem for the tea planters specially those of the Dooars. Intensity of infestation of the pest was low from 1980 to 1990 possibly due to DDT dusting as a routine practice to combat the pest problem during the early forties to seventies, (Mukerjea, 1962). During the period 1991 to 1995, the severity of infestation of H. theivora increased giving it the status of a major destructive pest of tea in the Dooars. It was noted that 70 to 83% tea estates in the Dooars suffered moderate to severe categories of H. theivora damage during year 2000 to 2004. In a similar obsertation on Nilgiri tea plantations, Sudhakaran (2000) mentioned that during 1950's the menace of H. theivora was rather low and its damage could hardly be regarded as causing loss of crop but after four decades it had reappeared i.e. during 1990's in an alarming way in South India. The phenomenal rise in H. theivora population might be due to several reasons. The changed agro-practices and large scale deforestation might have influenced its severity (Gope and Handique 1991). Whether changes, environmental complexity, anthropogenic activities and reduction of natural enemies of the target pest resulting from routine use of insecticides might were responsible to a large extent for severity of infestation (Das, 1957). Observations by Das (1965), Das (1984) and Gurusubramanian et al., (2005) on tea, and that of Entwistle (1964), Gibbs and Leston (1970) and Smith (1978) on other crops have substantiated that a higher 59 incidence of Helopeltis occurred in crops if the plantation area was located at the forest fringes or was interspersed with forest plants due to the presence of alternate or alternative silvatic hosts of H. theivora. The region wise variation of severity of attack by H. theivora in the Dooars was also documented in earlier studies of Andrews (1999), who reported that the pest was worst towards the east Dooars and showed an intense occurrence in the tea estates away from the hills. This is the area occupied by the grey sandy loam of the Dooars. Das (1957) further tried to relate local conditions and vegetation as influencing the incidence. The fact that areas of eastern (Kalchini) and central Dooars (Dalgong and Nagrakata) that are bordered by forests remain more or less permanently infested by H. theivora bears out the above conjecture. · . •!• Use pattern of insecticides in tea in the subdistricts of the Dooars In the Dooars, the control of insect pests is predominantly done by endosulfan, quinalphos, acephate, chlopyriphos, monocrotophos, cypermethrin and deltamethrin for last several decades, but subsequently oxydemeton methy~ lamda-cyhalothrin, etofenprox, alphamethrin, fenpropathrin, profenfos, thiomethoxam and irnidacloprid have been introduced in tea protection since 1999.