Stress induced changes in the pests of tea plantations of sub Himalayan Terai-Dooars region: An appraisal

Abstract

Tea plantations in the sub-Himalayan region of North Bengal spreads over the Darjeeling foothills, the Dooars and Terai plains. This crop is attacked by not less than two dozens of insect and mite species. At least four of the sucking and six of the chewing pests are well established as regular pests causing substantial damage tothe foliage-crop. Changing abiotic factors in the last 50 years have beenmarked by a reduction in the total annual rainfall and its pattern, rise in the maximum temperature, use of synthetic chemicals as fertilizers and pesticides. Matched withthis, there is also a change in the pest composition and occurrence in this region.Tea is presently experiencing, on one hand, new invaders hitherto Unknown from the crop and on the other, a change in the pattern of pest incidence, tolerance levels, and behaviour.The tea mosquito bug (Helopeltis theivora) studied from the Dooars and Terai showed a higher occurrence in late monsoon and autumn as compared tothe earlier experience of their dominance during spring and earlymonsoon. Insome plantations, jassids (Empoasca flavescence) and thrips (Scirtothrips dorsalis) are becoming regular pests of some tea estates ofTerai and hill slopes.While the new folivores like the Sungma caterpillar (Orygia sp.), Arctornis submarginata, and green weevil are the pests in making, the pattern and proportion of occurrence of the relatively new defoliating loopers, Hyposidra talaca andH. infixaria isunprecedented, the other looper, Buzura suppressaria, known for last hundred years fromtea takes a second position when comparedwith themore vigorous,multivoltine and unmanageable Hyposidra spp.

Variability of pests, when collected from conventional tea plantations, where they have a regular exposure to the synthetic pesticides, as compared to those from the organically managed plantations, was evident for many morphological, biological and biochemical traits. Darker pronotum and higher fat content in the body of pesticide-exposed population of H. theivora with a higher tolerance (high LC50 value) to endosulphan and delta/cypermethrin were observed. The corresponding variability was also noted in the levels of the defence enzymesmeant for detoxification of pesticides. Three of the principal defence enzymes i.e. general esterases (Est), glutathione-S-tranferases (GST) and cytochrome P450 (monooxygenases: Cyt 450) were mostly higher in the pesticide-exposed specimens/populations. Variability at the level of Est-band patterns could be witnessed. Similar increase intolerance level could be observed in pesticide-exposed looper population with an enhanced titre of defence enzymes. Similar findings of elevated levels of esterases and monooxygenases were recorded in E. flavescence (Jassid) and S. dorsalis (thrips). Other aspects of variability were noted even at the level of the defence enzymes based on the host plants fed by loopers, jassid, and tea mosquito bug. Mite populations (Oligonychus coffeae), exposed to acaricides showed greater tolerance (highLC50) compared to the unexposed ones. The esterase level of these mites also changed for better adaptation and tolerance to an acaricide-charged environment.

An interesting change in egg-laying strategy was noted in H. theivora when exposed to insecticides. The exposed females avoided oviposition inthe toppart of the shoot and first four leaves and shifted its laying activity in the lower young leaves to its associated shoot part located in the second tier of the bush. A clear case of behavioural defence demonstrated for the first time by a tea pest to avoid pesticide spray. So, the intricacies of pest problems in tea is based on multi-dimensional factors, partly regulated by the abiotic agroclimatic stresses and partly by the ambience of the tea plantation and synthetic chemicals applied for their control.