Beauveria bassiana: as a potential microbial biocontrol agent for tea mosquito bug, Helopeltis theivora Waterhouse (Hemiptera: Miridae) in Dooars and Darjeeling, India

Background In the present study, the efficacy of two isolates of Beauveria bassiana namely, BKN20 and BKN1/14 was evaluated against the tea mosquito bug (TMB), Helopeltis theivora, Waterhouse (Hemiptera: Miridae) damaging harvestable shoots of tea plants in the Dooars and Darjeeling regions of West Bengal, India. Results Laboratory study revealed that, in both isolates, BKN20 was more pathogenic than BKN1/14, exhibiting 76% mortality of the test insect. The BKN20 isolate was formulated as an aqueous suspension (5%AS), and evaluated against TMB through micro-plot trials in tea plantations. The Micro-plot field study revealed a maximum of 72.19% reduction in the shoot damage due to TMB in plots sprayed with a 1000 ml/ha concentration of BKN20 5%AS containing 2 × 107 conidia/ml, as compared to the synthetic insecticide (Thiamethoxam 25%WG), where a 63.12% reduction in the shoots was recorded. Furthermore, different concentrations of the formulated BKN20 5%AS were evaluated against the test insect at 2 locations of tea gardens in the Dooars and Darjeeling regions. The results from both locations revealed that 1000 and 1200 ml/ha concentrations of BKN20 5%AS (each concentration containing 2 × 107 conidia/ml) significantly (p < 0.05) reduced the TMB population and they were more effective than Thiamethoxam 25%WG (120 g/ha). However, non-significant differences in crop yields were recorded. The formulation BKN20 5%AS was found to be non-pathogenic to non-target insects, i.e. natural enemies present in the tea ecosystem. BKN20 5%AS had no phytotoxic effect on the tea leaves, with acceptable organoleptic attributes. Conclusion The BKN20 isolate could be commercialized as an alternative microbial insecticide to reduce the load of chemical insecticides in the tea ecosystem.

Infestations Of Two Major Pests Of Cocoa, Conopomorpha Cramerella and Helopeltis Theivora Under Natural Condition

The Cocoa pod borer (CPB), Conopomorpha cramerella (Snellen) (Lepidoptera: Gracillariidae) and Helopeltis theivora (Miller) (Hemiptera: Miridae) are the major pests of cocoa in the South East Asian Region. C. cramerella started to infest cocoa pod which more than 7cm in length, whereas H. theivora started to infest the pod at a very early stage of pod development. The present study aims to determine the relationship and the degree of damage severity of these two significant pests under natural condition. No insecticide spraying was carried out throughout sampling occasions; however, other management practices were implemented. Destructive sampling of fifty cocoa pods at various lengths was carried out at fortnightly basis.Percentage of infestation, number of CPB eggs, entry, and exit holes were recorded for an individual pod. The results obtained denoted that H. theivora begin attacking pods at a young age and the majority of the pods were infested when they reached 8 cm in length. The percentage of damage severity increased as the pod developed. In contrast to H. theivora incidence, the number of C. cramerella eggs were recorded higher when the percentage of H. theivora incidences were low. The highest mean number of C. cramerella eggs were recorded when the pod length is more than 150 mm. Although infestation of the C. cramerella was more serious, management of both pests must be started when the pod length is less than 8 cm. Integrated control approaches must be targeted to both pests at the appropriate time, to reduce significant losses.

Biology, feeding and oviposition preference of Helopeltis theivora, with notes on the differential distribution of species of the tea mosquito bug species complex across elevations

Cocoa (Theobroma cacao L.) is an important beverage crop and commercially grown as a plantation crop. With the changing climate, the tea mosquito bug species complex, viz., Helopeltis theivora, H. bradyi and H. antonii, is emerging as a major threat to cocoa cultivation in India. Among the species of this complex H. theivora is responsible for causing considerable damage. The present investigations were carried out to find a weak link in the life cycle of H. theivora so it can be managed effectively. Specimens of the tea mosquito bug were found to first appear during the first week of September in South India. Helopeltis theivora requires on average 29.28 days to complete its life cycle on cocoa. The highest level of natural mortality was recorded in the first-instar nymph. The total developmental period of the fifth-instar nymph was significantly longer than that for the other nymphal instars. The sex ratio reflects that the population is highly female-biased, which may contribute to the dominant nature of this species in the cocoa ecosystem. Observation of the feeding and oviposition behavior of H. theivora revealed that the insect prefers to feed and oviposit on developing pods rather than on leaves and shoots. Analysis of the species distribution of tea mosquito bug at different elevations revealed that cocoa gardens situated less than 300 meter above mean sea level are dominated by H. theivora, whereas gardens situated more than 300 meter above mean sea level are dominated by H. bradyi. This separation of species across elevation may be driven either by abiotic or biotic factors.