POPULATION DYNAMICS AND CROP LOSS ASSESSMENT OF MAJOR INSECT PESTS OF CUCUMBER (Cucumis sativus)
Cucumber (Cucumis sativus L.2n = 14) is one of the widely cultivated summer vegetable crops from the gourd family of Cucurbitaceae which is native to India. The climatic conditions of northeast India especially Assam are highly conducive for reproduction of insects, and this region has been considered to be a biodiversity hotspot. Moreover, this region has huge potential for the production of vegetable crops especially cucurbits, but insect pests like fruit flies, pumpkin beetle and sucking pests are major limiting factors for successful cultivation of cucurbits like cucumber etc. Therefore, understanding population fluctuation in the field in relation to losses caused by insect pests are a function of their population dynamics which needs to be thoroughly studied. In view of above factors, an investigation on “Population Dynamics and Crop Loss Assessment of Major Insect Pests of Cucumber (Cucumis sativus)” was carried out at Horticultural Farm and RNA laboratory of Agricultural Biotechnology, Assam Agricultural University, Jorhat. The results showed that during 2019 -2021 summer season, the red pumpkin beetle first appeared on 24th April 2019 and 7th May 2020 with initial population of 4.51 and 2.23 beetles per plant, respectively. However, maximum beetle populations were recorded on 8thMay 2019 and 3rd July 2020 with 6.15 and 6.52 beetles per plant, respectively. Similarly, during 2019-2021, the fruit infestation by fruit fly was recorded first time on 13th June, 2019 and 26th June ,2020 with per cent fruit infestation of 10.24 per cent and 10.75 per cent, respectively. However, highest level of fruit infestation by fruit fly was observed on 25th July, 2019 and 8th August 2020 with 46.86 and 41.3 per cent fruit infestation, respectively. The data on relative abundance of different insects of cucumber during 2019-2021 revealed that the relative abundance of fruit fly species, Bactrocera dorsalis (62.27%) was the highest followed by B. tau (15.68%), B. cucurbitae (10.73%), Dacus longicornis (2.47%), Coccinella transversalis (2.45%), Aulacophora foveicollis (2.20%), Micrapsis discolor (1.56%), A. frontalis (1.16%), and Coccinella septumapunctata (1.08%). During 2019 and 2020, the Simpson-Index diversity showed highest value for fruit fly (0.97 and 0.94) followed by predacious coccinellid beetles (0.67 and 0.71) and red pumpkin beetle (0.55 and 0.64), respectively. Whereas, Shannon-Wiener index diversity (H’) during 2019 and 2020 observed highest in fruit fly (1.65 and 1.85) followed by predacious coccinellid beetles (1.15 and 1.05) and pumpkin beetle (0.68 and 0.85), respectively. The results of yield loss studies during 2019-2020 revealed that the per cent fruit infestation was highest in untreated control plot (30.73%) compared to treated plot (26.8%) with fruit yield of 1223.91 kg/ha and 2311.83 kg/ha, respectively. The yield loss in untreated plot was recorded as 1087.91 kg/ha with 47.05 per cent avoidable loss in yield. Whereas, per cent yield increase over control during 2019-2021 (summer season) was observed as 88.88 per cent. However, economic injury level (EIL) of fruit fly with respect to cucumber was determined as 15.27 and 10.40 per cent with per cent fruit loss of 0.743 and 0.745 during 2019 and 2020, respectively. The optimal phylogenetic tree of 10 fruit fly specimens collected from different locations grouped into two clusters (Cluster A and Cluster B) of two specimens out groups namely B. rubigina and D. longicornis with branch length of 0.0724. In cluster-A, highest genetic distance of 0.047 was recorded between B. rubigina and B. dorsalis. Similarly, in cluster-B the highest genetic distance of 0.0682 was recorded between D. longicornis, B.tau and B.cucurbitae collected from different locations.