BIOECOLOGY AND MANAGEMENT OF Lepidiota mansueta Burmeister (COLEOPTERA: SCARABAEIDAE)
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Date
2014
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AAU, Jorhat
Abstract
Field and laboratory experiments were carried out at Majuli river island and White grub laboratory, Assam Agricultural University, Jorhat to study the bioecology and management of Lepidiota mansueta (Coleoptera: Scarabaeidae) during 2011 to 2013. Experimental results confirmed the biennial life cycle of L. mansueta. The incubation period was found to be varied from 13-18 days with a mean of 15.80 ± 1.70 days. There were three larval instars. The duration of first and second instar were relatively shorter and it ranged from 38-42 (mean: 39.45±1.47 days) and 53-65 days (mean: 61.70 ± 3.53 days) respectively. The third instar grub showed prolonged duration which ranged from 532-556 days with a mean of 542.65 ± 7.60 days. The pupal duration was found to be varied from 26-28 days (mean: 28.60 ± 1.90 days). The total duration from egg to adult ranged from 666-702 days with a mean of 688.20 ± 9.84 days. The mean length and width of egg was 4.96 ± 0.03 and 3.93 ± 0.04 mm, respectively. The mean length and width of first, second and third instar grubs were 13.30 ± 0.95 and 2.47 ± 0.51, 24.10 ± 1.32 and 6.93 ± 0.74, 48.30 ± 1.62 and 10.30 ± 1.02 mm, respectively. The mean length and width of the pupae were found to be 32.60 ± 1.04 and 11.50 ± 0.68 mm respectively. Sexual dimorphism based on adult morphometrics revealed that females were significantly longer in body length (48.4 ± 1.03 mm) than males (43.4 ± 0.51 mm). Although the head plus thorax length of both sexes were identical, the abdomen was significantly longer in female (22.7 ± 0.89 mm) than in male (17.8 ± 0.43 mm). The length of the female alimentary canal (115.0 ± 0.12 mm) significantly differed from the male (94.0 ± 0.08 mm). The mean length of fore legs (18.0 ± 0.12 mm) and mid legs (23.0 ± 0.09 mm) were significantly longer in male than female (Foreleg: 17.2 ± 0.13 and mid leg: 22.1 ± 0.12 mm), whereas the hind legs were identical in both the two sexes. In field conditions, none of the plants were damaged by both sexes of the beetles soon after emergence from the ground. Host specificity test comprised of thirty different plant species conducted in laboratory conditions also revealed that adults of both sexes of L. mansueta were non- feeding. The alimentary canals of dissected adults did not show any food materials in their guts which clearly indicated the non-feeding nature of adults. Further, scanning electron microscopy images of mandibles and maxillae of both sexes of the beetles did not exhibit any dentations which indicate that the mandibles and maxillae were not fitted for phytophagy but probably used for digging of soil for burrowing.
In field, the first and second instar grubs were observed from April-May and May-July during 2011 and 2012 respectively. The third instar grubs were noticed in soil throughout the sampling periods with a peak population (4.2 and 3.7 per cubic meter) during August, 2011 & 2012.
Habitat selection by L. mansueta grubs studied in five selected ecosystems at Majuli revealed that grubs were significantly more abundant in grasslands which were situated near water sources (8.0, 6.5 & 6.0 nos./m3 during 2011, 2012 and 2013). In open grassland, the mean population of grubs was 4.6, 4.7 & 4.2 numbers per cubic meter during 2011, 2012 and 2013 respectively. Studies on abundance of L. mansueta grubs in grassland and cultivated lands showed significant preference for the soil depth of 0-10 and 11-20 cm. In grassland, grubs showed significant preference for the soil depths of 0-10 (3.18 ± 2.49) and 11-20 cm (1.48 ± 1.06). In cultivated field, the mean number of grubs recorded in 0-10 and 11-20 cm depths were 0.84 ± 0.62 and 0.96 ± 0.66 number, respectively. The grub population recorded in 0-10 and 11-20 cm were found to be at par with each other but significantly differ from 21-30 and 31-40 cm soil depths.
Correlation studies between L. mansueta grubs and soil physico-chemical parameters revealed that among soil physical parameters, bulk density (r = -0.804) and sand content (r = -0.482) had significant negative correlations. Among the soil chemical parameters, soil organic matter content (r = 0.834), available nitrogen (r = 0.802) and soil microbial biomass carbon (r = 0.781) had significant positive correlations, while, available P2O5 (r = -0.466) had significant negative correlation on grub population. Multiple regression analysis between grub population and soil parameters revealed that 92.7 per cent grub abundance could be attributed due to the combined influence of soil physico-chemical parameters.
The tuber damage caused by the grubs of L. mansueta recorded in potato varied from 22.20 to 30.84 per cent with a mean damage of 28.95 ±1.39 and 27.33 ± 3.14 per cent during 2012 and 2013, respectively. Plant damage in sugarcane ranged from 14.81 to 26.46 with mean damage of 19.25 ± 3.76 and 20.27 ± 1.38 per cent during 2012 and 2013, respectively. The plant damage in green gram and corm damage in colocasia found to be varied from 12.94 to 20.47 and 14.51 to 19.12 with mean damage of 16.02 ± 1.74 & 17.55 ± 1.49 and 17.05 ± 1.33 & 15.52 ± 0.82 during 2012 and 2013, respectively.
Among the six insecticides tested for their effectiveness against L. mansueta grubs in potato, fipronil 5 SC @ 150 g a.i./ha recorded lowest per cent of tuber damage on weight basis (5.24 and 4.57%) and this treatment was found to be at par with fipronil 5 SC @ 100 g a.i./ha (5.91 and 5.09%), chlorpyriphos 20 EC @ 400 g a.i./ha (5.55 and 6.20%) and quinalphos 25 EC @ 400 g a.i./ha (6.14 and 6.72%) but significantly superior over rest of the insecticides. On number basis, fipronil 5SC @ 150 g a.i./ha registered lowest tuber damage (4.20%), and it was at par with chlorpyriphos 20 EC @ 400 g a.i./ha (5.25 %) and quinalphos 25 EC @ 400 g a.i./ha (5.66%) during 2012 and chlorpyriphos 20 EC @ 400 g a.i./ha (5.18%), fipronil 5SC @ 100 g a.i./ha (5.99%) and quinalphos 25 EC @ 400 g a.i./ha (6.43%) during 2013, but significantly superior over rest of the treatments. As regards to tuber yield, all the insecticidal treatments exhibited statistical parity except amamectin benzoate 5% SG @ 12.5 g a.i./ha. However, highest benefit cost ratio was recorded in chlorpyriphos 20 EC @ 400 g a.i./ha treated plots (2.50 and 2.62) followed by quinalphos 25 EC @ 400 g a.i./ha (2.23 and 1.85).
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