STUDIES ON SOME ECOLOGICAL ASPECTS AND MANAGEMENT OF COTTON FLEA BEETLE, Podagrica puncticollis Weise (COLEOPTERA: CHRYSOMELIDAE) ON COTTON (Gossypium hirsutum L.) IN METEMA DISTRICT, NORTHWESTERN ETHIOPIA

Abstract

Cotton flea beetle, Podagrica puncticollis Weise is among the most chronic and economically damaging insect pests of cotton in Northwestern Ethiopia. Although this beetle is known to cause significant crop losses, the ecological aspects and management of this insect pest have not been adequately assessed and documented. Thus, five studies were conducted in the area. The first study was conducted in Metema district to assess the cotton growers’ perception and knowledge of cotton flea beetle by means of questionnaire. For the survey 200 cotton farmers and 12 experts were interviewed. The results showed that farmers were well aware of cotton flea beetle and other pests of cotton and measured them the main problem to cotton production. Majority of the respondents (89.9%) were ranked cotton flea beetle as the most damaging insect pest during main cotton cropping season. About 75.5% of the farmers indicated that they protected their crops against cotton flea beetle using cultural practices and insecticides. The second study was undertaken to identify the host range and over-seasoning strategy of cotton flea beetle. The host range study of cotton flea beetle was undertaken in ten kebeles of the district. At least three fields were visited every 15 days, in each kebele for host plants as well as to determine population density and percent leaf damage by adult flea beetle, at different growth stages of cotton. The results revealed that a total of 11 host plant species of cotton flea beetle were identified in the cotton growing areas of Metema throughout a season. Indigofera longibarbata (Fabaceae), Hibiscus articulatus, H. cannabinus, H. vitifolius, Abutilon figarianum, Sida alba and S. urens (Malvaceae), Bidens pilosa and B. setigera (Asteraceae), Corchorus olitorius and C. trilocularis (Tiliaceae) were found to be common host plants of cotton flea beetle. The over-seasoning strategy of cotton flea beetle was investigated by focusing the crop fields (cotton, sorghum and sesame) and non-crop habitats (woodlots, patches of alternative hosts, crop field border edges, fallow lands and grasslands) nearby to cotton fields. During dry season, 10 m x 10 m plot for each habitat type was reserved randomly to sample number of aestivating beetles. On each sampling date, a 1 m2 sampling unit of leaf litter was collected from the reserved site of each habitat. Soil sample was taken to determine the depth at which the insect aestivate during the off-season. To know whether the adult beetle is active or not during the dry season observations were made on the emergence of flea beetle from the soil by using pot experiment. The over-seasoning strategy of cotton flea beetle, measured mainly in terms of the number of aestivated adult beetles recorded under the leaf litters of all studied habitats. The highest (14.8) average number of aestivated beetle per m2 of leaf litter was recorded in the Sterculia setigera tree, while the least (0.65) in sorghum grown fields. The adults of cotton flea beetles also survived the dry season at highest (up to18.8 adult beetles) densities underneath loose barks at the upper parts of the trunk of standing trees mainly on loose barks of S. setigera. The cotton flea beetle survived between 10 and 30 cm depths of soil in the aestivation sites from January to May usually in areas like woodlots and field border borders nearby cotton fields, and also within the cotton fields. The third study was undertaken to determine the flight pattern, cotton phenology-dependent distribution and natural enemies of cotton flea xviii beetle. The flight pattern of cotton flea beetle was monitored by means of white and yellow water traps installed in Gende Wuha Research Station of Gondar Agricultural Research Center, Metema district. Count of the adult beetle was done weekly. The results showed that significantly larger numbers of adult flea beetle were caught in yellow water trap than in white water trap. The flight activity of cotton flea beetle had two peaks. Numbers of adult beetle caught by all methods had significantly (P≤0.01) positive association with temperature, whereas negative relationship with rainfall. Also, an insignificant negative correlation was noted by relative humidity. The cotton phenology-dependent distribution of cotton flea beetle study was undertaken in 10 kebeles of Metema district. During that period, at least three fields were assessed in every 15 days in each kebele to determine population density and percent leaf damage of adult flea beetle at different growth stages of cotton. In phenology study, the number of adult beetles recorded on 50 cotton leaves revealed that the cotyledon stage recorded highest (5.12) number followed by first true leaf stage (4.74) and the least (0.02) number at second picking. In case of leaf damage, the highest (34.6%) leaf damage on 50 cotton leaves was recorded at cotyledon stage and reasonable (22.7%) severity of leaf damage at second several true leaves. Generally, the degree of occurrence and leaf damage caused by cotton flea beetle was decreased with the increase of the age of the cotton plants. Beside with phenology study, natural enemies of the pests were assessed and documented. The natural enemies associated with cotton flea beetle in cotton field, identified were spider (Lycosa sp.), ladybird beetles (Cheilomenes sp.), green lacewing (Chrysoperla sp.), black ant (Camponotus sp.), Predatory bugs (Coranus sp.), earwig (Labia sp.), field crickets (Gryllus sp.) and hister beetle. The fourth study was conducted to evaluate the reaction of cotton varieties to cotton flea beetle incidence. The treatments consisted of 12 cotton varieties laid out in randomized complete block design (RCBD) with three replications. The results showed highly significant (P≤0.01) difference among cotton varieties in the numbers of adult beetle they hosted and the injury they sustained in percent leaf area damage and number of shot-holes per attacked leaf at 15, 22, 29, 36 and 43 days after sowing (DAS). Cucurova, Local, Ionia and Acala SJ-2 cotton varieties showed highly susceptible response, while Candia, Sille-91 and Deltapine-90 were moderately susceptible varieties to cotton flea beetle. However, Bulk-202, Delcero and Claudia were relatively more tolerant varieties followed by CCRI-12 and Cuokra. Moreover, highly significant differences (P≤0.01) were observed among cotton varieties in seed cotton yield. Yield obtained from all cotton varieties varied from 602.36 to 1644.71 kg per hectare. The highest seed cotton yield was obtained from Bulk-202 and the lowest from Local variety. The fifth study was conducted to determine the effect of integrated use of insecticides, sowing dates and seed rates on infestation level of cotton flea beetle and cotton yield. The treatments consisted of two insecticides (Cruiser 350 FS for seed treatment and Carbaryl 85% WP as foliar spray, and untreated check), three sowing dates (early, mid and late) and three seed rates (low = 15kg ha-1 , medium = 20 kg ha-1 and high = 25 kg ha-1 ) laid out in randomized complete block design (RCBD) with three replications. Time of sowing cotton included at the onset of rains, and at 2 and 4 weeks later. The results of the study showed significant (P≤ 0.05) differences among treatments in number of cotton flea beetle per plant at 15, 21 and 30 DAS, number of shot-holes per damage leaf at early three stages and percent damage per 10 plants at early 4 stages of cotton plant. Commonly plots received treatment groups combined with insecticides showed good performance in reducing number of adult beetle, number of shot-holes and percent damage plants as compared to untreated check. Commonly, the rates of flea beetle number as well as damage observed on cotton leaf and on cotton plant as a whole declined with the increased age of the cotton plants and delayed sowing. Significant differences were also observed among treatments in some agronomic features xix of cotton. Similarly, plots that received treatment groups combined with insecticides also produced notably maximum number of bolls per plant and high seed cotton yield ha-1 . Seeds treated with Cruiser early sown with high seed rate showed significant differences and produced highest (35 per plant ) number of bolls and more (2451kg ha-1 ) seed cotton yield than the other treatments. But, untreated late sown plots plus received low seed rate gave the least (11bolls per plant and 914.67kg ha-1 ) results compared to all other treatments. The overall studies in this dissertation offer an opportunity to ecologically wide association studies, to investigate the population dynamic of cotton flea beetle, to develop long-term monitoring system for the population movements of this insect pest, to estimate the severity and timing of pest outbreaks, to enhance the decision support systems, to designing a suitable intervention and successful integrated pest management (IPM) strategy for the district.